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Old 18th October 2006, 04:44 PM   #1
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Default Zen amplifiers and global feedback

Hi all,
I started recently to study (and simulate using PSPICE) the "Penultimate Zen". Even if, after some tuning, at the simulator the amplifier sounds great, I would like to ask if nobody is worried by the large amount of global feedback applied.
I'm not religious about that, but I would like to hear some comments.
Please don't say: <I don't care about what Dr. Matti Otala can say about that, it's a sexy configuration and I don't care about TIM>.
Thanks
Teodoro Marinucci
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Old 18th October 2006, 07:36 PM   #2
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The Zen amp does not generate enough open loop gain
to have a lot of feedback - the 2S transconductance of the
power Mosfet compared to the 8 ohm load gives about 16X,
which is 24 dB. Typically this means about 10 dB of feedback,
which is quite low.

TIM is a description of a distortion mechanism in multi-stage
amplifiers, where the job of correcting output distortion tends
to overload the input stage. This input stage, being only an
impedance buffer, is pretty relaxed in its response.

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Old 18th October 2006, 09:10 PM   #3
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Default Thank You ! Nelson ...

Shall you have the time and the patience of what I "discover" during my simulations ?
Thanks
Teodoro Marinucci
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Old 18th October 2006, 09:53 PM   #4
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In the sense that it can be used to compare the quality
of sims versus reality, it is of interest.

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Old 18th October 2006, 10:01 PM   #5
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Theodoro,

Generally speaking, don't worry about large global feedback on one, or two stages amplifiers. Things goes wrong (really wrong !) when a third stage is added... (because of additive phase shift).
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Old 19th October 2006, 01:36 AM   #6
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Speaking purely for myself, I don't regard 10dB of feedback in a solid state piece to be a 'large amount' of feedback. That's way down in tube territory. Most tube circuits use something on the order of 10-12dB NFB. Solid state amps typically use 30 to 40dB or more.
It always amuses me to hear solid state people rant about the "poor" specs exhibited by tube equipment. Every once in a while, I toss out the suggestion to try getting good specs out of a solid state piece with a mere 10dB of feedback. Yes, Nelson, John Curl, and a handful of others manage to get decent specs out of relatively modest amounts of feedback, but most solid state designers can't. Those who are True Believers in high amounts of negative feedback would be better off complaining about the lower gain available from tubes...which in turn limits the amount of negative feedback that can be applied. At least that complaint has some basis in reality.
Of course, there's no law that says you have to use feedback. If you want, feel free to reduce the amount of feedback in the Zen circuit(s).
I haven't had much opportunity to play with electronics recently, but I've gotten solid state designs down as far as ca. 7dB NFB with decent specs. I'd like to get to zero, but that will take better parts and more fiddling time.

Grey
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Old 19th October 2006, 07:16 AM   #7
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Grey got the point, for many people it's not worth the trouble to linearize every single stage to have descent open loop performance.
Just design a circuit with the highest gain you can with the devices you have, and then the huge amount of global negative feedback will correct all the non linearities... (or maybe not)


Nelson, your answear about the TIM in single stage amplifiers made me wonder something: can we really talk about global feedback in a single stage amplifier?
Isn't global feedback (feeding part of the inverted output back to the input) the same as local feedback (e.g. more source degeneration, or a lower stage gain) here?
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Old 19th October 2006, 07:24 AM   #8
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Quote:
Originally posted by Bricolo

Isn't global feedback (feeding part of the inverted output back to the input) the same as local feedback (e.g. more source degeneration, or a lower stage gain) here?
include phase in equotation,and you'll have the answer
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Old 19th October 2006, 10:03 AM   #9
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Default First: the assumtpions

Nelson,
I'm starting to describe what I did. At any moment, if you think that my questions, my assumptions, my methodology, are too stupid, say "stop" and I shall stop.
My problem is that, even if I like to build machines delivering good music, I like also to understand (as far as possible to me) how they work. My problem is that my (little) culture is mainly about tubes, so I have some difficulty to understand solid state.
First: my assumptions.
I wanted to build 20+20W, class A, hybrid amplifier.
For the tube part I wanted to use the Aikido (www.tubecad.com) line stage. I have already the PCB, the tubes, all the simulations that I understand.
I started thinking about the final stage. I took into consideration the final stage proposed therein: It's a sort of Moscode amplifier, with a 40V positive rail, 1.2A idle current.
It is for that reason that I have "designed" a regulated power supply and I have now a (nice toroidal) power transformer able to deliver 42V, 3A (AC): it seemed already to me that having a 126VA transformer to deliver only 20W was really a waste.
I'm starting to appreciate your topology, and I would like to put it in the place I have already reserved for the final stage.
The circuit I'm using is:

Click the image to open in full size.

If you tell me "go on" I'll start describing the modifications I have made.
Thanks
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Old 19th October 2006, 10:11 AM   #10
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Default To Bricolo ...

Bricolo,
You sholud always bear in mind that "tube people" are really obssessed by (any kind of) feedback.
There are people disliking cathode followers because of the local, degenerative, feedback.
There are people putting 2200uF electrolytic capacitors to bypass a 50Ohm self-bias resistor.
There are people that, since Otala said that too much global feedback is harmul, once they only hear the word, become anxious.
Me, I'm trying to understand ...
Thanks for any contribution.
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