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Old 23rd January 2014, 08:13 PM   #1
nattawa is offline nattawa  Canada
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Default Meistersinger VFA-200, A beginnerís first try in LTSpice

Iíve just put together a full complimentary 200W VFA and would like to hear any comments and opinions. The circuit is nothing new, in fact I would believe others have done this before. Almost all stages are a straight copy from The Book (Cordellís) with some small difference in details, such as the TMC compensation and the VAS over-current protection. All transistors are from Bobís model file too.

The entire circuit was rather ďcarelesslyĒ thrown together and itís already offering very good promises in performance and stability in simulation (noted in the schematic and the pictures), at least as far as I could tell, especially surprising when I realized all the 2N5401 seem to have twice the Hfe than that of 2N5551. No fancy, advanced techniques or topologies were used or planned to use except a simple beta-enhanced VAS stage.

I will build this amp along side with Dadodís 200w CFA Vertical MOSFET version and hope mine will be a worthy competitor. My beginnerís worst fear is that there could be fundamental design flaws or big no-noes in the design that Iím not aware of. So Iím putting up the draft here and will appreciate all comments.
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Old 23rd January 2014, 09:10 PM   #2
dadod is offline dadod  Croatia
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Quote:
Originally Posted by nattawa View Post
Iíve just put together a full complimentary 200W VFA and would like to hear any comments and opinions. The circuit is nothing new, in fact I would believe others have done this before. Almost all stages are a straight copy from The Book (Cordellís) with some small difference in details, such as the TMC compensation and the VAS over-current protection. All transistors are from Bobís model file too.

The entire circuit was rather ďcarelesslyĒ thrown together and itís already offering very good promises in performance and stability in simulation (noted in the schematic and the pictures), at least as far as I could tell, especially surprising when I realized all the 2N5401 seem to have twice the Hfe than that of 2N5551. No fancy, advanced techniques or topologies were used or planned to use except a simple beta-enhanced VAS stage.

I will build this amp along side with Dadodís 200w CFA Vertical MOSFET version and hope mine will be a worthy competitor. My beginnerís worst fear is that there could be fundamental design flaws or big no-noes in the design that Iím not aware of. So Iím putting up the draft here and will appreciate all comments.
Looks as very nice VFA, complementary IPS, output triple.
Just two things. You have Cordell's book and read the part of indeterminate VAS quiescent currents when current mirrors used in such configuration. Cordell suggested solution in figure 7.10, with R14, R16 resistors.
The triple is prone to instability, you need some kind of compensation there. I used 220pF connected directly between drivers base and collector like in my TT amp, without that amp oscillate even is the simulation does nor show it. Some use the capacitor(or RC combination) to the predrivers.
BR Damir
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Old 23rd January 2014, 09:26 PM   #3
nattawa is offline nattawa  Canada
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Thanks Dadod!


R73 and R74 in my schematic are the two resistors in Bob's book that help define a stable VAS current. I will reserve base Zobel network components for output and driver transistors in the layout version schematic just like the way I did your CFA layout.
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Old 23rd January 2014, 10:52 PM   #4
Mark Johnson is offline Mark Johnson  United States
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Why use a very expensive and hard-to-buy 2SC3503C transistor for the bias spreader (Q13)? You don't need exceptionally low Cbc, nor exceptionally high Vearly, nor exceptionally large fT in this position; why not use a 2N2222 (TO-92) or BD139 (TO-126) and save significant money?

Why use low-beta, high-voltage transistors in the current mirrors (Q18,20) + (Q6,17) ?? You know their collectors are hard clamped, guaranteeing VCE < 3 volts at all times {even startup}, so why drop in a 150 volt transistor there? Why not use higher-beta, lower-voltage devices which are optimized not to exhibit quasi-saturation at low VCE? What's wrong with BC327 / BC337?
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Old 23rd January 2014, 11:06 PM   #5
alexf is offline alexf  United States
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The beauty of this circuit is that it is not critical regarding the transistor parameters. I agree there is little reason to use high voltage 2N5551/2N5401 devices in current mirrors, but that't what Cordell did in his book and it doesn't hurt much either.
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Old 23rd January 2014, 11:27 PM   #6
tvrgeek is offline tvrgeek  United States
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Have you thought any about protection systems?
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Old 23rd January 2014, 11:29 PM   #7
tvrgeek is offline tvrgeek  United States
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Cordell and Self are trying to teach you about amp design, not design one for you. Don't assume their device choice, or any SOP, is correct.
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Old 23rd January 2014, 11:31 PM   #8
tvrgeek is offline tvrgeek  United States
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Something I completely don't understand is the position of the Tian probe with TMC. In one of the other threads, it suggested your placement may not be accurate.
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Old 24th January 2014, 02:46 AM   #9
Dave Zan is offline Dave Zan  Australia
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Originally Posted by tvrgeek View Post
Something I completely don't understand is the position of the Tian probe with TMC.. your placement may not be accurate.
This is a useful place to test. I do not think it is the definitive test.
I am not sure if there is one definitive test for TMC, as there is for Miller, OIC/Cherry and some other schemes.
I believed I had this worked out for TMC too but need to recheck some results.
I knew my answer was not theoretically perfect but expected the inaccuracy would be too small to matter.
Not sure now, so take my previous comments with a bit of salt.

Best wishes
David
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Old 24th January 2014, 02:55 AM   #10
Dave Zan is offline Dave Zan  Australia
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Originally Posted by nattawa View Post
I’ve just put together a full complimentary 200W VFA and would like to hear any comments and opinions....
It's an attractive circuit but when I tried it I found the sensitivity to imbalance was not reduced as I expected.
Have you checked this? I only did a quick sim and have not rechecked yet, so I wouldn't swear to it but I would be curious to see your result.

Best wishes
David

Much easier to read if you turn the grid off and use a white field.

Last edited by Dave Zan; 24th January 2014 at 02:58 AM.
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