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Old 13th September 2016, 09:12 PM   #1
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Default Op-Amp Circuit Components: Any Ideas About This Circuit Section's Function?

This is from a car audio amplifier schematic (Alpine MRP-F450), but I'm hoping you guys have seen this before. I haven't.

It's in the line-level input (isolation [ground loop breaking]) front stages.

In the schematic section highlighted, there is a diode bridge connected to the dual rail supply voltages. However, I've never seen this before and have not had any luck finding a technical explanation anywhere.

I'm assuming it's maintaining the virtual ground as the op amps are connected in such a manner, and marked with "0V". However I don't see why this would be beneficial or necessary.

Ideas? Comments?
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Old 13th September 2016, 10:19 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by MartyM View Post
In the schematic section highlighted, there is a diode bridge connected to the dual rail supply voltages. However, I've never seen this before and have not had any luck finding a technical explanation anywhere.
That's a clamp (actually two clamps, one for each op amp), not a diode bridge.
They prevent the voltages at those two nodes (+ input of the op amps) from exceeding
either of the two voltages connected to the ends (+/- 14V). Each clamp works
in conjunction with its 560R.

Last edited by rayma; 13th September 2016 at 10:25 PM.
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Old 13th September 2016, 10:25 PM   #3
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Ok, that's what I was thinking but given it's line-level, and speaker level inputs are in another section, it seemed odd for an op-amp circuit to have that. I'm not unfamiliar with clamp circuits but I don't recall seeing it other similar designs.

Thanks for the response.

Last edited by Travis Clarke; 13th September 2016 at 10:27 PM.
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Old 13th September 2016, 10:28 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by MartyM View Post
Ok, that makes sense and was what I was thinking but given it's line-level, and
speaker level inputs are in another section, it seemed odd for an op-amp circuit.
Some op amps are very unhappy if the input voltage exceeds the supply voltage, hence the clamp,
which keeps the input within 0.6V of the supply.
http://www.analog.com/library/analog...amp-input.html
http://www.analog.com/library/analog...46-02/ovp.html

Last edited by rayma; 13th September 2016 at 10:31 PM.
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Old 14th September 2016, 02:26 PM   #5
AndrewT is offline AndrewT  Scotland
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Some opamps have the diode clamps inside the package.
With all these clamps (internal and external) one must use preceding impedance to protect the diodes from damage if there is ever an event that requires clamping.

You can also use diode clamps between the +IN and -IN

For best performance these diode clamps should pass zero current when not triggered. Some transistors wired in diode mode perform better than real diodes in this clamping duty.
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Last edited by AndrewT; 14th September 2016 at 02:29 PM.
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Old 14th September 2016, 02:30 PM   #6
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I think I see what's going on in this case. The speaker level inputs are connected to the same line level input paths (scaled via resistors) and the clamps are to prevent excessively high speaker level voltages, not the line level.

Yes, I understand the clamp resistors are needed.
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