I have been an owner of Don Sachs gear since he began, and he modified all my HK Citation gear before he came out with his own creations. I bought a Willsenton 300b integrated amp and was smitten with the sound of it, inexpensive as it is. Don told me that he was designing a 300b amp with the legendary Lynn Olson and lo and behold, I got one of his early pair of pre-production mono-blocks recently, driving Spatial Audio M5 Triode Masters.
Now with a week on the amp, I am eager to say that these 300b amps are simply sensational, creating a sound that brings the musicians right into my listening room with a palpable presence. They create the most open vidid presentation to the music -- they are neither warm nor cool, just uncannily true to the source of the music. They replace his excellent Kootai KT88 which I was dubious about being bettered by anything, but these amps are just outstanding. Don is nearing production of a successor to his highly regard DS2 preamp, which also will have a unique circuitry to mate with his 300b monos via XLR connections. Don explained the sonic benefits of this design and it went over my head, but clearly these designs are well though out.. my ears confirm it.
I have been an audiophile for nearly 50 years having had a boatload of electronics during that time, but I personally have never heard such a realistic presentation to my music as I am hearing with these 300b monos in my system. 300b tubes lend themselves to realistic music reproduction as my Willsenton 300b integrated amps informed me, but Don's 300b amps are in a entirely different realm. Of course, 300b amps favor efficient speakers so carefully component matching is paramount.
Don is working out a business arrangement to have his electronics built by an American audio firm so they will soon be more widely available to the public. Don will be attending the Seattle Audio Show in June in the Spatial Audio room where the speakers will be driven by his 300b monos and his preamp, with digital conversion with the outstanding Lampizator Pacific tube DAC. I will be there to hear what I expect to be an outstanding sonic presentation.
To allay any questions about the cost of Don's 300b mono, I do not have an answer.
People have gotten the weird idea that somehow the filament of the 45, the 2A3, the 300B, and the 845 are responsible for the ultra-low distortion, and the super-vivid tone color, of the DHT family. Wrong. It isn’t.
It’s the grid. DHT’s have a physically large grid, well spaced away from the whirling cloud of electrons called a "space charge". Surprisingly, electrons are not directly emitted, pass through the grid, then strike the plate. Instead, they whirl around in the space charge, find a passage through the venetian-bland repelling field of the grid, and are accelerated to the plate.
It’s the grid geometry that sets not only the DC characteristics of the tube, but also its linearity (especially high-order terms). This is the most critical part of the entire circuit. If you need pentode or beam tetrode characteristics, fine, you’ll have very low Miller capacitance, very high output impedance, and easy drive characteristics. This makes an excellent RF modulator, where distortion doesn’t matter.
But if low distortion comes first, and you’re not asking for 20 to 50 dB of feedback to linearize the whole amplifier, a true purpose-made triode should have the lowest distortion. Since there are already lots of octal sockets in PP power amps, why not make a special triode tube just for them? There is some design work to optimize the grid structure so DC biasing is the same as a triode-connected pentode, but the absence of all those other grid wires should help. If the design is good enough, it could rival the 300B without the hassle of direct heating and the complex filament circuit.
@lynn_olson- that’s a cool idea Lynn. There are certainly plenty of audiophiles out there with KT88 amps that are running them in triode mode that could benefit from these, not to mention folks with KT88 amps that don’t currently have a way to run triode connected.
i don’t currently have a KT88 amp, but would probably build one if such a tube were available.
Since nobody is reading this thread right now, I’m going to throw out my wish list, my note in a bottle, to the wilds of the Internet:
* I’d like to see LinLai or JJ or any of the other tube vendors, try something a little out of the ordinary. A true triode, using an octal KT88 socket, that biases up exactly like a triode-connected KT88. An indirectly heated triode, in a KT88 package, with only three elements ... cathode, control grid, and plate. With no screen or suppressor grid, and the control grid correctly spaced so the whole tube mimics a triode-connected KT88, so it can plugged directly into a KT88 socket in an existing amp and work right away.
What is the benefit over a standard KT88? Well, with no useless screen or suppressor grid, the one remaining grid can be optimized for lowest distortion ... in particular, the lowest proportion of high-order harmonics, like a direct-heated triode.
It’s not the direct heating of the M-shaped filament that’s responsible for the very low distortion of DHT’s (compared to triode-connected pentodes and beam tetrodes). It’s the clean, uniform grid structure, and the carefully chosen spacing from the cathode (or filament). So there’s no reason a purpose-designed true triode can’t be designed to fit a standard KT88 (or EL34) socket that has the same DC bias characteristics as it’s more complex brother, but also much lower distortion.
Literally, a simple plug-in improvement for all the hundreds of thousands of conventional PP-pentode amps out there. No change in bias, no change in cathode circuit, no change in fixed-bias operating point, just lower distortion, ideally approaching DHT performance if the grid is correctly designed.
You could call the new tube a TR88 to distinguish it from a KT88, while signifying it is plug-in compatible (thanks to the same DC biasing). Or TR34 if it replaces an EL34.
Just a quick update that Don and I are continuing to refine the Blackbird, with a bit of Raven and Karna Mark I thrown in. The chassis will be 18" wide to give a more spacious layout, a simplified build procedure, and visually match the Raven.
