F5 bias question

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audiorob

Member
Joined 2006
richD said:
Does anyone have a “ How to Bias the F5 for Dummies”?
I’ve read Mr. Pass’s article and it’s still not clear to me on how it is done.
Thanks
Click to expand...


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.
 
J

juluska

Member
Joined 2008
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
 
vdi_nenna

vdi_nenna

Member
Joined 2000
Paid Member
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?

3.- turn all the pots about 5-7 complete turns
Click to expand...

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

cviller

Member
Joined 2005
vdi_nenna said:
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.
Click to expand...

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

vdi_nenna

Member
Joined 2000
Paid Member
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
 
R

rade95

Member
Joined 2010
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.
 
cviller

cviller

Member
Joined 2005
Marre could still be right - depending on where your trimpot is on its scale, there can be a lot of turns without any noticable effect. Best is to nul both and try again. Alternatively, you can take some good pictures and post them - someone might be able to spot something.
If you really want help, you can print out the schematics, place your ground probe on ground and measure voltage around the bias areas, write it on the schem, scan it and share it here on the forum. Then all the smart guys here will be able to help you quickly. ;)
 
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