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Old 11th May 2007, 11:13 AM   #1
joad is offline joad  Sweden
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Default Adcom GFA-555 bias?

Hi, I have a Adcom GFA-555 but there is a humming sound from the right channel. Is there something I can do to find the problem?

Is it possible to change the bias voltage, and to what voltage?
I have 6 voltage pins on the main board markt V+ D+ B+ V- D- B-
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Old 11th May 2007, 02:22 PM   #2
K-amps is offline K-amps  United States
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Humming? Are you sure it is bias related. What is the DC offset for that channel ? Also is there any AC voltage on the outputs.
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Old 11th May 2007, 03:50 PM   #3
joad is offline joad  Sweden
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I have 0 volt ac on the speaker terminal.
but if i understod you correctly Dc offset on speker terminal? There i have 0.025 volt dc.
The measurments are on a cold amp. I can post later on a warm amp.

You can find the schmatic in this tread: Adcom 555II - Nelson Pass ? Thanks to Carlmart
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Old 11th May 2007, 04:46 PM   #4
K-amps is offline K-amps  United States
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Any AC voltage on output?
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Old 11th May 2007, 05:15 PM   #5
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Be careful Joad!

I came here a few years ago asking for advice on a 555. Since then I have built six amplifiers as well as a multitude of other audio related things. I have also lost countless hours of sleep wondering what to make next. Unless you need another habit - get out fast

-I'd take a look at the psu capacitors. Maybe you have a bad one on that side..

-maybe a bad bridge rectifier?

-Check the PSU. Make sure the dc voltages are about equal +/- on all the rails. Also look for anomolies measuring for AC at the same.

-all in my very humble opinion.
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Old 11th May 2007, 05:26 PM   #6
joad is offline joad  Sweden
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Zero AC voltage on outputs. The humming is gone. The humming are coming from someting else, preamp or Squeezebox or EMC on singnalcable. But i still wants to bias adjust the power amplifier. Because it hasent been done ever in its life. Do you know how to do that in easiest way?
Now with warm amp the DC offset is: In right channel 0.029VDC In left channel 0.027VDC

I will search for the problem in the other elektronics and return
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Old 12th May 2007, 02:05 AM   #7
K-amps is offline K-amps  United States
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I think you need 24mV dc across the 0.82 ohm OP emitter resistors minimum.

If they are 0.47 ohm then set at 18mV

For the 555mk.II it is more like 10mV

All these when amp is marginally warm. i.e. 30-40C on heatsinks.
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Old 13th May 2007, 03:02 AM   #8
anatech is offline anatech  Canada
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Hi joad,
I haven't checked the absolute value, but you do adjust them after running the amp at about 1/3 power for 1/2 hour, then letting it cool down until the bias settles down. If you don't do this the bias current will not be set correctly.

I have posted to procedure a couple times around here if you do some searching.

Watch you didn't burn out the signal ground reference resistors.

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