300b lovers

Thanks for the update, Don.

Yoder, there was a lot of progress sonically after the first prototypes. What we have now is actually simpler in some ways. We went for a pair of spacious monoblock chassis mostly to simplify assembly, and to improve cooling a bit.

HI

No Spatial is currently only making the Raven and Blackbirds... 

@yoder  I have played the mono 300b amps with 86 dB speakers and it drove them with no issue.  I am sure your 93 dB ones will be just fine.  It sounds like a 100 watt tube amp really.....  Yours runs a bit hot as it was the first build for proof of concept.  The final versions.. well they can play all day long at the show and you can put your hand on any transformer or the top panel and it is barely more than warm.  Lots of improvement since the first prototype build.  If you ever have trouble with that prototype send me a PM and I will of course try and help you.

Yes, Don and I are working on a medium-price (by high-end standards) amplifier, most likely a stereo integrated amp. Not much to say about it now, since it's mostly conceptual at this point. If people are looking for value, I think Spatial may still be making Don's previous Kootenai amp, which is a superb amp.

@donsachs 

The stereo prototype is a very fine looking and sounding amp, so the final versions must be amazing!  I went all in with this amp being my first 300b after reading up much on 300b's elsewhere prior to seeing this thread.  I did not comment much on the sound as I don't have previous 300b experience and am not great with such descriptions.  Also maybe not highly relevant to current models, but probably as close as I will get!  It does run a bit hot.  Not so much heat up the room to me, but the transformers are hot to the touch and it does need space around it to radiate.  Definitely not to be tucked into a small cabinet, but probably no 300b is.  To hide all these great looking tubes wouldn't seem right either.  The sound to me mostly matches up with how you previously described the prototype monoblocks (if I recall correctly something like SET-like tone and vocals, but the punch of push/pull).  I have yet to own a 300b SET, but for the tube tone I would say it does compare to one of my favorite SET examples so far - it compares favorably with my 45 tube monoblocks for tone such as guitar tone.  It does not get too soft or weak in bass response as I've experienced with some non-300b tube amps.  Bass is deep, powerful, and I think has very good pace.  Power wise with no measurements just judging by volume adjustment level, output compares favorably with a couple other well regarded 30 WPC tube amps that I have or had.  Right now I have 97.5 dB 6 ohm speakers in a medium sized room so no power challenge there.  I will have ~next year some 93 dB 7 ohm speakers in a large room and am curious to see how it will match up.  I only asked about more entry level options as that is usually my gateway to learning/hearing about the top level gear.  

@yoder 

Thanks for the kind words.  The original stereo one was a proof of concept of the circuit.  I hope yours is running fine and making you happy.  The final production mono block version is considerably better in pretty much every way, but your stereo one is a lovely amp.  It runs a bit hot for my taste, and I could not fit all of the power supply and signal path refinements into a smaller, stereo chassis.  That said, I was very impressed with the amp, which, of course, led me to keep refining things into the final version.  For those who are interested, yes, there will be a more "entry level" product coming.  I am about to start prototyping.  This is a 300b lover's thread, so not really appropriate to discuss in detail, but it will have kt88 outputs, push pull in triode, and DHT drivers.  Similar circuit ideas to the Blackbird and your stereo amp, and obviously scaled back a bit to fit in a stereo chassis.  Fully balanced of course.  Probably by year end.  I have no idea of price, but it will be in the reasonable range and about 30-40 watts/ch.

There are 3 or 4 sets of the final production Blackbirds in the world.  One is probably still in the boxes (go figure) as the person was building a new house or some such.  There is one guy who loves his and posted on the spatial audio circle I believe.   There are a few sets of the prototype ones in the smaller chassis with 6V6 drivers floating around and people like them.  But no one is really active on chat boards.  C'est la vie.   A review pair will make its way into someone's hands this year most likely.  The Raven preamp has had more people posting because it costs less and there are around 30 of the production ones out in the real world.

I have one of or the original stereo prototypes. I discovered it because of this thread though didn’t expect to be able to own one. I did see these amps (Blackbirds though they may not have referenced the name) brought up on another forum when I was looking up 6V6’s and it was brought up as a modern example/option using 6V6’s as drivers. I guess they didn’t know production version changed to KT88’s. I have some NOS matched pairs on order and am curious to see what differences they make. I still don’t know squat about tube design, but when I first got it I didn’t really know the difference between them and say similar looking 6SN7’s, but they seem like a great tube in their own right. 
 

There are a couple owners of the mono prototypes in another circle. I don’t think those owners will ever willingly let them go. The guy I got my stereo amp from definitely wouldn’t have sold it if he hadn’t gotten the monoblock prototypes. 
 

I still see Don Sachs preamps referenced and also notice Raven brought up as a top notch preamp option, even by non-owners.  There is definitely lots of talk about amps, but I see that people are more likely to roll through various preamps and DACs while holding onto their amp and speaker pairing once they’ve found a good one.  Other than not being many Blackbird 300b owners out there, there are many who don’t know about Don Sachs products, and many that do don’t associate with amps or especially 300b amps. Also experienced audiophiles I’m talking with in person who aren’t familiar with Spatial Labs. Some audiophiles don’t even visit forums at all to see such information if it is there. So only a few owners out there and I think there are a lot of folks out there that don’t know about the Blackbirds to chat about them. 
 

I hope Southwest Audiofest went well and generates some buzz. Has to be taken with a grain of salt due to all the variables, but always interesting to catch impressions from those who visit the feats. Too far for me, but I’m glad Gary and Lou got it going. I would definitely stop by and visit if ever at Axpona. 
 

$20k for amps is pretty unattainable for many consumers, but I’m glad there are those who can afford them so they can potentially be available on the used market some day. Are there any thoughts to a more entry level amp that could also pair well with the Raven as well as Spatial Labs speakers or similar, say 95 dB or better as previously referenced as a good sensitivity for ~20 WPC?

Certainly a great benefit in a phono preamp where you have to overcome the wall of noise in the very first stage, and yes, cascodes are more linear than pentodes. The EF86 mike-preamp pentode so popular in the Fifties and Sixties is now very expensive and hard to get, so cascodes make more sense today.

Don’t need all that gain in a power amp in the absence of feedback, but if we ever needed feedback, yes, that’s a good way to get it.

I completely agree about the benign nature of low-order distortion. Most of all, it reduces nasty high-order IM distortion which is objectionable and obviously electronic-sounding. I also agree about the heavy 2nd-order distortion of SETs, which limits their effective dynamic range. Clever design techniques can mask the 2nd-order distortion (like coupling caps with complementary sonics) but the IM distortion remains, and is audible with symphonic, choral, and heavy rock music. (Music with a sparse spectra, like jazz quartets or chamber music, doesn't expose IM distortion, but music with a dense spectra turns IM distortion into a wall of noise that goes up and down with the music.)

Since transistors are notorious for high-order distortion, cascoded differential sections are about the only way to tame the things, while also getting rid of nonlinear Miller capacitance. The classic John Curl topology.

The even-order distortion of the driver section is cancelled in the primary of the interstage transformer, reducing driver distortion even further.

@lynn_olson Actually if the circuit is fully balanced/differential from input to output, even orders are cancelled at every stage along the way. In this manner distortion is compounded less from stage to stage.

The result is the 3rd harmonic is the dominant distortion product rather than the 2nd. Many people do not realize that the ear treats the 3rd in much the same way as the 2nd; its the only odd ordered harmonic that is musical to the ear. The 3rd is very good as masking higher ordered harmonics.

