2600 repair suggestions needed

brad sanders radioactive at geocities.com
Sat Jan 18 03:09:44 CET 1997


>Okay!  NOW I'm getting confused....

>If someone could SPECIFICALLY suggest PROPER op-amps to use in place of the
>301's as I can't find them OR the LF411's locally, I'd like that.

Sorry; I didn't intend to confuse anyone. 

The TL081 is roughly equal to the LF411. The AD711 is an "upgrade" to
either. If this is going in the STOCK S/H module, and the opamp in the
module is "buffered" by a pair of FETs, then the ultimate performance
of that module is going to be pretty much at the mercy of these FETs.
Ironically, you might get even BETTER performance with the AD711 and
"shorted out" (bypassed) FETs.

No way would I use the 741 - or any other "bipolar input" opamp - in a
S/H circuit. The reason is input bias current, and thermal drift; the
bias current acts to charge the S/H cap, which causes (in this
application) pitch drift. For example, the OP07 has "average" 2-5nA
bias; the FET input AD711 (the "upgraded" LF411/TL081) has an input
bias current over 100 times lower: 20-50pA. 

Simple suggestion: use the TL081. In fact, order a tube of'em, and
use'em everywhere. Order 25 and you'll probably get a price break. 
If you want to get really "optimal" on the S/H module - and you want
to use the EXISTING module, rather than DIY a new one - you might
order one AD711 for use here. It's little "better" (in this app) than
the TL081, except in temp stability - so it might not even be an
upgrade unless you're doing LONG sustains, or playing under a fan (the
electric kind, not the fishy kind).


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