Pass mosfet citation 12 clone

Tekko · 16th September 2010, 12:48 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.

Frank Berry · 16th September 2010, 01:27 PM

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.

Tekko · 16th September 2010, 01:36 PM

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.

Frank Berry · 16th September 2010, 01:42 PM

You may need to add a relay to open the connection to the speakers when the amplifier is powered down.

Frank Berry · 16th September 2010, 02:21 PM

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?

Tekko · 02:55 PM

Yes i have, it gos to the positive rail with a resulting bang from the speaker.

Schematic:

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

Tekko · 16th September 2010, 03:26 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.

Frank Berry · 16th September 2010, 03:42 PM

Probably correct. Let us know the results after you mount Q4 to the heatsink.

Tekko · 16th September 2010, 04:05 PM

Q4 mounted to heatsink, quiescent current lingering around 110mA and doesent seem to be rising, atleast not yet.

Tekko · 17th September 2010, 08:38 AM

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.

16th September 2010, 12:48 PM	#1
Tekko diyAudio Member Join Date: Jan 2005	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: And no that opamp is not part of the circuit, i just dident bother with desoldering it before using the board.

16th September 2010, 01:27 PM	#2
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	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. __________________ Frank

16th September 2010, 01:42 PM	#4
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	You may need to add a relay to open the connection to the speakers when the amplifier is powered down. __________________ Frank

16th September 2010, 02:21 PM	#5
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	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? __________________ Frank

16th September 2010, 02:26 PM	#6
Tekko diyAudio Member Join Date: Jan 2005	Yes i have, it gos to the positive rail with a resulting bang from the speaker. Schematic: Heres a vid of this amplifier in action: YouTube - MosFet Citation 12 by Nelson Pass Last edited by Tekko; 16th September 2010 at 02:55 PM.


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16th September 2010, 01:36 PM	#3
Tekko diyAudio Member Join Date: Jan 2005	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.

16th September 2010, 03:26 PM	#7
Tekko diyAudio Member Join Date: Jan 2005	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.

16th September 2010, 03:42 PM	#8
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	Probably correct. Let us know the results after you mount Q4 to the heatsink. __________________ Frank

16th September 2010, 04:05 PM	#9
Tekko diyAudio Member Join Date: Jan 2005	Q4 mounted to heatsink, quiescent current lingering around 110mA and doesent seem to be rising, atleast not yet.

17th September 2010, 08:38 AM	#10
Tekko diyAudio Member Join Date: Jan 2005	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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