Electronics 101: Use of Negative Feedback; a Q

I've taken an interest in the past year or so of trying to understand the exact origins of this idea . . . that is, that Global Negative Feedback is a Bad Thing. I think that there are three opening points here that can be accepted as fact:
- The ubiquitous negative view of global NFB in high-end audio is not found in other analog electronic engineering disciplines
- There were many decades between the first patents of the electronic feedback amplifier, and when it started to achieve a pariah status in high-end audio
- It is extremely rare to find a discussion of global NFB in an audio forum or publication where it isn't linked with the promotion of some accompianying viewpoint regarding components or topology . . . i.e. a proxy for a tubes-vs-transistors discussion.

From what I have found, this idea of Global NFB being Bad really came into being in the very early 1970s, in Audio Engineering Society papers written by mainly by Matti Otala, sometimes in association with John Curl. This is the origin of the TIM acronym, meaning Transient Intermodulation Distortion. These days, TIM is more accurately described as slew-limiting distortion, and has been very, very throughly investigated and discussed within AES, including standardized measurement methods.

Matti Otala and others did put forth the viewpoint that global NFB was at least part of the root cause of slew-induced distortion, and in his papers proposed some topologies with greatly reduced open-loop gain (hence less global NFB) as a solution. But while the association of TIM and global NFB somehow remained, Otala's suggested circuit topologies remain obscure. The reason for this is that while slew-induced distortion is indeed affected by circuit choices that also affect the amount of global NFB . . . slew-induced distortion isn't CAUSED by feedback.

The other source of this Bad Feedback idea seems to be a article written by Peter Baxandall for Wireless World, from December 1978. This article contains a graph where Baxandall plots the amplitude of the second through the sixth distortion harmonics (Y-axis) as a function of the feedback ratio (X-axis). Here, at a glance that increasing the feedback up to about 20dB causes an increase in the fourth, fifth, and sixth harmonics while it reduces the amplitude of the second. Continuing to increase the amount of feedback further then reduces all the harmonics in a linear fashion.

This is the ONLY well-documented scholarly paper I have found from which I can imagine comes this idea of global NFB decreasing the amount of lower harmonics, at the cost of increasing the energy in the upper harmonics. But this article is just one in a series of six on power-amplifier design, and if you actually READ them and not just glance at the pretty graphs, it's obvious that this is NOT Baxandall's conclusion from this experiment.

Rather, Peter Baxandall unambiguously states his conclusion from this experiment is that global NFB needs to be INCREASED to reduce all the harmonics to a negligible level, rather than avoided so that the second harmonic can completely dominate the distortion signature. He also specifically emphasizes that "even FETs used without feedback generate high-order harmonics - and therefore . . . high-order intermodulation products".

It's rather unfortunate that in discussions of negative feedback on the internet, the seminal papers from which most of these ideas stem remain largely ignored. This is of course because they're copyrighted, and one simply cannot pop up a link to them. But if anybody's truly interested, the Otala papers (and responses to them) can be obtained from the Audio Engineering Society http://www.aes.org, and the Baxandall series has been re-printed by Jan Didden at http://www.linearaudio.net

it would appear that electrical matching concerns between an amp and speakers having fluctuating impedance stats as a function of frequency may be mitigated in whole or part by using NF.

Here's what I think is the most precise response this question:
- Due to a varying impedance-vs-frequency characteristic, the response of a loudspeaker will be affected by the output impedance of the amplifier, thus . . .
- If the amplifier's output impedance differs significantly from that which the speaker designer used for evaluation, then the response of the speaker will be different from what the designer intended, and . . .
- Although there are no specific standards, *most* loudspeakers are designed with low-output-impedance amplifiers in mind, and . . .
- The use of global or local Negative Feedback is the (almost universally applied) method of acheiving a low output impedance in an amplifier.

I believe that Ayre SS amps do not use NF in the circuit design, yet have relatively high DFs, thus suggested low output impedances. I don't know how Ayre achieves these results without NF, but they do.

