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Old 27th October 2003, 03:12 PM   #1
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Unhappy symmetrical 400 buzz ...ooops ...help!

Ok. Very quickly...what happened was in effect, I connected the speaker out + of both my anthony holton symmetrical 400's monoblocks. (don't ask...it was a mistake) Music still comes out of both monoblocks, but one has a buzz now. Worse, instead that amp also has -200 mV dc offset rather than the +3mV that it should have. ALSO, when I shut the amp down, the DC offset climbs from -200mV to -1.2V!

http://www.aussieamplifiers.com/sym-sch.htm

As I said, the other monoblock is fine, still silent, and only 3mv dc offset and no turn on thump or off thump. Any ideas? Where should I start looking for trouble? Does it sound like a driver transistor is going? Thanks!!

-Matthew K. Olson
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Old 27th October 2003, 03:19 PM   #2
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Do you mean that you shorted the + outputs?

Were the amps playing some music?

Were their power GNDs connected or not?

Let us know with more details... I have a idea but I need to know the answers to my questions..

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Andrea
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Old 27th October 2003, 03:44 PM   #3
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I think he means he connected both the amps together as in the +ive from one into the +ve of the other?
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Old 27th October 2003, 04:03 PM   #4
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One of the grounds was connected to the speaker....thats the amp that is buzzing I think. The other amp...which is ok...only was connected at the speaker + out. Yes...speaker + out was connected to speaker + out. yikes!

-Matthew K. Olson
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Old 27th October 2003, 04:06 PM   #5
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No music was playing. Imagine, one speaker was connected... - and + and as soon as I connected the other + ....POP...icky sounds...and I immediatedly disconnected it...this is embarrassing. Anyhway, we connected everything correctly...and the amps still played music...even cranked....but w/ a nasty buzz. Again, the buzz I think is coming from the amp that had been grounded to the speaker. Later!

-Matthew K. Olson
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Old 27th October 2003, 04:34 PM   #6
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I suggest you to check DC voltages at various nodes, since you have a fully functional comparison...

Were the inputs connected?

I'd also check R9

Cheers

Andrea
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Old 27th October 2003, 04:59 PM   #7
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R9 is a good idea :-D after checking the schematic...I was wondering if it was one of the lower value resistors...cool! I'll give it a check when I get home. Thanks!

-Matthew K. Olson
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Old 27th October 2003, 08:21 PM   #8
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Matthew,

The DC offset seems to increase up to a good fraction of a volt when I turn the amp off (normally). R9 could be damaged, I smoked mine once when it accidentally conducted speaker current. Besides that I am not sure what could have happened, but it can't hurt to check the bias on each output device. Once one of my output devices got damaged and its Vgs went down to 2.6 volts [!], I'm glad I caught that before it took out anything else.

It was nice meeting you, by the way! (It may be years before my amplifiers are anywhere as nice as yours.)
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Old 27th October 2003, 08:38 PM   #9
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Well, my DC offset is still negative after removing a bad R9, but at least its at -3mV...and never climbs when I shut the amp off....so thats good Any idea on why the dc offset is still negative? I haven't checked Anthony Holton's dc nodes yet....I'll work on that in a few minutes. Just wanted to post my results.

Kilentra...don't worry...you'll get there I have spent MANY hours reading and many many huge $$$ on learning how to do amps. Plus I have found a lot of good cheap sources. Some day I'll post all my sources for all the cheap stuff I've found. :-D Back to playing w/ a toasted gainclone (which was part of the sym 400 toasting too...don't ask) Later!

dc offset negative? comments?

-Matthew K. Olson
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Old 27th October 2003, 08:45 PM   #10
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Why would a negative value of offset bother you? Perhaps I'm missing something, but is there something about this design that guarantees a positive value for offset??
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