300b lovers

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.

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.

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?

@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.

@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.  

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.

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.

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.