No. They use feedback, even if they say they don't, or come up with a reason why their feedback is "different". This is akin to all the "LED Televisions" on the market - virtually all of them are in fact LCD televisions with LEDs used as a backlight. But they needed a new acronym to differentiate them from the previous models, and marketing departments aren't exactly known for their terminological precision. How many times have you seen "solid-state" on the outside of a television who's principal component was a picture tube? "Zero-feedback" is a similar label.

In reality, virtually every audio power amplifier without an output transformer (tube or transistor) uses some sort of unity-gain voltage follower circuit as the final stage to lower its output impedance. This type of circuit has 100% local negative feedback, hence the unity gain.

I assume from a lay person's perspective, in plain English, that means if a speaker was voiced to be driven by a "Voltage Paradigm" amplifer (as described in Ralph Karsten's White Paper, a SS amp), then using a "Power Paradigm" amp (usually a tube amp) to drive the speakers may affect the sonic presentation. This assumes of course, that the tube amp has a relatively high putput impedance if little or no NF is used.

Ralph really seems to favor a binary viewpoint of the subject with these two clearly-defined camps of amplifiers and speakers . . . but it's indeed correct that certain some loudspeakers produce far more variation in response with a high output impedance than others. This is an important subject to him because his circuit design preferences place practical limits on how low his amplifiers' output impedances can be.

As for the subject of how low an amplifier's output impedance *needs* to be, I actually think John Atkinson approaches this subject in a rational and practical manner when he measures an amplifier's response into an IHF speaker load, and the corresponding impedance/phase plots for the loudspeakers he measures. It's also possible that one may prefer a loudspeaker's response to be somewhat different that its designer, and that a higher-output-impedance amplifier may yield a pleasing result.

Personally, I can't say I've found such a modification to a speaker's frequency response to be a pleasing one, except for a few very rare occasions. But your mileage may vary . . .

Admittedly, I am not familiar with Atkinson's test involving an IHF speaker load. Care to explain what that test is all about?

Start with http://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements and then read the loudspeaker measurement section of some loudspeakers. Please don't take this as my endorsement of Stereophile's reviewers in general . . . but specifically Atkinson's measurement sidebars tend to be very consistent an well-reasoned, and he explains himself well.

in the case of the ARC Ref 150 which uses 14 db of NF and probably has an output impedance between .4 and 1 ohm (probably closer to .4 ohm because its "rated" and reported DF is 17), can one assume that the Ref 150 will behave "SS-like" if presented with a speaker load that was voiced to be driven by a high current SS amp?

In the above link, you'll see that Atkinson actually measured the output impedance as 1 - 1.4 ohms from the 8-ohm output tap, corresponding to a Damping Factor of 8 - 5.7 respectively. This amp actually has a somewhat higher output impedance than most of the high-quality, correctly-operating vintage tube amps that I've measured . . . most of which actually have lower output impedances than many modern "low-feedback" solid-state designs. I think with the ARC Ref 150, the interaction between the amplifier's output impedance and most loudspeakers' impedance curves will be a significant part of its sonic signature.

Short of shlepping a 75 pound tube amp to a dealer to properly audition speakers, it's helpful to know (or at least be able to reasonably predict) if a tube amp can "switch-hit" and function in a "SS-like" way if presented with a speaker load which was voiced to be driven by a low-impedance, high DF, high-current SS amp.

The loudspeaker/amplifier equation is complex, and schlepping an amp to the dealer (or demoing the speakers in your home) is absolutely the best way to be sure you'll be happy with it. After all, can you really study automotive specifications and get an idea whether or not you'll like a car without at least putting forth the effort to take a serious test drive?

If you really don't want to pair up your amplifier with speakers you're considering, and you want to be happy with the way they work together . . . then I feel the most logical solution is to purchase the amplifier, loudspeakers, and loudspeaker cables all together, or purchase active speakers.