JFET input, MOSFET VAS, LATERAL output = Perfect!! - Page 201 - diyAudio
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Old 18th September 2011, 01:50 PM   #2001
AKSA is offline AKSA  Australia
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Yes, enjoyed a geriatric snooze at 4pm today after a delightful coffee with my family, and a lecture from my daughter, whom we all call Mother Superior.....

Cheers,

Hugh
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Old 18th September 2011, 01:57 PM   #2002
mikelm is offline mikelm  England
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Quote:
Originally Posted by swordfishy View Post
Mike, how's it look with with 50r of degeneration and no compensation whatsoever (my setup).
the 180 transition is more or less in the middle of the two I posted
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Old 18th September 2011, 02:15 PM   #2003
mikelm is offline mikelm  England
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Quote:
Originally Posted by swordfishy View Post
Actually Hugh, given that you a using ever so slightly less degen (I am using 51.1R),
Guys,

The point I'm making here is that with a mosfet VAS the so called degeneration resistor is not doing any degeneration whatsoever ( if my spice is not lying )

The open loop gain does not decrease as the resistor value increases.

So I suggest that you try removing it and see what you think of the difference it makes to the sound.

If you decide to leave it there you can call it a stability impairing resistor - that's all it does !
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Old 18th September 2011, 02:16 PM   #2004
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my phase lag at 2meg hz is about 38 degrees
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Old 18th September 2011, 02:19 PM   #2005
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Mike,

I'll try that too...... thanks for the tip!

Hugh
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Old 19th September 2011, 12:24 AM   #2006
GregH2 is offline GregH2  Australia
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Thanks Mike,

I must admit I have never tried this as spice suggested that the THD was lower with it in place so I just left it there. You may like to confirm this though as I can't recall exactly what my circuit looked like when I was spicing this.

I'm sure it will sound better without it, the bjt version most definitely does. Unfortunately my boards are done, it is boxed and final as far as this amplifier is concerned. I am already working on the next design.

Now, can I ask, have either you or Hugh tried no compensation and a big choke yet?

I found for my version the secret was no compensation, then make a choke as big as necessary to get suitable overshoot performance. This may be big or small depending on what you like and are willing to tolerate. With my tinny speakers I found the big choke was best, but the sound was by no means bad with no choke at all!

Cheers,

Greg.
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Old 19th September 2011, 05:52 AM   #2007
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you can adjust the overshoot either by increasing the choke value or by decreasing the R value - even more variables to consider !
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Old 19th September 2011, 07:17 PM   #2008
mikelm is offline mikelm  England
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another variable . . . .

I just noticed that stability improves greatly as the drive impedance decreases so comparing what amount of compensation capacitance we are using really does not mean anything unless we control for factors like drive impedance & CLG . . .

another issue . . . .

I think the 500R in series with the miller cap sounds dreadful ! so forget that

Last edited by mikelm; 19th September 2011 at 07:23 PM.
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Old 19th September 2011, 11:49 PM   #2009
GregH2 is offline GregH2  Australia
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Oh dear, yet another thing!

Fortunately Hugh and I have very similar CLG! You're the odd one out Mike!

How about you Keantoken?
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Old 20th September 2011, 12:27 AM   #2010
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My CLG is defined by a 1k/47R feedback network. I don't think this is much different than you guys.

- keantoken
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