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

A

Adrian Tuddenham

John Larkin said:
Has the L.F. noise figure of a Nuvistor ever been bettered by a
semiconductor? For a long time they gave the lowest flicker amplitude
in F.M. oscillators and E.M.G. amplifiers.

I suspect so. I don't know what's your criterion for "lf"...[/QUOTE]

10c/s and lower. The range which most data sheets don't include.
 
V

Vladimir Vassilevsky

MooseFET said:
How about this:
Y = A*exp(B*t) + C*exp(D*t) ...etc not ringing

Y = A*exp(B*t)*sin(C*t) ... ringing

The first one can have multiple inflexion points in the step response as
well.

VLV
 
M

MooseFET

The first one can have multiple inflexion points in the step response as
well.

Pluto isn't a planet so we can have thins like that and perhaps not
call them ringing.
 
S

Sylvia Else

krw said:
Example? There are laws regarding what they can say.

They're not meant to blatently lie. But take for example, Nivea's Oxygen
Power products:

"NIVEA VISAGE Oxygen Power Reviving Day Cream with 15% pure oxygen
increases surface skin cell renewal. Your skin is revived from deep
within and more resistant to combat daily stress."

Sounds good, but what does it mean? Apart from the 15% pure oxygen (as
distinct from 15% impure oxygen, presumably) there's nothing there you
could pin them down on. Even "increases surface skin cell renewal" is of
dubious interpretation given that surface skin cells are dead.

Any strategy is to make a claim of an objectively measurable
improvement, but then include a small print caveat showing that the
claimed improvement is some trivial amount, and that even that was
supported by tests on a statistically insignificant number of people
(perhaps being the particular test group who provided the required
result, with the other test groups being ignored).

Sylvia.
 
K

krw

They're not meant to blatently lie. But take for example, Nivea's Oxygen
Power products:

"NIVEA VISAGE Oxygen Power Reviving Day Cream with 15% pure oxygen
increases surface skin cell renewal. Your skin is revived from deep
within and more resistant to combat daily stress."

Sounds good, but what does it mean? Apart from the 15% pure oxygen (as
distinct from 15% impure oxygen, presumably) there's nothing there you
could pin them down on. Even "increases surface skin cell renewal" is of
dubious interpretation given that surface skin cells are dead.

There is no law that says an avert has to mean _anything_.
Any strategy is to make a claim of an objectively measurable
improvement, but then include a small print caveat showing that the
claimed improvement is some trivial amount, and that even that was
supported by tests on a statistically insignificant number of people
(perhaps being the particular test group who provided the required
result, with the other test groups being ignored).

We're back to the best of the audiophools, OK. Audiophools go down
from there.
 
K

krw

krw said:
Sorry, I didn't see the original post.

[...]
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

Google for "transient intermodulation distortion" and "Matti Otala".

You should be able to answer a question that's been bugging me. Which
do audiophools spend more money for, speaker cables or HDMI cables?

Well, I left this discipline behind me about 30 years ago, so I don't
know what's "in" these days.

Discipline? There is no "discipline" here.

From Websters:

1: punishment
2obsolete: instruction
3: a field of study
4: training that corrects, molds, or perfects the mental faculties
or moral character
5 a: control gained by enforcing obedience or order
b: orderly or prescribed conduct or pattern of behavior
c: self-control
6: a rule or system of rules governing conduct or activity

The only possible definition that fits is #1, for the consumer.
But I'd say, since people don't seem to
change much, in my experience they favor things that escape scientific
explanations.

Particularly when there isn't much money to be made selling lamp cord.
 
S

Sylvia Else

krw said:
There is no law that says an avert has to mean _anything_.

Exactly, but if you're the average female punter hoping to improve her
appearance, claims like that might *seem* to mean something.

Sylvia.
 
S

Spehro Pefhany

Exactly, but if you're the average female punter hoping to improve her
appearance, claims like that might *seem* to mean something.

Sylvia.

It's about 5% less "pure oxygen" than fresh air. Prevents spoilage,
perhaps?


Best regards,
Spehro Pefhany
 
J

JosephKK

Definition: ringing = step response is not monotonous.


VLV

That is a far different definition than normally used. Compare with
the IEEE dictionary definition.
 
