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Old 1st April 2011, 06:03 PM   #1
neazoi is offline neazoi  Greece
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Default Which point to connect the global feedback?

hello,
Here are two schematics.
I would like to use the circuit on the left as a driver stage to drive the power amplifier displayed in the circuit on the right. i.e I would like to replace the opamp section and keep the power amplifier section.

The amplifier on the right circuit has a feedback loop which goes back to the opamp positive input. Where should I connect the feedbac wire if using the opamp circuit on the left?

Also what if I just do not use feedback at all?
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Old 1st April 2011, 06:16 PM   #2
lineup is offline lineup  Sweden
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The feedback resistors are R20 and R42.
They provide feedback from op output to inverted input pins.

I dont know if you can do what you want.
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Old 1st April 2011, 06:16 PM   #3
Mooly is offline Mooly  United Kingdom
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It's not quite that easy.

Firstly though... the opamp on the right. What type is it as I see pin 8 with no DC path and unconventional pinouts ?

To use the opamps on the left would be a major redesign due to DC biasing needed for the power amp stage.

Why do you want to do this ?
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Old 1st April 2011, 06:20 PM   #4
neazoi is offline neazoi  Greece
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The amplifier on the left uses the ne5532 which is a great cheap opamp. the amplifier on the right uses an old opamp lm387. This is the only reason actually.
can I just disconnect the feedback wire that goes from the output of the amplifier to the opamp?

Any alternatives? for the power amplifier?
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Old 1st April 2011, 06:57 PM   #5
Mooly is offline Mooly  United Kingdom
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The power amp stage has no voltage gain on its own, just a gain of approx 1. Also the input to the stage needs to be around 6 volts.

If you disconnect the feedback the amp won't work at all. It's all tied in with the biasing to get the correct DC conditions.
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Old 1st April 2011, 07:00 PM   #6
Mooly is offline Mooly  United Kingdom
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Also the connections look wrong... the feedback is positive as drawn. The two opamp inputs are reversed.
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Old 1st April 2011, 07:02 PM   #7
Mooly is offline Mooly  United Kingdom
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I guess you could rig up a 5532 and arrange midpoint biasing to the non inverting input and retain the feedback network although it would then have to be AC coupled to ground.

Do you know how to do that ?
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Old 1st April 2011, 07:44 PM   #8
neazoi is offline neazoi  Greece
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Quote:
Originally Posted by Mooly View Post
I guess you could rig up a 5532 and arrange midpoint biasing to the non inverting input and retain the feedback network although it would then have to be AC coupled to ground.

Do you know how to do that ?
Unfortunately no
I am posting the full opamp circuit for your reference. it is from a magazine, the author has provided a photo of the working machine too.
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Old 1st April 2011, 07:55 PM   #9
CBS240 is offline CBS240  United States
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Hi

My opinion is you should start from the top with something simple along the lines of the idea you seem to want to implement. Here in post 26 is a simple starting point circuit using the Op amp front end and power follower. Bi-polar supply will eliminate the output cap.
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Old 2nd April 2011, 07:49 AM   #10
Mooly is offline Mooly  United Kingdom
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This should allow a 5532 to replace the LM387. Make sure to get the pin outs correct.

To do it any other way such as eliminating that opamp altogether and using just those on the left would be too big a redesign as the L.C.R. filters would have to be incorporated while maintaining the original response.
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