Double transistors in the VAS Why?

GEirin · 25th February 2010, 10:47 AM

Hi all.
I`m interested to know the VAS of the Harman Kardon powers amplifiers.
Why has two transistors in paralllel? Q417-421 and Q419-423.
Has any advantage? Is better

TonyTecson · 25th February 2010, 10:59 AM

feed-forward feedback is taken from the front pair IIANM...
q417,419.....

bocka · 25th February 2010, 11:26 AM

Quote:

Originally Posted by GEirin

I`m interested to know the VAS of the Harman Kardon powers amplifiers.Why has two transistors in paralllel? Q417-421 and Q419-423.

Probably because of limiting the power dissipation of each transistor

AKSA · 25th February 2010, 11:34 AM

#3 is correct. Smaller devices, sharing the current, will have lower parasitic capacitances and be faster. This has profound impact on the sound quality.

Modern devices like the 2SC3423 will run higher currents - up to 15mA with ease - so two devices may no longer be needed.

Hugh

jacco vermeulen · 25th February 2010, 12:18 PM

If just so, then why use a parallel pair in the Vas and a higher power device with nearly identical specs as driver ?

BJTs for VAS Stages
(= $6000 msrp Class A amp)

Vas : 2SA1145/2SC2705
Driver : 2SA1360/2SC3423

http://www.toshiba.com/taec/componen...c//66/7580.pdf
http://www.toshiba.com/taec/componen...nc/66/7540.pdf

GEirin · 25th February 2010, 03:41 PM

Hi
Thanks, very much for yours replies.
My question: for sound quality, is better, 1 transistor or parallel pair in VAS?
Guillermo

Havenwood · 25th February 2010, 05:56 PM

What about the impedance (input and output) in VAS?
Zin and Zout decrease.

Zin lower is bad?

For parasitic capacitances, VAS Cascode fix this.
What is Vce optimal? 5V?

AKSA · 25th February 2010, 10:08 PM

If you use an EF to drive the VAS, you will pick up about 2dB lower H2 and 15dB lower H3 with no change to other artefacts at +20dBU and 1KHz. For very large output stages, a buffer on the VAS is a good idea too.

For domestic amps, I've never found any benefit with a buffer after the VAS.

Jacco,

Bloody good question.....

Cheers,

Hugh

25th February 2010, 10:59 AM	#2
TonyTecson diyAudio Moderator Join Date: May 2003 Location: Maybunga, Pasig City	feed-forward feedback is taken from the front pair IIANM... q417,419..... __________________ hard work erases ignorance.....

25th February 2010, 11:34 AM	#4
AKSA diyAudio Member Join Date: Sep 2001 Location: Melbourne, Australia	#3 is correct. Smaller devices, sharing the current, will have lower parasitic capacitances and be faster. This has profound impact on the sound quality. Modern devices like the 2SC3423 will run higher currents - up to 15mA with ease - so two devices may no longer be needed. Hugh __________________ Aspen Amplifiers P/L (Australia) www.aksaonline.com

25th February 2010, 05:56 PM	#7
Havenwood diyAudio Member Join Date: Mar 2008	What about the impedance (input and output) in VAS? Zin and Zout decrease. Zin lower is bad? For parasitic capacitances, VAS Cascode fix this. What is Vce optimal? 5V? __________________ - Music by John Williams -

25th February 2010, 10:08 PM	#8
AKSA diyAudio Member Join Date: Sep 2001 Location: Melbourne, Australia	If you use an EF to drive the VAS, you will pick up about 2dB lower H2 and 15dB lower H3 with no change to other artefacts at +20dBU and 1KHz. For very large output stages, a buffer on the VAS is a good idea too. For domestic amps, I've never found any benefit with a buffer after the VAS. Jacco, Bloody good question..... Cheers, Hugh __________________ Aspen Amplifiers P/L (Australia) www.aksaonline.com

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25th February 2010, 12:18 PM	#5
jacco vermeulen diyAudio Member Join Date: Oct 2004	If just so, then why use a parallel pair in the Vas and a higher power device with nearly identical specs as driver ? BJTs for VAS Stages (= $6000 msrp Class A amp) Vas : 2SA1145/2SC2705 Driver : 2SA1360/2SC3423 http://www.toshiba.com/taec/componen...c//66/7580.pdf http://www.toshiba.com/taec/componen...nc/66/7540.pdf

25th February 2010, 03:41 PM	#6
GEirin diyAudio Member Join Date: Jun 2004 Location: Mendoza	Hi Thanks, very much for yours replies. My question: for sound quality, is better, 1 transistor or parallel pair in VAS? Guillermo

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