A guide to building the Pass F4 amplifier

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Chiptech

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Einric said:
I would double check your resistor values and solder joints.
Click to expand...

Checked resistor values. I had a 27.5K value in R8 instead of 22.1K Replaced it.

All other values are good.

Check out the Mosfet pad shown in photo. I'm surprised this didn't result in a short.

Done for tonight.
 

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Ben Mah

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Zen Mod said:
check for uniformity:

-torque of mosfet screws
-voltage at/across all source resistors


re-check all solder joints

F4 is damn follower, if you have lower output - it's either problem with pre (swap channels test) or trivial problem with interconnects/contacts or ......... Gremlins

temperature issue - hope it'll be clearer when you check all source resistors
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Chiptech,

Did you measure the voltage across ALL of the source resistors as Zen Mod asked? Perhaps your mosfets are not as matched as you had expected.
 
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Ben Mah

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It is best if the three mosfets in each phase (positive and negative) are matched as close as possible. So swap mosfet from R16 with mosfet from R20 will get you better matching.

Since you said that the other channel, the right channel, is louder and hotter, measure the voltage drop across all of the source resistors there too to compare. If it is running hotter, it could be running higher voltages and therefore higher current.
 
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Ben Mah

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Well my thought is that if there are variations in the mosfets and if the bias is based on one source resistor, it can happen that one channel is based on the low mosfet and the other channel is based on a high mosfet.

For best match between channels, measure all three source resistors in a phase and take the average voltage for the bias.
 
C

Chiptech

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Joined 2005
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Ben Mah said:
It is best if the three mosfets in each phase (positive and negative) are matched as close as possible. So swap mosfet from R16 with mosfet from R20 will get you better matching.

Since you said that the other channel, the right channel, is louder and hotter, measure the voltage drop across all of the source resistors there too to compare. If it is running hotter, it could be running higher voltages and therefore higher current.
Click to expand...

R16 thru R18 are N Mosfets

R19 - R20 are P Mosfets

I don't think I want to swap R16 [N] for R20 [P]?
 
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