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

[abby]

Hi,

Can someone explain, in simple terms, why high feedback is considered
inferior to low feedback in audio circuits? To me, linear is linear
and
a high feedback op-amp circuit is linear. Apparently, that isn't
entirely
true.

Thanks,
Gary

 

[Bob Eld]

Hi,

Can someone explain, in simple terms, why high feedback is considered
inferior to low feedback in audio circuits? To me, linear is linear
and
a high feedback op-amp circuit is linear. Apparently, that isn't
entirely
true.

Thanks,
Gary

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.

 

[Vladimir Vassilevsky]

Hi,

Can someone explain, in simple terms, why high feedback is considered
inferior to low feedback in audio circuits?

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.

To me, linear is linear and
a high feedback op-amp circuit is linear. Apparently, that isn't entirely
true.

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
http://www.abvolt.com

 

[krw]

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

Oh, but they *can*.

 

[Phil Allison]

"abby"

Can someone explain, in simple terms, why high feedback is considered
inferior to low feedback in audio circuits?

** Entirely false assertion.

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

 

[Adrian Tuddenham]

Hi,

Can someone explain, in simple terms, why high feedback is considered
inferior to low feedback in audio circuits? To me, linear is linear
and
a high feedback op-amp circuit is linear. Apparently, that isn't
entirely
true.

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.

 

[nospam]

Oh, but they *can*.

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".

--

 

[Sylvia Else]

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

John

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

Sylvia.

 

[Bob Eld]

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

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 shit. Do your own
experiments if you don't believe it.

 

[krw]

And invariably their wives can too.

Dunno 'bout that....

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".

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

 

[Jamie]

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

Wait, you mean my 99.99% oxygen free monster speaker wire won't help me?

http://webpages.charter.net/jamie_5"

 

[Sylvia Else]

There are Bernie Madroffs in the world too.

Well, yes, but selling a few silver wires and gold plated CDs to people
is hardly in the same league.

Sylvia.

 

[Vladimir Vassilevsky]

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 want the mere 40dB of feedback at 20 kHz, then the amp should
have enough of phase margin at 2 MHz, which is not quite so simple for
the power BJTs operating at the high levels of injection. The
optimization of the multiple feedback loops for the worst case operation
requires careful approach. The other thing about audio is that it is
severely constrained in the cost. That means el cheapo transistors and
the single layer PCBs with the split grounds


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[nospam]

Dunno 'bout that....

I thought in reviews of hi-fi essentials like anti-vibration CD player
mounts and oxide free copper mains leads a statement along the lines of
"even my wife could hear the difference" was mandatory.

--

 

[[email protected]]

Hi,

Can someone explain, in simple terms, why high feedback is considered
inferior to low feedback in audio circuits?  To me, linear is linear
and
a high feedback op-amp circuit is linear.  Apparently, that isn't
entirely
true.

Thanks,
Gary

There were a few things in older transistor amp design that weren't
done well, and the lower feedback designs were more tolerant of these
things. Besides the Doug Self papers in Chris Jones post, dig up the
old Marshall Leach papers.
http://users.ece.gatech.edu/~mleach/
http://users.ece.gatech.edu/~mleach/lowtim/

If I were to roll an amp together, I'd be more inclined to go for
linear MOS on the output. Randy Sloan's amp book is a good reference,
even if you only want to read about such design.
http://tinyurl.com/b2ltfa

 

[krw]

I thought in reviews of hi-fi essentials like anti-vibration CD player
mounts and oxide free copper mains leads a statement along the lines of
"even my wife could hear the difference" was mandatory.

No, once the wife hears where the $1000 goes, all fidelity is lost.

 

[krw]

Well, yes, but selling a few silver wires and gold plated CDs to people
is hardly in the same league.

In many ways they're worse.

 

[krw]

Yup, high-end audio is mostly a useless waste of money, but at least
it's not outright harmful like cigarettes or opera tickets.

Unless the rent money goes into 5N copper.

 

[Sylvia Else]

In many ways they're worse.

If one were going to go after purveyors of that kind of gear, then for
consistency, one would also have to go after a large part of the entire
cosmetic and beauty industry. In common with the impugned audio
equipment vendors, they too are at best selling people the means to
delude themselves.

Which is not to say that would be a bad thing, but it illustrates the
true scale of the problem.

Sylvia.

 

[Sylvia Else]

No, once the wife hears where the $1000 goes, all fidelity is lost.

And of course, there's the problem that when the wife hears it, she
knows what's expected, and says "wow!" The audiophile then listens
again, says "you're right" and sends it all back.

Sylvia.