The circuit continues to be balanced throughout, with a tube lineup of a 6SN7, a pair of matched and balanced triode-connected 6V6, and a pair of matched and balanced 300B’s. The power supplies (both of them) have a slow-start circuit that protects against hot-start transients if the AC power flickers off for a second or two, as well as controlling tube warm-up.
Sonically, Don and I are prioritizing depth and realism of tone color, like 300B SET amplifiers, combined with a clarity and directness usually associated with high-performance Class A and Class D transistor amplifiers.
How would you compare the sonics of your 300B amp to the the Class D amp you make now? You’re the creator of both, so you’re in the best position to evaluate and compare. I only spent a half-hour of casual listening to the Purifi at the show (and Audio Group of Denmark), so I’m hardly an expert on the subject.
Its been a long time so my comparisons have a set of our M-60 amplifiers in between if that makes sense. The M-60s were overall less colored by distortion with wider bandwidth and a greater sense of palpability. Peter Moncreif had us do a direct comparison at the show.
So the M-60 compares to the class d in that they have a similar tonal balance- with no grain or edginess in the mids and highs. The big tell that we hear and that customers report is that the class D is more focused in that its easier to hear what’s going on in the rear of the sound stage, pick out details and that sort of thing.
And if you really want to get hardcore, make sure all the cathode circuitry, of each section, comes down to a single star ground on the main ground bus-bar.
If I can add to this, make sure that the grid circuit and cathode circuit of each tube employ a single wire that goes to ground for both of those circuits. If you are using terminal strips, to do this the grid resistor and cathode resistor would tie to the same point and then a single wire to ground is used.
Tubes amplify differentially, which is to say the grid and cathode are out of phase with each other. So if a single wire is used for ground and noise is injected into that wire, it will be rejected by the tube. If you use separate wires the tube will be more noise susceptible.
Yes, plenty good enough for the job. Don’s comments above are right on the mark.
What you want is isolation. Stage-to-stage, and isolation of the critical filament supply.
And if you really want to get hardcore, make sure all the cathode circuitry, of each section, comes down to a single star ground on the main ground bus-bar. So, star ground for 6SN7 cathode components, a few inches away, the star ground for all 6F6/6V6 cathode components, and another few inches away on the main bus-bar, all the cathode components for the 300B. The idea is keep all audio currents local to that stage, and to that stage only, and only have DC return currents on the main ground bus-bar. You would be surprised how few high-end components do this.
The presentation below is for advanced students. You know who you are:
if you can fit a toroid in there, then look at the antek site. They have small filament transformers and may have what you want. AN0205 is 25VA with dual 5V windings so will give you 2.5 A x 2. They are fine for what you want. they also sell transformer covers. If you want to regulate the filament supply for each 300b then you can get a 2 x 7V.
I found online only one filament transformer for 5V - Hammond 546-166MS. This transformer has only one 5v 3A tap. So I need 2 such transformers. I use Hammond in my DIY phono stage. But IMHO Hammond transformers are built cheap compared to Lundahl, Hashimoto, AN or even James Audio.
Is these Hammond good enough for this task?
You can do a lot of sleuthing just by listening to the spectra of noise. Magnetic induction is going to be pure 50/60 Hz and fairly hard to hear. Capacitive coupling is high frequency only, and will sound like buzz, usually harmonics of 100/120 Hz switch noise from the rectifiers and transformer secondaries.
Ground loop noise can be isolated by shorting the input plugs of your preamp or power amp. If the input is shorted, and the noise persists, it is inside the component itself, and is usually a design or layout error.
If the noise is the result of two components connected together, that is a ground loop. This can be confirmed by disconnecting the interconnects between them, turning both on (with volume down), and using a DVM to measure the AC voltage potential between the two chassis. Scrape through the paint or anodize if you need to, then measure.
The AC potential between the two should be less than 1 or 2 volts. If it is more, then you have a ground loop. This is caused by capacitive leakage from the power transformer to the chassis. It can cured by reversing the AC polarity going into the power transformer on ONE of the components, but this is not a DIY job.
What causes this is that consumer AC power is not balanced; instead, there is neutral, which is only 1 or 2 volts away from safety ground, and hot, which is 120 volts in North America and 220 to 240 volts elsewhere. Power transformers are not symmetrically wound; one side has lower capacitance to ground than the other, but unfortunately, the leads are not marked, so they can randomly assembled in production. Ideally, the low-capacitance side of the primary should go to HOT, and the high-capacitance side of the primary to NEUTRAL.
If all your components were assembled this way, you would never have ground loops. Unfortunately, the phasing of the power transformers is random. The capacitive leakage from primary to transformer case will let the chassis float to a high value relative to safety ground, which is the true ground. The only real solution are medical-grade power transformers, which have extremely small leakage to chassis.
Short of that, you can hire a skilled technician to wire all of the power transformers in your system for minimum AC HOT to chassis leakage ... which is a good idea from a safety perspective anyway. No more little shocks when you touch a component (which should never happen in equipment built to code). Safety code requires that the fuse, then the power switch (in that order), always be on the HOT side of the line.
