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Old 7th July 2006, 03:51 PM   #1
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Default Amp Distortion...

This isn't really a DIY question, but with the knowldge here I'm hoping someone will know the answer.

I've recently acquired a B&K EX442 amp. It's driving a pair of Apogee Centaur Minors (6ohm ribbon hybrid speakers). At any volume level I'm getting sever distortion, especially in the midrange frequencies. At higher volume levels the amp acts as if it is not getting enough power and shuts down. I checked all the fuses, inside and out and they are fine. B&K tech support was unable to help diagnose this problem, hopefully someone here knows or could lead me in a good direction.

Thanks,
Brian
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Old 7th July 2006, 04:13 PM   #2
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For what my opinion is worth (probably not much) every amp I've encountered with what you describe it was the vas or driver transistors going out.It happened most recently to one of my JLH amps,very harsh and distorted mids although the bass still seemed to be somewhat ok,turned out to be the vas transistor breaking down.
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Old 7th July 2006, 04:24 PM   #3
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Assuming the Apogees are ok, if both channels are affected and the amp has had an otherwise unblemished history then it could be the amp is unstable when connected to the Apogees. Instability could cause these symptoms. I am not familiar with B&K standard of engineering so this is just a guess.
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Old 7th July 2006, 04:43 PM   #4
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Doing a little digging on the setup, it is bipolar front end driving class ab mosfets. A couple of threads posted elsewhere were having gain issues which turned out to be faulty feedback caps.
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Old 8th July 2006, 11:25 AM   #5
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It would help a lot just to look at the output with a scope, feeding it a sine wave.
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Old 17th July 2006, 08:35 PM   #6
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After some further investigation, this amp is only getting unstable when connected to a load less that 8 ohms. On any pair of 8ohm speakers it plays beautifully. If I wire my Apogee's in series it plays great. However when I hook up anything that is below 8 ohms such as 4 ohm speakers or any of my 8 ohm speakers wired in parallel the amp gets extremely distorted, heats up very quickly and will even cut out (if pushed).

Hopefully this will help shed some more light on the problem.

Thanks again,
Brian
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Old 17th July 2006, 08:43 PM   #7
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Are the Apogee's considered a 'difficult' load?

Enquiring minds and all...
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Old 17th July 2006, 11:05 PM   #8
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Yes. Apogee's are considered a difficult load. They have both conventional as well as ribbon drivers. Overall impedence is 6 ohms but can drop to 3.

Thanks,
Brian
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