[sdiy] filter for noisy DC power supply

David Griffith dgriffi at cs.csubak.edu
Wed Oct 22 21:42:33 CEST 2014

On Wed, 22 Oct 2014, rsdio at audiobanshee.com wrote:
> On Oct 22, 2014, at 2:50 AM, David Griffith <dgriffi at cs.csubak.edu> wrote:
>> My gizmo is powered by an ordinary wall-wart power supply.  This supply 
>> is giving a hum to the output.  There is no hum when powered by a 
>> battery. Would someone please point me in the right direction of some 
>> simple filter circuits to clean up the hum?

> It's possible that your gizmo has poorly-designed ground. The battery 
> doesn't cause a problem because it is completely floating. The wall-wart 
> is possibly creating a ground loop. This is worst when power supply 
> ground currents run through the same traces as audio ground currents. 
> There's no way to filter that out - it requires changing the traces.
> Does your gizmo have balanced inputs and outputs? If they're unbalanced, 
> how do you handle ground without injecting power supply noise onto the 
> ground line? These can be tough challenges for audio design, but you can 
> probably find some articles online. Is this a gizmo that you designed?

The gizmo is a Behringer Microphono PP400 turntable preamp.  It has 
unbalanced inputs and outputs. I haven't yet bought it.  On Amazon, one of 
the reviewers says that it has a hum that he cured with a simple circuit 
inline with the power supply.  I'm trying to figure out what that circuit 
might be.

David Griffith
dgriffi at cs.csubak.edu

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