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 stage…(I divided it because the image is too heavy)
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 stage…(I divided it because the image is too heavy)
Attachments
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.
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.
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.
"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.
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?
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?
Attachments
Excuse my late, but I can post one picture a time, and because I'm new "under moderation"...so I can't reply immediatly!
Some measures of this amplifier (taken from Suono Magazine)
- frequency response 3,3 hz-75 khz within 1 db
- slew-rate 50V/uS
- output impedance 65 mohm at 100hz, 65 mohm at 10 khz
- power 33W/8R, 59W/4R, 90W/2R, 120W/1R, no stabilized supply
I think not so bed, with my massive power supply, it performs better on the low loads...
This is the parts list:
Some measures of this amplifier (taken from Suono Magazine)
- frequency response 3,3 hz-75 khz within 1 db
- slew-rate 50V/uS
- output impedance 65 mohm at 100hz, 65 mohm at 10 khz
- power 33W/8R, 59W/4R, 90W/2R, 120W/1R, no stabilized supply
I think not so bed, with my massive power supply, it performs better on the low loads...
This is the parts list:
Attachments
- Status
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