Thought on OTL tube amps

Ralph, thanks very much for the highly informative post.

I have one further question. In this thread, and I believe in another current thread, you alluded to PRaT as something that can be designed for, within the constraints of certain tradeoffs that depend on the type of amplifier. Could you elaborate on what technical factors and design characteristics affect the PRaT of an amplifier? That's something which has never been clear to me. Thanks!

Best regards,
-- Al

I said yes to most, but you are correct the Joule is not lean I could live with this one.

Sure. The issue is this: there are human hearing perceptual rules and there are the specs that we see on paper, and surprisingly they have very little to do with each other. So if you design your amp to look good on paper, it at best will sound like a good hifi.

What we really want is for it to sound like music. For that, we have to get the circuitry/stereo to obey as many of these perceptual rules as we know about. This often results in amps and speakers that don't measure so well. For example, the human ear uses the 5th, 7th and 9th harmonics to determine the volume or sound pressure of a sound. Electronics have the ability to distort these harmonics (enhance them); this results in such electronics sounding artificially louder than they really are. We see this all the time- 90% of the time if an audiophile's wife is screaming them to 'turn that down!!' its likely because these loudness cues exist in un-natural abundance.

You can design to minimize the distortion of these harmonics. The first step is to not use loop feedback, as it is known to **enhance** such harmonics. Of course, then to get rid of distortions you have to use every design trick in the book to try and not make distortion.

IOW what we want to do is engage the human limbic system rather than the cerebral cortex. So in addition to distortion issues, the circuit or system has to be fast enough; if too slow the processing of the sound moves to the cerebral cortex. IOW the experience of the sound becomes intellectual instead of toe-tapping.

This is a sort of basic introduction but I think is also the easiest to understand. Obviously without feedback the THD of the amplifier tends to go up, but THD is a description of a settled-out sine wave and really says nothing about the behavior of the amplifier with a constantly-changing non-repetitive waveform, which is what nearly all music is. The evidence right now is that negative feedback actually *increases* distortion with non-repetitive waveforms- quite the opposite of its intentions.

If you look at this from the field of Chaos Theory, what we find is that the formula that describes feedback in an amplifier is nearly the same as we see in Chaotic systems- and not what we want for reproduction. Chaos Theory suggests that amps with feedback exhibit a chaotic behavior which will include harmonics of the fundamental waveforms up to and exceeding the 81st harmonic! I find it interesting that as the science of math evolves, it is opening doorways to improvements in audio that many people have thought were far too cut and dried for there to be any serious evolution.

JWM

The Atma-sphere MA1 amp on my Soundlab A3 speakers is anything BUT lean sounding. I find it to be full of bloom and weight.

"...what we want to do is engage the human limbic system rather than the cerebral cortex "

That should be motto of every audio designer and it is ours, see - http://www.enjoythemusic.com/magazine/manufacture/0708/index.html

Yet, I cannot agree with Athmasphere that negative feedback is bad.

Our entire method of amplifier design is based on the approach which considers an amplifier as a control system. Because of this, our amplifiers are designed with an emphasis on very, very, very high speed control loops. The ultra-short transit time allows the amplifier to correct for many small errors; and the control loop can follow the input much more accurately. It does not (or very minimally) create odd order harmonics, it does not create pahse discrepencies and other garbage conventional NF does. These electrical characteristics result in a more detailed, transparent sound with less noise and louder yet cleaner musical reproduction.

Its very difficult to compete with OTL in harmonic richness for any solid state amplifier. I admit it. We are doing our best but its difficult. However, if don;t like heat, weight, tuning up, warming time and may be have difficult to drive speakers then - you are welcome to try our (and others) ss amps.....otherwise, everybody would own OTL amp...

Oh, almost forgot - there were a few well deserved praises to Jud Barber (of Joule-Electra) OTL designs but Jud does use very judiciously negative feedback....to control speakers even better.

I hope that after RMAF 2010, we will be able to add to this discussion of fidelity to true acoustic music we all strive to achieve and have in our systems.

Good Luck to Everybody.