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Old 25th February 2010, 09:47 AM   #1
GEirin is offline GEirin  Argentina
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Default Double transistors in the VAS Why?

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
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Old 25th February 2010, 09:59 AM   #2
AJT is offline AJT  Philippines
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feed-forward feedback is taken from the front pair IIANM...
q417,419.....
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Old 25th February 2010, 10:26 AM   #3
bocka is offline bocka  Germany
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Quote:
Originally Posted by GEirin View Post
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
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Old 25th February 2010, 10:34 AM   #4
AKSA is offline AKSA  Australia
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#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.

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Old 25th February 2010, 11:18 AM   #5
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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
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Old 25th February 2010, 02:41 PM   #6
GEirin is offline GEirin  Argentina
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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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Old 25th February 2010, 04:56 PM   #7
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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?
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Old 25th February 2010, 09:08 PM   #8
AKSA is offline AKSA  Australia
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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
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