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Old 2nd January 2009, 01:48 AM   #571
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here it is .. not sure if it is right (or wrong)
OS
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Old 2nd January 2009, 01:49 AM   #572
andy_c is offline andy_c  United States
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That single voltage source probe is only an approximation for loop gain measurements, and that approximation completely falls apart when trying to measure the Miller loop gain. That technique assumes the impedance from one side of the probe to ground is zero. This is a good approximation when simulating the global loop gain when one side of the probe is the amp output, but it falls apart in the Miller loop case.

Please read this post for how to use the full-up probe from the loopgain2.asc example in the LTspice Educational directory. If you use the simplified single-voltage-source method you've been using, the results will be very inaccurate.

Sorry about the diagram. Once I shrunk it down small enough for the posting software to accept it, it became hard to read.
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Old 2nd January 2009, 02:05 AM   #573
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I've been playing around myself, and after much fiddling I came to the conclusion whoever it was that designed the Cyrus One/Two knew what they were doing!
Similar to the english HH company's VAS (below)that I ported to FA2 I played with diode vs CM, never ran it with a CFP input.
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Old 2nd January 2009, 02:16 AM   #574
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In terms of distortion, the diode vs a diode-connected transistor doesnt seem to make much difference. I suspect that where it really matters is thermal compensation.

The CFP input stage as Cyrus have it seems to be the key - with this in place, H2 disappears and H3 practically disappears. Oddly enough, attempting to add degeneration resistors on Q1/Q2 ruins it!

This thing has crazy gain though. Careful compensation is definitely needed. The original Cyrus One amp also had a triple stage, although it's also quasi-complimentary. This seems to allow them to use 2SA872/2SC1775 in all stages apart from drivers and outputs, but if memory serves the rail voltage isn't very high.
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Old 2nd January 2009, 02:38 AM   #575
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Quote:
Originally posted by jaycee
Oddly enough, attempting to add degeneration resistors on Q1/Q2 ruins it!

Confirmed, although I can't get the sim to complete the FFT with no degeneration at all, but must use at least 1 ohm R's.

EDIT:Not worth the compensation effort for the reduction in THD. Way too much gain with the degeneration removed.
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Old 2nd January 2009, 04:22 AM   #576
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Nice Jaycee and MJ

One can see why the cyrus sounds as good as it does. The liitle bump in analisys might be cureable, had same small problem with my blameless design. BTW run the sim again but at high power and at 20 Khz and look at the figures again regarding the two current mirrors.

Tonight I put part of my blameless on simm too, to play around with maybe a cascode in vas or ltp but Ive had mixed results before.

Anyway I atach plot of what is possible with such a simple design, only couple trannys more than blameless, simm at 20KHZ and power shown. Its a little handicapped because I have fets and mosfet in the circuit and only simming it with a cfp driver ef. I use this but with some modifications that lower THD some more but only significant at 20KHZ and higher powers. Before you comment, no its not roenders outputstage, its been used since 1966, do your homework it is shown in a book I cant recall now and JLH has it in a paper dating from 80s. The amp is stabilized with just 10pf miller cap but I use alternative compensation real life, no difference in measured performance but the simm is having some problem with this other compensation, there was no such problem with the test on a scope back when I designed this.

Yep, this is not for the anti GNFB crowd, they would

Hope the pic comes out.
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Old 2nd January 2009, 06:29 AM   #577
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By homemodder - Anyway I atach plot of what is possible with such a simple design,
Your plot incited me to do a full distortion test on all my mods..
First is the Baseline test with visible H2/3/5..
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Old 2nd January 2009, 06:40 AM   #578
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Then I replaced the CFP input stage which I dropped earlier as a waste of parts ,(boy ,I was wrong)the CFP eliminated H3 fully.
Seems what little bit of loading occurs from VAS/Cdom has
much less effect on CFP stages. I next put the darlington
into play and that only reduced H2 by a measly DB , but since
this mod is a board direct replacement.. no biggie..

I also tried NO Re on my LTP .. no instability and 88 DB gain..
(can't do 100+ )

I guess I will "readd" my CFP .. only 2 trannies for no H3
is a good deal..
As good as it gets for this..(below)
OS
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Old 2nd January 2009, 10:36 AM   #579
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Quote:
Originally posted by ostripper
I guess I will "readd" my CFP .. only 2 trannies for no H3
is a good deal..

In simulation...
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Old 2nd January 2009, 10:49 AM   #580
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Yes , in simulation on LT , microcap , NI 10 ,and with the onset
of clipping. (trying to put a little more H3 into the picture).
They can't all be wrong..
OS
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