Why Is High Feedback Considered Bad In Audio? In Simple Terms

I think bad feedback is like driving a car with a steering wheel with say a 0.5 second response time. Sure you see the intersection to turn at, you turn the wheel lots (lots of feedback) but the car doesn't turn yet.. The drive route is distorted.

I'm still working on that analogy..

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

Audiophiles have the ability to identify exactly where the source of distortion is coming from, so they're the ones that truly know what types of circuit topologies give the best results.

I've seen many audiophiles in action. When presented with double-blind tests, they can not only hear differences between amplifiers but also differences in low-level and speaker cables. They are an amazing group of people. They have proved, for example, that if you freeze your cd collection, the cd player's laser and electronics will more easily reproduce accurate sound. They have also shown that by merely plugging in a certain type of digital clock into your home's wiring that the electron flow becomes coherent and thus allows the amplifiers to create superior sound.

It's best not to ask electrical engineers such difficult questions. I suggest you post this question in one of their newsgroups. You'll get a much more satisfactory answer.

Bob

Because audiophools are lunatics who think they can hear things that don't exist.

John

In simple terms, depending on the design of an amplifier, the higher the feedback the closer to instability the amplifier becomes. Such instability manifests itself as overshoot and ringing on transients and impulses causing a "gritty" or unnatural sound.

For feedback to remain negative, there must not be 180 degrees of added phase shift around the loop, forward gain times feedback. If that is the case, the total feedback becomes 360 degrees or positive feedback. The amplifier either slams into its stops or more typically oscillates at some frequency where this just occurs.

In the application of feedback the trick is to apply just enough feedback to get the loop gain to decay to unity before the phase shift accumulates to the mentioned 180 degrees as the frequency is increased. The amount of phase shift left when the unity gain point is reached is called "phase margin." If this number is too low, ringing and overshoot can occur. Lowering the amount of feed back is one-way to control this improving the sound of the amp.

It is true that lower feedback is less linear meaning the distortion for steady state signals is higher but the ear is much more tolerant of some low amount of distortion than the "grittiness" of ringing transients.

Fantastically accurate rendition!

...Jim Thompson

Because there are not too many people who can get the feedback right. There are many subtleties to consider. The deeper is the feedback, more careful you have to be.

Because it is simple to get 0.001% of THD at 1kHz at the purely resistive load by the use of the deep feedback. However that doesn't mean that the amplifier is any good with the realistic audio signal and the realistic speaker load.

The circuit is linear until something is not running into the intermittent hard limiting or oscillation in some special conditions. Those effects can be easily overlooked and unnoticed during the trivial THD tests.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

In article , snipped-for-privacy@highNOTlandTHIStechnologyPART.com says...>

Oh, but they *can*.

Only if you do it very, very wrong.

Don't do that.

Doing the math right is another.

Where can I buy a calibrated gritometer?

John

I suppose if the guy who designed your amplifier didn't have a clue that's correct... at audio frequencies (up to, say, 20kHz) providing *plenty* of phase margin is no problem with contemporary parts.

If you listen hard enough....you can hear God.. :P

D from BC myrealaddress(at)comic(dot)com British Columbia Canada

They have visions, too ;-)

...Jim Thompson

"abby"

Alluding to mysterious absent experts and asking US to explain what YOU mysteriously allude to them having said somewhere is absolute bullshit.

Pissss orrrrff !!!

..... Phil

In an amplifier with a distorted transfer characteristics [volts in / volts out], adding feedback can linearise the characteristic over the working range. This gives lower distortion figures over the working range but it means that the amplifier distorts sharply and very badly when it eventually does overload.

A slightly distorted waveform contains unwanted lower harmonics and intermodulation products and doesn't sound particularly good, but a sharply distorted wavform contain much higher harmionics which sound absolutely vile!

If you can be sure of keeping the amplifier within its working limits at all times, feedback is generally a good thing, but if you overload it (even slightly) it can make things sound a lot worse. Smaller amounts of feedback would be a good choice for guitar amplifiers or a small P.A. amplifiers which may spend significant parts of their working lives runing at (or somewhat above) maximum output.

An amplifier which has limited slew rate will not have that slew rate improved by feedback, but the type of distortion it produces can be changed and may sound a lot worse. Again, as long as you know that your signal isn't ever going to exceed the slew rate of the amplifier, heavy feedback may give useful improvements in other parameters.

Don't believe everything that audiophools tell you. Mostly they prefer simple, badly performing circuits because that's all they understand, and often they don't even understand those.

If you want to read something sensible, look here:

Chris

And invariably their wives can too.

Hubby just spent a thousand bucks on 4 pointy bits of rubber to stand his CD player on...

"Oh yes dear it sound much better than yesterday".

On a sunny day (Fri, 23 Jan 2009 23:10:41 +0000) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

There is actually a simple solution to that, one I thought of in the seventies, and was actually used in commercial designs. Put a simple transistor stage before the power amp (in the power amp), and make sure that transistor stage limits just before the power amp does. The transistor stage needs no feedback, and the power amp always works in the linear range. When clipping happens, in the transistor stage, the output looks nice.

Hey - don't knock it - there are people whose living depends on it.

Sylvia.

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Exactly. Where can you buy a gritometer? Which is the point, Audio nuts hear all kinds of things some real some imagined. Ringing is easily observable with a scope using square waves into the speaker load with crossovers. If you don't think it exists, or that it affects the sound quality, fine. Bit it does illustrate the point that simple harmonic and IM distortion measurements are not sufficient. Furthermore it can explain differences in perceived quality among amplifiers, CD players, speaker crossovers and other audio devices. In particular, high order filters are to be avoided at all costs because they ring like bells and sound like s*it. Do your own experiments if you don't believe it.

Dunno 'bout that....

You're not married, are you? ;-)