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Old 24th April 2012, 05:14 PM   #12291
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that why te secuence is. turn on: source, pre, power. turn of: power, pre, source
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Old 24th April 2012, 05:17 PM   #12292
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Jaja...

But a little thump should not generate smoke
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Old 24th April 2012, 05:25 PM   #12293
flg is offline flg  United States
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But, sometimes a big signal slams your gain device to the rail and it has a capacitance problem preventing it from turnin back off with the signal. This phenomina can also cause oscilation you might not ordinarily experience.
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Old 24th April 2012, 05:25 PM   #12294
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yes it can the thumb is DCV
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Old 24th April 2012, 06:02 PM   #12295
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So how about the following plan:
1. Rebias and run for a while without load

2. Connect 4ohm 20w resistors across speaker outputs and drive with low voltage source

3. Get preamp to the bench, and start in the correct sequence

4. If everything still work, test with full setup
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Old 24th April 2012, 08:03 PM   #12296
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Quote:
Originally Posted by studiostevus View Post
So how about the following plan:
1. Rebias and run for a while without load

2. Connect 4ohm 20w resistors across speaker outputs and drive with low voltage source

3. Get preamp to the bench, and start in the correct sequence

4. If everything still work, test with full setup
1. Successfully completed, biased to 0.1mV DC offset and 0.595V bias.

2. Started with 4Ohm resistor and a soundcard as input. No smoke, but I measure 22mV across the resistor and 0.59V bias. I probably need to rebias. Should I short input to GND?

EDIT: scrap that... on the scope I see output, nice and clean from my soundcard. The 22mV across the resistor can also stem from the soundcard... when I increase volume on the card (without playing signal) it corrects to 0V and into the minus mV range...

Last edited by studiostevus; 24th April 2012 at 08:09 PM.
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Old 24th April 2012, 08:26 PM   #12297
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Step 3:

3. I first measured for any DC voltage on the output of the preamp. There was none. Then I started the equipment according the right order (source, preamp, F5). I have deliberately waited until the voltage of the preamp had settled. So then I started the F5 and ..... SMOKE !!!!

I couldnt clearly see where it came from, as I was too busy with switching everything off. I could however see the voltage across the Speaker resistor (4ohms) climbing.

What is going on here? Step 2 went fine (with soundcard as input), but step 3 (preamp as input) went wrong again?? Oscillations?
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Old 24th April 2012, 08:58 PM   #12298
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Just tested the other channel with exactly the same setup: No problem.

So there is clearly one bad channel on the amp, and a bad channel which does not react with the soundcard input, but does react with the preamp input!

Needless to say, I am very confused
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Old 24th April 2012, 09:15 PM   #12299
Itsmee is offline Itsmee  England
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Have you checked your preamp output for DC?

Try the bad channel of the F5 with the other channel of the preamp - the one that worked with the good channel of the F5.
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Old 24th April 2012, 09:17 PM   #12300
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I did... No dc on the output of the preamp.

I only used 1 output of the preamp for testing both channels of the f5...
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