P3A turn on current surge

Fritz · 6th April 2003, 01:14 AM

Ive built a P3A and when tested on the bench with a variac it powers up just fine. When I connect the speakers and try to power up there is a huge AC current draw that looks like a short.
Unhooking either of the speakers allows me to power up as it would with no speakers. Once the voltage is up and the single channel is playing I can connect the second speaker and everything sounds fine. It seems that the current draw at startup is keeping the voltage from getting high enough to turn on the transistors, however I dont seem to be able to get ahead of the current draw enough to raise the voltage?

Any ideas would be appreciated.

Should I just cross my fingers and twist that knob?

Mattyo5 · 6th April 2003, 01:38 AM

Variac is a good idea....um...but if you do turn the knob and cross your fingers...please get pictures! :-P

You might wanna get some simple resistors at the outputs and see what happens there. Also...you might want to check for a short between the collector of the output transistors and the heatsinks. Hope all goes well!

-Matthew K. Olson

ps...email Rod before you power the thing up...he can help

Fritz · 6th April 2003, 09:14 PM

Thanks Mathew,

I may try the resistors.
I dont think its a short because it goes away once you get the voltage up. Im thinking an instabilty before the transistors are all "on". I used .39 instead of .47 ohm bias resistors and Ive e-mailed Rod to see if he thinks that is the problem.

I wont really twist and prey cause Ive had a lot of trouble getting this project going. I toasted several transistors initially possibly because it was testing fine on the bench and doing this under load.

Fritz · 8th April 2003, 09:23 PM

Answering own thread,

Rod told me to set the variac to twelve volts hook the amp up and then switch on. Theres a large transient surge and then it settles. So the variacs slow power up was creating the problem.

6th April 2003, 01:14 AM	#1
Fritz diyAudio Member Join Date: Aug 2002 Location: CA USA	P3A turn on current surge Ive built a P3A and when tested on the bench with a variac it powers up just fine. When I connect the speakers and try to power up there is a huge AC current draw that looks like a short. Unhooking either of the speakers allows me to power up as it would with no speakers. Once the voltage is up and the single channel is playing I can connect the second speaker and everything sounds fine. It seems that the current draw at startup is keeping the voltage from getting high enough to turn on the transistors, however I dont seem to be able to get ahead of the current draw enough to raise the voltage? Any ideas would be appreciated. Should I just cross my fingers and twist that knob?

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6th April 2003, 01:38 AM	#2
Mattyo5 diyAudio Member Join Date: Aug 2002 Location: Farmington, CT	Variac is a good idea....um...but if you do turn the knob and cross your fingers...please get pictures! :-P You might wanna get some simple resistors at the outputs and see what happens there. Also...you might want to check for a short between the collector of the output transistors and the heatsinks. Hope all goes well! -Matthew K. Olson ps...email Rod before you power the thing up...he can help

6th April 2003, 09:14 PM	#3
Fritz diyAudio Member Join Date: Aug 2002 Location: CA USA	Thanks Mathew, I may try the resistors. I dont think its a short because it goes away once you get the voltage up. Im thinking an instabilty before the transistors are all "on". I used .39 instead of .47 ohm bias resistors and Ive e-mailed Rod to see if he thinks that is the problem. I wont really twist and prey cause Ive had a lot of trouble getting this project going. I toasted several transistors initially possibly because it was testing fine on the bench and doing this under load.

8th April 2003, 09:23 PM	#4
Fritz diyAudio Member Join Date: Aug 2002 Location: CA USA	Answering own thread, Rod told me to set the variac to twelve volts hook the amp up and then switch on. Theres a large transient surge and then it settles. So the variacs slow power up was creating the problem.