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Old 20th June 2012, 09:50 PM   #31
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by tinitus View Post
I expect you have found it by now

btw, it was our good man Juma with the thread 'F5 meets Buzquito'

now that I think about it, he might have inspired Nelson to design the new F5 like frontend/driver for the BA3

let's just say that some things are boiled water for Curl , Borbely and Pass .

in whatever line-up you line them
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Old 21st June 2012, 04:06 PM   #32
juma is offline juma  Germany
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Quote:
Originally Posted by Paulv View Post
... the bias jfet:2.4 mA...
Sorry, but that way you'll make a popular mistake known as "starved JFETs" which is manifested through grainy & murky sound. To correct it, go for JFET's Id = 70-90% of Idss.
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Old 21st June 2012, 10:48 PM   #33
Paulv is offline Paulv  France
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thanks Juma, I am on the learning curve, reading lots of threads here and playing with LTspice, hoping that simulation is not too far from real life, I know that I will have to experiment, maybe audiotester (another unknown tool) will tell me if I am right,
I have an old scope too, but dead signal generator....

I think I will opt for a VAS circuit, with BJT(15mA bias),
Laterals, double die 150...500mA bias
looks more stable, less concern with thermal compensation
Thanks to all here, the following schematic owes to you too Juma

Paul
(and sorry if I should not post in Pass Lab)
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Old 21st June 2012, 11:20 PM   #34
Paulv is offline Paulv  France
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Default and the fft analysis

2 volts RMS output (first watt )
9 V RMS
18v RMS

if it could only be true!!!!
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File Type: jpg first watt fft.jpg (147.6 KB, 192 views)
File Type: jpg 9vrmsfft.jpg (151.6 KB, 181 views)
File Type: jpg 18vRMS.jpg (154.1 KB, 173 views)
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Old 22nd June 2012, 01:03 AM   #35
fab is offline fab  Canada
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Quote:
Originally Posted by juma View Post
Sorry, but that way you'll make a popular mistake known as "starved JFETs" which is manifested through grainy & murky sound. To correct it, go for JFET's Id = 70-90% of Idss.
If you look closely at his schematics of post #23, the source resistors are 10 ohms which is clearly within the 80-90% Idss range. The 2.4ma that he indicates may be simply its spice model of Idss of 3ma or so.

However, in Paul post #33 he made the mistake with 22 ohms as source resistors..
Maybe it gives about 50-60% Idss or so thus is it so bad?

Last edited by fab; 22nd June 2012 at 01:16 AM.
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Old 23rd June 2012, 03:56 PM   #36
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Quote:
Originally Posted by Paulv View Post
thanks Juma, I am on the learning curve, reading lots of threads here and playing with LTspice, hoping that simulation is not too far from real life, I know that I will have to experiment, maybe audiotester (another unknown tool) will tell me if I am right,
I have an old scope too, but dead signal generator....

I think I will opt for a VAS circuit, with BJT(15mA bias),
Laterals, double die 150...500mA bias
looks more stable, less concern with thermal compensation
Thanks to all here, the following schematic owes to you too Juma

Paul
(and sorry if I should not post in Pass Lab)
Hi Paul

The bias of the output mosfet is depending on the VAS current in this type of configuration thus voltage across your 120 ohm resistors. Depending on your LAteral mosfet VGSoff, the VAS may have to be quite different than 15ma. In order to keep at least the 15ma, you may have to change the 120 ohm value.

And for the VAS transistors, the ones you have selected are quite good for this job. However, if you compare with a mosfet VAS you will find a drastic change in the sound (more "alive", if I try to describe it...) although the electrical performance is worse than with good BJT in VAS (as you indicate also more stable in temperature). Some may like it more with good mosfet VAS (like me) or not. It is a matter of taste here..

Fab

Last edited by fab; 23rd June 2012 at 03:58 PM.
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Old 23rd June 2012, 05:28 PM   #37
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Quote:
Originally Posted by fab View Post
If you look closely at his schematics of post #23, the source resistors are 10 ohms which is clearly within the 80-90% Idss range. The 2.4ma that he indicates may be simply its spice model of Idss of 3ma or so....
I think that current through input JFETs in that sch. is determined by J3 and J4 (they both act as constant current sources).
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Old 23rd June 2012, 10:43 PM   #38
fab is offline fab  Canada
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Originally Posted by juma View Post
I think that current through input JFETs in that sch. is determined by J3 and J4 (they both act as constant current sources).

My mistake Juma, of course you are right.
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