From: orville%weyrich@uunet.uu.net (Orville R. Weyrich)
Subject: Re: Setting HiFi system gain in amp feedback loop?
Reply-To: uunet.uu.net!weyrich!orville
Organization: Weyrich Computer Consulting
Date: Mon, 23 Nov 92 09:32:20 GMT

In article <1ecd3vINN16q@huon.itd.adelaide.edu.au> francis@cs.adelaide.edu.au writes:
>A curious question has been nagging me.  When one looks at a
>HiFi system the signal levels are as a rule fairly low,
>certainly domestic equipment is designed with levels
>significanlty lower than professional equpiment, and a
>domestic system is very rarely run with the volume knob
>wound up all the way.
>
>Essentially the domestic system has fixed gain in the power
>amp, but a considerable amount of attenuation in the signal
>chain, in the form of the volume control.  In a domestic
>system the only role for the volume control is setting the
>output level to the power amp.
>
>Another alternative is possible.  Rather than use passive
>attenuation to set the output level, why not run the signals
>to the power amp at the line levels of the rest of the
>system and then vary the gain of the amplifier?  This
>presupposes an amplifier designed using a fairly normal
>feedback system.  
>
>The charateristics of many passive components are not quite
>linear at very low signal levels, keeping the signal level
>high again helps amerliorate this a bit.

Huh? passive == resistor, right? Are you saying that resistors
are not linear at low levels?

>The amplifier should have improved charateristics at low
>gains.  The ratio of open loop gain to closed loop gain will
>increase at "ordinary" power levels and hence those
>distortion elements decreased by the feedback system will be
>correspondingly lower.  


If you are really serious about reducing distortion, you could run
"sensor" wires to the speakers like high regulation power supplies do
(but woe is you if the sensor circuit gets broken!)

When I was young and idle :-) I used to think about embedding a
feed-back sensor into the speaker itself to put the electromechanical 
portion into the feedback loop. I even have a Heathkit THD analyzer
that I never built, because it needed a low distortion signal
generator to set it up.

>Disadvantages must include a potential loss of stability as
>bits of the feedback loop have long wires attached to them.
>Also if the potentiometer became worn there would be a real
>danger of the feeback loop opening suddenly, leaving the amp
>producing its open loop gain straight into the speakers.
>Very nasty.

Agreed. Current limiting [clipping] would help, as would a circuit 
that shuts down the amplifier if the DC conductivity of the feedback
loop were broken. And also, using separate amplifiers for the bass
and treble would help -- nothing worse than clipping the output
of heavy bass notes to generate high energy harmonics that blow
tweeters.



orville

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Orville R. Weyrich, Jr.                        Weyrich Computer Consulting
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