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

[Richie Burnett]

Switch-mode power supplies are used in almost every bit of commercial 
equipment youwill encounter these days.  Early SMPSUs gained a bad 
reputation for EMI due to poordesign and poor understanding of how to 
control emissions.  If you pick a goodquality modern SMPSU there is no 
reason why it can't work well alongside sensitiveanalogue electronics. 
There are a few things to consider:There are two types of switch-mode power 
supply.  Flyback and Forward converters.Flyback power supplies are usually 
low-power (<70W), compact, lightweight and *DIRTCHEAP* to manufacture.  They 
are also the most noisy in terms of radiated andconducted emissions because 
they lack a smoothing choke at the output.  Their inputand output currents 
are both chopped up, so they tend to spew RF out both ports aswell as 
radiate magnetic field from the gapped flyback transformer itself. 
They'reokay for thigs like shaver and phone chargers but avoid these for 
anything thatrequires low ripple and low EMI.  Particularly the cheap 
chinese made ones!Forward converters are usually rated at higher power 
(>70W) and are bigger, heavierand more expensive.  Whilst their input 
current is discontinuous, the output currentis smoothed by a large choke so 
they produce far less ripple at the output.  Theyalso typically radiate less 
magnetic fields because they don't use a gappedtransformer.  Forward 
converter type SMPSUs are the type of choice for low noiseand sensitive 
applications because they are inherently quieter by design.100mV is a lot of 
residual ripple at the output of a SMPSU!  However this figure isprobably 
quoted at full load, so might be better under light loading.  As othershave 
said, forget the idea of using linear post-regulation to clean up HFripple. 
It doesn't work and burns up a lot of power as heat.  If the output rippleof 
a SMPSU is too high, a passive LC post-filter should strip off what 
remains.Most HF noise left at the output of switched-mode power supplies is 
usually common-mode noise.  You can heavily filter this just by passing all 
of the output wires(including the 0v line) several times through a 
high-permeability ferrite toroid.If you look inside commercial kit, you will 
often see a ferrite sleeve or toroidmid-way along the wires from the 
power-supply to the main circuit board.I hope this helps.  I used to be 
chief EMC engineer at a previous company so learnta lot of this stuff the 
hard way over the years.-Richie,> Some PSU thoughts.
>
> In order to minimize the size (and heat) of the JH Living Vocoder,
> ( http://www.jhaible.de/vocoder/living_vocoder.html )
> I'm considering the possibility of using a switched-mode power supply
> instead of a transformer. Something like these devices:
>
> 
> http://www.tracopower.com/products/ac-dc-power-supplies/encapsulated-modules/
>
> 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. The supply would sit
> physically very near the vocoder circuitry though, practically on the
> free lab area in the corner of the pcb.
>
> Can anybody recommend for or against such switching supplies with
> analogue audio? Would it be a sensible idea at all, or would I regret
> that I tried?
>
> /mr