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Old 16th September 2010, 01:48 PM   #1
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Default Pass mosfet citation 12 clone

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:
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And no that opamp is not part of the circuit, i just dident bother with desoldering it before using the board.
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Old 16th September 2010, 02:27 PM   #2
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I've got the parts to convert my Citation 12 DeLuxe to the MOSFET Citation 12 but haven't done the conversion yet. I may do as you have done and build the MOSFET version and leave the old Citation 12 as-is.
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Old 16th September 2010, 02:36 PM   #3
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Theres only one problem, when turned off and trhe caps drain and the voltage drops low enuf, this thing bottoms out the speaker with a loud "KASCHUNK" as the voice coil hits the back of the pole piece.
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Old 16th September 2010, 02:42 PM   #4
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You may need to add a relay to open the connection to the speakers when the amplifier is powered down.
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Old 16th September 2010, 03:21 PM   #5
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I just realized. The amplifier might be going into oscillation as it powers down.
Have you looked at the output with a scope while the amp is shutting down?
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Old 16th September 2010, 03:26 PM   #6
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Yes i have, it gos to the positive rail with a resulting bang from the speaker.

Schematic:
Click the image to open in full size.

Heres a vid of this amplifier in action: YouTube - MosFet Citation 12 by Nelson Pass

Last edited by Tekko; 16th September 2010 at 03:55 PM.
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Old 16th September 2010, 04:26 PM   #7
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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.
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Old 16th September 2010, 04:42 PM   #8
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Probably correct. Let us know the results after you mount Q4 to the heatsink.
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Old 16th September 2010, 05:05 PM   #9
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Q4 mounted to heatsink, quiescent current lingering around 110mA and doesent seem to be rising, atleast not yet.

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Old 17th September 2010, 09:38 AM   #10
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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.
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