Quasi output: different configurations

GEirin · 6th May 2011, 12:03 AM

Hi all.
I've seen different configurations of quasi outputs. I would like to know which is correct, and each has particular characteristics.
1- Naim amplifier
2- Quasi amplifier
Thanks

nigelwright7557 · 6th May 2011, 01:04 AM

Both are correct.

However I found with my quasi amp i needed a small capacitor from the lower driver transistor collector to the base to stop oscillation on the output.

GEirin · 6th May 2011, 01:19 AM

Hi nigelwright7557.
Thanks for you reply.
Ok. I have seen this cap in different amplifiers (Arcam A60). My question is, if the transistor down resistor must be in the collector (Naim) or emitter (Quasi)?
The different configuration quasi, have some effect on the sound?

GEirin · 6th May 2011, 10:59 AM

Hi all.
My question is, if the transistor down resistor must be in the collector (Naim) or emitter (Quasi)?
The different configuration quasi, have some effect on the sound? Some advantage?

sreten · 6th May 2011, 12:30 PM

Hi, the two baxandall diode arrangements above seem identical to me, rgds, Sreten.

magnoman · 6th May 2011, 12:52 PM

The one on the right, with parallel transistors seems to be using the emitter resistors to help balance the current between these parallel transistors. These resistors will also have an effect on the local feedback of the "pair" (maybe for the better but one would have to analyze that).
The resistors to the load I think are more likely for bias current setting and its temperature stability.

Hope this helps
-Antonio

GEirin · 6th May 2011, 07:17 PM

HI.
Thank you very much for your responses.
Now, I have another question, why is not the Baxandall diode in quasi mosfet amplifiers?
Regards
Guillermo

sreten · 7th May 2011, 01:35 AM

Quote:

Originally Posted by GEirin

HI.
Thank you very much for your responses.
Now, I have another question, why is not the Baxandall diode in quasi mosfet amplifiers?
Regards
Guillermo

Hi,

The short answer is bias voltage is far higher and Fets need far higher drive
voltages than transistors at high currents, essentially adding the diode does
little in this case, it doesn't affect the symmetry of the output stage much.

rgds, sreten.

nigelwright7557 · 7th May 2011, 01:49 AM

Quote:

Originally Posted by GEirin

HI.
Thank you very much for your responses.
Now, I have another question, why is not the Baxandall diode in quasi mosfet amplifiers?
Regards
Guillermo

The baxandall diode is and isnt used in quasi amps.

Add the diode increases the gain of the lower driver.
This is fine but I found the increase in gain could also cause instabilty and oscillation on the output. I found I had to add a small capacitor across b-c on the lower driver transistor.

AKSA · 7th May 2011, 04:40 AM

Nigel,

You can always put a 22nF across the diode to smooth the gain transition..... seems to stabilise Quasis very well.

Hugh

6th May 2011, 01:04 AM	#2
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	Both are correct. However I found with my quasi amp i needed a small capacitor from the lower driver transistor collector to the base to stop oscillation on the output. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

6th May 2011, 12:30 PM	#5
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	Hi, the two baxandall diode arrangements above seem identical to me, rgds, Sreten. __________________ There is nothing so practical as a really good theory - Ludwig Boltzmann When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

7th May 2011, 04:40 AM	#10
AKSA diyAudio Member Join Date: Sep 2001 Location: Melbourne, Australia	Nigel, You can always put a 22nF across the diode to smooth the gain transition..... seems to stabilise Quasis very well. Hugh __________________ Aspen Amplifiers P/L (Australia) www.aksaonline.com

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6th May 2011, 12:52 PM	#6
magnoman diyAudio Member Join Date: Nov 2010 Location: The City, SanFrancisco	The one on the right, with parallel transistors seems to be using the emitter resistors to help balance the current between these parallel transistors. These resistors will also have an effect on the local feedback of the "pair" (maybe for the better but one would have to analyze that). The resistors to the load I think are more likely for bias current setting and its temperature stability. Hope this helps -Antonio

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