Op-amp heresy.....

Chris Crosskey chrisc at zetnet.co.uk
Fri Jan 24 22:07:02 CET 1997


Hi guys, 
  I've been thinking about op-amps, mostly because of the thread running
at the moment, and a thought struck me from a few years ago. Back then 
my friend Paul (currently re-creating a Moog 55 by himself) had built a 
fairly large Penfold system, half a dozen VCO's etc, and he was unhappy
with the tuning stability and linearity of the oscillators. I had a look
at the circuit and suggested using a TL071 in place of the 741 in the 
lin-log converter stage. He did and the improvement was massive. He then
went through the rest of the modules over the course of the next month
or so doing it to all the 741's in there. There was one module where he put
the 741's back though. The audio mixers. He reckoned that with a TL071 it
was just a bit too clinical whereas with 741's it became somehow warmer.
That has got to be some sort of distortion effect, but it was a marked 
effect, you could hear it on tape easily. Maybe we shouldn't be in too much
of a hurry to bin the old horrors after all. Also, another friend with a 
VCDO (a Digisound module that played back sampled single-cycle waveforms
from an EPROM) said the same but with reference to the output buffer amp.
Basically if you used a better op-amp you were getting clock breakthrough
on the output, whereas with the 741 it wasn't a problem. I've just exchanged
emails with someone who was suggesting a lo-pass filter for use in hybrids,
the solution is fit a worse op-amp in the output buffer...
chrisc



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