On Wed, Feb 18, 2004 at 08:14:19AM -0800, Martin McCormick enlightened us thusly
> Thanks very much to you and to Bob Paddock. I haven't
> started actually building anything yet, but I hope I can either find a
> single-chip audio AGC device with a wide input level range
> or an amplifier with a voltage-controlled gain adjustment kind of like
> the old LM370. I do have some of those, but they add distortion to
> the audio and, if I remember correctly, one needs to cascade a pair of
> them to get 10 or 20 DB of gain adjustment. I haven't looked at those
> in a while, but I remember that they do work all be it roughly.
>
> If one has an audio amplifier with a DC gain adjust, then all
> that's left is to rectify the audio and filter that signal to produce
> the AGC voltage.
>
> I think the NE571's virtue was it had a rectifier on board and
> the adjustable gain stages all in one chip.
>
> The other possibility is a more complex arrangement using a
> PIC that contains an A/D converter plus a digital pot. I don't doubt
> that that can be made to work, but it will be slightly more expensive
> both in parts count and in their actual cost. It will probably need
> lots of tweaking to get it right, also.
Here's one for the nuttier ideas ever expressed on this forum, but you
might love it :-).
A guy I know made just such an adjustable digital pot as follows back in
1979. He took his signal, ran it through an 8 bit a/d, inverted all
lines, and ran it out through a d/a. Adjustment was provided by tweaking
the reference voltage to the d/a. Whether the inversion is needed or
not for your application, I don't know. Slap a bit of smoothing (0.22uF)
on the output and you have audio quality.
The application was output voltage amplitude on a 50 HP inverter done
with ~5000 LM1458 opamps and 200 cmos chips. It set the voltage, and
could reduce it quickly in an overload. The designer moved on fairly
sharply to avoid constraint by the men in white coats, but not before he
did some rather ingenious stuff which was never capitalised on - 0Hz -->
sinewave generator, 3 phase sinewave generator, dc current transformer,
analogue pot,
I think he used adc800 & dac800, but there's much smarter A/Ds around
now.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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