The 300B filament circuit is a very delicate circuit node. The high voltage windings of the B+ transformers see switching pulses of hundreds of volts with very steep rise times. It only takes a few pF of winding-to-winding capacitance to transfer that 120 Hz switching noise straight into that expensive 300B. Unless you know that the low-voltage winding is electrostatically screened (with copper foil), don’t do it. Use a separate transformer just for the 300B alone.
You wonder where low-level buzz comes from? Winding to winding stray capacitance. Whenever you hear buzz instead of low-frequency hum, that’s a capacitive coupling, not magnetic. The spectrum gives it away.
I would NEVER use vintage caps in a power supply. Never never never. Use modern parts. They’re not that expensive, and used in air conditioners all over the world. Vintage is OK in a crossover, where failure is no big deal. In an amp, just say no.
Not sure I see the merit of mixing films and electrolytics. If you have the space, use the industrial parts, and bypass caps to personal taste. In terms of location, the cap bank can be several inches away from the tube socket, but the little caps (0.1 uF or less) need to be close by, an inch or less.
Hi Lynn and Don,
What do you think? Is it a good idea to use driver-input power transformer taps to feed 300B filament? This power transformer should be less pushed then the 300B power transformer.
I don't use only electrolytics capacitors for B+. My DIY friend taught me to use a mix of electrolytes, polypropylene and vintage industrial oil capacitors (Siemens MKV, Tesla, KBG-MN and new Obbligato). All this capacitor bank is bypassed by 0.1-0.22uF PIO and 0.001-0.01 uf Soviet silver mica. This bunch of capacitors doesn't smare sound. The only drawback I can hear is the relatively long warm up time (at least 2-3 hours). Probably a 200+uF set of good quality vintage industrial oil capacitors will sound better. But it will be more expensive.
Since this is (mostly) a 300B thread, I’m still curious about the general circuit of Ralph’s 300B amp. He’s been doing this a long time, so his design choices are of considerable interest.
I have my own way of doing things, and that was strongly influenced by my research when I was writing for Glass Audio and Vacuum Tube Valley. John Atwood, in particular, showed me the Bell Labs archives and other primary sources. Charlie Kittleson, the magazine’s founder, had a treasure trove of working 1930’s electronics, which sounded very different than anything I’d heard before ... not like the Fifties sound at all. They clearly had different priorities back then.
John Atwood and I have a lively interest in early technology, particularly early monochrome and color TV in the USA, the UK, France, Germany, and Russia. Developments in color TV filter technology went on to influence Neville Theile in Australia, modern crossover design (Laurie Fincham in the UK), and the time-switching technology used in the GE/Zenith FM Stereo multiplex system.
I tried to paste my screen shot showing the 300 B line stage in the chassis. Hybrid so not a purist design like yours. Being a biased owner with other Amplifier ownership over some years it is a worthwhile audition if convenient.
Yes, it is quite surprising how sensitive these two circuits are to parts choices. Much more so than any I have ever worked with. I am used to coupling cap differences, and I have my favourite types, but there are several film caps in here that make as much or more difference. Also, I have been through several versions of anode loading, and it is clear in this circuit that a custom designed interstage transformer walked all over the other choices in the amps. We developed a new twist on the regulated supplies for the preamp, and now we will extend that idea to the power amp and then we are done. As Lynn said, I think we are reaching the limits of what the circuits can do. Also, I am trying the excellent Monolith Magnetics transformers as soon as they arrive. I will see if this pushes the amps over the top! As I believe I said earlier in the thread, this is pretty much a cost no object project to see what is possible with these circuits. They sound unlike anything I have ever heard or worked with. The better you make them, the less they sound like anything at all, which is about the highest compliment I can give an amp or preamp.
Sadly, only one prototype was ever built, but it did get demonstrated at EMI and the BBC in 1975. After that, loudspeakers, Tektronix, and various magazines.
Believe it or not, there’s a programmer in the UK who actually built this in software a couple of years ago. Now, that’s impressive. The 1975 hardware prototype took nine circuit boards plugged into a backplane ... the UK programming genius took it to the next level, and made it into an eight-band decoder, all working in parallel, thanks to the wonder of modern DSP.
These pictures give me a bit of a chill. It isn’t a Raven, not quite, but it’s pretty dog-gone close. Both date from the Twenties, and they come out of Bell Labs. In the first, note the archaic nomenclature for the direct-heated tubes and the weird little capacitor ... and what the heck is it doing, dragging the lower grid off-center?
OK, I didn’t know how much Don was keeping under wraps, so I was a little vague about our continued progress.
The new full-size chassis of the Blackbird (compared to the models at the show) gives us the freedom to "open up" the Blackbird ... ultra performance caps in critical locations, a bit of Raven tech in the front end, and more rigorous isolation between the high-voltage power supply and the audio circuitry. All of these need more room, which is why the production chassis will be 18" wide. Sonically ... well, I haven’t heard it yet, but they’re all good things that move in the same direction as the past year of collaboration.
One the things about the Raven/Karna/Blackbird that is frustrating, but also very gratifying, is the circuit is extremely transparent and revealing. The frustrating part is that parts quality is revealed in a relentless glare, at least in the critical nodes of the circuit. This is the downside of any zero-feedback design; there is no clean-up crew of servo feedback to tidy up afterward. You hear things as they are.
