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F5 bias problems
F5 bias problems
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Old 13th February 2013, 02:32 PM   #11
IZHAKKATZ is offline IZHAKKATZ  Israel
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Join Date: Jul 2004
Location: Israel
Thank you all for helping out!

Here is what I've got so far:

1-When measuring R12 (0.47 ohm) it starts at 1.5 ohm and quickly drops to 0.9- 1.0 ohm
2- When measuring R between outer legs of both trim pots resistance shifts up and is not stable.
3-When measuring V between outer legs of mosfet: Q3= 1.4 MV Q4= 9.5 MV
4-I've adjusted both trim pots 1 turn at a time and still P2 has no effect on both bias and offset
5-I'm not using a speaker protect circuit
6- the LED that should show power is on, does not turn on
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Old 13th February 2013, 02:42 PM   #12
cviller is offline cviller  Canada
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1.- 0,47 is typically a challenge for normal multimeters, so don't worry about that.
2 - as explained before
3 - i don't think you get much useful from measuring there - you could try referencing to gnd instead
4 - could be many things
5 - OK
6 - so that is at least a very tangible issue you could try to resolve.

But I would restate what Zen Mod said - post pictures
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Old 13th February 2013, 05:11 PM   #13
AudioSan is offline AudioSan  Norway
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did you check your PSU? do you have +/-23-25V? if you dont have the +23-25V then the LED will not light up.
and post the schematic that you use. there is many differt ones out there. so R12,Q3 don't tell us anything without knowing wich schematic you'r using.

Last edited by AudioSan; 13th February 2013 at 05:18 PM.
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Old 27th March 2013, 12:28 PM   #14
IZHAKKATZ is offline IZHAKKATZ  Israel
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Join Date: Jul 2004
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Hello good people

Just for future reference if this should happen to anyone else!
Problem solved! It turn out I used 47K ohms instead of just 47 ohms in R 13 & 14 (what an idiot)

Thank you all for trying to help out and offcours thank you Mr. Pass for such a beautiful sounding amp !

Just one short question, I'm using a shielded power cable with 3X 18 awg wirers

Is 18 awg too wimpy for a power amp? Does it matter which side of the cable the shield is connected?
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Old 27th March 2013, 01:32 PM   #15
pdhanwada is offline pdhanwada
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Join Date: Jun 2011
Default It happened to me too

It happened to me too. But I realized it pretty early during soldering. I had unsolder wrong one and solder the right one.
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