BTW, any properly functioning analog tape machine will have a 3rd harmonic as its dominant distortion product as the tape approaches saturation.

Mathematically this type of distortion can be described as a 'cubic non-linearity' as opposed to the 'quadratic non-linearity' of an SET. As Daniel Cheever pointed out in his paper from 1989, its important that the harmonics fall off on an exponential curve. Both an SET and a fully balanced amplifier can do this (its part of the reason people regard SETs as musical despite their many failings). The advantage of a balanced circuit is harmonics fall off on a higher exponential rate so higher ordered harmonics are at a lower level than seen in an SET; its inherently lower distortion.

This allows the distortion signature to be innocuous.

The advantage is greater power output with lower distortion. So at any power level an SET can make, in a circuit using the same tubes the PP amp can have vastly lower distortion and so be smoother with greater detail, since distortion obscures detail. 

By the way, cascoding the input section is how you get both voltage resistance and a hundredfold reduction in Miller capacitance. Since Miller capacitance in transistors is grossly nonlinear, this is a very good idea. Tubes exhibit Miller capacitance too, but it is an order of magnitude lower, and it is stable and predictable instead of being nonlinear. There are cascode tube circuits as well, but they offer no improvement in linearity (unlike transistors), and are mostly seen in phono preamps and FM tuner input sections. In the tube universe, pentodes behave similarly to a pair of cascoded triodes, and are more commonly used when a cascode is called for.

We've been using a differential cascode circuit for decades. It has several advantages over pentode or cascade operation; one obvious one being a reduction in the need for a coupling capacitor. Differential circuits benefit from the devices (tubes in this case) having a lot of gain. That increases the differential effect so distortion cancellation is improved and noise is reduced. A differential cascode circuit can have a lot of gain.  

This circuit can have a very high CMRR even in a tube embodiment. Its linear enough open loop that you can run it without feedback (something you can't do with pentodes), but if you want to do it, its possible to operate it in ultra-linear mode, where the plate Voltage of the top tube is applied through a divider network to the grid of the same tube. You can do that with a pentode too, but you can't run a pentode zero feedback and the amount of feedback available in UL mode is limited. 

Since a cascode circuit is lower distortion, another advantage is it can be used in a circuit with feedback and result in less higher ordered harmonic generation than if a pentode is used.

By the way, there’s a very intelligent discussion of the latest John Curl/Parasound amplifier design here:

Parasound Discussion

Note they are talking about fully isolated input+driver sections with 117 volt rails. That might sound insanely high for a transistor amp, but overshoots in input sections of transistor amps are a big deal, and these are very high-powered amps with serious voltages appearing in the feedback loop.

By the way, cascoding the input section is how you get both voltage resistance and a hundredfold reduction in Miller capacitance. Since Miller capacitance in transistors is grossly nonlinear, this is a very good idea. Tubes exhibit Miller capacitance too, but it is an order of magnitude lower, and it is stable and predictable instead of being nonlinear. There are cascode tube circuits as well, but they offer no improvement in linearity (unlike transistors), and are mostly seen in phono preamps and FM tuner input sections. In the tube universe, pentodes behave similarly to a pair of cascoded triodes, and are more commonly used when a cascode is called for.

If you’ve been curious the kind of things two designers discuss with other, the previous posts give a simplified example. Sonics, different circuits and what they sound like, and reliability. It's also an example why collaborations between designers can result in designs that are better than any one person can do.

One variant we did not try was a center-tapped inductor to load the 6SN7 plates, and direct coupling between the input and driver tubes. That requires a high B+ voltage for the driver, but that’s no problem when the input + driver have their own power supply that’s fully isolated from the output section power supply.

It’s a good question if this is better or worse than a special interstage transformer. The center-tapped inductor shares many of the design challenges of an interstage transformer without necessarily having any advantages. This falls into the category of "build it and see". A minor challenge with a 450 to 500 volt regulated B+ supply, but nothing impossible.

Another similar option, instead of a center-tapped inductor, are paired current sources and direct coupling between input and driver tubes. This has an ugly disadvantage that a small DC imbalance in the input tube turns into a 5 to 10 volt offset for the driver tubes, which is intolerable because the driver section is then grossly imbalanced with different operating points for each driver tube.

To prevent this, it would require something like an opto-isolated DC servo circuit to balance the plate voltages of the input tube. Doable but kind of nasty, adding a lot of pointless complexity that adds nothing directly to the sound ... basically, another point of failure. The alternative would be manual adjustments (with limited range) on each current source and a meter so the user could hand-adjust DC balance.

Alternatively, the center-tapped inductor, because it has a moderate DCR of a few hundred ohms, limits the amount of plate-to-plate DC imbalance to a volt or so. However ... small as that is, that’s more than an interstage transformer, where the output going to the driver grids is always perfectly balanced, with zero DC offset from grid-to-grid.

What the interstage transformer does is offload circuit complexity to the ingenuity of the transformer designer. The circuit schematic looks simple, but what’s going on inside that transformer is very complex, requiring sophisticated modeling tools to fully understand.

I am still puzzled why capacitors measure so well, yet are so audibly colored. And why on Earth do they require hundreds of hours of active operation, not just polarized but signal going through them, to finally stabilize sonically? What’s going on inside them? My only guess is a (very) slow electrochemical process that subtly alters the dielectric properties of the plastic film.

It may strike some readers as weird that Don and I are taking a minimalist approach to a PP amplifier, more like a SET than a typical PP. The signal path is simple:

Optional SE/Balanced input transformer -> Balanced 6SN7 -> Interstage #1 -> Balanced KT88’s in triode in Class A mode -> Interstage #2 -> Balanced 300B’s in Class A mode -> Monolith Output Transformer -> Loudspeaker.

Similar to a fancy Japanese-style SET done twice. The hard part are the interstage transformers, which are not easy to find off-the-shelf, and ideally should be designed for the specific tubes in the circuit.

Don and I tried many variants to get rid of Interstage #1, since that operates at the highest impedances (thanks to the 6SN7) and is hardest (almost impossible) to design. The first version was simple RC coupling to the driver stage, with 6V6’s as driver running at 24 mA each. It sounded pretty decent and measured quite well, as you would expect from RC coupling. But Don wanted more ... so we tried paired dynamic loads for the 6SN7, which reduced its distortion about three times and was noticeably clearer sounding. But ... and there’s always a but, isn’t there ... there was just a faint trace of solid-state coloration from the current sources. Not much, but there. This was a version the folks at Spatial really liked, and we built some of the early "shoebox" format amplifiers in this format.

My grumble was the insanely long "burn-in" time for the super-deluxe coupling caps between the input and driver tubes. 50~100 hours. I am wary of burn-in times this long, since I suspect the part might be chemically unstable and never actually settle down, always changing its sonic presentation over time.

We tried another version, replacing the current sources with 100 Hy custom inductors. Was it any better? I’d say different, with deeper tone colors, no solid-state coloration at all, but losing a bit of snap and attack compared to the current sources. All expected ... no transistor sound, but unwanted stray capacitance in the high-value inductors, and question marks about linearity in the bass region.

Our transformer designer saw the high plate impedance of a balanced 6SN7 as a personal challenge, and insisted he could design an interstage transformer just for us. I was skeptical it could be done ... I only knew of one other transformer that could do that, from Tribute in Europe, and that thing was quite large and a one-off project done for the Karna amplifier. It might still be available from Tribute, for all I know. Tribute transformers are pretty special and the equal of any Japanese confection.

