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Old 27th July 2003, 12:59 PM   #1
sobazz is offline sobazz  Denmark
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Default Oscillating P3A

Hello everybody

I'm in the process of finishing a couple of P3A 30w mono blocks. The first amp works absolutely flawlessly, but the second has some stability issues.

During the bias adjustment I discovered a significantly higher heat dissipation. The DC offset at the output was fine: approx. 15mV. Not as good as with the first amp; it's offset was only a couple of mV.

I do not have the resistors recommended by Elliot, so I haven't measured the output of the ps yet. I will do this tomorrow.

Not only does the output transistors dissipate more heat, but there's also a crackle or "clicking" noise. It's only audible in a quiet room, but it definetly shouldn't be there. The clicks are repeated at a frequency of about 1hz. Is this noise an indication as to where the problem lies?

I've resoldered all joins on the amp board. Didn't help.

Any help is appreciated.
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Old 27th July 2003, 01:48 PM   #2
MikeW is offline MikeW  United States
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Check the input transistors and the 22pf. caps around them.
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Old 27th July 2003, 03:33 PM   #3
paulb is offline paulb  Canada
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Yes, check all the caps carefully. I had a similar problem with a higher offset; it was a bad solder connection on the feedback electrolytic cap.
If you have access to a scope, check the output for oscillation, highly recommended.
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Old 27th July 2003, 06:10 PM   #4
djk is offline djk
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http://sound.westhost.com/project3a.htm

If the bias isn't set, then C5 will pump at a 1hz rate.

At low levels the amplifier will work without C5, disconnect it.
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Old 27th July 2003, 06:25 PM   #5
sobazz is offline sobazz  Denmark
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Unfortunatly I don't have a scope. I'm trying to find help for the debugging in the region of Denmark where I live. Hopefully someone responds.

I've changed Q1,Q2 og Q3. No effect. Improved a couple of solderings. No effect.

The bias is already set. Changing it does not affect the noise. I'll try to bypass C5.
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Old 27th July 2003, 06:59 PM   #6
sobazz is offline sobazz  Denmark
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Bypassing C5 has no effect.
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Old 28th July 2003, 06:44 AM   #7
sajti is offline sajti  Hungary
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Quote:
Originally posted by sobazz
Bypassing C5 has no effect.
No bypass. Remove it, and keep the place empty.

Sajti
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Old 28th July 2003, 03:07 PM   #8
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Hm...if he is using the exact same type of parts for both sides...why does he experience this in only one channel?? Maybe a few good "special" output transistor or parts?? Could be PSU?? Just a thought...
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Old 28th July 2003, 06:21 PM   #9
sobazz is offline sobazz  Denmark
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Well, I've got an update for you, guys.

After mingling around with the second amp a while I decided to try it with the first amp's ps. Using another supply did not affect the problem.

I assembled the first amp again and tested it. Suddenly the "clicking" was present in this amp too. I decided to complete the assembling of the second amp and today I tried both using my Harman/Kardon HK680 as preamp.

It seems that the problem was related to a bad interconnect cable that maybe got too close to my computer - I don't know.

Bottom line: these amps are QUIET! Very quiet. Almost quieter than my HK680.

Thank you for your suggestions.
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