Blameless, Dx was wrong, it is a very good amplifier - Page 13

destroyer X · 2nd November 2007, 08:55 AM

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

AndrewT · 2nd November 2007, 10:26 AM

Hi,
is that 754mVbe or across some other part of the circuit.
It seems far too high for a real Vbe measurement.

Giaime · 2nd November 2007, 01:15 PM

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.

GEirin · 2nd November 2007, 07:37 PM

In the Ultra Low Amp Design (Lars Clausen) thread, in post 30, Jam said T8 is Buffer. Then, T7 is Vas?
GEirin.

Giaime · 2nd November 2007, 10:29 PM

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!

GEirin · 2nd November 2007, 11:57 PM

I have seen this used in Yamaha AX892 and AX596.

2nd November 2007, 07:37 PM	#124
GEirin diyAudio Member Join Date: Jun 2004 Location: Mendoza	HI In the Ultra Low Amp Design (Lars Clausen) thread, in post 30, Jam said T8 is Buffer. Then, T7 is Vas? GEirin.

2nd November 2007, 11:57 PM	#126
GEirin diyAudio Member Join Date: Jun 2004 Location: Mendoza	Giaime I have seen this used in Yamaha AX892 and AX596.

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2nd November 2007, 10:26 AM	#122
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, is that 754mVbe or across some other part of the circuit. It seems far too high for a real Vbe measurement.

2nd November 2007, 10:29 PM	#125
Giaime diyAudio Member Join Date: Oct 2004	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!