[sdiy] Arp 2600 replacement op amps

[Michael Bacich]

On Oct 1, 2006, at 5:07 AM, James Dunn wrote:

> Any advice gratefully appreciated.

Advice?  Umm, don't do it.  All of those 2600 LM301 circuits are  
designed to depend on the LM301's unusual non-frequency compensated  
characteristics, and they won't work right with any other amps -- not  
without a lot of other modification and experimentation, anyway.

If you want your 2600 to have less noise, you can get a certain  
amount of noticeable improvement by replacing any carbon resistors in  
the audio path with metal film resistors.  It will be a lot of work,  
though.  It will yield more audio improvement than replacing all the  
opamps would, and it won't effect the circuit operation --  as  
replacing the opamps certainly would.

BTW, you can greatly improve the signal to noise of the 2600's   
reverb section by simply increasing the gain of the reverb send  
amplifier.  This will increase the volume of the reverb, which will  
allow you to lower the reverb noise level by lowering the reverb  
return level.  Here's how:

Try decreasing the value of R273 on the 4-2 board.  It (in  
conjunction with various other parts) determines the gain of the  
reverb spring driver.  Right now, it's 100K -- you could roughly  
double the gain by replacing with a 47K (or by simply paralelling  
another 100K resistor across it).  I would probably not go much lower  
than 47K, however, as doing so may cause audible distortion or  
unwanted artifacts in the reverb.  As far as reducing the reverb  
return gain, all you need to do is turn down the two reverb sliders  
on the front panel.  They are essentially reverb return level pots.   
A benefit of this increased reverb spring drive mod will be that  
you'll have potential for a much higher overall reverb level, which  
would allow for even more dramatic reverb effects whenever you're in  
the mood for that sort of thing.

This mod will not help the noise level of the 2600 when you're not  
using the reverb, but if you. like most of us, enjoy using the 2600  
reverb, this mod will give you MUCH more sonic improvement than  
replacing a boatload of opamps ever will.

Michael Bacich