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Old 17th July 2012, 07:28 PM   #1
timhum is offline timhum  United Kingdom
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Default cure for motorboating sound

I have just finished making a amp for my son to busk with, it has a couple of mic inputs and two guitar inputs into a 20 watt BTL amp all powered from a lead acid gel battery and the modules were from CPC.
It sounds well enough but when the volume is turned up and before distortion sets in there is a Da da da da sound which continues until the volume is backed off again. I think it is loud enough before this tedious interference but to get the full value from the system I wonder what can be done to stop this noise.
Any ideas?
Thanks in anticipation,
Tim
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Old 18th July 2012, 03:24 AM   #2
gootee is offline gootee  United States
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I guess that the obvious solution would be to lower the gain or increase the input attenuation until the motorboating doesn't happen with the volume control at max.

If you want to try to make the amplifier stable at the maximum volume control setting, with the present gain and input attenuation, then you will probably need to provide a schematic and some photos of the circuitry.

Is it AC coupled? Is there an input filter? Is there any capacitor in or near a feedback loop? Maybe there's a "response speed" (i.e. RC time-constant) mismatch between a filter upstream and something downstream or in a feedback loop.
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Old 18th July 2012, 04:19 AM   #3
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This can also happen due to a ground loop or if output circuit is powered at the end of the connecting wire.

Gajanan Phadte
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Old 18th July 2012, 05:48 AM   #4
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Quote:
Originally Posted by timhum View Post
I have just finished making a amp for my son to busk with, it has a couple of mic inputs and two guitar inputs into a 20 watt BTL amp all powered from a lead acid gel battery and the modules were from CPC.
So there is a mic input module and a separate 20 watt amp module powered from the same gel battery?
My very first project was to assemble some modules like this - and it motor boated You need to decouple the power supply to the input modules. A smallish resistor and a decent electrolytic capacitor should be fine. You haven't provided any details so it's a bit hard to be specific but a resistor that drops a volt or so should be adequate and a capacitor to make the RC time constant lower than the roll off of the amp. Is that sufficient detail?

Best wishes
David
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Old 18th July 2012, 07:07 AM   #5
timhum is offline timhum  United Kingdom
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Thanks very much for all your helpful replies guys. In spite of my scanty detail you have all given me things to act on. First I will decouple the supply to the mic and guitar input modules in the manner Dave Zan describes since he has come up against this problem before. I shall certainly post the results!
I have looked at the circuit for the power amp and it is the BTL version of the standard implementation of the TDL2005 amp, on this link
Power audio amplifier schematics with Tda2005
The only difference is that the zobel networks are missing on the version I bought as a kit. The inputs are AC coupled and the outputs from the input modules pass through a 10 K pot and 47K resistor where they join at the input to the master volume pot thence to the power amp.
Again, my grateful thanks to all who posted, thank you for your time and helpful attitude.
Tim
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Old 20th July 2012, 07:18 PM   #6
timhum is offline timhum  United Kingdom
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So I did as David suggested, did a few sums and found the parts in the oddments box. It worked like a charm and I have a very happy son.
Thanks again for all your replies,
Cheers,
Tim
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