Noise suppression

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Noise suppression

Postby DrZarkov » Fri Mar 08, 2013 7:12 pm

I've made a telvisor, which is working good. Except that the motor causes some noise in the (big) loudspeaker. The coil of the loudspeaker is less than 10 cm from the motor.

What is the best way to suppress the noises? A capacitor between the two contacts of the motor? If yes, which one? Another solution would be a sheet of copperfoil between motor and loudspeaker. But that is not very elegant...

Replacing the loudspeaker is not an option, a smaller loudspeaker would work (I've tried it), but of course would not have that good sound.
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Postby Steve Anderson » Fri Mar 08, 2013 7:52 pm

I would guess that the motor noise is getting into the supplies and/or earth. First try a 100nF disc-ceramic capacitor across the motor terminals (as close and as short leads as possible). I am assuming it's a DC-brushed motor.

I doubt it's some form of direct coupling between the motor and loudspeaker, this can be proven by temporarily feeding the audio into an external speaker a metre or two away, its size doesn't matter the noise will probably still be there.

Next would be to make sure all the audio stages supplies are well bypassed/decoupled. I know I go on about this but it is vital to getting a satisfactory result.

Big fat earth leads to a substantial metal chassis help no end.

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Postby DrZarkov » Sun Mar 10, 2013 9:15 pm

Thank you, Steve, the capacitor did the job. :)
Audio and video amplifier do have separate transformers, the only connection between the two could be on the electronics of the the video-source.
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Postby Steve Anderson » Sun Mar 10, 2013 10:24 pm

Volker, good it's fixed. Wise move to keep different signals as separate as possible. This applies more to the motor circuity than audio/video which should be OK from the same supply.

Brushed motors are one of the electrically dirtiest electromechanical devices ever conceived...of all types.

...and the reverse, a DC generator, basically anything with a segmented commutator. Those with slip-rings (e.g the alternator in your car) are much better.

The often-touted bicycle dynamo used in reverse has its problems, but there are no brushes, it's electrically silent...unless driven with a square-wave of some sort (which also applies to inverters), but that's quite easy to mop up.

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Postby M3DVQ » Mon Mar 11, 2013 1:29 am

When I built my televisor I resorted to a completely separate power supply for the motor. I got away with a big fat shared earth rail in the end iirc, but you could even isolate the motor driver board completely by opto-isolating the control signal from your sync circuits.
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Postby Steve Anderson » Mon Mar 11, 2013 12:09 pm

M3DVQ wrote:...you could even isolate the motor driver board completely by opto-isolating the control signal from your sync circuits.

Yes, I was going to mention that in my previous post, but for some reason I didn't. A cheap 4N25 will do the trick, it's plenty fast enough for this task.

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