But the transparency is also a gift, because "minor" substitutions are immediately apparent in the first minute of listening. I think Don, Cloud, and the rest of the Spatial team will agree on that point. I feel we are fairly close to the upper bound of what the circuit can do, but I keep being surprised.
The Raven, which I had never heard before, took me aback ... that was not what I was expecting. It is super fast and resolved, with sounds flying out of dead-black space. You can practically see the shapes of the notes as they fly by. No exaggeration, no tipped-up HiFi sound, no artificial edge sharpening, but boy, it’s all there. If it was on the recording, you will hear it.
I find it kind of shocking that a late-Twenties Bell Labs/Western Electric telephone repeater, built with modern ultra-wideband parts and quiet MOSFET cascode power supplies, sounds like that. There ain’t nuthin’ retro about the sound at all.
Actually, I have already improved the preamp considerably with a few subtle changes, and the power amps will get improvements to the power supply with the larger case, as well as higher quality and physically larger film caps in what we now know are 3 really important places. I could not fit some things in the previous case that Lynn and I agree should be added. I would expect the whole system to sound noticeably better than what was at the show. Same tonality, but a bit faster, with even better imaging and separation of elements in the sound stage, and tighter and deeper bass. Experiments are already showing these improvements. Production by the end of the year is the goal. When we have production models there will be photos posted on the Spatial Audio Lab site. I really appreciated meeting people at the show and getting their opinions!
Alex, I wish you every success with your revised 300B amplifier. An isolated power supply for the input+driver, and replacing all the electrolytics with arrays of 440VAC (630VDC) industrial motor-run caps, will make a difference that will astonish you. Not joking here. It will definitely take it into the top class of SETs.
Be prepared for a pretty large and heavy chassis for an 8-watt amplifier. 18" x 18" and 50 lbs or more would not be out of line.
As for cap-value tuning, set the RC (or LC) frequencies between 3 and 4 Hz. The beat of most music is between 1.3 and 2 Hz, and you do NOT want any of the RC networks interfering with that. Anything slower than the beat of the music will give a kind of seasick, unsteady feeling, so don’t go there.
Leave the banks of electrolytics to the DC-coupled transistor guys. A lot of their tuning (and amplifier sonics) comes down to the brand of the electrolytic. (Oops, did I let the cat out of the bag? Sorry, guys.)
Don accomplished a miracle of miniaturization for the show, but we’re dialing it back a bit for the production models (to simplify assembly). We’re expecting 18" wide, the exact same width as the preamp, and maybe 16" to 18" deep. Weight ... yeah, maybe 50 lbs or so. Depends on what the transformers weigh. Specs and overall performance will be the same as the show models, so if you like what you heard, that’s what you’ll be getting.
This most genial, thoughtful, educational, respectful and generous thread of all time deserves to be a "sticky". Not only is it full of wonderful history, a person can learn a lot from it.
A little ways up @donsachsused the expression "nutshell" in reference to one of Lynn’s posts. Hah! I think it is more like an advanced panel discussion between Don, Ralph and Lynn and some of our other technical gray-beards. Or maybe it can be likened to a graduate seminar in esoteric Amp design.
Wonderful. My nomination for thread of the year, 2023.
How would you compare the sonics of your 300B amp to the the Class D amp you make now? You’re the creator of both, so you’re in the best position to evaluate and compare. I only spent a half-hour of casual listening to the Purifi at the show (and Audio Group of Denmark), so I’m hardly an expert on the subject.
The thread is called "300B lovers", so tell us what you think about a 300B (of your own creation) vs your latest Class D amp. At the risk of thread derailment (I ask the forgiveness of the Audiogon moderator), and then I’ll return to the walk down Memory Lane.
P.S. Karna and I actually lived in a house on Memory Lane when we were in SW Portland, up in the West Hills, just off Sunset Highway. It's a real place.
Ralph, I sincerely invite you to build your own 300B amplifier.
@lynn_olsonI did just exactly that some years ago and played the amplifier at a CES in the late 1990s. The 300bs were driven by a cathode follower direct coupled to the grids of the power tubes so the bias was obtained from the driver tube which in turn ran fixed bias. In this manner the capacitance of the grids was a non-issue.
The very first one, the Amity, designed by yours truly, and built by Matt Kamna on an open breadboard-style chassis. Matt has since gone on to co-found Whammerdyne, a company that makes 2A3-based SET amplifiers, and exhibited in the Songer Audio room at the PAF show this year.
Here’s a picture of Gary Dahl (seated) and Gary Pimm, shortly after Gary delivered the newly built Karna amps to our living room in Silverdale, Washington. 2003.
The dual B+ power supply chassis is external, connected by the aviation-grade Amphenol connectors at the rear of the chassis. The glowing VR tubes are at the front of the chassis. The EL34 on the left side is part of the high voltage current source that feeds the VR tubes. The audio tubes are 5687 input, 45 drivers, and 300B power tubes. Interstage transformers are under the chassis.
This picture gives an idea of what Don accomplished over the past year, reducing this behemoth four-chassis prototype to something that could be practically built, and then exhibited at the Pacific Audio Festival.
