[sdiy] filter for noisy DC power supply
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!
Benden Sound Technology
More information about the Synth-diy