hum on digital recorder but only when using mains adaptor

Oct 24, 2011 4 Replies
D

I have a digital recoder similar to this.

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It works fine on - batteries. Then I build my own -3V DC power unit for it. It still works fine with that but there is hum *if I hold the recorder while recordin*. If I don't actually touch the recorder the hum is not there. I guess it's mains hum as there is no hum when run on batteries..



The psu I built is spot-on the correct voltage and easily provides the current demands of the recoder (500mA at most). The transformer I used is screened and the screen to tied to ground. One side (+) of the DC output is also grounded. It uses a LM337 regulator chip an has the usual configuration. There is a 1u tant capacitor across the output (as per design notes for this chip).



Any ideas how to cure(!) this hum problem appreciated.


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K

This is usually caused by a "ground loop" between the power supply, the unit, and you. Often, this is caused by poor isolation in the power supply. Try using an isolation transformer or a better power supply.

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P

"dave"

** Your body is the source of the hum and the recorder is built in a poorly shielded plastic case.

In any room where there is AC wiring installed, human bodies have an AC potential of up to several volts. When you hold the recorder, some of this voltage is injected into the electronics - probably the internal mic wiring or mic preamps.

When the recorder is powered by batteries, there is no injection when held because it is all at the same AC potential as you.

If you attach a metal plate to the back of the recorder and connect that to the DC supply common, then long as you hold onto that plate with your fingers it should fix the hum.

... Phil

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P

"Charles"

** The only difference between the "Charles" lunatic and parrot is parrots are smarter.

... Phil

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D

Thanks - will give it a try. I did think it was not a good idea to make such a sensitive device in a plastic box - I guess alumin. one would send the proce right up.

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