But ... a couple months went by, and Don got a special care package from our transformer designer. It was a pair of special custom Interstage #1 transformers, made just for us. Very simple wiring, compared to all the other variants we had tried, just six wires from the primary and secondary, as simple as it gets. In it went.

And that was the winner. Burn-in time was much faster, an hour or two, no super-exotic caps with their temperamental burn-in times, the circuit was way simpler, and the depth of tone colors was much deeper than any of the other versions. Obviously better than the inductor or current-source loads for the 6SN7, and no stinking coupling cap.

In fact, the IT coupling revealed the pretty obvious "cap coloration" of all the many coupling caps we had tried. The brutal fact is all caps have a sound, even though they measure extremely well. Worse, there is zero subjective correlation between sonics and the standard Df and Da measurements, not to mention they really do change sonics quite a lot over the first hundred hours. And nothing about that shift over the first hundred hours is measurable ... at all!

You hear the cap coloration by its absence. Go to true direct coupling, or IT coupling, and it is gone. After you’ve evaluated the sonics of twenty or more different brands and types of caps, you realize they all share a common coloration, with some much worse than others, but they all have something going on that is veiling the sound. And whatever it is, it can’t be measured with the instruments we have now.

I have no idea what it is. And I am taking about a coloration much more obvious than a cable swap between components. I’ll go out on a limb and say caps are the dominant sound of most tube electronics, whether preamps or power amps. Swap the coupling caps, and you have a brand-new amplifier, with brand-new colorations of its own. It makes you realize why there is a whole industry of guitar-amp tuning ... the tone coloration combinations are limitless.

But ... get rid of the caps, all of them in the signal path, and you are in a different sonic world. Do transformers have a sound? Well, if they are cheap transformers, yes they do. Murky and dull. The best, though, are very clear and have no cap sound at all.

That snap and clarity is what solid-state enthusiasts crave. Hey, I get it! The best solid state is very good and is free of cap coloration, as it should be. But ... solid state has its own sound, too. Unfortunately. Traces of it intrude on tube circuits if they have dynamic loads, or regulators that are not well designed. Don and I have gone to some trouble to use B+ regulators that are well behaved and have very high noise rejection (130 dB) and very low stray capacitance (using cascoded MOSFETs).

So I’ll go out on a limb (again) and say most audiophiles have never heard tube electronics without coupling caps. Ever. They think tube electronics just "sound that way" and tolerate burn-in times of hundreds of hours, with the sound changing hour by hour, sometimes better, sometimes worse, and sometimes just weird.

This plagues exhibitors at hifi shows, too, because the transport process results in some of the caps needing a re-set at the show, so the sound on Friday and Saturday can be remarkably different. And the culprit? Not the cables. Not the speakers. The coupling caps buried deep in the circuit, temperature cycling up and down, and going through unknown electrochemical changes deep inside.

And guess what? Solid-state electronics have caps too, not in the direct signal path, but as filter caps, typically large-value electrolytics. And you can bet they have a sound, too.

That’s a subtle aspect of feedback theory that is often overlooked. The summing node must be distortionless, and also free of overshoot or slewing artifacts. Applying input signal to a grid, and feedback to the cathode, impresses tube distortion (of the input tube) on the entire feedback loop.

Of course, we can get in trouble with a differential circuit as well, since the summing node is spread across two grid/cathode circuits, not one. Assuming perfect match, the nonlinearities of the pair should cancel. In practice, we should expect 2~3% gain mismatch in the differential pair, so there will be a residue of mismatch and associated nonlinearity, but not much.

If I understand your previous post correctly, the Zout of a Circlotron amplifier will be high, maybe in the 10 ohm or higher range. Or there is local feedback somewhere in there, reducing it to "normal" levels of an ohm or less.

As you can see, Don and I are taking a brute-force approach to distortion reduction.  Drivers that run at 40 mA per tube, and balanced deep Class A operation. The even-order distortion of the driver section is cancelled in the primary of the interstage transformer, reducing driver distortion even further. This presents a highly symmetric drive to the paired 300B grids.

@lynn_olson You might want to study the Wiggins Circlotron patent for the answer, or look at the setup/spec sheet of one of ElectroVoice's amps that used the Wiggins patent. Here is an example. 

I recently restored one of these (A20C model) that I had inherited about 20 years ago. Like you, I was very curious about it since EV and the patent claimed no zero crossing artifact if the amp is biased class B. I wanted to see if that was true. It is! With a sine wave input even at 1 milliWatt the output is excellent. Its first Watt performance (and sound) is really quite good.  

In the Features paragraph at the link you'll see that the primary impedance of the output transformer is 1/4 that of conventional PP amplifier circuits. That applies to an OTL in the same manner. Since the Circlotron allows for a fully differential/balanced Voltage amplifier and drive circuit, its symmetry means that distortion will be lower. So you can run it without feedback (you can do that with a totem pole circuit as well but its not nearly as easy).

We were the first to manufacture a Circlotron OTL and as such they were also the first really reliable OTLs as well because they were unconditionally stable with any output or load condition. I think that is due to the symmetry of the design and the lack of feedback, which can be a destabilizing factor especially if the amp operates near its phase margin limits (which can change with the load).

The Futterman OTL circuit employed positive feedback to equalize the drive to the top and bottom tubes of the totem pole output circuit, so that design seems to need negative feedback to really function right.

We've run feedback as well but do it with balanced feedback loops mixed with the audio in a manner similar to an opamp. This is because the cathode circuit is unavailable (there's a CCS circuit there after all); after doing that it took a while but I finally realized there's an advantage doing it that way because the feedback signal is not distorted by the tube as it would if the feedback were received in the cathode. In this manner there is less higher ordered harmonic and intermodulation generation caused by the feedback (meaning it sounds more relaxed).

I am convinced feedback to the cathode of the input tube is a reason why feedback has gained a bad rap in high end audio. Funny how traditions like that hang on for so long.

Ralph, you’re the acknowledged OTL expert. How does an OTL amp work without feedback? I’d like to know. The last I checked, tubes designed for series regulator use like the 6080, 6AS7, or the Russian 6C33C have a Zout on the cathode side somewhere around 100 ohms. I’m not an expert on this class of tubes, but I wouldn’t expect any tube to have a Zout in the 8 ohm or less range.

(For the reader following along: a Zout of 8 ohms gives a damping factor of 1, a Zout of 2 ohms a damping factor of 4, and so on. Zout in the 0.1 ohm range, typical of transistor amps, gives a damping factor of 80. It should be mentioned that damping factors much greater than this are kind of pointless, since the speaker cable, which is in series with the DCR of the lowpass inductor in the crossover, will typically have a DC resistance of 0.1 ohm or so. I don’t expect speaker cables to use superconductors any time soon, so we’re stuck with copper or silver at room temperatures.)

I’ll admit this is kind of an idle curiosity since I will never design an OTL amplifier. The amps I’m interested in use transformers to solve various circuit-design problems. But I’m always curious how things work, whether solid-state, vacuum tube, or Class D, or hybrids of all three (like ZOTL’s).

