-=> Roy J. Tellason said to Wayne Chirnside
 -=> about "BAG OF CHIPS" on 01-11-04  20:17.....

 RJT> them off a split supply.  I've got a little circuit here that's a
 RJT> charge pump to give you a little bit of -5V from a +5V supply -- I'll

There used to be a chip for that. IIRC it used only two external
caps. Because it used switching FETs for the rectifier the output
was very close to 100% of the input voltage, but reversed in
polarity.

But I think the output current was limited to 5mA or so...

Of course you could derive the neg supply from the transformer
that's driving the rectifier for your positive supply.

 RJT> bet that would do the trick and if I feed it unregulated voltage then I
 RJT> could probably have pretty stable dual 5V supplies,  which should be
 RJT> more than adequate for line-level audio signals. 

Yes indeed!

 RJT> I wasn't thinking of switching in bunches of different resistors to do
 RJT> gain control,  but rather some sort of pulse-width modulation.  Just
 RJT> gotta pick a high enough frequency and figure out what sort of
 RJT> filtering needs to be at the output end of things to make this work.

Back in the 1960s I saw a high quality audio compressor using
that technique. I'm pretty sure the PWM frequency was about
100kHz, and because they were limited by how short theycould make
the switching pulses, they used two stages with an extra low pass
filter between them.

Audio in --- LPF --- chopper --- LPF --- chopper --- LPF --- out

100KHz with 0.3uS pulse width gives 30:1 = 30dB attenuation. Two
stages will give 30+30 = 60dB. The two chopper stages are driven
from the one PWM control stage.

IIRC the filters were all 15KHz second order active using
discrete transistors.

I can't remember the exact configuration of the choppers, but I
think they were balanced circuits, possibly using audio
transformers. It's a long time ago...

They used to do some wonderful things with choppers in those
days. We had a precision DC multimeter that used an optical
chopper and an AC amplifier, capacitively coupled, with four
germanium transistors, to feed the output demodulator, which IIRC
was a mechanical one!

Nowadays you'd get better performance using a DC amp with silicon
transistors or even a low offset op-amp.

 RJT> He sure published an awful lot of stuff.  And much of it is in those
 RJT> little books that you can get at radio shack,  maybe that circuit is in
 RJT> there? 

I've also seen some books written by R. A. Penfold that give very
good value for money! They were published by
   Bernard Babani Publishing Ltd.,
   The Grampians,
   Shepherds Bush Road,
   London W6 7NF,
   England

I bought two of his on the PIC microcontrollers that gave me a
lot more good guts than the Easy-PIC'n book, but together they
cost about 1/3 the price!

I don't have a list of all their publications, but I remember
seeing several at Jaycar's shop in Adelaide.

Check out www.jaycar.com.au for an online cat.

The 2003 printed cat shows them costing about $25 Aus each (I
think I paid under $10 each), while Easy-PIC'n is $85 Aus. So
they are still cheaper.

From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... A good many things go round in the dark besides Santa Claus - Hoover
___ Blue Wave/386 v2.30

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