What is wrong with negative feedback?

The main "problem" is that that it's widely misunderstood by both those circuit designers that use it, and those that eschew it. It's also very much out of vogue these days. What negative feedback is, in essence, is the technique of trading circuit gain for circuit bandwidth and linearity.

But the vast majority of audio circuits use some form of negative feedback, regardless of whether or not they're advertised as "zero feedback". It's interesting to notice that many who shun feedback also prefer triodes . . . as most triode circuits have a good dose of negative feedback (based on the tube's internal characteristics). In fact, when a given circuit or active device (tube or transistor) displays the combination of less gain and improved linearity, it's likely there's some kind of negative feedback mechanism that's making it that way.

The common audiophile response is then "well that's LOCAL feedback, which is good!" . . . this usually is explained by lack of "delays" and such. But actually in many cases (i.e. typical solid-state amp), most of the "delay" (really a phase lag, NOT a pure delay) is in one stage (the Miller-compensated voltage amp), and most of the nonlinearity is in another (the output stage), so the stability consequences of global feedback are usually very similar to that of just local feedback around the voltage amp . . . but the global feedback arrangement of course works so much better. Incidentally, this is the main cause for higher-order/frequency distortion products in solid-state amps that use feedback - the distortion rises with frequency because the Miller-compensation technique shifts feedback from global to local as frequency increases, and takes the output stage out of the feedback loop. So the problem is really not with the feedback, but more the lack of it . . . that is, it's not available equally at all the frequencies that need it.

And it's also a misconception that local feedback results in automatically better stability . . . instability in cathode/emitter follower circuits from certain source impedances is a very well-documented condition. In fact, this is the whole reason for the invention of tetrodes and pentodes . . . triodes exhibit poor stability at the limits of their gain and bandwidth, as a result of their internal feedback mechanism. The addition of the screen-grid mostly eliminates the feedback, thus the increase in gain, and decrease in output impedance and linearity.

Oh and as for positive feedback . . . "bootstrapping" networks are extremely common in all kinds of analog circuitry . . . does that count?

Negative feedback extends bandwidth, lowers Harmonic and Intermodulation Distortions and lowers output impedance.

Unfortunately, if not used wisely, is increasing TIM - Transient Intermodulation. In time domain it will show as just small overshoot on fast changing signal like square wave. In frequency domain it shows as exaggerated odd harmonics that our ears are very sensitive to (especially higher order - responsible for perception of loudness). In really bad case it can momentary saturate output transistors that will stop responding for a short time since charge is trapped at the output transistor junctions. Our brain fills small gaps like that but it will make us tired. Whole thing (overshoot) happens because of limited bandwidth that is causing delay thru the amp. Delayed signal when summed (in opposite phase) with input signal that is changing rapidly is coming too late and amp for a moment has much higher gain. Class A amps don't require a lot of global feedback and gain (without feedback) is often as low as 200 but class AB has gains reaching 4000.

How amp should be designed? I would pick the most linear transistors I can find. I would use a lot of local feedbacks. I would measure bandwidth without global feedback and would limit bandwidth of the input stage to that bandwidth (necessary condition). Harmonic distortion would be probably 5-10%. I would use just enough feedback to get distortion below 1%. That would be great sounding amp that nobody would buy because of poor spects (distortion, bandwidth).

No feedback (or low feedback) design might sound more alive because distortion gives this effect (like distorted vs clean guitar) but mostly it would sound pleasant and not tiring instead of sounding brightly Hi-Fiish.

TIM was discovered in 70s. Before that designers went crazy with negative feedback - still claiming that it has to be sounding better than tubes. Logic says that if you see numbers like THD=0.000001% something else has to give. I believe that spects are pretty much useless since amp with greatest spects might sound the worse. People often use amps exact power doubling with 4 ohm load vs. 8 ohm load as a sign of great amplifier. I'm not so sure. It will show that power supply is strong but it will also show that a lot of negative feedback is used (since power supply is most likely unregulated).

Loop feedback in any form is supposed to reduce distortion. It is questionable whether it increases bandwidth, and in some models (see the link I provided) it reduces 'output impedance'. You'll see why I use the quotes if you look at the link.

The *big* problem is that loop feedback, in the process of doing all this stuff, exacts a penalty. This comes from the fact that any circuit that can amplify is doing so at speeds that are easily measured on rather pedestrian test equipment. This time is called Propagation Delay- the time it takes for the signal to propagate from input to output.

Now feedback is created by taking the some of the signal from the output, and applying it to an earlier portion of the circuit, which has a propagation delay. So you can see that the feedback signal is arriving ever so slightly too late to do its job right. The fact that it is too late causes the amplifier to become less stable as frequency increases, and there can be inharmonic noise created at the point where the feedback is returned.

This causes feedback to inject a low level harmonic distortion noise floor composed of harmonics up as high as the 81st harmonic into the output of the amp, and it has two audible artifacts.

The ear uses naturally-occurring odd-ordered harmonics to figure out how loud the sounds are. They are the 5th, 7th and 9th harmonics and they get enhanced (distorted) by feedback by a small amount. However, because these are loudness cues to the human ear this small amount **is easily audible** and audiophiles use the terms 'hard' 'harsh', 'bright', 'brittle', 'chalky', 'clinical' and so on to describe this distortion. Keep in mind that this is the case when the distortion of these harmonics may only be 100ths of a percent!! This is why two amps can measure the same frequency response on the bench but one will be bright and the other is not.

The 2nd problem is that the harmonic noise floor, through another hearing principle called 'masking', will block the ear's natural ability to hear into the noise floor of the playback system (the ear can hear 20 db into a natural noise floor like tape hiss or the wind blowing). Any information below the noise floor is not heard by the ear or not detected as easily. Since ambient soundstage information exists at low level, one of the more obvious effects of feedback is to foreshorten the soundstage depth and width.

Amplifiers in particular that use no feedback tend to have a different voltage response in dealing with the loudspeaker and the designer of the speaker has to accommodate this behavior. IMO, a speaker that requires an amplifier with feedback, due to the issues above, will never sound like real music. Speakers that *are* friendly to zero feedback amps at least have a chance.

see
http://www.atma-sphere.com/papers/paradigm_paper2.html for more information

I have read Roger Modjeski address this issue, and his opinion seems to be that negative feedback got a bad name as a result of designers who had no formal training in electronics misusing and not being competent to properly implement negative feedback. In the right hands, it is a good thing. I think it would sort of be like a chisel in the hands of an artist versus a non-artist. The artist makes great things with the chisel, and the non-artist cuts his hands.

I listened in a group setting to a Berning amplifier with adjustable feedback: No feedback, low feedback, and high feedback. We all agreed that the low feedback setting was the best, followed closely by no feedback, and distantly by high feedback. The latter setting was the only setting I would consider unlistenable. The no feedback setting was a little too soft and mellow for my tastes, but I could see someone enjoying that type of sound, and I would not kick it out of bed. For me the low feedback setting provided the best of both worlds - detail, controlled bass, without being irritating.

There are good sounding components using feedback and no feedback, which is simply more proof you need to listen to the component, because the component really is an extension of the skills and philosophy of the designer, and there are good skilled designers employing both methods.

"There are good sounding components using feedback and no feedback, which is simply more proof you need to listen to the component, because the component really is an extension of the skills and philosophy of the designer, and there are good skilled designers employing both methods."

I think this is the bottom line practically for most.

The caveat is you cannot listen to a single component alone, only a system with all the required components (source, amp, speaker/room) together. A single listen can only tell you what each component is capable of, not how good or bad each piece is or sounds. To sort it all out requires doing your homework and listening to as many combos as you can over time.