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Old 15th September 2011, 08:48 PM   #16041
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Quote:
Originally Posted by ThorstenL View Post
Well, in first order views everything that makes a sound really does sound the same, so forgive me, but invoking "first order" gets you an F from Thorsten "The Professor" Loesch (the "The Professor" is an old nickname of mine, my best academic qualification is a diploma, but several of them in quite widely distributes areas).
Cop out, you can guess by inspection and I know it. Keep flailing behind the curtain.

Somewhere around 1500 to 1800 Ohms depending on the transconductance of the input after the bias trim. Oh yes it changes 10% or so with the 1Meg resistors. I used a gm of 5mS.

PMA (anybody)use your sim, stuff DC current into the high Z node and see. jcx did.
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Last edited by scott wurcer; 15th September 2011 at 08:53 PM.
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Old 15th September 2011, 08:50 PM   #16042
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I realize that this may not make things more clear, but Thorsten has brought up something that has been addressed in the JC-3 to some extent. The type of compensation used in the JC-3, is called Miller Feedback compensation. One advantage of this kind of compensation is that it provides a lower drive impedance at high frequencies due to the feedback effect of the comp. cap(s) (highly linear only, please) that help to remove the problem of nonlinear caps in the output driver stage and also pads the intrinsic non-linear capacitance of Ccb in the voltage drivers, to make it essentially more linear. Everything is important.
I think there is much said here, without looking at the JC-3 for what it is, and why it was designed that way.
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Old 15th September 2011, 08:58 PM   #16043
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Quote:
Originally Posted by john curl View Post
I realize that this may not make things more clear, but Thorsten has brought up something that has been addressed in the JC-3 to some extent. The type of compensation used in the JC-3, is called Miller Feedback compensation. .
Why are they missing from that WW schematic?
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Old 16th September 2011, 12:05 AM   #16044
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OK let's walk through it, two equations is all you need. You try and stuff current into the high Z node. This current is supplied by the input stage (I = gm*Vin). But by inspection Vin = Vo*(R2/(R1+R2)) where R2 is the input resistor and R1 is the feedback resistor. The output resistance is (R1+R2)/gm*R2 or 1800 Ohms where R1 = 80k and R2 = 10k and gm = 5mS. There are no "real" missing terms that make a SIGNIFICANT difference.

EDIT - Add the 500k resistor you get 1580 Ohms. Night and day you decide.
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Last edited by scott wurcer; 16th September 2011 at 12:28 AM.
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Old 16th September 2011, 12:25 AM   #16045
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You are correct, Scott.
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Old 16th September 2011, 03:37 AM   #16046
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There is a difference in the circuit initially used to make the line driver for the JC-2 and to convert it to the input stage for the JC-3. One of the changes was removal of the extra compensation caps that were originally in the JC-2. It just speeds up the amplifier a little bit, but I don't remember the details. This does make the compensation cap(s) that are inherent in the Ccb of each driver, not padded with a parallel linear cap, so they are slightly more non-linear. However they are summed together and are working in opposite voltage, so there should be first order cancellation of the voltage dependent total capacitance.
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Old 16th September 2011, 02:32 PM   #16047
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Quote:
Originally Posted by janneman View Post
I am eagerly awaiting a Letter to the Editor

jan
Sorry I'm late to this "party". This is from 25 pages ago or so...

Gee, Jan, you sure didn't try to argue with Malcolm Hawksford about this exact same topic when you interviewed him for AudioXpress a couple of years ago. Malcolm said:

"The sharp switching
edges from the DAC output can only
be perfectly reproduced with an I/V op
amp that has infinite bandwidth and no
limit on slew-rate. Any practical circuit
will have nonlinearity and slew rate limits
such that a transient input signal can
slightly modulate the open-loop (OL)
transfer of the op amp.
Modulating the OL transfer function
means you modulate the circuit’s
closed-loop (CL) phase shift. What is
interesting is that it looks remarkably
similar to correlated jitter; they share
a family resemblance. It also is
similar to what people have been talking
about as dynamic-phase modulation in
amplifiers. Whenever an amplifier stage
needs to respond very quickly, it tends
to run closer to open loop and therefore
is more susceptible to open loop nonlinearity."

You didn't argue with him then. Why argue with Nelson now?
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Old 16th September 2011, 02:41 PM   #16048
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Charles are you seriously coming back after all this time to try to find out why I didn't do something that you think I should have done three years ago?
Wow! I'm flattered!

But, to answer the question, I don't see what I should have argued about. Malcolm made a factual statement that as far as I can see is correct (if he indeed did say what you quoted; I haven't checked it). What point should I have argued?

I also don't get the reference to Nelson; that discussion was about the 'sameness' or not, of degeneration and loop feedback, and related to Bruno Putzeys article in Linear Audio Vol 1. Letters to the Editor, commenting on articles, are almost always enlightening; you should send one!

jan
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Old 16th September 2011, 02:42 PM   #16049
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Quote:
Originally Posted by john curl View Post
There is a difference in the circuit initially used to make the line driver for the JC-2 and to convert it to the input stage for the JC-3. One of the changes was removal of the extra compensation caps that were originally in the JC-2. It just speeds up the amplifier a little bit, but I don't remember the details. This does make the compensation cap(s) that are inherent in the Ccb of each driver, not padded with a parallel linear cap, so they are slightly more non-linear. However they are summed together and are working in opposite voltage, so there should be first order cancellation of the voltage dependent total capacitance.
I still think the circuit is fine. The number I picked for gm was arbitrary the actual JC-3 gm might be much higher, the larger it is the lower the impedance anyway.
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Last edited by scott wurcer; 16th September 2011 at 02:49 PM.
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Old 16th September 2011, 02:46 PM   #16050
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Quote:
Originally Posted by Charles Hansen View Post
"The sharp switching
edges from the DAC output can only
be perfectly reproduced with an I/V op
amp that has infinite bandwidth and no
limit on slew-rate. ?
Why does the amplifier have to see the edges? There are ways to prevent that.
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