[sdiy] filter for noisy DC power supply

Steve Lenham steve at bendentech.co.uk
Thu Oct 23 10:41:27 CEST 2014

On 22/10/2014 10:50, David Griffith 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?

I personally wouldn't assume that there is a ground fault, etc, etc - 
some devices (esp. cheap ones) simply don't have very good power supply 
rejection. Example: the EDP Wasp synth uses an LM386 amp IC run directly 
from the DC power input voltage to drive the speaker and H/P output. 
With a regulated PSU all is well, but with a "normal" unregulated 
adaptor...hummmmmmmmmmmm. However, the line output is powered from a 
regulated +5V and has no such problem.

My favourite simple hum rejector is as follows. It will not cope with 
massive levels of input ripple (around 1Vp-p max) but, on the plus side, 
drops very little voltage. If you need to cope with more hum then you 
need to look at proper regulators, but must then address the issue of 
how many volts you can afford to lose.

Connect an appropriately-rated NPN transistor inline with the positive 
supply line (input to collector, output to emitter). Connect a 1K0 
resistor from collector to base and a 100uF capacitor from base to 
ground (neither of these values is hard and fast - they can be adapted 
to what is available). Done!


Steve L.
Benden Sound Technology

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