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

Several of my own P.A. designs (all transistor) have soft clipping in the driver stages. On the occasions where I have had to run them overloaded[1], they sounded much louder than their numerical wattage would have suggested and there were no complaints about distortion.

[1] When the client says it's only a small job and it turns out to be a much bigger one, for which I really should have brought more kit.

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~ Adrian Tuddenham ~
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Except that people aren't poisoned and don't have have life-threatening allergies triggered by gold-plated CDs. The cosmetics and perfume industies are literally getting away with murder and (in Europe at least) are virtually unregulated.

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~ Adrian Tuddenham ~
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Except that article in Spectrum where some audiophool explained how to replace all of the safety-rated mains capacitors in your stereo with less safe ones hand made by audiophools.

Chris

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ A typical audiofool ignorance.

The amount of nonlinear distortion = deviation of the transfer curve from the straight line. Soft clipping = more distortion if compared to hard clipping to the same ceiling.

If the amp has to enter the area where it clips the signal, one has to limit the windup of the integrators in the feedback path. So the recovery from the saturation will be quick.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Gaussian and Bessel (linear-phase) filters don't ring, no mater what the order. Sharp-cutoff (Butterworth, Chebychev, elliptical) filters do.

You can make a sharp-cutoff (delay equalized) analog filter that doesn't ring, but it's a pain. Digitally, it's easier.

Since ears aren't phase sensitive at high frequencies (especially above 20 KHz!) a little ringing doesn't matter anyhow.

John

Wrong.

Gaussian and Bessel filters do ring, although not nearly as much as the other filter types.

True.

BTW, there is no connection between the "ringing" and the linearity of the phase in the filter.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

I am just as scathing of audiophoolery as you are; this comment was based on 40 years of practical P.A. experience as a designer and as an operator.

Soft clipping gives less audible distortion on a low fidelity P.A. loudspeaker system. If an amplifier has to wound up to too high a level in order to deal with an awkward situation 'in the field', it is infinitely preferable to hear soft clipping in the top 6dB of the signal than to endure the harsh distortion from flat-topping.

With soft pre-clipping, the power stages are less likely to saturate at all.

There are other reasons for using as little feedback as you can get away with on a P.A. amplifier:

Most designs need an output transformer to feed the floating 100v line, so a tertiary winding, close coupled to the secondary, is the only way to give meaningful feedback. There may be considerable phase shifts due to resonance between the the self-capacitance and inductance of the 100 volt winding.

The load is unpredictable and the amplifier must remain stable under all sorts of conditions from an open overhead line several hundred yards long (during set-up) to a short circuit (when some idiot drives a nail through the cable). If there is significant leakage inductance between primary and secondary, it won't take much feedback from the secondary to give instability under some conditions of loading.

My comments about soft clipping applied to difficult underpowered P.A. situations where turning the volume down isn't an option and you are forced to choose the least of several evils. I am not advocating soft clipping for high quality sound reproduction (where adequate amplifier power is the obvious first step).

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~ Adrian Tuddenham ~
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www.poppyrecords.co.uk

[snip]

I haven't rolled my own power amplifiers for 30+ years now.

In the past, though, it was my standard procedure to have way more than adequate _real_ audio power... like 100W per channel in my living room ;-)

But, I also utilized soft clipping in the driver stages to ensure I didn't rail the output stages.

I always used RF-grade BJT's in my designs. If I were to build something again I'd still use BJT's... since I have, in my secret pocket, a way to A-B bias reliably ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

No, the users of cosmetics delude others, which is exactly what the manufacturers advertise.

That people are vain? Maybe. OTOH, I'd rather see a dressed-to-the-nines woman walking down the street than one with unbrushed teeth, just out of bed, and in an old torn housecoat.

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?

Commercial audio amps is one of the things that I design as a business. In our days, that's mainly Class D.

Loud sounds, big guns, shiny cars and such attributes are often associated with weak personalities :-)

Limiting the signal upfront the amplifier can be useful. However I disagree about limiting in the driver stage.

Using the RF BJTs in the linear amp would be nice from the purist point of view. It would accomplish the unconditional stability with the deeper feedback. However the audio applications are severely constrained by the cost rather then by the performance. So the main difficulty is not to get to the ultimate perfection, but to make it reasonably good while minimizing the cost.

BTW, the main push towards class D is because of the material savings due to the reduced size of the heat sinks.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

When I was rolling my own, I was listening to classical music (I'm a clarinetist myself), and rarely was there anyone present besides my wife and myself.

Weak personality? Shirley you jest ;-)

Before the feedback path.

For myself, cost was not an object... and the power transistors were free.

For the neighbor kid's 400W guitar amp I used fans ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

In your opinion, would a filter that has no overshoot in the time domain be capable of ringing?

Chris

"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.

Terms in the second link... vividness tonal accuracy

[snip]

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

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

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

Well, I left this discipline behind me about 30 years ago, so I don't know what's "in" these days. But I'd say, since people don't seem to change much, in my experience they favor things that escape scientific explanations.

I know some who used certain cactus to, um, "enhance" their appreciation.

be

Consider this response to a step:

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Ringing but no overshoot.

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.

Sylvia.