Vintage Synth Repair

> The mod I was reading about suggests
> the  411 as a low noise replacement
> for the 301.  I admit my 2600 is a bit
> noisy, most notably in the mixer and
> reverb section.

The LF411 isn't exactly a low noise opamp.
Typical noise voltage is 25nV/SQRT(Hz).

The LM301 has a typical noise voltage of ca.
15nV/SQRT(Hz), which actually makes it
_better_ than the LF411.

Both are not exactly low noise opamps. For
comparison, a decent mic amplifier chip
like the SSM2017 has less than 1nV/SQRT(Hz).
Of course this will _not_ fit in here; just
to show the difference. There are opamps with
similarly low noise, whouch _would_ fit in,
but don't ask me for part numbers.

Now this is only part of the sory, though.
If you have a high impedance circuit (large
resistors in your circuit), BiFet Opmaps
like the LF411 might be less noisy, because they
have very low noise *current*. (This adds
to the noise voltage, depending on the feedback
resistors around your opamp.) So depending on the
circuit, you might even get a little noise
improovement with a 411 instead of a 301.
BUT then I'd really suggest a low noise BiFet
Amp like the TL071 (if you want an inexpensive
replacement). This has a noise voltage similar to
the LM301, but - like the LF411 - does not contribute
much noise current.
If you want to go a little further, try one of the
plenty modern opamps offered from Analog Devices
or TI (formerly Burr Brown) which have noise voltage
in the range of a few nV/SQRT(Hz). Expect to pay a few
dollars, though, and check if you like the sound.
Replacing an opamp with a better one will not automatically
lead to a better sound - it might, or it might not. 
You have to try it with your own ears. I'd try a
cheap TL071 first. (needs no external compensation
capacitor, so remove this if you swap the opamps.)

JH.