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Old 8th September 2003, 03:38 PM   #1
iceman is offline iceman  Sweden
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Default strange sound

Hi

I built this amp, and yesterday when i play it sounds
wonderful. But today there's a strange sound in the left channel.
It's sounds like the speaker was broken.The sound only appear when there is music on between the song it's completely quit.
Anyone who think he knows whats wrong ??
Please help me !
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Old 8th September 2003, 03:40 PM   #2
iceman is offline iceman  Sweden
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Default picture

I forgot, here's a picture from inside the amp:
Attached Images
File Type: jpg p000560.jpg (38.5 KB, 333 views)
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Old 8th September 2003, 03:44 PM   #3
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How can they write
Quote:
TRY SOMETHING SIMPLE FOR A CHANGE
on the website
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Old 8th September 2003, 04:29 PM   #4
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What do you mean Jan?

Ismannen, how about your rectifier bridge? Way too small.

Have you checked all voltages? You haven't blow any transistor. Bias, do you run in class AB or just B?

I gather that you will mount the transformer better?

The supply wires seem to be _very_ thin.

Your ground and connection of the smoothing caps. It's rather critical the connection between the caps, rectifier, speakers and the signal ground.

The 230 V wires should go loose and on top of other wires. Over longer distances the 230 V wires must have additional insulaltion, or cut the wires and solder in good 230 V wire.

Not smart to place signal input near the transformer. I would also recommend to have the speaker connections well separated from the inputs.
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Old 8th September 2003, 04:38 PM   #5
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Peranders
Quote:
What do you mean Jan?
Just a reflection to the text on the amp website.....

However I go along with you in your reply to iceman.
Bridge rectifier diodes and Supply wiring seems very tiny.
Also check each resistor etc. for correct value (a common error) and the Vbe for all transistors etc.
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Old 8th September 2003, 05:52 PM   #6
PMA is offline PMA  Europe
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Let me give an input to this swedish/danish thread. I have encountered similar problem when repairing A-75 amp. One of the mosfet driver transistors was broken and there was a "gap" in music at low levels followed by horrible distortion above "threshold" level.
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Old 8th September 2003, 05:58 PM   #7
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Iceman,

I have seen the schematics. Was not there :
1) too low quiescent current (cross-over distortion)
and / or
2) thermal runaway in the circuit, resulting in one of the power or driver transistors burnt?
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Old 8th September 2003, 05:58 PM   #8
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That may also be the problem, or that one or more transistors has different pin configuration than the original listed.....
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Old 8th September 2003, 06:45 PM   #9
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My first statement would be to check your soldering. Often an emitter lead, or base lead will come off and the amp will still work but will be cutting into class B since there's only half conduction.

I have this nice Harman/Kardon amp with PP output stage in CFP and when you leave one power transistor out of the circuit it'll still work and sound fine, but run hot since the design is pecuiliar enough that is biases the single transistor in class A when the other one is missing. I haven't reverse engineered it enough to figure out why this class A thing occurs, however, I find it very interesting. The only problem is, the emitter resistor gets quite hot when this happens because the Q current goes through the roof.

If you're wondering how I discovered this, I was trying some new transistors MJ21193/21194 in there to evaluate their sound (which was very nice.) However, during the changeout, I forgot one of the transistors for some reason. I can't believe I didn't notice that before I turned the amp on. (I'm just happy I didn't find a way to destruct the indestructible amplifier.)
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Old 8th September 2003, 07:22 PM   #10
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Default Thanks for replies

Peranders:Yes, all voltages are OK(-+24,2 V). I run it in AB
About the transformer; I will mount it better.
I just dont know yet if i gonna use just this transformer, or if it's overkill to have 500VA 18-0-18 to this amp.

I gonna test the transistor tomorrow.I use TIP35C/36C
The strange thing is that it sound OK yesterday, the only thing i do today was that i fix the cables etc..

About the rectifier;I gonna change it to a new one soon when I put the headphone-amp in the case. The new is built with
MUR2020.

Again: Thanks for all replies
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