Subject: Re: [sdiy] Custom Prophet 5 Power Supply (was - bad voltages) n00b question

[Tom Wiltshire]

On 12 Nov 2007, at 20:05, Nathan Reeves wrote:

> what is "noisy" about power supplies?  what is the noise?  does it  
> have to do with the stepping down of the voltage in the xfmr?
> maybe a question for different list? lol
> cheers
> nate

The short answer is "Mains hum".

As far as I know (and I'm not an expert in the area) there are two  
major ways that noise gets from the power supply into the rest of the  
circuit;

1) Poor regulation of the output DC voltage. The point of a power  
supply is to convert AC mains voltage into DC voltage (at whatever  
level) for the circuits inside the synth. If this conversion is done  
badly, there will be some 'ripple' remaining on the output DC  
voltage. Effectively, it's a DC voltage with a small AC voltage added  
to it. The AC part you can hear as mains hum.

2) Electromagnetic effects. The transformer works by using the  
primary coil to convert electricity into a oscillating magnetic  
field. This magnetic field then induces a current flow in the  
secondary coil. Trouble is, it'll induce a current flow in any other  
coils (or even wires or circuit board traces) that are within its  
magnetic field.

Right, now I'll shut up and let an actual electrical engineer have a  
go....

Regards,
Tom