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Old 25th March 2013, 09:17 PM   #1
shrub0 is offline shrub0  United States
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Default Portable PA system blues

I'm trying to use a Lepai T-amp
Lepai LP-2020A+ Tripath Class-T Hi-Fi Audio Amp w/Power Supp 310-300

with a Kemo preamp
Kemo M040N Preamplifier Module 320-555
to create a 12V portable PA amplifier. Whenever someone raises their voice into the mic or a bass guitar string is slapped it makes the speakers make a continuing medium pitch thump thump thump noise and the only way I can stop this is to turn the lepai amp off. I've tried replacing the kemo preamp but that didn't seem to help.

How can I stop this from happening? Do I need an input resistor in series with my 1/4 jack input? Is it a problem that i'm taking the mono out of the preamp into both channels of the Lepai T-amp?

Thanks for any help,
Mark
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Old 25th March 2013, 09:50 PM   #2
JMFahey is offline JMFahey  Argentina
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That's called motorboating, usually caused by poor grounding or filtering.
Also possible if the PSU is underrated.
You also need some extra filtering for the preamp, you just can't feed it straight from the same point that feeds the power amp.
Can't go further without getting more details on your installation, but that's about it.
What happens if you feed some audio straight into the power amp, no preamp involved?
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Old 25th March 2013, 11:12 PM   #3
shrub0 is offline shrub0  United States
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Okay so I should install a voltage regulator for the preamp? If I send music to the amp like from an ipod I never have this issue. Thanks for a quick repsponse.
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Old 25th March 2013, 11:15 PM   #4
shrub0 is offline shrub0  United States
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Everything is grounded to the negative terminal of the battery. Should I ground some things the metal input plate instead?
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Old 26th March 2013, 01:30 AM   #5
Enzo is offline Enzo  United States
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How large is your battery. My first reaction was that your loud signal caused a sag in the power supply that started the motorboating.
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Old 26th March 2013, 04:17 AM   #6
shrub0 is offline shrub0  United States
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5AH battery Genesis 12V 5Ah Sealed Lead Acid Battery 140-460
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Old 10th May 2013, 02:00 AM   #7
shrub0 is offline shrub0  United States
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So i tried running just one of the channels. To no avail. It will still motorboat. Any suggestions? Should I put a capacitor inline with the battery like in car audio?
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Old 10th May 2013, 08:11 AM   #8
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Have you got an oscilloscope (actually, the power supply variations can be watched on an analogue multimeter, but some digital ones just about average it out. besides, watching a meter woggle backwards and forwards is more fun than watching numbers change). Yes, stabilising the preamp supply should eliminate the problem, as long as the voltage on the battery never drops below the operating voltage of the preamp. A series diode and additional reservoir capacitor will probably be just as effective, (if not more so) and much easier to build.
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Old 10th May 2013, 12:08 PM   #9
JMFahey is offline JMFahey  Argentina
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I have been reading about the Kemo modules.

I find them a bright idea, but perhaps poorly implemented.
The idea is cool: offer prebuilt "building blocks" so you combine them in different ways.
Problem is that since they are encapsulated you can't tweak things ... specially internal grounding, gain, etc.

As I said in post #2, you probably need extra +V rail filtering, you can't feed it straight from the same terminal as you feed the power amp.
Combine that with the fixed gain of 100X and the volume pot at the input (so the module runs always at full blast, even with volume on 0) and it's a miracle that it does not oscillate even more.

Try adding this simple RC filtering, with only 12V available you don't have margin for much regulation but this should do.

For best action I would solder this RC filter straight at the battery terminals and send the "clean" 12V to the module.

And .... for the next time consider building your own preamp all by yourself.
No need to mess with etching a board, just some perfboard is fine for such a simple project (single Op Amp gain stage)
And it will be customizable to your exact needs.
Good luck.
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Old 12th May 2013, 03:21 AM   #10
shrub0 is offline shrub0  United States
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Thanks jm, I purchased the components and will have time tomorrow to solder everything in. I have a power switch for the pre amp should I insert your rc circuit before or after the switch?
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