The Raven and Blackbird are non-feedback amplifiers, with the cathodes bypassed so local feedback does not apply, either. So there are no stability criteria or load stability issues. The distortion is simply the distortion of the matched pairs used in the preamp or amplifier. It has similarities to a SET amplifier in terms of a simple harmonic structure, but the pair-matching and balanced operation reduce distortion by about 30~35 dB ... without feedback or any associated stability or settling-time issues.

@lynn_olson Our OTLs are zero feedback too. We get a similar reduction in distortion over SETs.

Slewing, by contrast, is part of the amplifier running out of current, not voltage. Specifically, current available to charge a capacitance. Now, 80 pF isn’t much capacitance, but tube circuits are inherently high impedance (compared to solid-state) and operate at fairly low currents (again, compared to solid-state).

The load the driver sees in our OTLs is considerably higher than just 80pf; more like 200pf. We tie the cathode resistors of the driver tube to B- which in our smaller amps is about -300V. We put some current though there too, so the 6SN7 in question has to have a -GTA or -GTB suffix to handle both the current and Voltage to which is subjected. But they hold together in that circuit for years and even decades.

Driving a 300b in an SET has a lot in common with sitting on a park bench by comparison. We built an OTL using four 300bs about 25 years ago using the same kind of driver circuit. It worked fine. There's no problem at all using this technique as seen in the schematic that was posted above.

To recap, there are different challenges associated with low and mid-frequency distortion vs high frequency distortion. HF distortion is very often caused by nonlinear current delivery into a capacitance, and stray capacitance is everywhere in audio design. Sometimes you can reduce the capacitance using various methods such as cascode circuits, or pentodes (which are electrically similar), or take the alternate approach of increasing the drive current severalfold.

The 300B is a bottleneck in many SET amplifiers. The 80 pF load isn’t so bad, but the 300B requires 70 to 80 volts to clip it, and if the driver circuit is A2 capable, 100 volts. And ... the 300B has lower distortion than many, if not most, driver circuits, which defeats the entire purpose of using an expensive DHT like the 300B.

What looked like a simple problem is not simple at all, if you want to hear what the 300B actually sounds like, instead of a distorting driver stage. You have to deliver extremely low distortion into a capacitive load, over a range of hundreds of volts (if using PP output devices). This is no longer trivial. The common RC coupling seen in many amplifiers may not be up to the task.

We found transformer coupling with dedicated power tubes, themselves operating balanced Class A mode, gave the lowest distortion. Transformer coupling also allows A2 drive, with the 300B smoothly transitioning into the positive-grid region with no glitching. Although the 300B is not rated for A2 operation, we’ve found no indications of harm, although steady-state operation into A2 might overheat the grid, so not suitable as a guitar amp.

Now if feedback enters the picture, the design criteria all change. Forward gain goes up by as much as 10~20 dB, different parts of the circuit get optimized, and stability at high frequencies, particularly transient overload, become important. Nested loop feedback (2nd-order or higher) gives even lower distortion, but long settling times (after transient overload) can be problematic (because the different loops have different recovery times). You can have even more fun with modern feedforward techniques, but now we need serious computer modeling to pull that off and still have a stable amplifier.

Something both transistor and tube amps share are performance limitations set by current available to drive a capacitance. In a transistor amp, that will be the dominant pole capacitor associated with the second voltage-gain stage. The current available to charge that capacitance sets the slew rate of the entire amplifier. It should be mentioned that slew rate is similar to hard clipping; in slewing, linear operation has ceased, and there is no relation between input and output. As in hard clipping, there is a large region that is pre-slewing, with increased distortion but still a relation between input and output. With sinewave stimulus, the region of maximum dV/dT (rate of change) is actually around the zero crossing, which of course generates lots of high-order distortion harmonics.

Hard clipping and slewing have quite different origins; hard clipping is the result of one or more stages getting too close to the power rail, abruptly shutting off the gain stage. This can be either hard or soft, depending on the amount of feedback as well as the shutoff characteristics of the active stage. Transistors typically have a quite narrow shutoff range, with 0.7V being typical. Tubes are usually considerably broader, around 10 to 30V, depending on the device. This is also why tube rectifiers have a softer switching characteristic than solid-state.

Slewing, by contrast, is part of the amplifier running out of current, not voltage. Specifically, current available to charge a capacitance. Now, 80 pF isn’t much capacitance, but tube circuits are inherently high impedance (compared to solid-state) and operate at fairly low currents (again, compared to solid-state). The appearance on a scope are triangle waves, instead of flat-topped sine waves.

The somewhat arcane descriptor often seen in op-amp specifications is "large-signal bandwidth". This is another way of seeing slew rate: you measure output just below clipping and increase the frequency until the output begins to decline (which is the result of massive slewing). This measurement is simple enough for an op-amp and done all the time, but it can destroy a solid-state amp, so it usually calculated from quick measurements of slew rate.

Small-signal bandwidth is quite different and is usually measured well below clipping ... 1 watt is a common reference point. This is the result of various lowpass functions in the amplifier ... in a transistor amp, there is often a simple RC filter at the input that scrapes off unwanted RFI, which causes distortion in the audio band in solid-state circuits. Tubes are less prone to this but it is still not desirable to amplify AM radio signals at 500 kHz. In the tube world, the dominant lowpass is often set by the output transformer, which behaves like a 2nd-order (or higher) lowpass filter around 50~100 kHz. If a feedback circuit is wrapped around an output transformer, there needs to be compensation in the feedback network that phase compensates for this ... that’s the shunt capacitor you see around the feedback resistor ... but it must be tuned for that specific transformer, not just anything.

The Raven and Blackbird are non-feedback amplifiers, with the cathodes bypassed so local feedback does not apply, either. So there are no stability criteria or load stability issues. The distortion is simply the distortion of the matched pairs used in the preamp or amplifier. It has similarities to a SET amplifier in terms of a simple harmonic structure, but the pair-matching and balanced operation reduce distortion by about 30~35 dB ... without feedback or any associated stability or settling-time issues.

The approach Don and I take are borrowing elements of a modern SET and classical Western Electric designs from the 1930’s, getting the distortion as low as possible at the device level. This is where balanced operation and transformer coupling come in. The transformers of the 1930’s didn’t have much bandwidth, but they didn’t need it, since signal sources usually topped out at 8 kHz. Nowadays, of course, we need at least 30 to 50 kHz, which is where custom-designed transformers come in, using design tools not available in previous decades.

I have mixed feelings about cathode follower drive: the output impedance of the CF is low (probably 100 ohms or so), but the peak current available is no different than anode drive. Considering the load is dominated by the Miller capacitance of the 300B (about 80 pF), the CF will definitely extend the small-signal bandwidth, but will have no effect on the large-signal bandwidth (also known as slew rate) which is determined by the (linear) current available to charge a capacitive load.

@lynn_olson I think you have the highlighted bits wrong.

A CF driver can deal with a lot of capacitance in the grid of the output tube. We use a single 6SN7 section to drive 14 such grids (in our MA-1 amplifier) and it does it with no worries even in class A2 (or AB2, if the amp is subjected to a low impedance load) where grid current is present, with good linearity.

The peak current available is higher because the coupling is more efficient and the output impedance of the CF so much lower. When AC coupling (anode drive) it is very difficult to get the driver to be able to handle grid current in the output section (transformers are good at this though)!

