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Old 26th October 2008, 11:25 PM   #1
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Unhappy Gainclone with 16vdc offset

Hello all... I am building a gainclone amplifier using the non-inverting configuration, 17v rails and 1000uf filters... The power supply consists of a 12-0-12 tranformer and 8 fast recovery diodes. Everything connects to a custom PCB that I made. When I measure the DC offset I get approx 16.3 VDC from the speaker terminals. This is not the first one I have built however it is the first time I have experienced such high offset values, seems like an internal short of some sort. Any insight would be appreciated. thanks
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Old 27th October 2008, 03:24 AM   #2
lineup is offline lineup  Sweden
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can see 2 issues possible:

1. some resistor at either +input (signal) or -input (feedback pin)
is not properly in contact with GROUND (0V)

2. you have oscillation at output

I should check for these.
That you have attached components that effects input pins in a proper & correct way.
And of course that you hava a good PCB, without shortcuts.
And that you have one Zobel filter across output.
Usually something like 100nF in series with 4.7-10 Ohm resistor.

Even if another problem, I think you could find it simply by measuring voltage level at each pin of LM3875 (a guess ).
Without signal and maybe with some resistor as a DUMMY LOAD.

Does not have to be 8 OHM dummy Load.
For 17 Volt DC,
you can use >=1200 Ohm 0.5 Watt resistor
or >= 560 Ohm 1 Watt resistor
... this will let them take max 50% of rated power and they should survive worst case
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Old 27th October 2008, 06:15 AM   #3
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If everything is connected correctly, rail level DC at the output indicates a blown output transistor = IC.
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Old 27th October 2008, 12:53 PM   #4
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I had this on an lm3875 once. The chip blew itself apart with a loud crack before as I was trying to identify the problem.

I required a stiff drink and a trip to the laundrymat after that one!
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Old 27th October 2008, 06:48 PM   #5
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Quote:
Originally posted by lineup
can see 2 issues possible:

1. some resistor at either +input (signal) or -input (feedback pin)
is not properly in contact with GROUND (0V)

2. you have oscillation at output

I should check for these.
That you have attached components that effects input pins in a proper & correct way.
And of course that you hava a good PCB, without shortcuts.
And that you have one Zobel filter across output.
Usually something like 100nF in series with 4.7-10 Ohm resistor.

Even if another problem, I think you could find it simply by measuring voltage level at each pin of LM3875 (a guess ).
Without signal and maybe with some resistor as a DUMMY LOAD.

Does not have to be 8 OHM dummy Load.
For 17 Volt DC,
you can use >=1200 Ohm 0.5 Watt resistor
or >= 560 Ohm 1 Watt resistor
... this will let them take max 50% of rated power and they should survive worst case

Quote:
Originally posted by pacificblue
If everything is connected correctly, rail level DC at the output indicates a blown output transistor = IC.

Quote:
Originally posted by justblair
I had this on an lm3875 once. The chip blew itself apart with a loud crack before as I was trying to identify the problem.

I required a stiff drink and a trip to the laundrymat after that one!

Thanks for the assistance... I kinda figured as much...
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