Tremolo control - input signal seems to be riding the output

[atferrari]

LTSPICE - .asc file, model and sub attached.

In this basic tremolo control, I cannot avoid what seems to be the input signal riding the output.

As is, even if the JFet is cut (??) I still get the equivalent of the input signal, at the output.

Sure it is more than 100% modulation; done on purpose to put in evidence that the JFet is cut.

Around this for many hours, I supose I am wrongly biasing the opamp but cannot see where or how.

Comments will be appreciated.

 

[crutschow]

What exactly do you want the output to look like?

As the FET is configured the U2 op amp gain can vary from 1 (follower) to about 6.3 with the FET going from off to full conduction.
It doesn't sound like that's what you wanted.

Edit: If you want the output to go full-off then add a 10kΩ resistor in series with the + input to U2 and connect the drain of J1 to the junction of that resistor and the + input.
Connect R5 to Vref.

 

[atferrari]

What exactly do you want the output to look like?

As the FET is configured the U2 op amp gain can vary from 1 (follower) to about 6.3 with the FET going from off to full conduction.
It doesn't sound like that's what you wanted.

Edit: If you want the output to go full-off then add a 10kΩ resistor in series with the + input to U2 and connect the drain of J1 to the junction of that resistor and the + input.
Connect R5 to Vref.

My bad; at a certain point is a voltage follower. Yes!

I want the output following the modulation imposed by the signal at the gate, as you say, output full-off.

Not sure I understand your proposed changes (much less why). I will try to work it out.

Gracias Carl!

/Edit to add:

No time yet to play with LTSpice but what you are suggesting is to stop messing with the gain setting and implementing an attenuator for the input. Am I right?

Edit/

 

[crutschow]

My bad; at a certain point is a voltage follower. Yes!

I want the output following the modulation imposed by the signal at the gate, as you say, output full-off.

Not sure I understand your proposed changes (much less why). I will try to work it out.

Having an output gain of one as stated in your first sentence doesn't seem compatible with your second requirement of the "output full-off".

My change in the circuit was to allow reducing the signal gain to zero to get that full-off.