Has anyone ever tried a Beta Enhanced emitter follower VAS?? - diyAudio
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Old 26th November 2006, 08:19 AM   #1
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Default Has anyone ever tried a Beta Enhanced emitter follower VAS??

And if so what were your impressions?? I have built the example in Doug Selfs blameless and so far have found it to be quite lucid and musical, my current source consists of two transistors and a resistor of 68,1 ohm for a 9-10ma bias..I was using a bootstrapped vas stage prior and found the bass to be too mushy by comparison, and only after comparison.. So far after using mr selfs input differential idea and vas idea with an alteration in the current source i dont understand why he gets so much flack in the diy community, so far his ideas have proven to me to be sonically satisfying in a non fatiguing way, is it his focus on distortion measurements??..


Thanks
Colin
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Old 26th November 2006, 09:26 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I like Self for the technical solutions to achieve examplary technical performance.

I do not like his absolute reliance on low distortion to exclusion of all else. Sound quality is important to most of us and he completely avoids that.

I think the Miller compensation capacitor is an example of where he aims for low distortion and automatically gets bad sound performance on varying and multi frequency signals. I also believe that the mirror loaded LTP causes a similar problem but to a lesser extent. But both these need to be examined in detail for the importance of each to the overall performance. Self is the only writer I have seen that is prepared to release all his research data showing why and how to minimise the errors. Use his techniques but be selective.

Technically superb and all the data to back it up.

Personally I believe fig 4.17 e or f is superior to 4.17a which he uses in all his amps.

I further believe that d followed by e or f would be better. But in all of these Cdom should be removed and alternative compensation added to maintain stability.
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Old 26th November 2006, 12:32 PM   #3
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Can someone show the schematics of the figures being alluded to?

Thanks.
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Old 26th November 2006, 02:10 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi Sam,
send me a digital camera and permission from D.Self & I'll post the diagrams.
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Old 26th November 2006, 05:42 PM   #5
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The schematics can also be found on Selfs web site. The same schematics as 4.17a-f in the book appear in section 5.2.2 in the link
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

I suppose the first post in the thread refers to case c.
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Old 26th November 2006, 06:25 PM   #6
mikeks is offline mikeks  United Kingdom
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Default Re: Has anyone ever tried a Beta Enhanced emitter follower VAS??

Quote:
Originally posted by vynuhl.addict
Has anyone ever tried a Beta Enhanced emitter follower VAS?

Thanks
Colin
Poor clipping characteristics.
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Old 21st April 2010, 10:46 PM   #7
tessier is offline tessier  Canada
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Hi

Do we need to use a fast transistor with low cob for both the two transistors of a Beta Enhanced emitter follower VAS, or only for the first transistor ?

And most of the times that type of VAS need a high value of compensation capacitor (67 to 100 pF) for a good amp stability, does it made that VAS a bit slowish ?

Thanx

Paul
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Last edited by tessier; 21st April 2010 at 10:52 PM.
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Old 21st April 2010, 11:53 PM   #8
AKSA is offline AKSA  Australia
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Yes. Both devices should be fast. I use C1845 for the first, and C3423 for the second, the latter running at much higher current, of course.

The value of the miller cap is no higher nor lower than a single VAS if the EF is fast, which it normally is since it is only passing 1mA or so. The dimensioning of the miller cap depends upon the speed of the rest of the circuit devices, and their topology, as well as the VAS devices, which in truth play a minor role.

The size of the miller cap has direct bearing upon the sonics of the amplifier. It should be chosen very carefully.

Hugh
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Old 22nd April 2010, 01:12 AM   #9
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Default Please hugh...what is mushy?

What Colin said?..... bootstrap good or bad in his mind..... down 2006..... then he felt in love by Maya amplifier.

We, strangers to the language, we do not know those idiomatic expressions or not often used words.
I had not the knowledge know the difference of to tell and to say.... in my mind is the same..but yesterday, watching a movie i found to tell was used meaning an order, something rude...and to say meaning something more soft.

regards,

Carlos
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Last edited by destroyer X; 22nd April 2010 at 01:15 AM.
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Old 22nd April 2010, 06:35 AM   #10
latala is offline latala  United Kingdom
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Yes I tried this technique
First time I saw it as in Linsley hoods 75 watt amplifier
I used it back in about 1975 in one of my amplifiers The results were beyond belief distortion measured fom 0.1with a standard Vas to below 0.01% with the enhanced Vas
You have to be carefull to ensure that your amplifier is stable as the advantages can be lost if you end up over compensating to reduce hf gain
regards Trev
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