The large signal advantage is several: no blocking distortion at overload since there’s no coupling cap (so overload recovery is instantaneous) and the Voltage amplifier sees a very high impedance load so it has a much easier time doing its job (so it can be lower distortion). This allows for the coupling cap used between the Voltage amplifier and driver to be a small value, which is advantageous because there’s less inductance associated with the coupling cap, so it can sound better and also offers better layout options. This aspect helps with HF bandwidth but also helps if you want LF bandwidth since the coupling cap value is so small.

In our MA-1 we have full power to 2Hz using a 0.1uF capacitor.

Another advantage is as a fixed bias scheme, its extremely stable since the impedance controlling the power tube’s grid is so low. Put another way its more reliable.

My advice, since its obvious you’ve not tried it, is to do so. If for an SET I would limit the LF timing constant of the Voltage amplifier’s coupling cap since SETs have such terrible problems with elliptical load lines at low frequencies.

Sorry, have been absent from this thread for quite some time.   Not spending too much time on here.  I will be in Dallas at the SW Audio Fest next weekend in the Spatial Audio room and the Raven and Blackbirds will be driving their best open baffle speaker.  So if anyone wants to chat.....

@theclipper

There are 3 or 4 sets of the final production Blackbirds in the world.  One is probably still in the boxes (go figure) as the person was building a new house or some such.  There is one guy who loves his and posted on the spatial audio circle I believe.   There are a few sets of the prototype ones in the smaller chassis with 6V6 drivers floating around and people like them.  But no one is really active on chat boards.  C'est la vie.   A review pair will make its way into someone's hands this year most likely.  The Raven preamp has had more people posting because it costs less and there are around 30 of the production ones out in the real world.

As for my history with tube amps...   I helped my older brother with his Dynaco kits when I was about 8 or 9.  Then I got on to the SS bandwagon with everyone else of my generation, but I never forgot how that Dynaco gear sounded on my brother's AR speakers with AR XA turntable.   I built various SS kits in my youth... anyone remember SW Technical Products?  Then Hafler kits, preamp and amps and learned to mod them.  Then had kids.....  I finally bought another tube amp in about 2004 or so, a modded Jolida integrated and it sounded far better than my Krell integrated that I had at the time.   Then I started restoring vintage tube gear, met Jim McShane, became his main tech and restored probably 500 pieces of vintage tube gear including probably 75 citation II amps and 40 or so citation I preamps.  Plus tons of Macs, Sherwoods, Marantz, Fisher, etc...   Learned a lot about circuits and started making my own gear.  

I think @atmasphere has fooled around with MANY vintage tube pieces as well as I gather.  You learn a lot....  And here we are today...... So many tubes, so many circuits, so little time.  Always room for personal taste and disagreements.  That's wonderul as long as all folks are polite, and this thread is testament to that!

As for the driver tube on the Blackbirds... It has a huge effect on the sonics of the amp.  I didn't try an EL34 because over the years I have heard many EL34 amps and never liked that tube compared to a good 6V6, 6L6, or KT66, KT77, KT88.  We all have our taste.  I tried a number of those and the KT88 was best by a good margin to my ear.  I have some rather esoteric DHTs as drivers in my own Blackbirds, but you have to find matched pairs and they are not in modern production, which is what you need for commercial production of tube gear.  I happen to have good sources and good stocks of these tubes, but again you want the customer to be able to buy tubes that are in modern production from more than one source.  There are exceptions to that rule of course, but you better have a good stock of tubes available if not using current production.  So the Blackbird uses tubes you can easily buy without me (or Spatial Audio) being involved.  I am not into the "find a vintage tube, use it, buy a 1000 of them and then mark them up 100%" model of business.  The exception is that we use the VR tubes, but those are everywhere and cheap.  Flea bay has millions and they last a long time.  Same with damper diode tubes.

Interesting SET that harks back to the original Western Electric 91A, which also used feedback to get the desired performance.

I have mixed feelings about cathode follower drive: the output impedance of the CF is low (probably 100 ohms or so), but the peak current available is no different than anode drive. Considering the load is dominated by the Miller capacitance of the 300B (about 80 pF), the CF will definitely extend the small-signal bandwidth, but will have no effect on the large-signal bandwidth (also known as slew rate) which is determined by the (linear) current available to charge a capacitive load.

The small-signal benefits would be useful in a high-gain feedback topology as shown above, since it would improve phase margin at high frequencies, which in turn improves distortion at high frequencies. And the circuit shown above is definitely high gain compared to typical zero-feedback SET amplifiers; performance would be dominated by the feedback loop, not the 300B.

(Note for those puzzled by the schematic: there are two ground lines, not one, with the upper ground line at the top of the schematic. It winds its way down to the lower ground line, which ultimately connects to chassis ground at the RCA input at the lower left side of the schematic.)

@alexberger Yes, that's how its done. Note that the EF86 input tube is wired in triode mode. Its quite linear that way; my Neumann U67 microphones use the same tube in the same manner sans feedback.

Hi @atmasphere

I just found schematics with a cathode follower 12BH7 300B driver.

Is it similar to the schematics you were talking about?

https://www.diyaudio.com/community/threads/300b-schematic-recommendations.340133/page-2

Just don't need pentode input and global feedback thought input tube cathode.

Well, I know more people than Whitestix bought the Blackbirds. My guess they are not on this forum (yet).

Has anyone purchased a set of the Blackbirds? Kind of surprising there still hasn’t really been any real world feedback on them yet (tho maybe I missed something).

The takeaway is that is impossible to "overdrive" the 300B. By contrast, a charmer like an EL84 can be driven with a whisper ... even a 12AX7 biased at 1 mA will sound good as a driver (which doesn’t work with any other power tube). A classic Mullard circuit is ideal for a pair of EL84’s since they are so easy to drive. 6L6's take a bit more muscle, so 6SN7's are a better choice.

The 300B is the opposite. A high voltage, high current, and ultra low distortion driver is mandatory, otherwise you never hear the 300B.

A simple and very effective way to drive 300bs is to use a cathode follower driver, direct-coupled to the grid of the 300b as it fulfills the requirements listed above. This requires a B- supply but you can control the grid so well that it can be driven class A2 (grid current) and you can easily overdrive the tube using a single 6SN7 section. This also allows for a much smaller coupling capacitor; 0.1uf (at the grid of the 6SN7) will allow -3dB bandwidth at 2Hz. This frees up the Voltage amplifier/driver of conventional design from a highly capacitive load. The downside might be that the power tube has to have its bias set correctly (so a provision to measure current is needed), which is done by setting the bias of the 6SN7. 

Doing this I've been able to overdrive 300bs (even multiples!) quite easily. The CF circuit, without the typically large coupling cap that often gives CF circuits a bad rep, has a tight grip on the grid of the 300b; so much so that driving class A2 with the substantial grid current that tube needs is no problem. You can easily drive the grid +15V WRT to the cathode with good linearity. 

The cost of a B- power supply is insubstantial when compared to the cost of a good inter-stage transformer and you get less distortion with greater bandwidth. You also don't have to introduce a power tetrode or pentode into the circuit.

A nice feature of this approach is the bias setting is very stable so might only need checking once or twice a year.

The cathode circuit is quite sensitive to (subjective) parts coloration ... not surprising, because both grid and cathode are the two input nodes for vacuum tubes. The difference is the grid circuit has very low current flow (but not zero) while the current flow through the cathode is nearly the same as plate current (the full audio signal). This means the full audio signal flows through the cathode resistor and the bypass capacitor, and the tube amplifies any errors in the cathode circuit the same way it amplifies any errors in the grid circuit.

