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-   -   F5 bias question (http://www.diyaudio.com/forums/pass-labs/145767-f5-bias-question.html)

richD 12th June 2009 04:24 PM

F5 bias question
 
Does anyone have a ď How to Bias the F5 for DummiesĒ?
Iíve read Mr. Passís article and itís still not clear to me on how it is done.
Thanks

audiorob 12th June 2009 04:44 PM

Re: F5 bias question
 
Quote:

Originally posted by richD
Does anyone have a ď How to Bias the F5 for DummiesĒ?
Iíve read Mr. Passís article and itís still not clear to me on how it is done.
Thanks


Maybe the scope of this issue could be narrowed if you could tell us
more specificly, what problem you're having (what is going well, and
what isn't). In general, you might find the Zen 5 paper helpful, as the
Zen 5 amplifier's bias is set by the same method.

juluska 12th June 2009 04:59 PM

don't worry, it's easy:
1.- Turn off your amplifier
2.- put the pots to 0 ohms position (measured between legs 1-2 and leg 3)
3.- turn all the pots about 5-7 complete turns
4.- then turn on the amplifier (with no speaker connected) and begin adjusting one channel, with one multimeter in the speakers output and other multimeter in R11 or R22.
5.- Now all you have to do is to turn P1 and P2 in half turns alternately until achieve a value near 0V DC at the output and .59V volts across R11 or R12

richD 12th June 2009 07:27 PM

Thanks juluska, I think I can handle it now!

juluska 12th June 2009 07:38 PM

Quote:

Originally posted by richD
Thanks juluska, I think I can handle it now!

take care with heat, it runs really hot but you can touch the heatsinks for 4-5 seconds.

richD 12th June 2009 11:37 PM

Thanks again. I have not build it yet. I just want to make sure I understand everything before hand.

vdi_nenna 20th August 2009 09:13 PM

Hi juluska,

What you're saying is you can monitor R11 or 12 for .59v?
One doesn't have to monitor both at the same time? Let's say I monitor only R11. R12 will end up at .59v also?

Quote:

3.- turn all the pots about 5-7 complete turns
Also, in the above are you saying that there should be some resistance above zero before turning on the amp? What happens if I start the amp at zero on the pots?

I ask because I've had problems with my F5 build.

Thanks,

Vince

cviller 21st August 2009 12:02 AM

Quote:

Originally Posted by vdi_nenna (Post 1903730)
Hi juluska,

1. What you're saying is you can monitor R11 or 12 for .59v?
One doesn't have to monitor both at the same time? Let's say I monitor only R11. R12 will end up at .59v also?

2. Also, in the above are you saying that there should be some resistance above zero before turning on the amp? What happens if I start the amp at zero on the pots?

3. I ask because I've had problems with my F5 build.

1. Exactly - you might not end up with precisely the same voltage on both R11 and R12, but that is not as important as DC offset.

2. That suggestion just saves you a few turns while the amp is ON. Do it the way you feel most comfortable.

3. Is it working now?

vdi_nenna 21st August 2009 05:29 PM

Hey thanks. Not trying to hijack this thread...but here goes.

Not fixed yet, just got the replacement parts yesterday. I planning on removing the active components, marking them e.i., value and channel, then test them outside the circuit to see what went wrong. It's bugging the $#!+ out of me! Will also recheck all resistor placement and values. Something has to turn up. It's too simple a circuit and nothing burned up! I'm betting on a semi gone bad somewhere, but keeping all possibilities open.

thanks,

Vince

rade95 4th March 2013 12:46 AM

Hi guys, like the other diy-er, I also have a problem adjusting F5 BIAS on one of the channels. Only turning P1 makes voltage rise on R11 and R12 while turning P2 makes no difference. I power on with 2 turns on P1 and P2, then turning P1 by 1 or 2 turns suddenly makes voltage rise on R11 and R12 but if I then turn P2 by 2 turns, voltage stays the same. If I have say 0.35V on R11 I get about 0.53V on R12. P ch MOSFET IRFP9240 gets hot really fast and R11 starts getting hot also R12 but not quite as hot. Could it be that P ch JFET is not conduction or N ch MOSFET maybe bad? Any advise would be appreciated. I took R15/R16 and TH1/2 out of the PCB.


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