Lectric fx STFU (DOD 230) Noise Gate Question

[idy]

I recently built a noise gate on the PCB by lectric-fx, the STFU. It is a component for component copy of the DOD 230 (with one change I can't quite figure out) and works fine... with a few questions.

I used a green LED and a photocell in place of the vactrol, selecting for on resistance under 5k and dark over 5m. (He suggested under 10k and over 1M).

Lectric adds a trimmer to the second opamp stage to make allow level to be adjusted. I had to sub the suggested 25k up to 250k to get unity. 100k wasn't quite enough...

The real question has to do with the first opamp stage. The original has a non-inverting buffer, the output and the inverting input connected. The STFU has...a 1M resistor in the feedback loop! The input resistor is 10k, so I am thinking this buffer has been changed to a gain of 100? No way! I am putting a boosted guitar signal in and it isn't clipping, so the gain can't be that high, no? I tried jumpering the resistor and sure enough the level drops a bit, but is this really a gain of 100? Why not?

It's been a while, I'll have to figure out the image posting thing again to get the schematics up...

[Scruffie]

The reason you had to use a much larger trimmer was your LDR, the output is an inverting gain set up and your LDRs on resistance with the LED lit up must have been too high. It says to experiment so try some different LED's and LDR's.

The input opamp is still a non-inverting buffer, there's nothing hanging off the inverting input to set the gain of that 1M resistor, if you look at gut shots of the original unit you'll see that resistor is there.

[idy]

 Thanks!
I'm happy to know the schematic is the issue and the stfu is "normal...
And I'm willing to believe that tinkering with the lot/led will smarten things up...
But I stick at the op amp...
The co figs I know are:
Non invert buffer: out shorted to - in.
Invert buffer:- input resistor = feedback loop resistor, + input to ground/vb.
Invert gain: this one has the "leg" from the feedback resistor (and - in) to ground/vb, a voltage divider.
Non invert gain: ratio between input resistor and feedback r is gain.

This circuit is the last configuration, no? Not a buffer....?

[Scruffie]

You have inverting and non-inverting gain set ups mixed up.

It's a buffer, the 1M between the inverting input and output is a hold over from old opamps.

[idy]

Thank you.