Double transistors in the VAS Why?

[GEirin]

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]

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

 

[bocka]

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]

#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]

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

http://www.diyaudio.com/forums/solid-state/98631-bjts-vas-stages.html#post1165309
(= $6000 msrp Class A amp)

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

http://www.toshiba.com/taec/components2/Datasheet_Sync//66/7580.pdf
http://www.toshiba.com/taec/components2/Datasheet_Sync/66/7540.pdf

 

[GEirin]

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

 

[Havenwood]

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]

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