Which resistor do I measure to set bias?

DJNUBZ · 13th May 2011, 09:51 PM

Could someone tell me which resistor I need to check the voltage on to set bias in this circuit?

http://www.saturn-sound.com/images%2...20amp'.jpg

Is it R7?

Is there any way to adjust DC offset in this amp?

nigelwright7557 · 13th May 2011, 10:13 PM

R14 or R15 to measure bias current.

Tekko · 13th May 2011, 10:14 PM

R14 or R15.

And no, on a single supply amp, the output will and should always be at half the supply voltage.

If you have dc after the output coupling cap, its leaky and need to be replaced.

DJNUBZ · 13th May 2011, 10:21 PM

If it has old caps will they reform? I have about 200mv on one side.

Tekko · 13th May 2011, 10:26 PM

That depends on how old. If a few years old, yes they can reform, but 20+ years old, not likely.

200mV suggest severe leakage, i'd dispose of those caps before even attemting to bring the amp anywhere near a outlet. And that goes for every single lytic in the thing.

DJNUBZ · 13th May 2011, 11:03 PM

So after the amp warmed up a bit more that offset went away.

So after looking over the circuit board again it seems like what I have is a variation of that schematic. R14 and R15 would be large resistors but there aren't any on the board. In fact it looks like the transistors go straight to the outputs. There are no large resistors anywhere.

13th May 2011, 09:51 PM	#1
DJNUBZ diyAudio Member Join Date: May 2003 Location: Chicago	Which resistor do I measure to set bias? Could someone tell me which resistor I need to check the voltage on to set bias in this circuit? http://www.saturn-sound.com/images%2...20amp'.jpg Is it R7? Is there any way to adjust DC offset in this amp?

13th May 2011, 10:13 PM	#2
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	R14 or R15 to measure bias current. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

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13th May 2011, 10:14 PM	#3
Tekko diyAudio Member Join Date: Jan 2005	R14 or R15. And no, on a single supply amp, the output will and should always be at half the supply voltage. If you have dc after the output coupling cap, its leaky and need to be replaced.

13th May 2011, 10:21 PM	#4
DJNUBZ diyAudio Member Join Date: May 2003 Location: Chicago	If it has old caps will they reform? I have about 200mv on one side.

13th May 2011, 10:26 PM	#5
Tekko diyAudio Member Join Date: Jan 2005	That depends on how old. If a few years old, yes they can reform, but 20+ years old, not likely. 200mV suggest severe leakage, i'd dispose of those caps before even attemting to bring the amp anywhere near a outlet. And that goes for every single lytic in the thing.

13th May 2011, 11:03 PM	#6
DJNUBZ diyAudio Member Join Date: May 2003 Location: Chicago	So after the amp warmed up a bit more that offset went away. So after looking over the circuit board again it seems like what I have is a variation of that schematic. R14 and R15 would be large resistors but there aren't any on the board. In fact it looks like the transistors go straight to the outputs. There are no large resistors anywhere.

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