OTA filter noise?

[loss1234]

In a basic  13700 2 pole filter State Variable filter, which i am trying to box up, I am getting a lot of noise (Well only with a guitar through an amp).

I am trying to give the OTA as much gain as I can going in but I don't want it to clip. (i am quite sure the noise is coming FROM the OTA)

I am wondering:

1. should I try using a different kind of output buffer? (it's a TLO62 right now)

2. should i try putting a cap and resistor to ground on the output to filter it out

3. I am using a DC to DC converter to get +/- 15 volts...could this be adding to the noise?

4. WHen i add a bunch of BYPASS caps to the rails, the noise doesnt go away, but it does die down a bit,,,,,does that say anything about the source of the noise??

5. at the end of the day, if you only hear the noise in quiet passages and the rest of the time it sounds awesome, how much should i worry?

thanks!!

[Cliff Schecht]

1. I've been using the TL082 as a buffer after my OTA filters with great results.
2. I like the cap in the feedback loop of the output gain amplifier. 100 pF is usually adequate.
3. What frequency is it switching at and what does the +/-15 V outputs look like on a scope (if you have one). If the switching is above about 25 kHz, then it's probably not contributing any noticeable noise.
4. Yes, it does. I would check that the switcher isn't the source of the noise by finding a well filtered PSU and running your SV filter off of that. You want at least one 0.1uF bypass cap on every IC voltage supply (+ and -). You could also try more filtering at the spot where V+ and V- enter the board.
5. Meh. Noise is noise. It's especially tricky with an OTA because the input voltage has to be cut down to near the noise floor, amplified a lot, then cut down and amplified some more. I've designed quite a few voltage controlled filters with the 13700 and noise wasn't a significant problem, if one at all.

I must ask, where did you get your basic SV filter circuit from?