[sdiy] 2600 op-amp question

[Joachim Verghese]

Hi,

> First, I'm going to replace the 301 op-amps as several people suggest.
> I'm trying to decide what to replace them with.  I've been thinking of
> using LF351s, but I'm wondering if anyone has better suggestions.

The LM301 is definitely no HiFi part, but then a synth like the
ARP2600 has very little in common with HiFi equipment. ARP obviously
knew what they were doing (well, most of the time), and frequently
employed techniques such as feed-forward compensation to improve
the performance of the LM301.

The circuit board layouts were designed to accommodate the bipolar
LM301, and therefore issues like RF-pickup might not have been taken
into consideration. FET-input op-amps such as the LF411 or LF351,
with their teraohm input impedance, might lead to problems in some
cases.

Bottom line: Don't change the design unless you know exactly what
you're doing.

Personally, I guess the op-amps in the reverb amplifier could be
worth replacing with some modern low-noise audio type, but for the
basic VCO-VCF-VCA building blocks I'd probably stick to the LM301.
All of this is IMO, of course.

> Tim Smith (who worked at ARP) knows his way around a 2600, and
> suggests which opamps should go into which cuircits.

Tim Smith used to be an (independent) ARP-authorised service engineer.
One of the business ideas of his company, Weyer-Smith Labs, is to
replace op-amps in synths, it seems.

-joachim