Behringer A500 power amp problem, please help? - diyAudio
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Old 26th February 2007, 05:59 PM   #1
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Unhappy Behringer A500 power amp problem, please help?

Hi guys

I have two of these, fine power amps Click the image to open in full size.

I use them as hifi poweramps, monoblocked with a Rotel pre amp.
I noticed last week, (and this may have been the case for a long while), that when playing piano music. with the gain on these anps at half, a minor sibilance, bell like distortion, and a like a "sucking of air* sound around piano music.

On the rare time I played a CD with some piano music at the beginning, i put it down to a bad recording.

Seems it does it with every stand alone, piano, harpsichord, etc very strange. And they are also very consistently at the same level of gain, distorting, slightly. I dont listen to piano concertos , but rarely, but its ruined that, im at a loss, have sprayed switch cleaner on the pots, that is all, no difference.

If i take the gain potentiometers on the A500,s very low, and turn the preamp up full, no problem. Also i put the gain on full, it sounds fine again, but because the gains up full, the noisefloor is higher, and so its a bit noisy.

I just wonder if anyone would have an idea, as i tried both amps, on both the same gain level, and both are the same, could it maybe be a fault of the potentiometers, or just poor design circuitry?. The preamp works ok, as i used it with another pro amp i have, made by Samson.

if you had Any ideas i would be grateful?
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Old 26th February 2007, 07:47 PM   #2
mikeks is offline mikeks  United Kingdom
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Oscillation. You'll need to increase the size (*2) of the input-stage degeneration resistors.
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Old 26th February 2007, 07:51 PM   #3
KISS is offline KISS  United States
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You didn't mention the age of the amp. I would surmise that problems like this are related to leaky capacitors that are likely in the signal path. Many times I see 1 uf polarized caps in the signal path of amplifiers when they really should be non-polar ones. Problems with capacitors generally occur after about 5 years.
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Old 26th February 2007, 09:41 PM   #4
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my amps are just out their year long warranty by 2 months.



typical

Ive included an internal pic, of the Amp, its laid out like a quadClick the image to open in full size. but as far as the decent sound, i think with components, thats where the similaritys end
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Old 26th February 2007, 11:26 PM   #5
KISS is offline KISS  United States
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Another possibility is TIM distortion or Transient Intermodulation Distortion. I found this on the web:

http://www.hometheaterhifi.com/volum...04-part-2.html

Maybe it's time you build a Leach AMP. ~30 years old and still strong.

My dbx 4bx, when it was working had derivative processing which they dubbed impact restoration. It made LP's sound more like CD's and you could dial the amount of "attack" that could be heard. You could actually adjust the control to hear the hammers hitting the strings on a hammer dulcimer or a piano. I would occaionally take some amps that I serviced and hook iit up to my system and some couldn't take the processing. It may have actually sounded like what you are describing.

So, it could very well be a lousy design.
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Old 27th February 2007, 09:17 AM   #6
Bonsai is offline Bonsai  Taiwan
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Can we see a circuit diagram - might help us isolate the problem and propose a cure.
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Old 27th February 2007, 05:30 PM   #7
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by mikeks
Oscillation. You'll need to increase the size (*2) of the input-stage degeneration resistors.
I have actually worked on an otherwise identical amp (Alesis), so i know what i am talking about.

http://fileshare.eshop.bg/downloadsm...sis_RA500.html
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Old 27th February 2007, 08:32 PM   #8
AKN is online now AKN  Sweden
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Hi Mikeks,

Quote:
I have actually worked on an otherwise identical amp (Alesis), so i know what i am talking about.
http://fileshare.eshop.bg/downloads...esis_RA500.html
Alesis RA500 may look the same as Behringer A500 on the outside but not on the inside.
I'm pretty certain that Behringer A500 has "flying rail" topology with OP front end (I guess), I think similar to QSC USA 400.
On pics of A500 you can clearly see only two supply leads from transformer to each amp (no center tap) that confirms floating "flying rails" topology with AC coupled output.
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Old 28th February 2007, 02:08 PM   #9
geminni is offline geminni  Yugoslavia
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I had the same thing with some of my amplifiers. The problem was that the stand by curent (or how is it called in enlish) was too low. I had to adjust VBE a little bit more. I realized that it happend allso when the output transitors are not mached then it allso needs a little bit more "push"
I was very nervous when i heard that augfull piano sound! It can allso be heard with piccolo, organ instruments or any other "breathing" instruments.
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Old 28th February 2007, 05:11 PM   #10
KISS is offline KISS  United States
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english translation: Stand by currrent = bias current. Stand by current could have different meanings depending on context. Interesting fix.
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