Replace a transistor with an op amp?

[ViolenceOnTheRadio]

Ok so here's the schematic for the active bandpass filter I'll be using.
My question is, can I swap the transistor with a simple op amp multiplier?



I was thinking if instead use an op amp as a multiplier, I can then use a gain knob to boost the strength of midrange shunt to ground or passed through by the circuits level control.
Is my hypothesis correct?

[teemuk]

If you have a way of making the OpAmp a voltage-controlled switch then yes. Otherwise, no.

Despite the FET being there that filter is still passive design. The JFET is just a remote controlled switch that either presents low enough series impedance to effectively ground the circuit, toggling the T-notch filter, or alternatively presents high enough series impedance to essentially lift the circuit off ground, turning the filter off.

[ViolenceOnTheRadio]

Does this mean I can eliminate the Jfet, run the pot directly to ground and simply wire in a bypass switch to engage/disengage the filter?
I'm going to go out on a limb here based on what I know about the amp and assume it's just used to turn the filter off.

Thanks for the explanation though, I had trouble making sense of what I thought was amplifying the signal directly to ground.
Now since I made that assumption, would using an active bandpass filter with say a gain of 25 X outputted to ground indeed increase the level of that bandwidth pulled out of the signal?
I'm intrigued by the possibility of a gain control giving me a wide range of dB cut from a fixed band.

[ViolenceOnTheRadio]

Never let it be said I failed to beat a dead horse...but maybe this question will kill 2 birds with one stone

This filter is controlled by a foot switch.
Am I correct in assuming that this is the true purpose of using the Jfet to engage/disengage the circuit since the length of the cable would be a huge power drain if it were simply wired to the bypass switch?

[pjwhite]

You could replace the JFET with a switch, but putting the switch at the end of a long cable would not be a good idea.

The JFET is not there on account of any power consumption considerations, it is there to isolate the audio circuit from the digital switching circuit (footswitch).  The long length of cable connecting the footswitch is likely to pick up all kind of noise and hum.  But by using the JFET as a saturated switch, this noise is not passed through to the filter circuit.

If you wanted to replace the JFET with something else, I would suggest a relay, located close to the filter circuit.

[amptramp]

Maybe an open-collector comparator would do the job - something like an LM139.  Just take one input ot half voltage and pull the other one up through a resistor that can be grounded by a remote switch.  Depending on which input you used, it could be normally open or normally closed.

[Ice-9]

The FEt is indeed there to work as a switch, it can be removed and a spst switch used as a replacement but as said in another post, not on the end of a long run of cable. This circuit snippet looks like it might be from a Marshall amp.