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

[Harry Bissell]

I vote with Achim... I've had some negative experiences with the Traco products as well.

A problem is that lower cost switching regulators usually try and skimp on the inductors
to save money, running them at higher flux levels (more near saturation)and this can result
in increased magnetic leakage from the units.

(linear supplies can do this a s well, but you can use toroid transfromers and reduce this to
almost nil...)

The remaining 50 - 100mV of supply ripple is nearly impossible to get rid of, at the high frequencies
it tends to go around anything you pot in its path.

Switching supplies ~can~ definitely be used, but it takes more skill to do so. A lot of folk just
can't be bothered and use the tried-and-true linear supplies

H^) harry

----- Original Message -----
From: ASSI <Stromeko at nexgo.de>
To: synth-diy at dropmix.xs4all.nl
Sent: Wed, 25 Jan 2012 16:10:41 -0500 (EST)
Subject: Re: [sdiy] Switched-mode power supply module in synths... good or	bad?

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.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Waldorf MIDI Implementation & additional documentation:
http://Synth.Stromeko.net/Downloads.html#WaldorfDocs

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Harry Bissell & Nora Abdullah 4eva