[sdiy] VCA controls

[Batz Goodfortune]

Y-ellow Y'all.
         LEDs. Lots of LEDs. Hmmmmm. There was a rather large mixer 
designed by one RA Penfold I belive, in an 80's issue of Practical 
Electronics from the UK. He needed peak LEDs all over the place and his 
solution to the supply-thumping problem was to reference the LED drive 
circuitry, not to ground but across the +/- supply. There were a couple of 
transistors to shift the audio reference half wave to the negative rail and 
then turn the LED on at a given level. I cannot remember how this works 
exactly and I'm not really inclined to go hunt it out. If my wonderful ACER 
Prisa 620U scanner actually worked under WINK I'd bother to scan it for 
y'all but it doesn't so there's little point.

Except to say that by dumping the current across the supply rails and 
giving all the audio adequate supply rejection, the problems of thumping 
with LEDs was eliminated.

It's possibly even easier to do this with an LM391x because the thing can 
be tortured into doing things it's own mother wouldn't recognize. Including 
multiplexing a single LM391x to drive n channels. I've done this. It works! 
Not only does it work but it'll give you a bar/dot mode in the same display 
for peak/average. A circus for which can be found on my web site. SOMEWHERE???

But the bottom line is that by not dumping dirty current to ground, or in 
some way floating the supply to the LED driver circuit, you can eliminate 
the problem. Except as already noted, with flashing LEDs. Nothing bar 
nothing will clean those little bastards up. Especially when they create 
enough RFI splatter to block out an entire district's  radio reception.

Hope this helps.
Hey why do I always have to be the serious one?

Be absolutely Icebox

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