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Old 13th September 2013
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I have found the solution.

It is not only the pulse width control voltages that have this unwanted randomisation. All of the CVs do, but it is most noticeable on the pulse widths. What got me onto the right track was the fact that all the CVs had worse noise when they were at high values. This points the finger towards a noisy reference voltage to the system's DAC.

The DAC used on the Mopho Keyboard appears to use a 5V reference derived from the +12V supply. Now, the 12V supply gets tainted by noise from the other parts of the electronics of the Mopho - which is normal behaviour of any electronic system including synths.

The problem is that the Mopho creates the 5V reference by simply dividing the +12V rail with a pair of resistors (R67 and R71). This divided down signal, complete with a portion of the noise from the +12V rail, gets sent to a voltage follower (U18b) and then sent to the DAC. Oh dear... because this noise now ends up on the CVs. The higher the CV the more noise gets through.

Solution: Filter the +5V reference voltage before it goes to the voltage follower. I did it with a decent 10uF electrolytic or tantalum cap soldered directly across R71. The reference voltage is now nice and quiet. The Mopho behaves better, not only with respect to the pulse wave, but also seems smoother in general tone too. An excellent and easy modification to do.