[sdiy] Linearized 2164 VCA Warning

[Harry Bissell]

the op-07 would not likely be a good swap for the TL07x series, its dog-slow
and has a way-lower input impedance. iirc, the op-07 is like .3V/us vs
13v/us for the TL07x

That said I'd use it to swap out a 741 if I wanted excellent DC performance.
In fact I'm fond of not using faster parts than I need. I like the 4558 for most 
synth work that does not need ultra high input impedance

H^) harry


----- Original Message -----
From: David G. Dixon <dixon at interchange.ubc.ca>
To: 'Andre Majorel' <aym-htnys at teaser.fr>, synth-diy at dropmix.xs4all.nl
Sent: Wed, 29 Dec 2010 03:15:37 -0500 (EST)
Subject: Re: [sdiy] Linearized 2164 VCA Warning

Hey Andre, I was just finishing up some work for my Parisian client when I
got your message (coincidence?)...

> > Given the exact nature of the problem, I'd rather add a
> > two-cent resistor and a couple of traces than replace a
> > one-dollar opamp with a five-dollar one to achieve exactly the
> > same end.
> 
> Precision op amps also have lower tempcos and lower long-term
> drift. Whether that matters in this circuit, I don't know.

It doesn't.  It's a VCA!

> They don't have to be expensive either. The LT1013 and OP07 are
> pretty cheap. But the cheap ones have low slew rates that might
> limit audio-rate modulation.

The one and only time I ever tried to replace a TL072 with an OP07, it got
so hot I had to shut the synth off (this was the current servo in my 2164
Expo VCO, and the current was bloody small).  I pried it out of the socket,
put the cheap-ass TL072 back in, and it's been oscillating happily ever
since (in tune and rock steady, I might add)!

I'll stick with the cheapos, thanks!

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-- 
Harry Bissell & Nora Abdullah 4eva