I second what Don just said. Isolation is the key. You can get away with a power transformer that has dual isolated secondaries, but this is a specialty item so don’t bother tracking it down. At the DIY level, just get another B+ transformer that runs at a voltage suitable for the IT-connected driver tube, and take it from there. In terms of rectifiers, damper diodes are the quietest of all, but they consume a lot of heater current at 6.3 volts.
The dual power supply approach is surprisingly rare in consumer equipment, even at extremely high price levels. It is the single biggest improvement you can make to any tube gear, from push-pull 6L6, EL34, and KT88 to SET amplifiers of any type.
Extremely large banks of electrolytics are popular over in transistor land, but they are frankly mediocre sounding caps, compared to good film caps of more moderate values from 50 to 200 uF. In the Karna, I used banks of industrial-type motor-run caps from ASC and GE.
These are precision parts designed for extremely severe duty outdoors. I prefer them to audiophile parts in that application. A minor audiophile tweak is to bypass the industrial array with a single 0.1uF cap of very high quality, such as copper foil. (Also use copper foil for the RC or LC coupling of the 6SN7, but be aware that wax caps are not suitable for under-chassis use.)
Location is important. Keep the wiring, especially in the cathode circuit, as short as possible. This is more important than the type of wiring, although if you want to go nuts, use industrial Litz wire for the critical audio path. Litz does require a solder pot to get rid of the enamel coating, a minor annoyance when working with it. A close second choice is tinned stranded, which is super easy to work with.
This approach, if done right, will take your SET performance to the mid to upper tier of Audio Note, in the $20,000 to $50,000 price range.
@alexbergerConsider using the tube rectifier for the front half of the amp and good diodes for the 300b. Do a C-L-C for each and use a good film cap as the last one after the choke. Separate power supplies allow the input and driver tubes to basically not know the 300b is clipping. When you push the 300b the power supply for the whole amp sags a bit. With separate supplies the input and driver sections are isolated from the 300b. You will hear a certain ease and clarity to the sound that you don't have now. The Blackbird uses a similar approach, but much more sophisticated power supplies, which I will not get into.
Hi Lynn,
Reading your previous post encouraged me to order a power transformer and a pair of 15H chokes for input and driver tube. It will give me not just PS separation but also reduces the load current to the 5u4g rectifier that I use for all tubes in this stereo amplifier (180mA total). It also gives me a freedom to increase drivers idle current and probably I will try 6v6 instead of 6f6. 6v6 needs more current.
Now I use very big capacitors for B+ - 2000uf for 300B and 1000uf for drivers. What is the advantage of separate power supply for drivers over big capacitors? How do behave big capacitors during the clipping of 300B?
Alex Berger, what you have sounds fine to me. The only refinement would be separate power supplies for the input+driver and the output section. And maybe a dedicated filament transformer for the 300B, whether AC or DC powered.
Ralph, I sincerely invite you to build your own 300B amplifier. (The name of the thread is "300B lovers", after all.) You’re smart enough to bring Allen Wright’s topology back to life, and give it unique improvements of your own. Seriously, if anyone can do that, it would be you, not me. The only way to explore the 300B sound is design your own amp around it, and I have no doubt it would turn out well. I think you would be quite pleased with the result.
Don, myself, and the Spatial team are developing the Raven/Karna/Blackbird architecture discussed in this thread, with really good suggestions from the whole team. Much appreciated, I can’t say that enough. You guys have taken it way beyond the 2003 thought experiment that Gary Pimm and I built.
What folks heard at the 2023 PAF show was a good preview of the full-on production model, which Don is charging ahead with as we speak. (Well, actually, I was just on the phone with Don, and he was at the beach, but tomorrow, OK?)
I use 1/2 of 6sn7 in the input stage. The driver stage is the 6f6 tube drives the 300b though interstage transformer. The coupling between 6sn7 and 6f6 is RC with Vcup Cutf capacitors.
@alexbergerThe 6SN7 can support fairly high current which is why it can make a good driver tube if used properly. It can also make an excellent voltage amplifier (1st stage of gain) since it is quite linear if used correctly.
SETs usually do not need much gain since the speaker used with the amplifier should be high sensitivity (if a 300b power tube, +100dB is a good value). The 6SN7 will allow plenty of gain for this. One section can be used as the voltage amplifier and the other section the driver.
You should be able to get plenty of bandwidth using a 6SN7 and an interstage transformer! The issue will come down to the quality of the transformer itself.
your choice of 6sn7 as driving tube is not good (high output Z, low max plate current...)
Now, your choice 1 of implementation is the better one, only your concern is invalid.
High output Z does not automatically leads to narrow bandwidth. Bad Interstage transformer causes it. High Z tube can't drive 300B into deep A2 so you never get 8W class A output from 300B that way. However, you can use Western electric's classic Ultra path design pattern, combining a CCS with a 0 DC current nickel 1:0.75 interstage transformer and achieve pretty good result if you bi-amp the speaker.
I have a question about the first 6sn7 stage in my 300B SET. What is the best way to make load and coupling?
1. The drawback of the interstage transformers is a narrow bandwidth in combination with 6sn7 that has high internal impedance. But low distortions.
