Amp is making loud "boom pop" noises every second or so.. - diyAudio
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Old 29th October 2012, 05:31 PM   #1
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Default Amp is making loud "boom pop" noises every second or so..

I went to adjust the volume on my preamp's metal volume knob (which is grounded to the speaker's circuit) and I got a shock (static electricity shock I presume) and right then the speakers started making a loud "boom click" "boom click" noise over and over.

The amp is wide open and I attenuate the signal going into it from my preamp.

I disconnected the preamp from the amplifier and the amplifier is still sending out this very loud "boom pop" noise every second or so. I even got another electrostatic shock touching the amp when I went to disconnect the preamp.

I wouldn't be surprised if I blew out my tweeters in the speakers because the amp is much more powerful than the speakers can handle--I managed this with attenuating the signal going into the amp.

So I gather some circuitry is now bad in my amplifier? It's a dual mono block behringer a-500 reference amplifier, with a torroidal in the middle.

What's most likely the problem with the amp? Can I fix it?
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Old 29th October 2012, 05:48 PM   #2
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Okay I've isolated the problem to the left channel of the behringer. So I guess this should make it easier to test for what's wrong on it since I can compare measurements with the other channel.
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Old 29th October 2012, 06:04 PM   #3
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Without seeing the schematic this is no more than blind guesswork, and it could be anything. Switch your scope on and start at the output of the futzed channel and work your way back to the input. When you no longer see the 'kick' on the scope every few seconds you know you've just gone past the culprit.

It genuinely could be any part in the left channel so there's little point making suggestions.

If you only haver a multimeter and not an osciloscope then this might be awkward.....
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Old 30th October 2012, 07:11 AM   #4
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by audiogeekess View Post
and right then the speakers started making a loud "boom click" "boom click" noise over and over.
This click, is it the click of a relay inside the amp or is it coming from the speakers?
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Old 5th November 2012, 12:31 PM   #5
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Originally Posted by jitter View Post
This click, is it the click of a relay inside the amp or is it coming from the speakers?
Thanks for responding. The noise is coming from the speaker of the affected channel. If you like I can record--with my iphone 4s--the sound that it's making over and over and share it with you.
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Old 5th November 2012, 01:31 PM   #6
jitter is offline jitter  Netherlands
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Please do...
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Old 5th November 2012, 08:12 PM   #7
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I would disconnect the speakers and install 8 ohm resistors higher in wattage than the amp power rail supply cap value V^2/8. You may put a speaker out there, just use a junk speaker from the charity resale shop. Protect the speaker with two >1500 uf capacitors back to back, so any DC is blocked by the capacitor.
If you can't afford an oscilloscope, obsolete VOM meters can respond fast enough to find slow AC problems. DVM meters average over 3-4 seconds and will smooth out your boom pop. I like a 100000 ohm/ volt VOM, but VTVM's are also useful. These respond to DC on the AC scale unless you put a capacitor in series with the + probe higher in voltage rating than your wall power supply (200 V peak western hemisphere). Some VTVM's come with an RF probe which has the capacitor already in it.
Then you can check your power supplies. You can't have a schematic, a Behringer repair shop owner has explained the depth of the legal language prohibiting him from releasing Behringer schematics to the internet. But you can look up where the collectors of your output transistors or PS inputs of your output modules, perhaps, and find out what your power supply rails are. They should be lower than the highest voltage rating large value capacitors in the amp. The rail capacitors usually come in pairs, plus and minus.
Zoom pop sounds like a capacitor repeatedly charging up and shorting out, but not always.
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Old 21st December 2012, 12:09 AM   #8
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Well I am having the same problem with the other channel now on the amp. It got damaged the same way.. I got charged up with static electricity and went to adjust the volume on my DIY attenuator and it shorted it out. I don't understand why it did it again because a couple weeks ago I went through the trouble of grounding the preamp metal chasis to earth.

Is it because none of my grounds are isolated? The negative side of speaker wires are grounded along with power supply etc.. all together. Maybe I wasn't supposed to wire it all to a common ground?

I don't know what to do now. I was going to buy another A-500 but I fear blowing it out as well. I blew both channels on this old amp within a couple months.
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Old 21st December 2012, 12:44 AM   #9
jaycee is offline jaycee  United Kingdom
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do you have the chassis grounded to a protective ground? it sounds like you may well have some ground leakage
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Old 21st December 2012, 12:58 AM   #10
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Originally Posted by audiogeekess View Post
Is it because none of my grounds are isolated? The negative side of speaker wires are grounded along with power supply etc.. all together. Maybe I wasn't supposed to wire it all to a common ground?
The "grounds" of the speakers should be handled by the amplifier, not you the consumer. Your only responsibility is to ensure the 3rd pin of the power cord goes to a valid earth connection in your wall plug. I don't know if you mean you found the speaker returns to be at earth potential because you measured it, or if you actively did something to connect the speaker returns to earth, or to each other. The latter two are not correct, those are the amp designer's perogative. My PA amp does some funny things with the speaker "returns" so that the two halves of the amp may be "bridged" to make a big mono amp. Yours may be the same or different.
The disadvantage of Behringer is the no disclosure of schematics. The other is that people are always on here asking questions about them, even when fairly new, which give me the impression that they blow up a lot. Any amp tends to fail after 20 years if time sensitive components are not changed. I might suggest something else next time if you can't fix this one, some brand you find you can get a schematic diagram for before you buy it. Also, no secret transistor part numbers is an advantage. You will see in my signature what brands of used equipment I have bought. This is true of those brands, except the Herald mixer which I traced out and drew myself on notebook paper. But then, I paid $15 for Herald, operational with good potentiometers (and a stupid hissy hummy design) .
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Last edited by indianajo; 21st December 2012 at 01:06 AM.
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