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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