2. RC - coupling has wide bandwidth but higher distortions.
3. LC - coupling looks good, but what is the optimal inductance for 6sn7? The issue can be not deep enough low frequency bandwidth and high LC resonance Q.
4. What are the drawbacks of SRPP with C or direct coupling? What is for and against it?
@lynn_olson@donsachs@atmasphere- Thanks for your responses. My system is bi-amped with a SS amp driving the lower three octaves, so I'm primarily concerned about upper bass on up. I'm pretty happy with my current DIY 300B PSET monoblocks, but I like to tinker.
I've primarily been building electronics based on other's designs (at least at the module level), but I've recently been trying my own designs (heavily influenced by others). I think I want to build a PP amp as my next amp, using either 300Bs or 2A3s. My main speakers are pretty sensitive (98db/w line arrays) so I don't need a ton of power. I'm still thinking about what approach I want to take.
Since he is no longer with us to defend his design, what do you think are the positive attributes of a differential output stage in a tube power amp?
@jaytorSince Lynn isn’t going there I’ll take this one. The advantage is the differential effect reduces distortion in the output section and makes the output section easier to drive since it will have a bit more gain.
There are a lot of differential output sections in well known tube amps- such as the Dynaco ST-70. What makes it differential is the use of a common cathode resistor. A Constant Current Source (CCS) can help performance but isn’t needed for the gain stage (whether an output section or not) to be considered differential.
Despite Lynn’s remonstrations, if designed properly a CCS in the output section of an amplifier will not limit current right up to the full power of the amplifier; in fact if the output section isn’t differential, using a pair of cathode resistors rather than a common one, the output power is unchanged or even reduced. I’ve seen applications where the use of the CCS actually increased the output power by a few Watts since the distortion was held in check to a higher power level.
What might not be obvious WRT an output section is you can set up the cathode circuit regulator to sense B+ variation and adjust the cathode voltage in response, which reduces distortion and increases tube life. This eliminates the benefit of a regulated B+ which would otherwise be a hefty lift in terms of execution and cost. IMO Lynn is missing a bet on this one and leaving performance on the table.
Yes we all have different tastes. I would say the Lynn and I overlap considerably. When we discussed our favourite rooms at the PAF we were in general agreement. I stand by what I said a million pages back in this thread. If I were to magically create a straight wire with gain that could drive any speaker load with infinite power and had no sonic signature at all, my guess is that half the people wouldn't like it. Heck, maybe I wouldn't like it. If I were to characterize what I favour it would be lush, not mush, an ethereal high end where cymbals and triangles hang in the air and decay, and tons of detail without brightness. I hate artificial, in-your-face detail created by bright sounding systems. I am an imaging freak as well. If I build an amp or preamp and the sound stage doesn't extend at least a few feet outside the speaker boundary and the vocal doesn't appear at the floor to ceiling interface, then something is wrong. I was a little dismayed at the show when our system could give great depth, but we had a shoe box of a room and the sound stage was constricted in height and width by the room boundaries. Same system in my room threw the sound stage I just described. I voice things with acoustic instruments and vocals. If you can get a piano right you can pretty much play anything else with correct tonality. If you can follow individual voices in a choir or instruments in an ensemble of some type, then your system can handle complex passages without breaking down.
Oh, and I love a good horn system. Not the cheap ones that are popular, built with junk parts and cabinets, but a good horn system.
So that is what I like. Lynn and I agree more than we disagree!
There was a funny incident a few years back at the Dallas Audio Show. Back then, it was a little bitty thing, just a few exhibitors, but very much a home-town thing where everyone knew other. New to me, of course, as a former West Coast guy fairly new to Colorado. Never been to Texas before.
I wander aimlessly down the halls, no real goal in mind, looking for interesting tube gear. I walked into one room, and whoa, that’s Nelson Pass! Now people joke about me being Mr. Natural, but Nelson really looks the part. You can’t miss him. Me, my only trademark at a show are the Hawaiian shirts I like to wear.
Nelson had actually built an open-baffle speaker around a Lowther and a 12" guitar speaker called the Tone Tubby that I had written about some time ago. Well, that’s different, but why not? As I turned towards the door, Nelson blocked the exit. how do I get in these situations? Me and my big mouth.
It turned out the two drivers were bi-amped with a simple low-level crossover. Oh, now I get it. Four knobs, two for level, two for the crossover frequency. Nelson wanted me to tune the thing ... by ear.
Now I really want to escape, but Nelson is still in the way. Fine, anything to get out. Twiddle, twiddle. Too little bass. Mo’ bass, man. Turn that knob up. A bit less Lowther, but not too dull. Mess with the crossover overlap some, so that mellow hemp cone transitions into the characteristic hard paper Lowther cone. A few minutes later, sounds OK, as good as I can get it right now. (Did not sound OK when I walked in.)
Escape permitted. Afterward, Nelson allowed as to how he saw that article I wrote about the charms of the Tone Tubby and wanted to build a simple open baffle around it, with a Lowther on top. So he figured if anyone could tune it on the fly, it would be me. Well, he had me there, but I allowed that he might have different preferences than I did, so feel free to mess with the knobs, although he might want to mark the current positions before changing anything.
These weird things happen to me at shows. That’s how I met Nelson Pass.
