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.
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.
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.
If you have access to a scope, check the output for oscillation, highly recommended.
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.
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.
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.
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.
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...
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.
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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