What are the pros and the cons of those 3 vas ? - Page 8 - diyAudio
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Old 6th December 2010, 03:59 AM   #71
AKSA is offline AKSA  Australia
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Gaetan,

It might be better to put the limiter on the collector rather than the emitter side, still measuring current, but set to a ridiculous current well short of the max collector current but much higher than the VAS operating current.

If you place it on the drive (base) side, there will be a transition zone of activation which could affect the sound quality.

Hugh
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Old 6th December 2010, 04:48 AM   #72
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Hello Hugh

I was not sure about this VAS, usually I don't use a current limiter in a VAS, so I did asked for the pros and cons of it.

Btw, I've just look in all my amps schematics and this VAS was coming from a Leach amp, the image I've post was the VAS alone (no amp schema) from a web site.

Thank

Bye

Gaetan

Last edited by gaetan8888; 6th December 2010 at 05:09 AM.
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Old 6th December 2010, 12:34 PM   #73
AndrewT is offline AndrewT  Scotland
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The Leach and similar amps that have current and/or IV limiting on the output stage can overload the VAS when the limiting activates.

The extra transistor is there to prevent the VAS being driven to destruction by the LTP trying to force the VAS to drive the current limited output stage when it is already overloaded.

The sum of the two resistor is the required degeneration that the VAS needs.

The BE resistor should be sized to drop ~2/3 Vbe (~400mV) when the VAS is passing ~ two times it's bias current.

If VAS Ib=10mA then VAS can go from 0mA to 20mA and not be overloaded. When running at upto this range of currents the Protection transistor must not interfere with the VAS feed from the LTP. If the lower resistor is 20r then 20mA of maximum VAS peak output current produces a Vbe of 400mV. The protection transistor will be fully active when the VAS current rises to ~30mA (~3times Iq).
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Old 1st January 2017, 12:24 AM   #74
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Hello

I know that there is different opinions about the listening result of a cascoded VAS in a amp.

But can we use a Hawksford cascoded VAS, without a buffer transistor, with low power amp (up to 40 watt) ?

I just want to know more about it.

Here is an image of a Hawksford cascoded VAS with a very little mod. I could use this one with a 30 watt amp ?

Thank

Happy New Year

Bye

Gaetan
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Last edited by gaetan8888; 1st January 2017 at 12:45 AM.
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Old 1st January 2017, 03:01 PM   #75
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I don't know when the Hawksford cascode was published but one similar to this was used in Gibbs and Shaw's 1970 Gemini circuit with remarkable reduction in distortion by reducing Early effect.
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Old 2nd January 2017, 05:06 AM   #76
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Quote:
Originally Posted by gaetan8888 View Post
...a Hawksford cascoded VAS with a very little mod.
Hi Gaetan
That "very little mod" completely defeats the point of the Hawksford cascode.
Have you read the Hawksford article itself?
Worth some effort even if it's written a bit obscurely.
Personally I find it easier to understand as an internal feedback loop, but perhaps that's just me.

Best wishes
David
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Old 2nd January 2017, 09:01 PM   #77
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Hawksford cascoded VAS I show in my image are from the book of Douglas Self, and my "very little mod" are just adding the 10 R resistor (the magic resistor suggested by Peufeu in his web site).

Thank

Bye

Gaetan
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Old 2nd January 2017, 09:03 PM   #78
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Quote:
Originally Posted by Dave Zan View Post
Hi Gaetan
That "very little mod" completely defeats the point of the Hawksford cascode.
Have you read the Hawksford article itself?
Worth some effort even if it's written a bit obscurely.
Personally I find it easier to understand as an internal feedback loop, but perhaps that's just me.

Best wishes
David
Hello

Can you email me this article ?

gaetan888 at gmail dot com

Thank

Bye

Gaetan
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Old 2nd January 2017, 10:03 PM   #79
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Quote:
Originally Posted by gaetan8888 View Post
Hawksford cascoded VAS... from the book of Douglas Self...
What edition is your book?
I have the 5th edition (Focal Press) and there is an emitter resistor for Q1.
That resistor is essential to make it a "Hawksford" cascode, as opposed to just a cascode.
It looked that you had removed this resistor for your "very little mod" but it seems D.Self or the book publisher messed this up.

Best wishes
David

Paper is here. http://www.aes.org/e-lib/browse.cfm?elib=5157

Last edited by Dave Zan; 2nd January 2017 at 10:08 PM.
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Old 2nd January 2017, 10:08 PM   #80
jcx is offline jcx  United States
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Hawksford "Slope Distortion" paper:
https://web.archive.org/web/20130124...%20cascode.pdf
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