"I've got a Mopho keyboard and it's a great little thing. But one thing puzzles me; when listening to any patch using a pulse wave, I notice the overall sound somewhat dirty sounding. It's not very nice really.
You can hear what mine sounds like here:
http://www.oakleysound.com/Mopho&Pulse.wav
I popped the hood and found that it is caused by the internal CVs to the voice chip that control the pulse wdith having some sort of randomised voltage added to them.
However, 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 set to 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 - but in particular from the -5V charge pump power supply which generates horrible 80kHz perturbations.
However, the real 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 10uF 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.
The Mopho Keyboard is primarily built from surface mount components using lead free solder. This makes it slightly more awkward to work on than older through hole designs. However, it is quite possible to do a nice job if you take care.
You'll need one 10uF, 16V tantalum capacitor. You can use a decent quality electrolytic if you wish, and in that case, a 10uF, 25V will suffice.
Solder the cap across the resistor R71. With the keyboard to the front of the unit, the positive terminal of the cap should be facing left. That is the negative end should go to the ground plane which is attached to the end of R71 that is nearest to the number 1 of its name.
For the Mopho Desktop:
This seems to be built in much the same way with the DAC reference being taken off the +12V line. I am guessing it too will have the same unwanted noise on the DAC reference. However, the board is different in several other respects and the parts numbering is different.
Now I haven't tried this but from online photographs I can assume that you will need to solder the 10uF cap across R52. This is situated, again above the DAC, near U12. The negative pin of the cap should be soldered towards the front of the unit, ie. the positive pin will be mounted nearest the R52 legending."