Designers have been assuming for a long time that the grid current in normal Class A or AB operation is negligible, but I don’t think that is true for DHT triodes. They demand very high performance drivers with very low distortion into a complex load, which is where RC-coupling falls short.

The primary merit of transformer coupling is its efficiency, with 95% to 97% of the driver plate current available to the grid of the DHT triode. This is NOT true of RC coupling, where 30% to 50% of the driver plate current disappears into a plate-load resistor, where all it does is heat up the resistor. A dynamic load like a current source is more linear, but the transfer efficiency (between tubes) is no better than RC-coupling, so the unused current goes into a transistor heat sink instead of a resistor. Dynamic loads are also more complex if good performance is desired, with cascoded stacked MOSFETs, with secondary protection diodes, as the most reliable and best option.

Transformer coupling is absurdly simple, with no need for a grid-protection resistor, no coupling cap, no plate load resistor, and no circuit board for the cascoded MOSFETs of a current source. Just wires going to tube sockets.

I suspect the 95% to 97% transfer efficiency of transformers is the reason for the vivid tone colors that are the hallmarks of any IT-coupled amplifier. You hear it immediately, which why Don and I hope more people can hear the Raven preamp with the matching Blackbird amplifier.

Another thing that drove me mad, my SET sounded too analytical and sterile. It took me 6 month to find the source of this issue. It was a Vishay Z-foil resistor in input stage 6sn7 cathode. When I changed it to a Shinkoh 2W resistor all this sound sterility was gone! Z-foil worked OK when I used RC coupling between input and driver stage. RC coupling softened sound. But  when I changed it to more transparent IT coupling, the extreme sharpness of Z-foil resistors showed up.

@lynn_olson how could Don Sachs have been building pp amps for decades when he first owned a tube amp in 2004?

Diar @lynn_olson 

Thanks for the informative answer.

I used a 6f6 driver in my single-ended amplifier, which I have been improving for many years. For some reason, Shindo used this tube in the driver of his 300V amplifier, and the person who made the first version of my amplifier followed in his footsteps. That guy (who built my amplifier) also tried to compensate for 300b 2nd order distortions by 6f6 driver distortions. But I don't think it worked. In the end I increased 6f6 current to make it work in a more linear range.

A year ago, I switched from RC coupling to interstage transformer. The sound improved beyond recognition.
Then I switched to a 6v6 tube, increasing the current from 26 to 30uA. Compared to 6v6, 6f6 gives a more compressed sound, with soloists pushed forward and a stage reduced in depth and width. 6v6 has a bigger and deeper bass and a more relaxed sound.

The KT88 driver tube will not be handled by either my filament power transformer or the interstage single-ended transformer (which is designed for 30uA nominal and 40uA maximum). So I want to try the 5881 tube in the driver. This tube should work plug and play in my amplifier.

The takeaway is that is impossible to "overdrive" the 300B. By contrast, a charmer like an EL84 can be driven with a whisper ... even a 12AX7 biased at 1 mA will sound good as a driver (which doesn’t work with any other power tube). A classic Mullard circuit is ideal for a pair of EL84’s since they are so easy to drive. 6L6's take a bit more muscle, so 6SN7's are a better choice.

The 300B is the opposite. A high voltage, high current, and ultra low distortion driver is mandatory, otherwise you never hear the 300B. You just get murk. 45’s and 2A3’s are a bit less temperamental, thankfully, with the 45 shining through with extremely low distortion.

I left the driver tuning up to Don Sachs, who’s been building PP pentode amps for decades ... and started out by restoring Citation I and II’s, which are notorious as the most complex amps and preamps of the Golden Age. By contrast, the Marantz amps and preamps were much simpler.

Don’s a big fan of the 6V6, 6L6, and KT88 (in triode mode). Kind of hard to argue with that ... some of the most famous amps ever made used those tubes. Anyway ... he tried about every well-known tube under the sun as drivers. I kind of thought something as petite as a 6V6 (which is equivalent to a 45 in ratings) would be optimal, but the KT88, running at fairly high bias, sounded best of all. Part of the reason this is relevant is the 300B is very, very sensitive to the driver tube, much more so than most tubes.

The 300B has a difficult combination of very low inherent distortion (bested only by a 45 triode), and a grid-drive requirement of 80 volts peak at very low distortion. In most commercial 300B amplifiers, all you hear is the distortion of the driver, especially if it is RC-connected. You never hear the 300B as it really is. All the usual complaints of dull, soggy sound are the result of a not-good-enough driver.

Get a powerful enough driver with enough current (30 mA or more) and transformer couple it to the 300B grid, and you hear a very different sound ... very fast thanks to the high slew rate, and very wide-open thanks to low inherent distortion. Yes, I know about the Sakuma-san 300B - 300B amplifier ... I met him and heard it at one the last VSAC shows in Silverdale.

The Sakuma sound is mostly about the Tamura interstage transformers he favors, along with unique tube combinations. Surprisingly, my own Amity and Karna amps sound nothing at all like Sakuma designs ... if anything, our designs kind of echo the Citation I and II ... a big, fast, American sound, like a track-ready V8 Corvette. Sakuma would be more like a Morgan, very vintage.

Sakuma is 100% right about interstage transformer coupling. All the flavor comes through ... this is the unique hallmark of any interstage coupled amplifier. You hear it instantly, as soon as you walk in the demo room. Other methods subtly degrade tone colors, and all capacitors have an annoying tone color that is always there. I wish I knew what causes it, but it is not there in the cap measurements, unfortunately. But is very obvious when it is gone, especially after you get the last cap out of the signal chain.

Don and I valiantly tried to use various types of RC coupling between the input section and driver, some better than others, but the cap coloration was always there, no matter how fancy the cap was. They were all colored sounding, in various degrees, and a little dynamically flattened. So we had our transformer designer come up with a one-of-a-kind interstage to couple the input to the driver tube, and boom, problem solved. Simpler circuit, too. No caps in the signal path any more, and all the tone colors coming through ... which is the whole point of any vacuum tube amplifier.

Hi @lynn_olson ​​​​@donsachs 

When did you play with the driver tube to your amplifier? Which tubes did you try except 6v6 and kt88?

Did you try 5881, KT66, 6L6, EL34?

If yes can you compare the sound of these drivers?

Hi @lynn_olson

I upgraded my 300b SET amplifier. I replaced 6f6g Torvac driver tube to 6v6 Psvane. 6f6 worked with idle current 26ma. 6v6 with the same cathode resistors gave 24ma. I changed cathode resistor to get 29ma. In result much better bass control and dynamics, better instruments separation and higher resolution.

The difference between 6v6 and 6f6 drivers is huge. It similar to changing RC coupling between driver and 300B to IT coupling!

Conclusion, the more powerful driver is better for 300b. I know some folks (including Sakuma) end by 300b driver for 300b.

By the way, the traditional definition of "efficiency" is: (Max RMS output power at stated distortion level) / (Total power going into the plate circuit). Power consumed by heaters, filaments, input and driver tubes, or regulators is not usually considered.

For a pair of output tubes ... 6V6, EL84, 6L6, EL34, KT88, 6550, 2A3, 300B, 845, or similar ... they can be set up to run in either Class AB, or Class A. Class AB operation typically has a higher B+ voltage and a lower quiescent (steady-state) current, while Class A operation has a lower B+ voltage and a higher quiescent current. For the same average power draw, Class A usually puts out half to one-third the output power of Class AB, which is why it is less common than Class AB.