J

JosephKK

John Larkin wrote:



Good point.


Definition of thud?

Leftist weenies thud, rattle or ring. Engineers talk numbers and
formulas :)


How about this:

HPF = 1 - LPF

If LPF is rings, then HPF rings, too.

VLV

And likewise if the LPF does not ring the HPF does not ring.
 
M

Martin Brown

Don't be too quick to condemn. There are a heck of a lot of snake oil
salesman out there.
"Our speaker wire is sung to by virgins in an oxygen free atmosphere"
etc.
They are good for a laugh from time to time "Emperors new clothes"
style.

But not everything in the audiophile realm is entirely hocus pocus.
There is some real engineering too.

The final mechanical transducers that turn electrical signals into
vibrations in the air are less than ideal loads and sufficiently
imperfect that some do sound a lot better in a real room than others.
And certain unlucky combinations of speaker and amplifier can sound
awful or even be marginally unstable. When you get to a system where
sound engineers mistakes become noticeable it probably isn't worth
going beyond that unless the objective is to waste money. Biwired
amplifiers make some sense from a network point of view.
Yup, high-end audio is mostly a useless waste of money, but at least
it's not outright harmful like cigarettes or opera tickets.

What's harmful about opera tickets? I'll have any you are throwing
out.

I suppose it depends what you call high end. There is a stratospheric
audiophool price ceiling if you are so inclined. But it is worth
having decent speakers and a better than average amplifier if you like
music. A fairly good system test is some music you know well. And if
you happen to know something like the minimalist Philip Glass
composition Songs from Liquid Days then Track 2 will easily separate
the sheep from the goats.

I was suprised to find that almost no sensibly priced home cinema
systems could render it accurately. Detail was lost and the
instruments became muddy and the tempo less well defined. Minimalist
music is rather good for this sort of thing because the short
repetative themes allow for easy and quick comparisons. And it falls
apart if the timing isn't accurate.

Regards,
Martin Brown
 
A

Adrian Tuddenham

John Larkin said:
10c/s and lower. The range which most data sheets don't include.

Yikes, that's serious 1/F noise turf.

What's the signal impedance? What sort of noise levels do you need?[/QUOTE]

I don't have any particular project in mind at the moment. I was just
interested to know if semiconductors had caught up with valves on all
fronts or whether there was a niche where they still had the advantage.
 
J

Jasen Betts

I don't have any particular project in mind at the moment. I was just
interested to know if semiconductors had caught up with valves on all
fronts or whether there was a niche where they still had the advantage.

magnetrons are still king in microwave ovens :)
 
M

Martin Brown

Add 'detail' to the list.


Add 'muddy' to the list.


Add 'tempo less well defined' to the list.


Add 'Minimalist music' to the list.


F*cking H*ll now the amplifier changes the timing.
Wel I guess with a digital preprocessor it can.

And some of them do. Try it and see. I suggested an experiment. Don't
take my word for it. Home cinema systems seem to do a lot of digital
processing internally now. The effect is not especially subtle so you
don't need magic ears to hear the difference. Same speakers, same CD
transport, different amplifier and an A B switching test.
LOL

Just hope you were kidding.

No I was trying to point out that some music is rather more demanding
of kit that others.

I can't believe that all electronics engineers have damaged hearing.

Regards,
Martin Brown
 
E

Eeyore

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

Sadly the excellent Hitachi lateral mosfets have now gone, but you can get
approx equivalents from Semelab/Magantec and Exicon.

I built an amp module using the Hitachi parts with 0.0005% THD ( yes -106dB
SINAD) @ 350W into 8 ohms btw. The final product THD was determined by
op-amp buffer stage performance @ 0.003%. You can get better op-amps now
though. It would also drive 600W into 4 ohms and around 900W into 2 ohms.

That used tons of feedack both local, overall and pole-zero. It also
sounded lovely. Quite a few made their way into recording studios as
monitor amps.

Graham
 
E

Eeyore

Jan said:
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.

There's an even smarter way which involves monitoring the actual feedback
signal.

Graham
 
E

Eeyore

Adrian said:
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.

I always ensure that the maximum power bandwidth is at least 45kHz before
slew rate limiting appears.

Graham
 
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