SWR biasing need help...

spirus · 6th November 2006, 01:03 PM

I replaced the fired-up 2sc3264 and now the amp is playing but its sound is a little ... not right?
I have no oscilloscope right now to check it via signal generator-> amp input-> amp out-> dummy load-> oscilloscope so the whole thing is done by ear...
I have this schematic but I cannot check whether its bias is right or not...
Does anyone know the check points and what volt I should measure via the DVM?
Thanks guys...

spirus · 7th November 2006, 12:30 PM

I assebled the whole thing
Voltage supply is +/-62V instead of +/-75V
I power one with a speaker, I get a terrible noise and the power amp is drawing excess current!!!
i unplug he output speaker jack and re-plug it and the damn thing plays like heaven...

shut down and power on without a speaker/load the amplifier draws no current while idle
shut down and power on with speaker, buzz!!!, unplug the speaker and re-plug, it plays excellent!!!

any ideas as what is happening after all?

mitwrong · 16th November 2006, 01:20 PM

Spirus:

Analysis your circuit, it has a soft start circuitry D7, with the cap, source from a/c bridge rectifier, when you power up the amp, the soft start cct may not function at that time, therefore the differential pair transistor not at stable condition, the output is out of control, noise and unexpected current flow thru the spk view the pwr. trs.
that means when you replace the power transistors, It must be a differ with the rest of the tr. with beta. ( hfe ) when U power on with spk connected, it provides a current loop, and the trs. locks the current loop, you then get noise out from spk. After you unplugged the spk. the current loop lost, when you plug back the spk. no noise, due to the differential pair got stable and get control of the output trs. then the unit works fine.

You better check the soft start cct, then the differential pair, also check the output trs. whether they got the same beta.

WE CONSISDER OUTPUT PAIRS HAVE THE SAME BETA

spirus · 18th November 2006, 09:33 AM

Dear mitwrong
I dunno...
at first I thought that the problem came from the amp going to oscillation so the zobel networks or the inductor is off..
Currently I'm deep inside a Leslie cabinet and trying to fix some things...
I will look into the SWR amp again as what you say seems logiacal enough, at least to me...

AnG3L · 23rd November 2006, 08:58 PM

Ussualy, to adjust the bias, or in this case, the current, you have to short circuit the input, put a DVM in amp between the + of power supply and the + of the amp, and adjust the trimmer until you read the current, ussualy 200ma, but for same cases it's 40ma, for other's 400mA... it will depend of the circuiot construction and the type of transistors used.

Hope this had helped you

7th November 2006, 12:30 PM	#2
spirus diyAudio Member Join Date: Jun 2005 Location: Athens	well... I assebled the whole thing Voltage supply is +/-62V instead of +/-75V I power one with a speaker, I get a terrible noise and the power amp is drawing excess current!!! i unplug he output speaker jack and re-plug it and the damn thing plays like heaven... shut down and power on without a speaker/load the amplifier draws no current while idle shut down and power on with speaker, buzz!!!, unplug the speaker and re-plug, it plays excellent!!! any ideas as what is happening after all?

16th November 2006, 01:20 PM	#3
mitwrong diyAudio Member Join Date: Oct 2006	SWR problem Spirus: Analysis your circuit, it has a soft start circuitry D7, with the cap, source from a/c bridge rectifier, when you power up the amp, the soft start cct may not function at that time, therefore the differential pair transistor not at stable condition, the output is out of control, noise and unexpected current flow thru the spk view the pwr. trs. that means when you replace the power transistors, It must be a differ with the rest of the tr. with beta. ( hfe ) when U power on with spk connected, it provides a current loop, and the trs. locks the current loop, you then get noise out from spk. After you unplugged the spk. the current loop lost, when you plug back the spk. no noise, due to the differential pair got stable and get control of the output trs. then the unit works fine. You better check the soft start cct, then the differential pair, also check the output trs. whether they got the same beta. WE CONSISDER OUTPUT PAIRS HAVE THE SAME BETA

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18th November 2006, 09:33 AM	#4
spirus diyAudio Member Join Date: Jun 2005 Location: Athens	Dear mitwrong I dunno... at first I thought that the problem came from the amp going to oscillation so the zobel networks or the inductor is off.. Currently I'm deep inside a Leslie cabinet and trying to fix some things... I will look into the SWR amp again as what you say seems logiacal enough, at least to me...

23rd November 2006, 08:58 PM	#5
AnG3L diyAudio Member Join Date: Nov 2006	Ussualy, to adjust the bias, or in this case, the current, you have to short circuit the input, put a DVM in amp between the + of power supply and the + of the amp, and adjust the trimmer until you read the current, ussualy 200ma, but for same cases it's 40ma, for other's 400mA... it will depend of the circuiot construction and the type of transistors used. Hope this had helped you