The classic synth geek circuit. You can still get AD633s from surplus places or Ebay. I Ebay-d 10 of them from China for something like $25US with shipping.
Super easy circuit. I like that and it Sounds pretty good, I still need to better null out the Y channel but still it's got the classic old school sound.
You can the Eagle Schematic files here and the Eagle Board file here. I have not tested the board layout yet, this circuit was so simple that I perfboarded it, ran it to a small panel, and hooked it up. I may make some boards for this down the road.
How this one works:
The TL072 or whatever dual op amp you have lying around is for buffering the incoming audio signal. My circuit assumes decent levels (say +/- 2V P-P at 1K impedence) vs. guitar signals or whatever, so you may need to put more gain stages in front of whatt I have here. The rest of it is pretty much stolen right out of the AD633 Spec Sheet. The final single stage op amp is to gain back up the output, but the "Z" trimmer assists with that as well.
I have AC coupled inputs with .1uF Caps in the schematic but for mine I used .27's and your mileage may vary. You may consider leaving out cap coupling or have it on a switch. For me I was only B-Ming audio and the .27's were in my junk box so that's what got used.
Still need to work on the Y null. I had an old PAIA B-M when I was like 13 and I remember it didn't have this much Y-In bleed, but maybe that's just my memory being bad after all these years.
here's my orig pre-eagle drawing just for S&G's.. >