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A guide to building the Pass F4 amplifier.
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Old 26th July 2021, 11:17 PM   #2791
Chiptech is offline Chiptech  United States
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A guide to building the Pass F4 amplifier.
Quote:
Originally Posted by andynor View Post
It indicates less current than through the right channel equivalent resistor. You need help from bigger Boyz, but as far as I can tell it might affect / be caused by what’s happening at the drain of Q2. An obvious and possibly unnescessary question is: sure that R5 is 1k and R22 750R? And is the voltage at the negative input of the PCB the same as at the negative rail of the PSU?

Good luck! Hope the AC is working…

Regards,
Andy
Andy,

The R5 and R22 are correct resistors.

The negative input is the same as the PSU 21.62 all around.

New system goes in tomorrow. Old system hasn't crapped out yet.....
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Old 27th July 2021, 12:29 AM   #2792
Chiptech is offline Chiptech  United States
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A guide to building the Pass F4 amplifier.
Just determined the R9 is 8.5K instead of a 10K from the current F4 schematic.

Would that be causing my problem of part of my problem?
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Old 27th July 2021, 06:23 AM   #2793
andynor is online now andynor  Norway
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That’s good news, Chiptech. It is always good to find possible explanations for errors!

I would replace R9 with the correct value. But I dunno it’s effect on your specific issue. What I would also do, is print the schematic from diyaudiostore, and measure every single resistor and mark the results on the printout. Then replace R9 and other parts that may be erroneous, and then try it again and see what happens
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Old 27th July 2021, 06:25 AM   #2794
andynor is online now andynor  Norway
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Also, please check that DC offset is zero volts / mV at the left chan speaker output.
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Old 28th July 2021, 04:04 AM   #2795
Ben Mah is offline Ben Mah  Canada
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A guide to building the Pass F4 amplifier.
Chip,

R9 might have been changed to 8.5k in order to be able to bias the Mosfets properly. It should not affect the front end so I see no need to change it if your Mosfets biased properly.

Looking at your voltages in your post #2788, I see that voltages with preamp connected and without the preamp connected are different. That is strange. I don't believe that should happen. Do you have another preamp or source that you can try?

Also when you provide voltages at resistors, please specify whether they are voltage drops across the resistor or at resistor referenced to ground. And if it is at a resistor referenced to ground, please show at which end of the resistor (mark on schematic). As shown, it is difficult to figure out what the voltages represent.

At this point it seems that one or both JFETs are not working.

I would like to bring up a previous comment of mine. You had bench tested the left channel and measured voltages that seemed correct, and then when you connected your preamp the JFETs seemed to stop working. You then replaced the JFETs, benched tested and measured correct voltages, connected your preamp, and again the JFETs seemed to stop working. So again, something about your preamp or your input wiring may be causing this.
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Old 29th July 2021, 11:54 PM   #2796
Chiptech is offline Chiptech  United States
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A guide to building the Pass F4 amplifier.
Ben,

Just checked using the Starving Student headphone amp, same result, no sound from left channel.

I can go back and get more specific on my measurements.

I also think it has something to do with my input wiring. Perhaps I should swap those wires out for new ones.

First I will provide better defined data.
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