A guide to building the Pass F4 amplifier

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Ok.

Here are the bias readings:

R16 - .158
R17 - .181
R18 - .189
R19 - .179
R20 - .187
R21 - .163

It appears I have some variance. What would we normally like to see?

Here are the bias readings for the right channel;

Here are the bias readings:

R16 - .191
R17 - .159
R18 - .000
R19 - .169
R20 - .162
R21 - .188

It appears something is amiss with the R18 Mosfet.

I won't get to taking the board off until tomorrow morning.
 
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Yes I'm using the schematic from post #1

Not sure your question about R12?
 

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Official Court Jester
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ignore R12 question, tired eyes

what is problem now?

still unequal gain ?

can you use some scientific method there - put some signal measurable with DVM set to AC ( so 1KHz) , and check what you're getting at various points of circuit

first check input ( set to - say 1V), then after buffer then output

you can use even your PC or laptop as signal generator - plenty of free progs around

is it possible that you mixed mosfets between channels ?

it seems to me that you have two distinctive groups ( same applies for both polarities) - one in 160mV, one in 180mV

if you make proper measurement and analysis, maybe you can relocate them to have proper matching in both channels
 
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R18 0.47 - is that Ohm or Volt? Ohm, I assume.

What is the voltage drop across R18? 0.169V?

Always provide units of measurement so there is no confusion.

For best match it looks like a swap of R16 Mosfets and R21 Mosfets between channels, unless there is a weird voltage reading at R18.

Then re-bias.

As Zen Mod mentioned, the best way to check gain in each channel is to use a known AC signal and apply it to each channel separately and measure the signal level at each channel.
 
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I'm stuck at R18 where I didn't get any V reading last night. I took out and tested that Mosfet and it shows it's working. I used a PEAK Atlas DCA75 Pro.

Would the poor matching at R18 Mosfet create the zero [.000V] reading I got last night? And would that create the imbalance between L - R channels? Or is something else amiss?

One learning is that I skipped over the chapter on matching Mosfets within N & P.s

I can swap the R16's and R21's and bias up and see where I stand.

Thanks.
 
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You need to figure out why you got the zero reading.

If R18 was connected properly and everything else was connected properly (good solder joints) and R18 was functioning properly, then zero voltage drop means that no current was flowing through the resistor and mosfet.

So is the mosfet faulty, or perhaps the voltage at the mosfet gate was not correct and the mosfet did not turn on? Or perhaps your voltage measurement across R18 was not correct?

So if the mosfet is good, the resistor is good, then you need to check that the voltage at the mosfet gate is correct. You can compare it to the voltage at the other two mosfet gates.
 
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Switch up Mosfets and rebiased everything.

Right channel is working.

But no sound out of left channel.

Here are new Bias values V

Left Right
R16 .191 .176
R17 .178 .196
R18 .190 .189
R19 .182 .183
R20 .201 .199
R21 .179 .183

I have swapped intereconnects and speaker cables, no change.

I'm missing something. Photos attached.

Appreciate another set of eyes on this.
 

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Set your multimeter to the lowest resistance setting and place one probe on the RCA positive (inner ring) and the other probe on the signal input positive of the board. Also check signal ground from RCA to board, speaker positive to board, and speaker negative to board.

If you measure very low or no resistance, you have continuity. If you measure resistance then there is a problem.
 
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Set your multimeter to the lowest resistance setting and place one probe on the RCA positive (inner ring) and the other probe on the signal input positive of the board. Also check signal ground from RCA to board, speaker positive to board, and speaker negative to board.

If you measure very low or no resistance, you have continuity. If you measure resistance then there is a problem.

Ben, thanks for the help. I checked all the connections and they are good.
 
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What about your input buffer on the left chan?

how can I check the TL431 short of removing it?


your left channel JFETs and associated resistors:

I checked all the resistors. Did not check the JFETS

All good joints and clean of flux?

Yep

Correct JFET in correct position?

Yep

Do the left channel JFETs measure the same as the right ones?

How do I measure the JFETS?

Remove them?

Thanks