Mismatched Bias Voltage in my Adcom GFA5500 Power Amp - diyAudio
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Old 24th December 2012, 09:06 PM   #1
dtm1962 is offline dtm1962  Canada
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Join Date: Apr 2003
Location: Sarnia, Ontario, Canada
Default Mismatched Bias Voltage in my Adcom GFA5500 Power Amp

I picked up an Adcom GFA5500 power amp recently. The amp ran barely warm (a few degrees above ambient).

So I checked the bias voltage and I had 0.5mV on the left channel and 8mV on the other.

The GFA5500 has 5 pairs of MosFets per channel for reference.

So I began to set up the bias voltage to design (50 mV - using P2) resulted in the left channel being extremely hot. So I started checking the bias voltage for each device (across the "bias" resistor) and it varied from 50mV on R52 (standard resistor for Bias setup) and then the next Mosfet read 90mV and then next 120mV etc.

SO I had to back off on the Bias to allow the 120mV bias to go back to 50mV (as per design), So now I have a spread of 5 to 1 in bias voltage for the outputs. Some measure 10mV and other 20mV and higher.

Is there any issues with having this Bias that is unbalanced (5 to 1 ratio) in regards to these 5 pairs of uneven bias voltage?

Is there anything I can do to ensure reliability of the amp with this uneven loading of the output MosFets?

Any input would be helpful.

Thanks in advance,
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Old 24th December 2012, 09:21 PM   #2
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Frank Berry's Avatar
Join Date: Sep 2003
Location: Midland, Michigan
The output MosFETs should be matched. They should carry nearly equal current.
You may want to contact Adcom.
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