EA Trem - how does this work?

[PenPen]

Ok, so I've been trying to get a trem pedal design working. I've been dealing mostly with the LFO right now, and my breadboarded LFOs aren't exactly working. So, for the sake of being on the same page, let's look at the EA Trem, since it's LFO is pretty close to what I'm trying to do.

My question is, how does this work? It appears that it SHOULD be causing a voltage swing on the gate of the FET, but I don't get how that could happen, since the 0.47uF cap is blocking it?? How does this LFO do anything at all to the gate of the FET? I figured I would be able to measure voltage changes on the one end (non-ground) of R7, but I'm not seeing anything at all.

[R.G.]

My question is, how does this work? It appears that it SHOULD be causing a voltage swing on the gate of the FET, but I don't get how that could happen, since the 0.47uF cap is blocking it?? How does this LFO do anything at all to the gate of the FET?

The impedance in that circuit is quite high. R7 and the Depth pot are high valued, so the 0.47uF cap with those has a frequency rolloff that is below the sub-audio range, suitable for the LFO voltage to modulate the gate of the FET.

I figured I would be able to measure voltage changes on the one end (non-ground) of R7, but I'm not seeing anything at all.

The gate of that FET is quite a high impedance. Your voltmeter loads down the signal there so you can't see it. It sometimes takes special measurement equipment to measure voltages in a high impedance environment. The 1M of your meter may well not be enough.

[PenPen]

Ah, thank you for the response, RG. My other thought, and I tried this, was to hook my amp to R7, reasoning that if there is anything going on there I should be able to HEAR the pulse, but again I wasn't hearing anything. Is this again the result of the impedance?

Essentially, I want to create the LFO so that it has a higher than usual speed. The problem I anticipate with this is the possiblity that the FET won't be able to change state fast enough to reflect the higher speed, hence I wanted to breadboard the LFO and test the speed of it by itself before hooking it to the gate of the FET, so I could tune the LFO without the interaction of the FET to skew results. Perhaps instead you could point towards which caps in the circuit set these parameters?

[R.G.]

I tried this, was to hook my amp to R7, reasoning that if there is anything going on there I should be able to HEAR the pulse, but again I wasn't hearing anything. Is this again the result of the impedance?

Your amp probably does not have frequency response down at the LFO frequency, and if it did, your speaker would not reproduce it enough for you to hear. it's not surprising that you could not hear it.

Essentially, I want to create the LFO so that it has a higher than usual speed. The problem I anticipate with this is the possiblity that the FET won't be able to change state fast enough to reflect the higher speed, hence I wanted to breadboard the LFO and test the speed of it by itself before hooking it to the gate of the FET, so I could tune the LFO without the interaction of the FET to skew results. Perhaps instead you could point towards which caps in the circuit set these parameters?

The FET will be able to follow the LFO just fine. LDRs are slow, FETs are not.

C5, 6, and 7 are the frequency determining caps. Halving these from 1uF to 0.47uF (closest standard value to half, anyway) will approximately double the frequency of the LFO's entire range. Cutting them to 0.1uF will move the frequency range up by a factor of ten. You must change all three together for predictable results.

[PenPen]

Thank you again, RG. This answers all of my questions and gives me a good starting point.

I had figured LDRs wouldn't be able to respond quick enough, I thought the FET may be able to handle it but wasn't certain. That was half the reason I decided to use that method for my trem instead of an LDR.

Again, thanks for all of the assistance!