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Old 19th March 2009, 11:45 AM   #21
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Quote:
Originally posted by Babowana



Giovanni, thanks for the reference. It gives me a clear picture.

I was wondering why R13 was there. It seems that now I understand the reason. It is to keep very low dc voltage at the output of the servo, and by doing so, to make the dc offset at the amp's output "drift vertually at 0V", isn't it?

Ciao,

><
Right.
Probably it could be removed, but in my experience op-amps likes it.
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Old 19th March 2009, 01:07 PM   #22
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Ciao Giovanni,
Found your circuits interesting. Have now done quite a few sims and with the attached version at 2A, you should get 30W into 4ohms with quite low THD. Dampingfactor should be ca 20 relative 8ohms.
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Old 19th March 2009, 01:13 PM   #23
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Thank to all of you for your help.
Following all your sucggestion I ended up with another version, THIS.

Improvements (really?):

1. No feedback on the input tube(s); the operating point has been choose in manner to grant a good linearity in the whole working region.
6N6P tube are already a really linear tube.
I can even add a few of cathode degeneration to make it more linear.

2. More juice to drive the output fet? Well, two tubes in parallel and double the current from the top CCS should help.

3. CCS to be defined: tube gurus suggested me to use a cascode CCS for better response, I have to decide yet which one (I have few cascode, high voltage CCS already built, must try few of them in this application).

4. P mosfet: choosen to "respect" the Aleph output topology and also because the IRF9530 fet has a much lower capacitance (ariund 800 pF).

Ciao,
Giovanni
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Old 19th March 2009, 01:40 PM   #24
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Hi again,
The lower device seems to be upside down. To make it work Source should be upwards and Drain to negative.

Wouldn´t it be more important to use the servo to keep output a 0V instead of using it for monitoring Iq?
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Old 19th March 2009, 01:51 PM   #25
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Quote:
Originally posted by revintage
Hi again,
The lower device seems to be upside down. To make it work Source should be upwards and Drain to negative.

Wouldn´t it be more important to use the servo to keep output a 0V instead of using it for monitoring Iq?
Thank you very much.
Eventually inverted the mosfet and corrected the voltage sensing on the output.

Here it is.

Ciao ciao
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Old 19th March 2009, 06:18 PM   #26
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Looking better, but still 5 more parts than needed in the Aleph.
Ditch R3, R5, R9, R10, and C2. Direct couple the source of Q4
to the emitter of Q3.

Values for R14 and R16 might need to be reduced by half each.
As the emitter of Q3 now sees the voltage span across both.

R7 is of debatable necessity in the emitter coupled circuit,
as the source of Q2 is a voltage follower of the emitter. I'd
at least reduce the value of R7 a bit. Maybe 100R is enough.
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Old 19th March 2009, 06:47 PM   #27
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Quote:
Originally posted by kenpeter

Ditch R3, R5, R9, R10, and C2.


So, you want to change Aleph current source to CCS?
What is the main reason . . . ?


Quote:
Originally posted by kenpeter

Values for R14 and R16 might need to be reduced by half each.
As the emitter of Q3 now sees the voltage span across both.


Why not just removing one of them instead of the two reduced. . . if necessary . . . ?



><
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Old 19th March 2009, 07:06 PM   #28
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Hey kenpeter,

Take a look at this paper:

http://www.passdiy.com/pdf/zen-ver2.pdf

Giovannis Aleph CCS is in fig 6.
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Old 19th March 2009, 11:27 PM   #29
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Quote:
Originally posted by Babowana
So, you want to change Aleph current source to CCS?
What is the main reason . . . ?

Why not just removing one of them instead of the two reduced. . . if necessary . . . ?
><
Its not CCS. Its just like Aleph, only direct emitter coupled.
It needs two resistors with the load connected roughly in
the center. Exact center seems to work fine in your case.

Its two amplifiers in parallel with a fixed voltage drop
spanning them. Top amp is merely an offset follower
of the bottom amp, which retains any SE character.

Aleph holds the top amp 1 Emitter drop higher than the
lower amp. That drop across both resistors then sets
the quiescent current. And simply let the magic happen...

Tap your NFB from the same center tap as the load.
And from the load's perspective, the resistors vanish.
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Old 20th March 2009, 03:38 AM   #30
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Quote:
Originally posted by kenpeter

Its not CCS. Its just like Aleph...


I see . . . ah ha . . . you mean this . . .

It looks like a "push-pull" retaining SE character of the lower mosfet . . . Maybe I'm not right . . .


><
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