I should mention that Allen Wright liked a very different sound than I do; he liked fast, snappy, and what sounded to me like thin bass. I like a big, lush, spectacular, CinemaScope sound, the sound I heard in 70mm theaters when I was growing up. (Which had Altec Voice of the Theater speakers behind the screen, along with Altec amplifiers.)
The same applies to my brief encounters with Nelson Pass. He likes it a lot thinner than I do, but with a different tuning than Allen Wright. Kind of hard to describe, actually, since this was all a long time ago. Allen liked the sound he was getting, and he liked his own amp, even at that meeting all those years ago. What I thought was a disaster seemed OK to him. In all honesty, it was a split decision.
I mean, I didn’t like it, nor did Gary Pimm, but we were on a different wavelength than Allen Wright. His designs, like mine, are tuned to his own tastes, and we found out they were surprisingly different. Similarly, I was surprised at Nelson Pass’ tunings, very different than my own.
As it is, Don and I have a bit different preferences, but at least we are still on the same planet, so we get along. From what I heard of Allen’s designs, no way, they are too different, no good way to reconcile the two approaches. But he was a really fun houseguest, and Gary Pimm and I had great discussions with him about everything under the sun.
I miss him very much. He was really funny and one sassy dude with total disrespect for the high and mighty poo-bahs in the industry, which I very much shared.
As you might imagine, Allen was pretty shocked at the direct comparison, since his amp had much more powerful tubes than mine, which had generic Sovtek 300B’s, good and tough, but nowhere in the same league as Vaic’s finest. I mean, a quartet of top-of-the-line 300B’s ain’t cheap, so I never went down that road.
And Allen had just given a presentation at the VSAC, only hours before, on the power of this secret circuit, which he did not fully reveal. It was a very large current source with heat sinks and all. Yes, he could have cranked up the current even more, but the heat sinks and power transistors set an upper limit on the current. It was already close to max output.
He expected that I, an old Tek hand, would be thrilled with Tek-scope type circuit. But I disappointed him. Driving deflection plates (at very high speed) on a CRT is one thing, driving a loudspeaker is quite another. And I’d been designing speakers for Audionics several years before joining Tektronix in 1979.
Scopes are about speed, and the load is a very well-defined capacitance. Cascode differential circuits are the right answer for that problem ... they’re very fast, ideally suited for square waves, and linear enough for the purpose.
Speakers are orders of magnitude slower and are inherently vile loads. The best speakers are the worst loads ... the ones that have near-resistive loads are planar-magnetics with very low BL product (which is magnetic coupling). As you raise BL product, efficiency goes up, they get snappier sounding as the coupling gets better, and ... they also get more reactive, for the simple reason the amplifier is in more intimate contact with the big, sloppy, electromechanical system. Few amplifier designers are aware of this ugly reality. They keep hoping for speakers that can never exist.
The worst thing speakers do is insert speaker colorations (through back-EMFs) into the feedback loop, where they do not belong. Feedback is great at correcting amplifier nonlinearities ... it’s fast and responds in microseconds, just what you want. Speakers have inherent high-Q resonances that are an inescapable part of an electromechanical device. The better the magnetic coupling, the worse it is for the amplifier, which has dirty spurious currents injected into the output node by the speaker.
My approach is to brickwall-isolate these back-EMFs to the final output stage, and not expose the rest of the amplifier to them. I think of the speaker load like attaching a vacuum cleaner motor to the output section ... a source of noise and garbage, nothing good about it. The amp has to ignore this racket and continue to do its job. Feedback amps can get into trouble when the error voltages get very large; this can saturate the input section, and induce additional distortion.
In a more conventional application, like a long-tail Mullard phase splitter, differential circuits have a subtle imbalance that is not obvious at first glance. On the top, or front, side of the circuit, there is the expected Miller capacitance, as per expectation. This is the inverting side ... grid goes down, plate goes up, just like you expect.
The non-inverting side can be drawn (and is better understood) as a cathode follower driving a grounded-grid stage. Rotate the other tube by ninety degrees and it becomes more obvious. This side of the circuit has very little Miller capacitance, making it ten to twenty times faster than the other side. The beautiful symmetry falls apart at (very) high frequencies. As mentioned earlier, it can never enter Class AB drive when one side cuts off, although this is not a problem if the diff-pair is not used as a driver. In a scope, you see clever bootstrap circuits and cascodes to give that extra push at high frequencies.
This is Nelson Pass’ speciality; high speed cascode differential circuits. If that’s your thing, he has an amp or preamp just for you. If you’re using transistors, this is an attractive path.
@lynn_olson- First, let me state that I've really enjoyed your, Don's and Ralph's discussion of amplifier design and tradeoffs.
I've been intrigued by Alan Wright's designs (I'm currently building a line stage preamp inspired by the RTP3D). You've done a good job explaining the downsides of a differential output stage. Alan was a big proponent of this approach so he obviously felt it had advantages. Since he is no longer with us to defend his design, what do you think are the positive attributes of a differential output stage in a tube power amp?
I should add I am completing a large-format 2-way speaker this summer, a collaboration with Thom Mackris of Galibier Designs, and an entirely separate project from Don Sachs and the Spatial Audio team. It’s a culmination of the extremely long "Beyond the Ariel" thread over on DIYaudio.com, and the first version was built by Gary Dahl, of Silverdale, Washington.
