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Negative Feedback in Audio Amplifiers

Recently, I had the experience of re-manufacturing the tweeters in a pair of speakers I had bought. Due to a "senior moment", I fried the tweeters in a pair of Quadral Chromium Style 50 speakers, and the local dealer decided to f*ck me over even though I admitted the mistake and paid for replacements. Why they decided to screw with me is another story, no doubt involving elements of the RCMP who would prefer not to be named. At any rate, I was left with a pair of drivers with unequal efficiency, and it annoyed the living snot out of me.

So I decided to take the two broken tweeters and rehabilitate them. If you look into the Quadral speaker line, you will discover that the tweeters use a filament suspended between two powerful magnets in series with a 5.1 ohm resister. The filament is apparently a piece of aluminum coated with a thin layer of titanium, or something to that effect. My replacement is somewhat thinner, and required the manufacture of unique tooling to shape.

The result has less mass and consequently improved the sensitivity of the driver, with obvious implications to the resultant sound quality.

My amplifier is an Onkyo M282, which is apparently an amplifier that uses negative feedback, and made in Japan. On the Web, there are a number of articles which articulate the controversy over NFB amp designs, with numerous manufacturers shunning the technique for various reasons. None of the descriptions or reviews take into account the most important factor in an audio system -- the interaction of the speakers with the amplifier.

I believe I have achieved such good results primarily because I reduced the "mass" of the tweeter, which in combination with the amplifier design, has helped produce a superior sound. The key to understanding the thinking behind this is the interaction of the amplifier with the speakers.

Speakers are an inductive load, AFAIK, which means they present some of the same problems as electric motors in the design of driver circuitry. In the instance of an audio signal, the drivers have a hysteresis contingent upon their ability to accelerate in the presence of a electrical impetus. In my nascent understanding of electronic circuits, this resistance to movement will interfere with the amplifier output in the same way a naive power supply will drop-out under load.

The negative feedback circuitry in the amplifier will detect this drop-out and drive its input harder as a result, thereby reducing perceived distortion.

In the available literature there is talk of the "delay" that results from the signal excursion that occurs from amplifier distortion before it is corrected. I would suggest that this "delay" is imaginary for the reason that the negative feedback circuitry in fact operates essentially instantaneously, and applies a continuous correction.

It is worth noting that distortion at the amplifier output is a result of two sources: firstly there is the distortion introduced by the gain stages in the amplifier, and secondly, distortion introduced by the mass of the drivers, which interact with the magnets and impose an inductive signal on the amplifier output. In either case, the negative feedback circuitry should nearly instantaneously apply a corrective factor to the input signal which will effectively coerce the drivers to conform to the shape of the input signal -- if the NFB circuitry is not too weak or slow.

My suspicion is that by reducing the mass of the tweeters in my system, I have relieved a burden from the NFB circuitry in the Onkyo M282, and thereby improved the resultant sound quality by an order of magnitude or so.

The apparent controversy over the effectiveness of NFB amp designs seems to ignore the amplifier-speaker relationship, and the role it plays in fidelity. An ideal speaker would have no electric mass, and would therefore have zero influence (or load, I guess) on the amplifier, and would obviously reproduce the amplifier signal perfectly. Loudspeakers must necessarily have mass, if only represented by the air they displace, and will therefore affect the amplifier by virtue of inductance. Consequently it is my feeling that negative feedback amplifier designs represent a good solution to the practical problem of signal reproduction.

In thinking about this subject, it has occurred to me that a NFB amp will perform better if the speakers it drives are more expensive than another, all things being equal. Even ambient noise in the listening environment might affect the negative feedback circuitry.

Perhaps obviously, I have no test equipment to use to measure the effects or phenomenon that is at issue. Just my common sense. But if you were here, you would be able to attest to the fidelity of the ultimate result.

Comments and constructive criticism is welcome.

Regards,

Uncle Steve

--
More than a century has passed since science laid down sound 
propositions as to the origins of the universe, but how many have 
mastered them or possess the really scientific spirit of criticism? A 
few thousands at the outside, who are lost in the midst of hundreds of 
millions still steeped in prejudices and superstitions worthy of 
savages, who are consequently ever ready to serve as puppets for 
religious impostors.   -- Peter Kropotkin
Reply to
Uncle Steve
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"Uncle Steve"

( snip appalling load of drivel )

** You are making a fundamental error.

NFB loops ( in audio amplifiers) compare only the INPUT and OUTPUT signals - so the output *replicates* the input signal at a greater magnitude.

This cannot and does not correct for non-linearities or other flaws in the load.

... Phil

Reply to
Phil Allison

You must know that inductive loads are different from resistive loads. How do you account for that difference?

All I am suggesting is that speaker efficiency is a player in NFB amp systems. Totally hypothetical, but you've not said anything that would invaldiate the hypothesis.

Regards,

Uncle Steve

--
More than a century has passed since science laid down sound 
propositions as to the origins of the universe, but how many have 
mastered them or possess the really scientific spirit of criticism? A 
few thousands at the outside, who are lost in the midst of hundreds of 
millions still steeped in prejudices and superstitions worthy of 
savages, who are consequently ever ready to serve as puppets for 
religious impostors.   -- Peter Kropotkin
Reply to
Uncle Steve

** You are clearly a demented half wit.

Fuck off.

