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Old 16th September 2013
  #18
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Right for those of you that would like to do this modification, here's a bit more information:

The noise we are trying to filter out is mostly coming from the -5V charge pump power supply. The current spikes at 80kHz or so are polluting the supply voltage and the +12V low drop out regulator can't keep it clean. It is possible to filter the +12V feed to the +5V regulator with a RC network of 10R and 220uF. This will remove most of the noise on the +12V line. However, this is a more tricky modification and it still doesn't remove all the noise on the +12V that then feeds into the DAC. So filtering the DAC reference voltage is the best idea.

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.



And a closer view:



Don't pull too hard on the cap once it is soldered in place because you might pull off the solder pads of R71.

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.

Tony