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Old 2nd November 2007, 09:55 AM   #121
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Default Interesting Giaime, return to a previous post... some time ago when you posted last

updated schematic.

I was also trying R75 and R76 adjustments into the simulator...also i found 2K into the simulator, to equalize the input transistors VBEs..... the interesting thing is that into the real world, measuring my Precision prototype, that is using "today" your front end, and i have found 754 milivolts to each transistor.... so...VBEs equalized using 1K5.

This is to remember those terrible things.... the simulator results are not the same related the real life.... the transistor parameters are not the real ones.... in special VBEs normall do not result the same as real life.

regards,

Carlos
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Old 2nd November 2007, 11:26 AM   #122
AndrewT is offline AndrewT  Scotland
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Hi,
is that 754mVbe or across some other part of the circuit.
It seems far too high for a real Vbe measurement.
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Old 2nd November 2007, 02:15 PM   #123
Giaime is offline Giaime  Italy
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Default Re: Hello Giaime.

Quote:
Originally posted by GEirin
What is your think about Buffer Vas of the LC amp1.
GEirin
Mmmm... I don't see any VAS buffer. There's a beta-ehnancer emitter follower T8 (btw, R16 looks way off), and then the VAS, directly connected to the driver transistors.

Maybe you talk about those ones (T3-T4) as VAS buffer, yes they work that way but with mosfet output stages, driver transistors are a must, so there is no innovation in incorporating them. Instead, using a buffer between the VAS and a CFP output stage like in my amp is a thing not seen very often, I think.
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Old 2nd November 2007, 08:37 PM   #124
GEirin is offline GEirin  Argentina
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Default HI

In the Ultra Low Amp Design (Lars Clausen) thread, in post 30, Jam said T8 is Buffer. Then, T7 is Vas?
GEirin.
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Old 2nd November 2007, 11:29 PM   #125
Giaime is offline Giaime  Italy
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Hello!

yes, T7 is the VAS. You can look at T8 as a "beta ehnancer" transistor, in fact the VAS gain is proportional to VAS transistor beta, and by putting an emitter follower before it, you raise the current gain by a factor equal to T8 beta.

The problem is that R16 is way too high: it won't let pass even a bare mA even on full 45V rails, which is too low for T8. Go for 33k or so.

Btw, people use to ground the other end of R16, instead of putting it on a CCS feed. You can try both ways!
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Old 3rd November 2007, 12:57 AM   #126
GEirin is offline GEirin  Argentina
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Default Giaime

I have seen this used in Yamaha AX892 and AX596.
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Old 18th January 2014, 05:15 PM   #127
Bigun is offline Bigun  Canada
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This simple buffer is a wasted opportunity - look at my TGM8 (warning: blatant plug !!!) where I've used the buffer to bootstrap the input device - all dc coupled. The 'Ingram input stage'
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