300b lovers

Sounds like @atmasphere is early on their journey with a PP 300b.  Certainly far enough along to demonstrate a prototype at a show, much as we did in Seattle last summer. 

We started prototyping with DHTs seriously about 20 years ago. Back in the 1990s we built a 300b-based OTL which we showed at CES. 

The trick with the CCS is doing a good one. A lot of the circuits that you see on the web leaver performance on the table. We've had CCS circuits in our amps for the last 35 years.

'A lack of solid state coloration' is matter of avoiding the distortion signature that is endemic in so many solid state amps. This has to do with proper design of the feedback loop and a whole lotta loop gain in an amplifier design. The 'solid state' sound is just bad feedback application and is part of why feedback has garnered a bad rep in high end audio. 

Feedback has been known to give tube amps a 'solid state' sound too; this is because the feedback signal has been distorted prior to mixing with the audio signal its supposed to correct. Its like having an incorrect map to guide you through town. Norman Crowhurst wrote about this problem nearly 70 years ago but did not suggest a solution, despite it being rather simple. Peter Baxandall 'rediscovered' the same problem in solid state amps about 20 years later but he suggested more feedback, which doesn't solve the problem.

Its only been in the last few years where we've seen self-oscillating class D amps where this problem has been solved pretty consistently. You can do it in class A/AB solid state amps too if you're willing to work out the 3rd or 4th order feedback loops that need to be involved. Most designers just use a resistor which won't cut it...

@donsachs 

It is undoubtedly a different topology than ours and I wish them well.  I am sure they will get their version to where they want it to be as we have with ours.  They will sound different of course due to the preferences of the designers.

Hi Don, I just want to say that I appreciate your decorum and professionalism.   No doubt that the two 300b push pull amplifies will sound differently precisely for the reasons you mentioned. I’ve greatly enjoyed the many insightful and educational contributions from you and Lynn Olsen in this terrific thread.

I wish you and Lynn the best. Also Atma-Sphere with their endeavor as well.

Charles 

Once and always, Charles sets the bar for thoughtful and positive responses to topics on this forum, which in 10 months regarding this thread, have always been very upbeat and positive.  When we are treated to a master class in tube design philosophy, freely given, participants clearly appreciate the generous sharing of information by the masters: Don, Lynn and Ralph.  It has been a real education for me. 

@whitestix 

When we are treated to a master class in tube design philosophy, freely given, participants clearly appreciate the generous sharing of information by the masters: Don, Lynn and Ralph.  It has been a real education for me

Very well put.👍

This thread you initiated has miraculously avoided the all too prevalent trolling and disruptive deterioration. Bravo!!!

Charles

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.

Don and I go to some trouble to avoid even local feedback in the Raven and Blackbird ... all cathodes are bypassed in every stage, we do not use the Ultralinear connection in the triode-connected KT88, nor global feedback. Any distortion that is present is the result of first-order effects in the tubes themselves, not anything else. It also mandates selecting tubes that have a minimum of high-order distortion terms in the intended operating range.

This overlaps with the philosophy of some SET amplifier designers, but we prefer to use complementary pairs to cancel most of the distortion without recourse to feedback (local or global). The transformers, if correctly balanced, cancel out most of the distortion before passing the signal to the following stage, thus, each grid-pair is fed a cleaner signal.

This comes in particularly useful for the 300B grids, which can be a difficult nonlinear load for the preceding stage. Unlike RC coupling, when one grid demands current, both phases of the driver pair are available to deliver current to the 300B grid that needs it. This enables seamless and jump-free transition to A2 (positive grid drive) when the signal demands it.