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Old 1st October 2003, 03:28 PM   #1
Cicero is offline Cicero  Italy
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Default Zero NFB amplifier schematic..good?

I built this amplifier about seven years ago.
It's the zero-feddback, class-A, low power version of a project published on an italian magazine (Suono) in 1992.
I used a +-40V power supply, 200VA trafo and 2x33.000 microfarad capacitors, instead the original +-28V ad the amplifier sounds, to my ears, very very well.
The power is about 55 watts/8 ohm and much more on the lower loads (160 watts/2 ohm).
The only problem i've noticed is a little tendency to oscillate with a long, output cables; but it happens only with the MJE 340/350 driver transistor instead the original MJE 15028/29 (that i couldn't find). I replaced them with 2SA968/2SC2238 and solved all the problems.
What's your opinion? I'm going to build another couple.
This is the signal stageÖ(I divided it because the image is too heavy)
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Old 2nd October 2003, 07:49 AM   #2
Cicero is offline Cicero  Italy
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I'm new at this forum, and yesterday I've to wait some hours for looking my post...but it became too late and I came back home!
This is the power stages, not so common:
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Old 2nd October 2003, 01:48 PM   #3
Cicero is offline Cicero  Italy
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Try again..hope to be more lucky!
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Old 2nd October 2003, 03:50 PM   #4
jam is offline jam  United States
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Cicero,

Welcome. I think you missed something.

Could you post the output stage and component values?

Regards,
Jam
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Old 2nd October 2003, 04:25 PM   #5
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Default Why an opamp

Your schematic look's like an ordinary opamp. As we know an opamps has in short a differential input stage, then high gain second stage, and follower. An amplifier characteristic depend on negative feedback loop. As I see, you going to decrease gain of first and second stages by using an emitter resistors ( a local negative feedback ), but circuit is very complex. All nonlinearities muliplies. If you want to build a non feedback amp, i suggest to use not more then two stages amplifier, eg. differential input uses an irf610 with source resistors, and complementary or single ended source follower. It's really hard to control bipolar output stage without nfb ( turn off time ). You may use a high power version of borbely j-fet line preamp ( J-fets change to mosfets, and a regulator to first stage supply ). I have built something similar and with rgulated ( lm 317 ) positive supply of the first stage I get a +- 4 mV offset voltage. I do not use a cascode circuit, I think it is not necessary. Sorry but i don't know how to put an picture into this forum.
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Old 2nd October 2003, 04:50 PM   #6
Stefano is offline Stefano  Italy
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Default The Musical Box

This is the M. B. 30w class A zero-nf amp enginereed by Fabrizio Montanucci on the Suono magazine
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Old 2nd October 2003, 05:22 PM   #7
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Smile Psychic Powers?

"Your schematic look's like an ordinary op amp."

Local degeneration is feedback so I won't argue the no feedback dispute. But......there is no way you can look at this amp and judge it as a classic op amp topology with loop feedback, without knowing the degeneration and load resistor values you have no way of knowing the open loop gain. It looks possible that this amp could use R15 and C7 ll C8 for DC feedback only and establishes the required gain through degneration only without overall AC loop feedback.

"It's really hard to control bipolar output stage without nfb"( turn off time )"

I have seen several amps with bipolar outputs without loop feed back and think that turn off time may really only be an issue inside a negative feedback loop.
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Old 2nd October 2003, 05:45 PM   #8
jam is offline jam  United States
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Fred,

This circuit has some simalarities to the one I sent you. I think if you replace the diffs with jfets you might get away without any dc feedback. Still tlis form of of dc feedback is possibly not a damaging as using an op-amp (servo). I bet you if you could get the time constant low enough ( juggling resistor and cap values) it might be pretty transparent.

I wonder if there is much to be gained by cascodeing the second stage?

Regards,
Jam

P.S. Can some one please post the complete schematic?
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Old 2nd October 2003, 06:09 PM   #9
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Unhappy What Schematics?

You sent me an Email amplifier schematic? I don't remember any amplifier schematics from you...... Are you sure it was to me? Maybe you sent it to Jocko. People get us mixed up some times........
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Old 2nd October 2003, 06:11 PM   #10
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Quote:
People get us mixed up some times........

How come ...... ?

Regards

Charles
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