Simulation - Pass MF-12 MOSFET Quasi Comp (Original) Modification for HK Citation 12 - Page 2 - diyAudio
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Old 31st May 2011, 12:39 PM   #11
Ar4 is offline Ar4  Latvia
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In Nelson Pass website there are PDF file with schematics available, both for comp and quasi.

Also, with Tina TI simulations for paralleled Mosfet pairs max power is ~130w @ 8 ohm with 40V rail. Could that be true?

Last edited by Ar4; 31st May 2011 at 12:42 PM.
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Old 1st June 2011, 12:58 AM   #12
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I am working on the schematic now. So far so good!
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Old 1st June 2011, 02:48 AM   #13
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I think I did it!!!! I am going to re-write it on paper so it looks nice and legible and I will post a pic. It might not be until tomorrow evening. I will then put it into LTS.

I am amazed at how close it is to the complimentary schematic now that I compare it to my circuit. You said that all along Pete.

This is cool. Progress being made!

EDIT: Sorry Pete, I see you have already put the comp. version in LTS. I will modify that one.

Last edited by MikeBerry; 1st June 2011 at 02:52 AM.
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Old 1st June 2011, 12:39 PM   #14
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Here is my finished schematic!! The two unmarked transistors are a dual six lead "custom" transistor. I have no idea what it is. I believe it is mentioned in the PASS DIY article. It is SO much easier to see how my circuit works with a good schematic!!

Pete, could you please explain why the current regulating diodes are used instead of the resistors? Could you also explain what the C1 bootstrapping cap was used for and why it is not present in my circuit?

My current power supply filter caps are 17000uf 50v each. The original schematic calls for 6000uf 50v caps. Are 17000uf caps necessary, and if not, what is the lowest uf I could use without hurting the performance of the amp? I was thinking about replacing these as I am sure they must be over twenty years old.
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Old 1st June 2011, 10:12 PM   #15
llwhtt is offline llwhtt  United States
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That's what I came up with too. My only question is the 1.62 resistor (R15), is that supposed to be 1.62K? With 1.62 Ohms the adjust pot is pretty much useless. I too wonder about the lack of the bootstrap capacitor, maybe it's not needed with CCS diode. If you want to change your CCS diode symbol it's just like a normal diode symbol but with a circle instead of a triangle, the circle on top, line on the bottom in this case.

Craig
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Old 1st June 2011, 11:10 PM   #16
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Thanks! Let me double check on the resistor, but you are probably right. The BIAS pot definitely does something because I learned what happens if you try to BIAS it too much! I poofed both the 2N5551's! I will change the symbol too. I am going to work on putting the schematic in Spice tonight after my son is in bed.

Do you have any suggestions on transistors if I ever have to replace the "custom" dual transistor? Thanks again!
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Old 1st June 2011, 11:39 PM   #17
llwhtt is offline llwhtt  United States
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I think the original used the custom dual or a pair 2N5087s. Nelson used MPSL51s for the input diff. and MPSL01s for the VAS and bias. Those are still available at surplus places as I ordered a bunch for my PASS A40 project. I imagine there are several other devices still being manufactured that would be suitable though I've never looked for any. Did you find any info on those CCS diodes. Mouser has some in stock, just Google the part numbers and see what turns up. I used them in my A40 instead of the FETs and resistors.

Craig
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Old 2nd June 2011, 12:15 AM   #18
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I will check out the 51's and 01's. Is there any point to replacing the custom dual and the two 2N5551's? I found info. for both of the CCS's at mouser. You can still get the 1N5297's. I never had any luck finding the 1N5305's in stock anywhere. That is OK because they are currently (pun intended) fine. Sorry, that was bad, but I couldn't resist. Dang, I did it again.

You are correct the resistor is 1.62K. I am going to start working on the schematic in Spice.

Mike
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Old 2nd June 2011, 12:33 AM   #19
llwhtt is offline llwhtt  United States
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No reason to change unless there's a problem with reliability such as adjusting the bias you wiped out the 2N5551s. Maybe a tougher xsistor would not do that. Have you checked out all of the Citation 12 articles on the PASSDIY website? I know he did some updates to cover parts no longer available, not sure about the MF12 though.

Craig
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Old 2nd June 2011, 12:58 AM   #20
PB2 is offline PB2  United States
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Quote:
Originally Posted by MikeBerry View Post
Pete, could you please explain why the current regulating diodes are used instead of the resistors? Could you also explain what the C1 bootstrapping cap was used for and why it is not present in my circuit?

My current power supply filter caps are 17000uf 50v each. The original schematic calls for 6000uf 50v caps. Are 17000uf caps necessary, and if not, what is the lowest uf I could use without hurting the performance of the amp? I was thinking about replacing these as I am sure they must be over twenty years old.
Consider that if you have a constant voltage across a resistor, then the current is constant since: I = V/R and we want a constant current source in the "tail" end of the diff pair. If the transistors are matched and the Vbe's are the same then the current will divide evenly between the pair. The bases should be close to ground, and the emitters are at the same voltage since they are tied together. The emitters are just a diode forward drop below the base so nearly all of the rail voltage is across the tail resistor and since the base signal is small the current variation in the tail is small even with a resistor. However, consider that the main supply is not a perfect voltage source and is modulated by mainly the current drawn by the output stage. We can consider this modulation as a noise source on the rail and it will tend to modulate the current in the diff pair tail. What the diode does is to regulate and nearly keep the current constant, and in so doing reject the noise on the rail.

The bootstrap cap combined with the resistors forms a fairly good constant current source. Remember that the output stage is roughly unity gain so the output voltage is equal to the VAS voltage, the bootstrap cap is large so the voltage on it does not change much at audio frequencies holding a constant voltage on the lower resistor and therefore current to the VAS. The VAS is the main voltage gain stage of the amp, and the gain for this stage goes up as you increase the load impedance, a current source by definition has an infinite impedance so it provides the most gain. This boosts the open loop gain of the amp so that there is more feedback gain to correct for errors in the output providing lower closed loop distortion. The bootstrap cap is far from an ideal current source and the diodes just do a better job. Constant current sources can also be made with transistors, and FETs, just google them for more info.
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