Most customers want more power if they have a choice. That was true in the Fifties and it is still true today ... partly because most loudspeakers are very inefficient (less than 1%) and need all the power they can get.

Some fixed-bias amps have a separate servo circuit that monitors the bias of each tube, so user does not need to adjust the amplifier. This servo circuit needs to be very reliable, though, since a failure would destroy the output tubes, and possibly damage the bias circuit, as well.

Cathode bias, which is nothing more than a power resistor bypassed by a (very) high quality capacitor, acts like local feedback at DC, and more like fixed bias at audio frequencies. It is not suitable for Class AB amplifiers, though, since the total current going through the pair (or more) of output tubes varies with the power delivery (the efficiency actually goes up as output power increases). By contrast, Class A operation has more or less constant current draw from the output pair, but it is significantly less efficient than Class AB.

With use of RC cathode bias won't have such issue but we need to watch out when using fixed bias.

@kmtang When setting the bias for any fixed bias amplifier, its good practice to check the bias after an hour of operation. IOW all power tubes regardless of type will see higher bias current over time as the tube warms up. So usually after the amp has been on for about a minute or so you set the bias to about 85-90% of the bias current spec. That way it has less of a chance of damaging the tube as the tube heats.

I have tried two types of LinLai 300B tubes for some time with shunt regulator at the cathode, i.e., fixed voltage at cathode. I biased it 70mA with 420V B+. The anode-cathode voltage is approx 350V. 

Once the 300B tube got warmed up for about 30mins, the idle current start drifting higher. I had to reduce the idle current to 60mA which will be stable over long operating time. 

The same happens to the EH and the Russian Gold Lion 300B tubes as well. I don't have the WE ones for testing. With use of RC cathode bias won't have such issue but we need to watch out when using fixed bias.

Johnny

Hi, I used the "shunt regulator" to replace the RC cathode self bias circuit in my 45, 2A3, 300B, and 845 SE power amps. The sonic improvement is exceptional. My friends tried it and they all agreed it is real worth to try. 

I haven't try using fixed bias in my amps as it is a bit difficult for such modification.

Johnny

Both were used to play the lacquer once for quality control, then off to the plating plant.

Usually you don't want to play the lacquer at all- any playing degrades it. But you can cut outside the 12" diameter since the lacquer is 14"; in this way if there is a troublesome cut you can test that part of the recording by cutting it there and then playing it to see how it went. If OK then you can proceed with the regular cut.

I found that if we spent enough time with a project there was usually no need for processing of any kind; there is usually a way to cut a playable groove, even with out of phase bass (if its not too bad). Compression is really only used to speed up the mastering time; mastering time is expensive.

Well, keep in mind most LP’s were cut with spherical styli in mind ... specifically, the Stanton 681A with spherical styli. Over in Europe, it was the Ortofon SPU with a spherical stylus. Both were used to play the lacquer once for quality control, then off to the plating plant.

Ellipticals were fairly rare in the Sixties and often poorly cut, with obvious asymmetries. In college, I had an ADC elliptical cartridge that destroyed several of my records until I wised up and bought the Stanton (as recommended by the early Stereophile magazine).

We didn’t see Shibata or Fine-Line profiles until CD-4 quadraphonic records, with their 30 kHz FM carrier on each groove, required for adequate CD-4 playback. That was 1971 or so, if memory serves. The trick with Fine-Line profiles is azimuth needs to be *exactly* right, within one degree, or mistracking gets pretty bad. With sphericals, azimuth hardly matters, and even ellipticals are moderately tolerant of misalignment. But not Fine-Line profiles. They need to be exactly on the money.

One of my minor inventions with the Shadow Vector quadraphonic decoder (Patent #4,018,992) was an electronic crosstalk cancellation scheme, which electronically rotated the axis of the two generators so they were precisely at 45/45 degrees. That gave about 45 dB of measured separation, with an optional second-order corrector which operated above 10 kHz. That corrected for cantilever twisting at high frequencies, a problem I noticed happening with many cartridges.

Shadow Vector quadraphonic decoder

I was not thrilled to discover many $2000 to $15,000 cartridges had visibly rotated cantilevers ... not by much, but by about 2 or 3 degrees, which made azimuth adjustments extra tricky. With a Fine-Line stylus, nearly mandatory at that price point, you have to get the stylus exactly square in the groove, regardless of the generator axis. Which is where electronic compensation comes in ... if the generator is not at 45/45, you can rotate it electronically, and get the separation back with no penalty.

P.S. The crosstalk cancellation is very simple. Each channel has an adjustable amount of crosstalk from the other channel, with plus-phase crosstalk on one side of the pot, and minus-phase crosstalk on the other side. In the center of rotation, zero crosstalk. One pot for each channel, a test disc, two quick adjustments with a meter, and off you go. 45 dB or better separation from any record or cartridge.

P.P.S. That EAR schematic looks kinda sketchy to me. I would not use it. I suspect the errors and the wonky drawing style are intentional.

Still out in the weeds dept.;

Many phono preamp designers over the last 70 years have ignored the significance of a magnetic cartridge whose output is in parallel with the capacitance of the tonearm cable.

Any time an inductor is in parallel with a capacitance there is an associated resonance whose frequency is set by the values of the cap and the inductor. With high output MM cartridges this peak is generally about 20dB and centered just inside or just above the audio band. If the phono section does not have good HF overload characteristics, this peak will overload the input of the phono section resulting in ticks and pops.

Just to boggle your mind a bit more, cutting a lacquer master from the two-track master tape meant the engineer "riding the controls" as the cutterhead neared the center of the record.

We never had to do this with any of the LPs we cut. We used the Westerex 3D cutter with 1700 series electronics, all stock. We found that the old myth about loss of bandwidth towards the end of the LP was just that. We cut 30KHz tones in the inner grooves that played back fine on our Technics SL1200 with Grado Gold (we used that setup to know if the groove we'd cut was playable on an average machine). I think a lot of that myth got started in the 1960s when stereo cartridges really just weren't all that good.

Hi @lynn_olson

I use EAR834p schematics in my current DIY phonostage.

The second ecc83 miller capacity for RIAA equalization.

It is also using small capacitors in the RIAA equalizer. So I use air capacitors in this RIAA.

I also use the LCLC power supply filter plus 2x 0A2 voltage regulators.

In general I like the sound of this phonostage.

The main drawback of this schematics is output cathode follower that drives feedback and output RCA cable. When I tried different ECC83 tubes in this position - speed, dynamics and bass were changed dramatically.

If I reuse this schematics for the new project I am thinking about adding an output buffer. It can be 6SN7 with the output transformer.

https://www.diyaudio.com/community/threads/ear834p-is-not-what-it-seems.313042/

I did something more similar to this (just without a fixed bias for the second ECC83):

http://www.romythecat.com/Forums/ShowPost.aspx?PageIndex=2&postID=9161#9161

A quick note on the complex variable equalization seen on the Citation I: people who had extensive 78 and pre-1955 LP collections had records with wildly varying equalization curves. By the time of the stereo LP in 1958, the world had settled on the RIAA system, and all stereo LP’s use RIAA equalization (to the best of my knowledge).

Some early (mono) LP’s had the EQ marked on the label or the record jacket, or the record company was known for having a house curve, but 78’s were notorious for being all over the place, with no industry standard at all. Even the speeds vary, with 78 rpm merely being the industry average.