The woofer is an Alnico-magnet 416 (15" midbass) from Great Plains Audio, the successor to Altec Lansing, using Altec staff and tooling. It’s in a low-diffraction (4" radius curved edge) 4.2 cubic foot closed box. My version will have Bubinga (African rosewood) veneer on all sides.
The high frequencies are from an Athos Audio Yuichi A290 wood horn, with a to-be-determined 1.4" exit monitor-class compression driver. Crossover will be around 700 Hz, most likely Altec-style 2nd-order. The RCF 850 and 18Sound drivers are candidates. I also have a pair of Altec/GPA 288’s in house as fallbacks.
Efficiency will be a true T/S value of 97 dB/meter/watt. With a 27 watt/channel amplifier, headroom should be, in the timeless words of Rolls-Royce, "adequate". Alternatively, sufficient for a studio monitor application.
A 20-watt amplifier and 97 dB/meter loudspeaker was pretty typical for a serious high-end system in the mid-Fifties, so it’s not as weird as it sounds. It’s only weird in the modern context of 200 to 500-watt Class D amplifiers and 85 to 87 dB/meter audiophile speakers.
Most audiophiles do not realize how stupendously inefficient speakers are. By way of reference, 92 dB/watt/meter is about 1% efficient, or put another way, 100 watts of electricity is converted to one acoustic watt (which is plenty loud).
So where does the other 99% of these pricey watts go? Voice coil heating, which isn't great considering how tiny voice coils are, and how poor thermal coupling to the outside world is. First the voice coil has radiate its heat to the magnet, which is the closest thermal sink, then the warmed magnet has to transfer its heat to the inside of the enclosure.
Since the goal is to create X amount of acoustic watts, not a clumsy form of room heating, even small gains in efficiency are worthwhile, since less voice coil heating is occurring for given acoustic output.
Aside from outright failure, another problem with VC heating is copper's change in resistance with temperature. The resistance goes up with temperature, which might be acceptable, excerpt the time constant is fairly slow, on the order of several seconds, This creates a dynamic slurring which is pretty audible.
I can see 88-89 for smaller speakers. I have a little 5 Watt tube amp I designed for desktop or a bedroom system and I use a pair of Fritz Carbon 6s with it, which are 88dB and I never run the amp out of gas (but I never play it that loud either).
But if they are going to be large there's no reason they should be hard to drive. I keep telling people that if you want to get the most out of your amplifier dollar investment, its best served by a speaker that is higher impedance and easier to drive, on account of the simple fact that the harder the speaker is to drive and the lower the impedance it is, the more distortion the amp is going to make. IOW a simple way to make any amp sound smoother and more detailed is to have it drive a higher load impedance (if all other things were somehow equal, which they never are...).
Yes we agree. If you cannot drive a loudspeaker in a normal room to adequate levels with 60 watts/ch, then you really need to reconsider your speaker choice!
The least efficient speaker I ever owned was a pair of Joseph Audio RM25si Mk2, which were 89 dB and a true 8 ohm easy load. With the 60 watt amp I could play them to FAR higher levels than I would ever listen to in a very big living room. Now my speakers are generally in the 95-97 dB range so I could do the same with 10 watts. The 27 watt 300b monos hardly know the speakers are connected.....
There are so many just brilliant speaker designs out there that are 89-90 dB+ and easy loads. Why on Earth people get huge 84 dB poorly behaved speakers that require 200 watts/ch or more is beyond me.....
The previous point about Class A operation in a differential stage still holds: what happens when more than 100% of the current programmed in a current source is exceeded?
It won't if the circuit is properly designed!
The real question is what happens when the drive to the differential gain stage exceeds the range of that gain stage. The answer is one of the devices saturates while the other goes into cutoff. Picking the right amount of current in the constant current source (if there is one, differential amplifiers do not need a CCS to work... the first circuits we built employed a bipolar power supply; the cathode resistor had the entire B- Voltage dropped across it; this limited current to the same extent that any cathode resistor might in any single-ended circuit) is the key to making sure that the design isn't limited by the CCS. Instead you want it limited by other parameters- the tubes themselves, the plate load, etc. The addition of a CCS increases differential effect- thereby increasing gain and decreasing distortion, as well as improving bandwidth, assuming that the CCS does not impose a bandwidth limit.
@donsachsI get it. I was trying to point out the difference between what sounds 'louder' and actual sound pressure; as you know from playing tube amps the two are not always the same. IMO this is one of the bigger failings of SETs with zero feedback since, more than any other kind of amplifier made, they tend to sound louder than they really are due to how they make distortion.
60 Watts isn't a whole lot less to our ears than 250 Watts is due to the logarithmic nature of our ears. So as long as the 60 Watts can adequately drive the load it can do quite well. This is the same reason we didn't try to build a super high powered class D amp. It was more important to get it right than it was to make a lot of power- as it is, it makes 200 Watts into 4 Ohms (250 at clipping). If your speaker really needs more than that kind of power to really fly, its borderline criminally inefficient, since to merely double the sense of volume to the ear, you need ten times the power. To my understanding there are no 2500 Watt amplifiers that sound like music.
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