Pass mosfet citation 12 clone


2005-01-01 3:33 pm
I just got my mosfet citation 12 clone working after some trouble with dc offet prolly caused by the wrong transistors.

2N3904/6 were the closest i had to the MPSL01/51, hfe pretty much the same.

Here are a few pics:



And no that opamp is not part of the circuit, i just dident bother with desoldering it before using the board.


2005-01-01 3:33 pm
I have a feelin that Q4 should be attached to the heatsink, bias current is slowly creeping up, i st it to 130mA and now a few songs later its up at 180mA and climbing.

I just need to dig out some ribbon cable and stick it to the heatsink to counter thermal runaway.


2005-01-01 3:33 pm
There is another thing i am considering, replacing the 18k resistor on the input stage with a current source. Not sure if that would make a difference tho.

If it posed a significant difference in performance, i think pass would have included it in shcematic, unless he just did the mods the board allowed for.

What do you guys think, would a current source in place of the 18k resistor do any significant difference or would it just be a waste of time ?

I don't have the equipment to measure the difference if any, but i may try it just for fun.


2005-11-08 11:27 am
If your PS voltage is +/-35V (as the sch. shows) then you have a bit less than 1mA through each LTP transistor when you use 18k to supply the LTP.
With CCS (as you drew it in post #13 - red LED and 330R) there will be about 1.5mA through each transistor in LTP and that changes everything. Put a trim-pot across the R8 (330R in your sch.) in order to adjust the current through LTP and that will settle working points in the amp.
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