Simple ribbon controller for V/Hz synth

via nicolas3141 on this electro-music.com thread: "Continuing my simple synth series. Design goals: simple and educational circuit, 9V battery powered. V/Hz with CV output of about 1-5V giving at least two octaves of reasonable linearity. Stripboard layout shown with no cuts required. If you have a 5 or 6V reference voltage already available you can use that instead of C1 and U1. If you are building it into your synth you can use the main power supply instead of the battery and you won't need the stereo socket with the power-on-when-plugged-in earthing arrangement. Multiple output sockets on this can be very handy in a modular synth.

Finding suitable tape is the hardest bit. I have tested quite a few video/audio/computer-backup tapes and found that pro-grade tapes from the 80s or earlier are the most likely to be suitable (amongst older pro-grade tapes I found I needed to test about a dozen tapes to find two or three suitable ones). Modern consumer grade VHS is not so likely to be suitable. I think you want somewhere between 10K and 100K per 10cm of tape. Most tapes are more like 1M or more over that distance which is too high to be useful. I have found one oddball that was about 3K per 10cm.

One thing to note is that the contact resistance can be quite high. For example if you get a reading of 100K over 10cm, 25K of that might be the probe to tape contact resistances. So 1cm would read about 30K. This doesn't usually matter too much with a three wire voltage divider type circuits like this, but will affect the linearity slightly.

I have some quarter inch tape from a computer backup cartridge and some half inch tape from professional TV betacam that both give me a reading of just under 100K per 10cm. Both work well with 25-30cm of tape as the ribbon, but the betacam tape seems to wear out much much faster. I stick the tape down with double sided sticky tape. Long and high resistance ribbon controllers are more likely to pick up hum and noise. C3 is there to quite that down, but does slow the response slightly. You may need to adjust the value of C3. Use the smallest value that still kills the noise.

You only need to find one tape that works and it will supply you and everyone you know with ribbon controllers for the rest of your life including regular replacements of the ribbon if necessary. So if there is anyone in Christchurch, New Zealand who needs some suitable tape, feel free to contact me.

Cheers,
Nicolas"

Be sure to see the electro-music.com thread for updates.