PC Power Supply for DIY

[D. Schouten]

> I can get an old IBM PC for $10.  Is the power supply useful for 
> powering a
> synth DIY project?  I would think so.  Would I have to keep the 
> noisy fan?
> 
> Fred

As a switch mode power supply (SMPS) designer for years I'd say, 
never use a *PC* power supply for your synth DIY projects. The main 
reason for this, is that SMPS's are always designed to their specific 
application, in this case powering a PC. This means that the power 
supply is optimized for power delivery at +5V (the internal feedback 
loop is referenced to this output voltage).
The +/- 12V outputs have a relatively bad regulation, typically 
about +/- 5%. Note that for example some PC add-on cards even use 
onboard postregulation to keep the voltages within certain limits.

An other 'bad' thing is that *PC* SMPS's have a ripple of about 100 
to 200mV at the switching frequency (typ. 50..100kHz), riding on 
their output voltages. This is not very desirable in audio 
applications.       

And then the minimum load conditions. SMPS's, and especially (cheap) 
PC powersupplies, need a certain load level (typ. 20% of full output 
load) to work in a stable mode. Some PC power supplies even blow up 
if switched on with no load! In a PC, the power supply is always 
loaded with some power consuming devices like floppy drive, hdd etc.

So, to keep things simple, reliable and clean, just buy or build a 
lineair power supply.
But I have to say that I've built two synth DIY projects with my own 
custom designed SMPS's. With ripple specs comparable to the best in 
lineair land, and a power density a lineair power supply can only 
dream of.
How about a +/- 15V, 3 Amps supply on a 2.5 x 2 inch PCB, including 
transformer, huh?

Daniel...