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Old 26th January 2012, 02:59 PM   #1
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Default discrete op amp

Does anyone have any suggestions on improving this discrete op amp circuit? I know it's old and probably considered primitive today, but I thought I'd ask anyway. I don't have the component values yet, sorry. It's basically a differential pair driving a single ended output wherein the output transistor has a constant current load. One improvement I can see is to put a small capacitor in parallel with R6 to provide a high frequency pole. I probably won't run it at +/- 32 volts though.

I was going to use it only as a buffer in a preamp.

Yeah, I know, Jim's Audio. If you hate them, please keep it to yourself.
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Old 26th January 2012, 03:10 PM   #2
raul_77 is offline raul_77  Europe
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Hi,

Not improve your eschematic, but you can find in this web page some other discrete opamps. This are also easy and cheap:

VU-Meter

Happy days
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Old 26th January 2012, 03:30 PM   #3
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Hi,

Not improve your eschematic, but you can find in this web page some other discrete opamps. This are also easy and cheap:

VU-Meter

Happy days
Thanks. very interesting.

I think maybe I don't need to bypass R6 because of C4 and C6. I think C4 is called a "Miller" capacitor, but I could be wrong.
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Old 26th January 2012, 04:21 PM   #4
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I donīt like the output via transistor collector, too high impedance point to put a long wire, and the CCS, mmmhh, I would prefeer emitter follower instead, and if possible, bootstrapped to increased linearity and gain.
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Old 26th January 2012, 04:27 PM   #5
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Originally Posted by Osvaldo de Banfield View Post
I donīt like the output via transistor collector, too high impedance point to put a long wire, and the CCS, mmmhh, I would prefeer emitter follower instead, and if possible, bootstrapped to increased linearity and gain.
Yeah, it's not ideal, I agree. I'd prefer a push pull arrangement on the output, but I already bought the boards.

The preamp I have in mind would have to drive two outputs at the same time. One to a balanced output via a transformer, the other to a single ended output. I was going to use this as a buffer between the preamp and the single ended output. I don't have the capability to make my own circuit boards or design my own circuit.

When I first glanced at it before buying, I thought it was an emitter follower but after studying it some more I realize I was incorrect. Oh well, what ever.
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Old 26th January 2012, 05:30 PM   #6
DF96 is offline DF96  England
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C6 is a Miller capacitor. C4 appears to be injecting noise from the negative rail, maybe as some sort of cancellation technique? Very crude circuit, high distortion, high output impedance; you might be better off using a 741!
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Old 26th January 2012, 05:41 PM   #7
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In my opinion, high impedance to drive a transformer, you will have a lots of problems like hum, resonances, high end cut, etc. I would prefer some push pull low Z driver.
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Old 26th January 2012, 05:44 PM   #8
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In my opinion, high impedance to drive a transformer, you will have a lots of problems like hum, resonances, high end cut, etc. I would prefer some push pull low Z driver.

This isn't driving a transformer. Sorry for the misunderstanding. I have a good low impedance circuit for that. This drives the subwoofer single ended outputs only. So, I don't need high bandwidth. It's just going to be connected to a pair of RCA connectors.
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Old 26th January 2012, 05:53 PM   #9
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C6 is a Miller capacitor. C4 appears to be injecting noise from the negative rail, maybe as some sort of cancellation technique? Very crude circuit, high distortion, high output impedance; you might be better off using a 741!
Well, not to throw stones, but the Pass B1 buffer isn't exactly complex and elegant.

I suppose I can delete the C4 capacitor. Offhand I'd say it shunts HF to -32V rail, but what do I know. It's 5pF from what I can gather by looking at the board.
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Old 26th January 2012, 06:09 PM   #10
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... Sorry for the misunderstanding. .
Donīt worry.
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