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Serge VCFS - OPA2604 vs OPA2134?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Serge VCFS - OPA2604 vs OPA2134?
Hey all, I've been doing a lot of Random Source Serge DIY lately, and really getting in-depth with the filters and ResEQ. I put together a VCFS the other day and tried using two OPA2604s instead of the usual two OPA2134s, the first time I have attempted this. It made relatively little sonic difference except that when the Frequency knob was set full CCW, the High output produced silence along with a positive offset of (if I recall correctly) approx +5, while Band produced a -5V offset or so. When freq was swept a little clockwise, or if I fed positive CV to the CV in, these outputs would produce a loud pop which was definitely undesirable before starting to filter as expected.

I checked the joints and everything was cool, so eventually I just swapped out two 2134s. What do you know, no offset/popping issue, everything working great. So, 2134s it is!

Anyway, I was wondering if anyone else out there has a similar experience, or even better, might know what difference between these two chips might cause such a response?
not much help, but I did test both after getting some fake ones from china, they were very different, the real 2134 vs 2604, the ebay/ alibaba no, but I think I prefered the 2134. I think that was in the VCFQ.
Pick the one that sounds good to you.
Either one is overkill, but the difference could be in the speed of the opamp, they could be oscillating at very high frequencies, and/or operate outside of spec limits (the OPA2134 is unity gain stable, not sure if the OPA2604 is, and I am not sure what gain they are used in this circuit).
Having said that, I quite like the OPA2134 for "hifi"-esque applications, it does seem to imbue a slightly warmer sound in some circuits, but I wonder if you'll hear the difference in a VCF.
the OPA2604 has a higher bias current and a larger offset voltage. without seeing the schematic, its hard to say, but these are often culprits when looking for saturated amplifiers. in particular, if turning the frequency knob full CCW reduces the gain block current (which would make sense), then the bias current being 100pA could be the root of the problem.

the OPA2604 is slightly less stable (depending upon how you define stability), but that usually ends up causing oscillations.
I've used 2134s in filters with no issues. I haven't used the 2604, but I have a feeling that Guest is correct with the bias and offset. I tried using OP275s in a VCFQ once, and they did something fairly similar to what you describe. Those have a much higher bias current and offset voltage. It tended to make the response "lumpy" and distort a bit at some settings. hihi In that specific case, I just put in the LF353 that was originally recommended, and it worked perfectly after that.

I do use 2134s in other filters, and they work great. IMO I'd just stick with those.

The places that I usually use something like the 2604, is when I need to drive a higher impedance load for an output module or something like that. They work pretty well for that. (but then, so does the LME49720 which is better in many ways, but still might not be great in a filter).

Other than the 2134, I find that LF353, TL07x, etc. work pretty well in filters.

There will be quite a bit of trade-off between different types of op amps. For example, the OP285 (which I believe is laser trimmed) has a low offset voltage, but higher bias current.

Something like the OPA1642 might work ok, but it's SMT only as far as I know. It's also overkill in the extreme.
Thanks for the info, Guest and J3RK! Yeah, unfortunately the schematic for R*S's VCFS is not available but I could see bias current and offset voltage causing funkiness for sure. Good to know this for future builds!
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