Did a vero layout for the Oberheim VCF (Voltage Controlled Filter) / Maestro Filter Sample Hold effect today. Haven’t had the time to build it yet, but will try to source the parts I’m missing. I’ll get back with an update once I’ve built the thing.
Note that this layout has yet to be verified.
Update (17.10.2010): Finished building and testing the circuit. Decided to swap the S/H speed pot compared to the schematic, so that turning it clockwise increases speed. The direction switch seemed to be a bit strange in one of the two positions, but this might be a testing glitch or a mistake when populating the board. I guess the layout is verified at this point.
Update (03.11.2010): In the process of boxing this now. An optional indicator LED has been added to the layout visualizing the Sample and Hold speed setting.
Update (18.12.2010): I finally finished this effect. This has got to be the most troublesome build I’ve done so far. I’ve completely wired it up four times now, and this last time everything worked properly. In my first attempt the speed pot wires of the sample and hold effect injected a really annoying control signal ticking even to the bypassed signal. My second attempt saw me reorient the board inside the enclosure and use shielded wire for the output signal. This solved the ticking noise, but the filter/auto-wah effect just refused to work at all. My third attempt gt me nowhere closer to solving the auto-wah, but on the fourth attempt, tonight, I finally found the problem; the attack pot had gone bad. Replaced it, wired it all up, waited for the moon and all the stars to align, carefully put the back of the enclosure on… and it works!
The gut shot is an intermediate state, before I got all the wires hooked up, but shows the inside layout. Notice I placed a small charge pump circuit on top of the right-most pot using a bit of double-sided tape. The charge pump enables me to also get -9v DC internally so I can run the effect on a regular +9v DC power supply and still be able to feed the circuit the required +9v/-9v. I also need to get some knobs attached to those potentiometer shafts.
I’ve got the reverse sweep switch on the left and a SH speed visual indicator switch on the right-hand side. I didn’t find the reverse sweep all that useful myself, but it doesn’t hurt to have the switch present. I also decided to make the filter switch “stompable” so I can go between auto-wah and sequencer mode on the fly. I wired this up with a dual LED similar to how I did the Bad Cat Hot Cat emulator (see that post for a wiring schematic).
The speed indicator LED is also worth mentioning. My friend Jon thought it would be great if we could have a visual indicator of the speed setting on the sample and hold effect. This was readily available by connecting the positive side of a LED to pin 7 of IC4, but by doing so we also altered the tuning of the sequencer. Therefore we added a toggle switch to lift the LED from pin7 giving us the option of turning on the visual indicator LED while adjusting the speed and then turn it off again before playing.
Based on my own experiences with this thing I’m hesitant to recommend others try to build this, at least if you’re just beginning, but of course your mileage may vary…
Update (30.06.2011): Here’s a new vero layout for the Oberheim Voltage Controlled Filter / Maestro Filter Sample & Hold. This one is based on the tonepad schematic and uses an LM13600 instead of a pair of CA3080. I was also able to squeeze in both the input buffer I left out of the first layout and an onboard charge pump.
Update (20.10.2011): Fixed dead layout links.
Update (12.03.2012): At last! I got around to building the version 2 vero tonight and it works quite well actually. I tried a few different transistors for Q1, and the 2N3904 didn’t do anything at all, but I reasonably good results with both 2N2222A and 2N3703. I’m going to do a more thorough search for Q1 when I box this, and the layout may still have a few of the pots wired in reverse, so I won’t mark it as verified just yet.