I am looking at a schematic of an amplifier system featuring what I think is some kind of feedback / damping loop on the speakers woofer. I've attached the schematic showing the "damping resistor". I am wondering if someone who is able to read the diagram accurately could just describe how this system works and what its function is. I have highlighted the section i'd like to know about which starts from the negative terminal of the loudspeaker. The application is part of an active loudspeaker unit.

I believe the function might be an attempt reduce distortion?

Edit: now sorted out the schematic Schematic

  • \$\begingroup\$ No schematic as far as I can tell. It's not rocket science embedding one. \$\endgroup\$
    – Andy aka
    Commented Sep 24, 2016 at 23:52
  • \$\begingroup\$ Schematic has been edited out of the question. Whether rolling it back will restore it or the linked image has gone too, I can't say. Bad form, downvoted until it's restored. \$\endgroup\$
    – user16324
    Commented Sep 25, 2016 at 1:07
  • 1
    \$\begingroup\$ R75 looks to be a current sensing setup. I would guess that the idea is to limit the power output if too much current goes through the speaker. But, just guessing so no answer. \$\endgroup\$
    – JRE
    Commented Oct 19, 2016 at 15:26
  • \$\begingroup\$ Looks like the highlighted section monitors the current through R75, and reduces the input as needed to compensate if the current through the speaker is too high. \$\endgroup\$ Commented Oct 19, 2016 at 15:27
  • \$\begingroup\$ Rolled it back. It made no sense. If the question needs deletion, the OP should flag for moderator attention \$\endgroup\$ Commented Dec 30, 2018 at 22:03

1 Answer 1


R75 senses the speaker current and the bit in yellow essentially provides a variable (R2) amount of positive feedback from the load current to raise the output voltage with the load current, by injecting current into the inverting input of U3A.

Essentially this can be thought of as giving the amplifier a negative output impedance (Output voltage rises with load current), intended to cancel the drivers winding resistance and thereby raise the damping factor of the system.

It is almost but not exactly a current drive arrangement.

I would note that turning the damping control up too far could potentially compromise amplifier loop stability.

SW1 disables this loop by dropping the gain of U3B to zero leaving the thing setup as a conventional voltage amplifier with a little additional output resistance.


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