Mystery Circuit of the Day #1

[nickb]

Here's a puzzle.

As seen in a ADA MP-1 MIDI controlled preamp is this odd arrangement. The opamp with pins 1,2 & 3 serves to linearize the LDR U5, but what does U9 section with pins 5,6 & 7 do? As far as I can see it's output is stuck low, always. Doesn't make much sense, or does it?

[Reader]

My guess is that it does nothing and is just an unused section of the IC. Perhaps the designer configured it that way to keep it from oscillating or otherwise misbehaving.

[Gingertube]

Power on mute/soft start?
Cheers,
Ian

[loudthud]

Power on mute/soft start?
Cheers,
Ian

I'm thinking this way with the feedback to the positive input providing hysteresis.

[nevetslab]

I'd go along with that too ^^^^^^^. The positive control voltage input looks like it would drive U3A negative, turning off the diode in the VacTEC. Odd circuit.....

[Mike Sulzer]

intended as a current source?

[Tony Bones]

It looks like a negative impedance converter to me, but I don't understand what function it provides.

Think of it as giving R93 a negative resistance. Without that op-amp block, if we raise the voltage at the right end of R93, then current would flow to the left as expected. But with the the circuit, raising the voltage at the right end of R93 causes current to flow to the right, as though R93 had negative resistance. Obviously, this can be accomplished only by raising the voltage at the left end of R93 even more. (There was a time when I could have explained how it works. Today I'll ask you to use Google, but I remember that that's what it does.)

Anyway, I notice that the circuit not only drives the left end of R93, but also runs up to U5 which is some kind of optoisolator? Or maybe U5 is driving it? I don't know, but it looks like a negative impedance converter to me.

[nickb]

Thanks for all the clever ideas

I think the award goes to Tony Bones. I believe the purpose is to ensure that the LED is switched fully OFF when the control input gets high enough. Without Tony's suggestion, I doubt I'd ever have seen it.

PS: Tony take your award

[Tony Bones]

PS: Tony take your award

[ATTACH=CONFIG]50895[/ATTACH]

Thanks for the hat!