So the super complex switching on the Citation is mostly aimed at the record collector with a lot of mono records, both 78’s and LP’s. Some folks even kept their old mono preamp, just to play their old records with varying equalization. For that matter, tone controls were very important in the Fifties, and were still important in the Sixties.

Few speakers were flat, and record companies usually had a house sound, or actually several house sounds, depending on the musical genre. Elvis on a 45 single was not going to be treated like an RCA Red Seal classical record, and the early Beatles sound was very different on the US Capitol release than the Parlophone release in the UK. Tone controls were standard on all hifi equipment back then.

Just to boggle your mind a bit more, cutting a lacquer master from the two-track master tape meant the engineer "riding the controls" as the cutterhead neared the center of the record. Different engineers would have their own interpretation of what the master should sound like ... so yes, there can be many "master records", not just one.

P.S. If I'm reading that schematic right, the Center Channel output, although correctly summed from Left and Right, is actually out of phase with both of them. (The 12AT7 inverts phase.)

Before getting lost in the weeds on balanced vs single-ended phono preamp design, it might be useful to review the general types and look at their advantages and disadvantages.

1) The most common is a high-gain stage with RIAA frequency-shaping feedback wrapped around it. This dates back to an early Fifties RCA application book. Today, it’s what you get when you buy a $200 solid-state preamp ... a modern high-gain opamp with a feedback loop wrapped around it. It has the merit of low cost and simplicity, and if done in the Fifties style seen in many preamps, a traditional sound many like.

The drawback is using a low-current device like a 12AX7, which typically runs at 0.5 mA current, which does not have enough current to drive a moderately long cable and the reactance of the feedback loop at the same time. This leads to the preamp creating slew distortion with record pops and mistracking, which exaggerates their audibility.

2) A new/old approach is splitting the RIAA equalization in two, using it as a passive filter between the first tube (for the first filter) and second tube, and a second passive filter between the second tube and the third tube. The RIAA filter is usually split in two to avoid overload and noise problems that build up with a single passive RIAAA filter with a 40 dB attenuation loss between tube sections.

This passive-filter approach requires a judicious balance between noise buildup (mostly a problem in the first section) and overload, which can easily happen if the stylus starts mistracking (which is much more common than you might expect).

3) One of the more offbeat new/old approaches is a passive LCR filter between sections, using well-shielded inductors as part of the RIAA network. This is usually a pretty exotic part, and the first stage needs enough linear current to drive the highly reactive LCR network. I have heard this type of preamp and was startled by its naturalism and lack of phono preamp coloration. But they are exotic and difficult to design.

I should add that phono cartridges are often blamed for phono preamp coloration, which mimics mistracking and common types of cartridge coloration. Most phono preamps, whether solid-state or vacuum-tube, are actually quite colored and prone to HF distortion, making many records sound shrill and distorted. The best ones reveal surprisingly quiet record surfaces as well as open and natural high frequencies.

Before doubling the complexity of the phono preamp by using a balanced circuit, it first has to have a noise floor lower than the tape hiss recorded on the LP record, and more seriously, be free of slewing distortion and overload. This is subjective, but I hear clear and obvious overload on most preamps I hear at hifi shows. The exhibitor may blame the phono cartridge or the record, but a preamp swap will reveal the distortion is actually in the preamp itself, not the cartridge. Although phono cartridges are often flawed, many phono preamps make them sound much worse than they really are.

It may be a crude standard, but above all else, components should never audibly overload on any record, no matter how badly it is mastered. It does no good to have an expensive hifi system that can only play a handful of audiophile-approved discs that have been very carefully mastered. It should be the other way around: the preamp should accept ANY disc without breaking up, distorting, or becoming shrill. That’s much more important than pushing the noise level 3 dB lower than any record ever made.

@alexberger , @lynn_olson mentioned earlier:

Ralph brings up a very good point about feedback: the underlying theory assumes a distortionless summing point. (The summing point is the comparator input between signal input and the sampled output.) Any distortion introduced at this point of the circuit will be amplified without correction, and there is a real possibility of introducing new, higher-order terms that are not present in the forward path of the physical amplifier. Norman Crowhurst mentions this in passing in his Audio magazine articles in the late Fifties.

Actually I read about this in one of his books. The point is that feedback applied to a cathode is going to generate higher ordered harmonics and IMD because the cathode is non-linear, even on a 12AX7. If you can, the thing to do is apply the feedback to the grid of the tube rather than the cathode. This gets tricky if you have two stages of gain as you see in the schematic above! It might also mean you have to have a feedback capacitor to block DC, which isn’t likely to treat the feedback signal very well. You see this technique being used in the line section of the Citation right after the tone controls.

You can do this in an amplifier too, wrapping the feedback around the entire amp circuit. Admittedly tube circuits are often lacking in the Gain Bandwidth Product to prevent distortion rising with frequency, but if the feedback is handled properly to start with overall its a better chance of it working right.

But I think to make balanced first amplification stage of the phonostage can be very helpful.

SUTs can have a balanced output if you like- they don't care. Transformers are very good at going back and forth between balanced and unbalanced. You will have to be careful about loading the SUT properly to maximize its performance. Why stop with a balanced input- balanced (differential) throughout gets you greater power supply immunity and lower distortion overall, as well as lower noise if the gain stages are properly executed.

Hi @atmasphere ,

I have balanced connection from the cartridge to SUT. But from SUT to phonostage it is SE connection. In my next phonostage project, I plane to put SUT close to the first tube inside the phonostage.

But I think to make balanced first amplification stage of the phonostage can be very helpful.

Hi @atmasphere ,

If you run EQ via feedback, you run into the same problem that Norman Crowhurst wrote about nearly 70 years ago.

Which problem? Can you explain?

I saw Cintation 1 schematics. It is difficult to understand capacitors values and how do switches exactly work from these schematics. The first pair of ecc83 works for gain only (10000 voltage gain with a feedback) . After that we have a passive RIAA. The second pair of ecc83 has feedback that looks like the second part of RIAA (that is active).

I rebuilt a number citation 1 preamps over the years for customers.  I didn't care much for the line stage section with the anode follower, and miles of wire and switches, but the phono section was among the best of the vintage gear.  

In my next phonostage I want to make an external power supply with a separate filament transformer. But I haven't decided which RIAA schematics to use. There are basically two types of tube phonostages. In one type RIAA is implemented in feedback and another type is passive RIAA.

@alexberger You might consider that since the cartridge is a balanced source that you could have a balanced phono section too, or at least a balanced input. If you run EQ via feedback, you run into the same problem that Norman Crowhurst wrote about nearly 70 years ago. You could avoid it by applying the feedback to the grid of the tube rather than the cathode (you'll need a series resistor with the input to allow the mixing to occur, similar to an opamp circuit). You'll have to recalculate all the EQ values since feedback to the grid behaves quite differently (higher impedance).

Passive feedback can work quite well. Just because you have passive EQ does not mean that you can't run feedback in the associated gain stages. H/K did this with their Citation 1 back in 1958.

The advantage tubes have over solid state in a phono section has to do with the fact that most cartridges are inductors; when in parallel with the capacitance of the tonearm cable, an electrical resonance is formed. That resonance can overload the input of the phono section causing ticks and pops that sound like they are on the LP surface. If your tube phono section is properly designed (easier because the operating voltages are higher), this won't happen; you may discover that LP surfaces are actually a lot quieter than the digital crowd would have you believe.