Home-made Modular Synth teething trouble II

[Bill Layer]

Hi Phil,

  However, my power supply is regulated
>so I cannot see why this should be.

Regualtion has absolutely nothing to do with this. I wish it was that easy.

>Can anyone explain briefly why I need these capacitors, what they are
>doing and why I may need more than one capacitor over one supply rail.

They will be acting as local de-coupling for the modules. A small series
resistor would improve decoupling performance as well. 
>
>Are these capacitors acting as filters, and therefore different
>capacitor sizes filter different (unwanted) supply frequencies?

Yes they are acting as filters; I've never met a cap that didn't. Those
*filter* caps in your power supply are effectively sending noise (At a
cutoff frequency determined by the source impedance of the power
transformer/rectifier and it's relationship with the cap value) to ground,
but many people still think of them purely as power reservoirs to 'fill in
the gaps' of the rectified waveform. While that thinking is fine for
learning purposes, it ought to be abandoned when getting serious about PS
design.

It is important to select the right type of cap for the job. Electrolytics
are good at low frequencies, film caps at medium-high, and ceramics for RF.
Probably a combination of electrolytic and film types would work in your
situation, allthough you may find that just a small film or electrolytic
cap provides sufficient decoupling for each module. 

Hope this helps.


Bill Layer
Sales Technician
<b.layer at vikingelectronics.com>

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