.... Phil

Reply to
Phil Allison

Talk to your hifi supplier, he'll give you an objective assessment.

FGS

Reply to
Bruce Varley

than

This common misperception makes the hifi scam business profitable.

Reply to
Gib Bogle

Only at some frequencies. The electro-mechanical interaction that you mention causes a single drive unit to be inductive below the resonant frequency, resistive at resonance and capacitive above resonance. At much higher frequencies it looks more resistive and may become inductive again.

When several drive units are combined with a crossover things get more complicated with multiple changes between inductive and capacitive impedance.

There is some delay through the amplifier, but so long as the input bandwidth is limited to something sensible the signal is changing slowly enough for this not to cause any problems.

If you feed a very sharp edge into a badly designed feedback amplifier there can be momentary distortion which might be audible. However, in a well designed amplifer there will be a passive input low-pass filter to smooth off such signals and prevent this effect. The fashion for specifying bandwidths that are only audible to bats can make things worse than tailoring audio equipment to what is actually audible to humans.

Don't believe everything in "the literature".

John

Reply to
John Walliker

Sometimes I think Phil's responses are over the top, but not this time !

MK

Reply to
MK

Audio is such nonsense.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

I bet he will. Right after he's finished telling me all about the dangers of DC current coming out of my amp. I'm going to be needing another one as I appear to have broken him.

I'm pretty sure God doesn't have anything to do with it.

Regards,

Uncle Steve

--
More than a century has passed since science laid down sound 
propositions as to the origins of the universe, but how many have 
mastered them or possess the really scientific spirit of criticism? A 
few thousands at the outside, who are lost in the midst of hundreds of 
millions still steeped in prejudices and superstitions worthy of 
savages, who are consequently ever ready to serve as puppets for 
religious impostors.   -- Peter Kropotkin
Reply to
Uncle Steve

When I've properly broken in my new drivers I'll make a youtube video so you can hear the results for yourself. No need to thank me, it will be a public service.

Regards,

Uncle Steve

--
More than a century has passed since science laid down sound 
propositions as to the origins of the universe, but how many have 
mastered them or possess the really scientific spirit of criticism? A 
few thousands at the outside, who are lost in the midst of hundreds of 
millions still steeped in prejudices and superstitions worthy of 
savages, who are consequently ever ready to serve as puppets for 
religious impostors.   -- Peter Kropotkin
Reply to
Uncle Steve

te:

What I really don't get is the big fat cables. If the load is inductive, then adding more series R to the circuit just improves the L/R time constant. Sure you've gotta output more voltage from the amp, is that a big deal? (But I know jack $@#t about audio.)

Hey maybe we could market a line of 'fast' (thin) speaker cables.

George H.

.highlandtechnology.com  jlarkin at highlandtechnology dot com

Reply to
George Herold

How does that happen? Remember, the only thing that matters is the power at the speaker and all the amp can see is the other (near) end of the cable.

Speed isn't the issue. .67C is fast enough for audio. Even 50% better doesn't help (though if you can do better, I'd like to hear about it;-).

Reply to
krw

If you're in a room with walls, moving your head a quarter of an inch will change the acoustic transfer function more than practically any change you could make in amps and cables.

Flat flex; that's been done. Some people even try to make 4 or 8 ohm transmission lines. Loonies.

I've designed super-low impedance (specifically, low inductance) coax to drive NMR gradient coils. My customer was using 10 meter cables that had more inductance than the gradient coils that we were driving.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

It is pretty simple: The thin cables do not look as expensive as the gold-plated fat ones with transparent insulation.

--

As far as I understand, a speaker creates a larger back-EMF from 
the mechanical movement of the coil in the magnetic field than 
from the self-inductance of the coil. This means that the 
mechanical (acoustic) environment is determining in the impedance 
bumps seen by the amplifier.
Reply to
Tauno Voipio

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OK I know jack about speakers. But if you are just driving an inductor from a voltage source, and you've got voltage 'headroom' to burn, then adding series R can make it faster. (brute force and wasteful.)

George H.

Remember, the only thing that matters is the

Reply to
George Herold

Ahh OK. So it looks mostly resistive. Then lower R in the cable at least does no harm.

Reply to
George Herold

You sure your name isn't "Vapidio"?

Reply to
MrTallyman

Voice coils have a low mechanical impedance if driven from a low electrical impedance. They have more resonant woopie-doos if they are driven by a high impedance source. It's the "damping factor" thing.

Some people like the resonant bass boom that they get from a higher-z amp.

I don't think that negative amp impedances, or remote sense, are popular. Maybe there's money to be made there.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

The problem is that the voltage is fixed because amplifiers amplify the voltage so adding extra resistance just reduces the swing of the speaker. If the resistance of the cables is excessive you might impair the low frequency response.

--
Failure does not prove something is impossible, failure simply 
indicates you are not using the right tools... 
nico@nctdevpuntnl (punt=.) 
--------------------------------------------------------------
Reply to
Nico Coesel

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