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

[Ben Bradley]

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.

This older thread gives an interesting explanation that I've not
seen anyone post here yet:

http://groups.google.com/group/sci.electronics.design/browse_thread/thread/423f1d41bbec461a

 

[Ben Bradley]

I'll add it. Is that measured with a thermocouple?

Warmth is measured by the number of 12AX7's in the signal path.

 

[krw]

Some products have that stated purpose, it's true, but a great many make
claims (usually expressed alongside weasel words) as to benefits that
are not supportable.

Example? There are laws regarding what they can say.

 

[Adrian Tuddenham]

On Fri, 23 Jan 2009 17:08:00 -0800, John Larkin
[snip]

stunning (everything is stunning)
thin
tight
tight (bass)
timbre
tipped-up
transparency
truth
unfussiness
wire direction

"Toasting". A process of processing a cable which improves the
fidelity of such cable. Cables that are toasted are directional.
Unfortuantely, I can't find the original reference to toasting cables
which was most amusing to read. The testimonials proved the worth of
toasting.

http://www.noteworthyaudio.co.uk/Services.html#4
http://www.ultraaudio.com/opinion/20030801.htm

Terms in the second link...
vividness
tonal accuracy

It's all coming back to me now...

Anyone, besides me, remember when cactus needles were all the rage for
stylus use ?

For playing worn 78s they do have an advantage: a soft stylus contacts
most of the groove cross-section, thereby averaging out the roughness.
The S/N ratio can be considerably better than if you played the same
record with a hard polished stylus which only made contact at two
points.

If the flexible stylus is connected to a pickup with massive moving
parts, it will additionally form a low-pass filter, which may remove the
frequency band where the S/N ratio is poorest or may resonate and make
the scratch sound worse. A soft stylus should be in a pickup with low
moving mass to give a better frequency response - any filtering that may
be required can be applied electronically further down the chain.

 

[Adrian Tuddenham]

...Nuvistors were fun, too.

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.

 

[Adrian Tuddenham]

...Nuvistors were fun, too.

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.E.G. amplifiers.

 

[MooseFET]

Highpass filters ring by definition. The step response goes up and then
down.

That is not ringing.

Ringing implies some sort of cyclic action.

 

[[email protected]]

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.- Hide quoted text -

- Show quoted text -

"In particular, high order filters are to be avoided at all
costs because they ring like bells and sound like shit."

There is no reason a higher order filter should ring, unless you
design it that way. I can believe that they may sound different,
depending on where the corner frequency is.

George

 

[[email protected]]

Highpass filters ring by definition. The step response goes up and then
down.

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

"Highpass filters ring by definition. The step response goes up and
then
down."

What? I don't get this. Do we have a different definition of ringing?

George

 

[Vladimir Vassilevsky]

On Jan 24, 6:36 pm, Vladimir Vassilevsky <[email protected]>
wrote:


That is not ringing.
Ringing implies some sort of cyclic action.

Definition: ringing = step response is not monotonous.


VLV

 

[Vladimir Vassilevsky]

Some steps are certainly more boring than others.

Good!

When Larkin is wrong, he never admits it ))))


VLV

 

[Vladimir Vassilevsky]

John Larkin wrote:

If a thing rings, it rings forever.

Good point.

A simple CR doesn't ring, it thuds.

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

 

[Vladimir Vassilevsky]

On Sun, 25 Jan 2009 07:40:38 -0800 (PST), MooseFET


That's what they taught us. If a thing rings, it rings forever.

Apparently they didn't teach you FIR filters.

VLV

 

[MooseFET]

Definition: ringing = step response is not monotonous.

Are you writing your own dictionary?

http://www.google.com/search?hl=en&...:ringing&sa=X&oi=glossary_definition&ct=title

 

[Vladimir Vassilevsky]

^^^^^^^^^^
http://dictionary.reference.com/browse/ monotonous

Well, when it comes to the arguments like that... OK, keep loosing your
face

http://en.wikipedia.org/wiki/Monotonic_function

VLV

 

[Eeyore]

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.

It is only considered 'bad' by MORONS who like their 2-3% THD valve amps
and other such CRETINS.

Professional audio doesn't suffer from fools like that (mind you, they're
trying to break the door down).

Graham

 

[[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

Abby,
The phenomenon you refer to (increased distortion when feedback is
applied) is called “transient intermodulation distortion” (TIM). TIM
was a hot topic in the early 1970s. You don’t hear much about it
nowadays because it s causes have been identified and, thanks to
faster semiconductors, have been eliminated. Here’s what happens: If
the slew rate capability (maximum rate of change of voltage with
respect to time) of any stage of an amplifier is lower than the actual
slew rate of a signal, then the output can not keep up with the
input. The result is a very large error exists between the input and
the feedback. This error causes the input stage to saturate. When
the output finally “catches up” to the input, it takes awhile for the
input stage to come out of saturation. During this period a large
intermodulation distortion component appears at the output.

In the 1970s, power transistors were quite slow, preventing the output
from keeping up with the input. Today, TIM is easily prevented simply
by designing the open loop amplifier so that all stages have adequate
slew rate capability. As long as this is the case, negative feedback
will always result in lower distortion, and no distortion artifacts
will occur that were not present in the open loop amplifier.
Regards,
Jon

 

[MooseFET]

He has his own working definition of the word "ring." Anybody can do
that. Even "monotonous."

How about this:

Y = A*exp(B*t) + C*exp(D*t) ...etc not ringing


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

For C <> 0 and both series converging

 

[JosephKK]

Consider this response to a step:


........................................**************
....................................****..............
..................................**..................
..............................**.*....................
...........................*.*..*.....................
.....................**...*.*.........................
..............**....*..*.*............................
.....**......*..*..*....*...........................
....*..*....*....**...................................
...*....*..*..........................................
..*......**...........................................
.*....................................................
*.....................................................
......................................................

Ringing but no overshoot.

Have you ever seen a real filter act like that? For that filter
(after carefully and accurately characterizing the components) does
the spice simulation agree with the experimental results?

 

[MooseFET]

Have you ever seen a real filter act like that?  For that filter
(after carefully and accurately characterizing the components) does
the spice simulation agree with the experimental results?

Not exactly like that but sort of the same but harder to do with ASCII
art. The squiggles where much faster. In the intended application it
worked ok. I'm sure that spice would agree with the results if the
SRF of the inductors was accurately modeled etc.