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Old 17th November 2009, 08:20 PM   #1
GEirin is offline GEirin  Argentina
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Default VAS buffer

Hi all.

Mr Self has 2 VAS buffer type. (Attached bellow).
1- VAS with CCS + buffer with CCS
2- VAS + Buffer with CCS

Im very interested in to know the pro and cons the each type of VAS.
Thanks
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File Type: jpg Self Vas + Buffer.JPG (20.2 KB, 579 views)
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Old 18th November 2009, 10:32 AM   #2
GEirin is offline GEirin  Argentina
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OK.

Grundig SV 2000.
T404 VAS
T405 Buffer

Is better?
VAS NPN and Buffer NPN?
Separate CCS, one for VAS other for Buffer?
Opinions, please reply.
Thanks
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File Type: jpg GrundigSV2000.jpg (72.6 KB, 461 views)
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Old 18th November 2009, 09:30 PM   #3
forr is offline forr  France
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A few weeks ago, I was wondering in a post about the reason why such buffers are almost never seen on schematics, at leat on the recent ones. No answer at all.
Is there some rebuttal towards single ended buffers ? In that case, why not to use a White buffer (with a clean power supply) which has the nice feature of a well defined quiescent current.
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Old 18th November 2009, 10:03 PM   #4
AKSA is offline AKSA  Australia
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Geirin,

I prefer (f), because it is simpler (one CCS), and should lean more towards even order distortion. I have not simmed it, however.

Both work well.

Hugh
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Old 18th November 2009, 10:44 PM   #5
GEirin is offline GEirin  Argentina
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Hi Hugh
Thank you very much.

A question:
Which transistors you suggest to use for VAS and buffer?
VAS: BC546?
Buffer: 2SA1358, 2SA1360?

Guillermo
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Old 19th November 2009, 08:10 AM   #6
AKSA is offline AKSA  Australia
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Guillermo,

I like 2SC1845 for VAS (110MHz, Cob 1.6pF, beta 600, super low noise, 50mA but watch the dissipation, only 500mW, don't run it over about 150mW), and 2SC1360 for the buffer. However, since the buffer is an EF, and intrinsically fast, you could use any relatively high voltage device here, such as a 2SA1837, BF470, or even a cheap BD140 if operating at low voltage.

Cheers,

Hugh
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Old 19th November 2009, 08:57 AM   #7
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forr,
Quote:
A few weeks ago, I was wondering in a post about the reason why such buffers are almost never seen on schematics
Because they do more harm than good. Is that a good reason for you?
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Old 19th November 2009, 10:48 AM   #8
GEirin is offline GEirin  Argentina
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Hugh
Thank you very much for you help.
I will try to use this VAS type in my amplifier project.
regards
Guillermo
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Old 19th November 2009, 11:07 AM   #9
forr is offline forr  France
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Originally Posted by Lumba Ogir View Post
forr,

Because they do more harm than good. Is that a good reason for you?
Not at all. Because you don't give the slightest idea of harm it could introduce and some very good reasons have already been mentionned wihout any caution in D. Self's litterature.
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Old 19th November 2009, 11:41 AM   #10
AKSA is offline AKSA  Australia
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Lumba,

With respect, you are wrong. I support Sebastien. The buffer makes the VAS act as a voltage, not a current source, and this makes for less correction as it negotiates the erratic load of the output stage.

I notice better sonics, too, a stronger performance all round, and more resolution.

Hugh
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