Is Zero Global Feedback Important?

JLeeMD -- Mon, 05/16/2011 - 10:16

Some amp manufacturers such as Ayre and Simaudio make a big deal about having no global feedback in their designs, citing superior time domain performance. The argument certainly has logic. Is this a real issue or just marketing hype?

Mullard EL34 -- Wed, 05/18/2011 - 06:40

Global negative feedback is simply one tool available to audio designers, but global negative feedback is not inherently amusical or evil; Soulution, Pass Labs, Audio Research Corportion, etc., employ global negative feedback in their highly-regarded state-of-the-art designs. Nelson Pass (design guru at Pass Labs) has posted a very enlightening article on his Web site that discusses the impact of global negative feedback within a high-end audio design context (http://passlabs.com/pdfs/articles/distortion_and_feedback.pdf).

JLeeMD -- Wed, 05/18/2011 - 11:07

I tried reading Nelson Pass' article but found it too technical for this MD! I started to wonder about the merit of zero feedback when I was reading TAS review of the Simaudio 600i where RH was clearly impressed by the 600i's ability to communicate musical expression. But as you point out, Pass Labs and Audio Research amplifiers use feedback and they too are frequently commended for their musicality.
Now, what about YG Acoustics claim about the importance of phase coherence? I know that it is but one tool as is the case above, but the argument has compelling logic.

Mullard EL34 -- Thu, 05/19/2011 - 07:23

Sorry that the Nelson Pass white-paper seemed a bit technical. In essence, Nelson's position is that judicious use of negative feedback (regardless of whether the feedback is global in nature or locally-applied) is relatively benign. Too much negative feedback leads to the unwanted generation of high-order harmonic distortions while concurrently reducing the overall distortion levels. Sparingly applied negative feedback offers many sonic advantages without penalties.

Be cautious about being lured into over-simplfication. Einstein's tenet that "Everything should be made as simple as possible, but no simpler" certainly applies to audio design. Soulution designer, Christoph Schurman, views negative feedback, properly applied, as a core aspect of Soulution product designs. Christoph's design ethos revolves around speed (which requires high-bandwidth, which challenges stability) and cocomittant precision. Referencing the highly-regarded Soulution 710 power-amplifier, Christoph notes, "The signal passes through my voltage-gain stage in approximately 10 nanoseconds with a max amplitude deviation of 0.1dB." For Soulution, to truly push amplifier design forward meant devising a lower-gain topology that could guarantee complete measured stability, regardless of load behavior or feedback. Obviously, Soulution also highly values phase coherence! However, rather than throw the baby out with the bath-water, Christoph has chosen to reconcile the use of negative feedback by ensuring high-speed circuitry that eliminates any discontinuity between the design requirements for phase coherence and the use of negative feedback.

Don't eliminate products from consideration because of marketing hype about the technologies employed in a product's design. Just listen to the products with a unbiased set of ears and see which products best convey the musical essence of the recorded performance.

JLeeMD -- Thu, 05/19/2011 - 11:35

Thank you so very much for distilling the subject into a paragraph that even I can understand! I'm sure the same lessons apply to speaker design as well-regarded speakers (i.e., Magico) are not 1st-order crossover/phase coherent designs. I guess I'm a sucker for marketing hype!

Mullard EL34 -- Thu, 05/19/2011 - 14:55

Most of the better audio designers (Nelson Pass, John Curl design consultant), Charles Hanson (Ayre), Bill Johnson (Audio Research), Bob Cordell (design consultant), etc.) adhere to Einstein's principle against over-simplification; instead, these major design talents tend to take a best-of-breed, eclectic approach to balancing their designs, all in an effort to deliver the maximum musical truth via physical (and therefore flawed) instrumentality. Simplicity exerts a very powerful allure for all of these leading designers, but overall circuit/topology elegance usually triumphs over raw simplicity. Of course, marketeers just love to be able to ascribe their product's sonic success to a single aspect of their design; it makes it easier to win customer mindshare if they can offer a simple mantra for why their product is superior. ;-)

As always, trust your ears.