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Old 29th March 2012, 09:41 AM   #1
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Default Best VAS?

What's the best Vas stage in power amp? Simple one or two transistor? Darlington? Cascode? Complementary sysmetry?...
Of course, ''the best'' are: sounding, gain, bandwidth, speed, distortion...
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Old 29th March 2012, 09:53 AM   #2
AndrewT is offline AndrewT  Scotland
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start by looking at Greg's list of potential VAS devices.

You can ruin a VAS simply by picking an unsuitable device.
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Old 29th March 2012, 09:55 AM   #3
AKSA is offline AKSA  Australia
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I like a single 2SC3423 in common emitter, carefully compensated either with simple miller or TMC.

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Old 29th March 2012, 10:58 AM   #4
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Oh, excuse me! I want to know the topology circuit, not vas devices. Exactly is best Vas topology circuit. Of course, i know vas's transistor must have high Ft, high gain, low Cob, high Vce...
I have been seen GoldMund schema, JFET cascoded input, and simple Vas, but it's have extremely good spec: 250V/uS slew rate, -3dB @ 3MHz bandwidth,...
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Old 29th March 2012, 11:36 AM   #5
godfrey is offline godfrey  South Africa
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Here's a VAS design I came up with a while ago that I like the look of, although I haven't tried it. It's fast and offers clean clipping with rapid recovery.

There's a few key points:
A) None of the VAS transistors ever saturate or switch off.
B) If Vbias is set correctly, it provides symmetrical current limiting.
C) Due to (A) above, voltage at the base of Q2 stays almost constant, even when the output is clipping.

Regards - Godfrey

edit: p.s. Here's a version with a current mirror, hastily cut out from a complete amp schematic before I run out of edit time. It needs clipping diodes too.
Attached Images
File Type: gif vas.GIF (5.5 KB, 1315 views)
File Type: gif vas2.GIF (5.5 KB, 1258 views)

Last edited by godfrey; 29th March 2012 at 11:52 AM.
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Old 29th March 2012, 11:42 AM   #6
dadod is offline dadod  Croatia
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I am goin to try this one in the next amp.
dado
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Old 29th March 2012, 12:22 PM   #7
Bonsai is online now Bonsai  Taiwan
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Some ideas there Godfrey. I especially like the collector resistor R1. This doubles up as a current limiter as well.
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Old 29th March 2012, 12:33 PM   #8
AndrewT is offline AndrewT  Scotland
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Does R1 act similarly to Puefue's "magic" resistor? i.e. trying to maintain constant power dissipation.

Does the compound current route R9+R2 as the input collector load, change the way the VAS works?

Would a pair of 25V Zeners from the +-30Vdc supply rails work as the voltage limiting +-25V references?
Or.
Would it be better to run the next stages @+-25V and then the VAS limit would match the next stages capability?
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Last edited by AndrewT; 29th March 2012 at 12:39 PM.
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Old 29th March 2012, 01:13 PM   #9
Bonsai is online now Bonsai  Taiwan
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My Ovation 250 uses collector resistors as Godrey shows in a beta enhanced cascade config. However, I did not implement the anti saturation networks as he has done. For my reduced implementation, I can say it sounds pretty good (note: my overall configuration is balanced, but Godfrey's enhancements can be applied just as easily)
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Old 29th March 2012, 01:30 PM   #10
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In principle its a cascoded darlington. How does it perform linearity wise compaired to beta enhanced. I have found them to be inferior although there aint much in it. Also its not a straight forward comparison.
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