[sdiy] op-amp swap works wonders for digital delay

[anthony]

So I have this DOD (Digitech) PDS2000 Delay/Sampler. I hadn't been happy 
with its tone when used with my tube amp, so I looked at all the IC's inside 
to figure out what might give me better tone if I switched with a different 
make/vintage.

The first thing I noticed were the TL062's that were used for the input and 
output amps. I'm not really a big fan of BiFET op-amps in audio paths. Heck 
I'm not a fan of much of the regular bipolar op-amps for audio either.

I do like: RC4558, JRC4558DD, uPC4570, JRC4580, M5218, M5220 (I have a hard 
time finding these, but they're great for low impedence loads and trusty old 
4136's (I have tons of these and always use them in Buchla clones). Oh I 
like LM301A's too, especially for low-pass filters. The LF356 is my favorite 
cheap BiFet op-amp.
But I digress...

I have TI, JRC and NEC makes of the 4558's. None of them disappoint. I used 
2 TI units to replace the TL062's in the PDS2000. But for the heck of it I 
replaced an ON LM1458 with an older TI version and an RCA CD4066 for a 
National CD4066 and a Toshiba 74HC4040 for a National 74HC4040. I have 
nothing against National Semiconductor - I just thought I'd change some of 
the existing parts for older vintages to see if there was much difference. I 
like RCA & Toshiba 4066's the best.

Anyway the unit soudns much better. It would have been better form to just 
try the op-amp switch to see how that swap affected the tone, but I ended up 
with an obvious audible improvement nonetheless. I highly doubt switching to 
HC4040 changed anything audible, but the RCA 4066 possibly could have - 
maybe even negatively. But I like what I got so what the heck...

aa