I made a simple mixer for the synthesiser . It's a summing amplifier with two inputs, of which one is AC-coupled with a 47uF cap, so that the created high pass filter will still pass 0.1Hz signals through. My first idea was that this could be used to add some signal on top of the voltage passed on to the VCO, to create some pitch vibrato. This idea worked, but still failed, since the synthesiser gets totally out of tune when doing this. The problem is that what I thought of as unity gain from the summing amplifier, isn't really unity gain, since I'm using 5% resistors. The small change in gain can throw of the tuning, there is as little as 200mV difference between two semitones.
Also, the simple keyboard uses a wild variety of potentiometers, who's resistance will be a part of the feedback network of the summing amplifiers. Thus the keyboards output needs buffering. I'll get back on the topic as soon as I'm done doing that.
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| Summing amplifier mixer, schematic |
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| Summing amplifier mixer, actual board |
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