[sdiy] Switched-mode power supply module in synths... good or bad?

[ASSI]

On Wednesday 25 January 2012, 21:04:39, Mattias Rickardsson wrote:
> It would then probably be a +/-15 V supply, regulated down to +/-12 V
> with the linear regulators on the Living Vocoder pcb in order to get
> rid of the switch ripple. I expect the ripple (on the order of 100 mV)
> would be way too strong to use directly.

These TRACOs are pretty noisy IIRC, so you might want to look further.

Check the datasheet for ripple specification and get a PSU that has low 
ripple to start with (they usually have different circuit topology and are 
more expensive, though).  Fixed frequency switching is a definite plus and 
higher switching frequencies make the filter components smaller (that may 
come at the expense of more radiated emissions).  A linear regulator will 
_not_ be effective to attenuate the ripple in the 100kHZ+ region you have to 
expect, you should use a passive low-pass for that, maybe in combination 
with a voltage follower output or shunt transistor.  You need a very good 
voltage reference for these and again passive filters to take care of any 
noise above audio.  Again some PSU have those filters and transistors 
already included and unless you have at least a good scope they'll do much 
better than you can achieve yourself — in fact often better than typical 
linear regulators with noisy references.  Unless the Vocoder produces 
pronounced load transients, just use a 12V PSU directly.  Some further 
reading:

http://cds.linear.com/docs/Application%20Note/an70.pdf
http://cds.linear.com/docs/Application%20Note/an75f.pdf 
http://cds.linear.com/docs/Application%20Note/an83f.pdf


Regards,
Achim.
-- 
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