Trouble in voltage controlled low pass filter circuit

Detailer · 2022-10-26T19:39:43.907Z

Hi all, I am building my first synth and I came across this excellent book called Make Analog Synthesizers by Ray Wilson. In the VCLP filter I am having trouble understanding how the jfet will work since a positive voltage is present at Vgs turning the fet into a diode.
Thank you all

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trackerjack · 2022-10-26T19:59:18.539Z

If you ignore the Mod Input for a second, the gate is biased to a negative voltage.

R44 + R40 > R45

In my limited understanding, the jfet turns into a variable resistor…

Edit: … looks like a T-type low-pass. Astonishing how little you can get away with for a basic voltage controlled filter really. Might give it a try on the dusty old breadboard…

Bunker · 2022-10-26T21:39:20.486Z

Welcome to the forum! Silly question but have you had a look over at his site? He gave excellent technical descriptions of his circuits.

I leaned so much there back in the day. RIP Ray.

Detailer · 2022-10-26T22:22:54.089Z

I went to the music of outer space website and the explanation is the same as in the book. As far as I’m concerned, if one ignores the mod input , the voltage at the input of the n-fet transistor will be (R45/(R45+R40+R44))*BP which will be positive. I cant understand how can the voltage at input of a fet can be positive and the transistor act as voltage controlled resistor.
I did not know the sir Ray Wilson passed away , what a legacy , RIP.

trackerjack · 2022-10-27T08:34:40.405Z

No.
BN, BP and virtual ground form a bi-polar supply for the op-amp.

Even if BP is at 12V and BN is 0V (haven’t seen the whole schematic) the gate is biased below the virtual ground which is connected to source.

And yeah, absolutely devastated when Ray passed away a few years ago. Very kind and helpful guy.

Detailer · 2022-10-27T12:04:01.611Z

trackerjack , thank you so much , I did not understand that the source is connected to 4.5 V

Bunker · 2022-10-27T15:35:13.210Z

Looking at the schematic there’s a path to -ve via r38, r39 and r43 that’s in parallel to r45. Fag packet calculation tells me the gate should be at around -3 volts in respect to GND (Although the x14 node may alter this?). The maths assumes the supply rails (BP and BN) are +/-12v as a lot of Rays circuits used 12v instead of 15v rails.

Source is connected directly to GND (not virtual gnd) in the schematic which should be 0v rather than 4.5v.

trackerjack · 2022-10-28T09:20:43.841Z

I’m guessing it’s one of the 9V single-supply / pedal circuits.
So virtual ground at 4.5V checks out.

Bunker · 2022-10-28T15:03:50.811Z

I don’t think it is a single supply looking at the schematic tho? There’s no mention that the non inverting input is fed from a rail splitter etc? And BP and BN connect to the +ve and -ve supply terminals of the op amp so it look s like a standard bipolar set up in which case the non inverting input is tied to GND

I know the LM324 can be used in single supply circuits but in the configuration shown you’d still need to bias the non inv. input to half supply. I’ve had a look on Rays site but I can’t see this specific circuit? If it’s in the book only then maybe the OP could confirm the supply specs?

trackerjack · 2022-10-28T15:18:34.914Z

Perhaps OP can post or link to the complete schematic.
I’m expecting the power section looks like this:

9v_supply

BP and BN designators for Battery Positive/Negative?

Bunker · 2022-10-28T15:20:59.055Z

Yeah that makes more sense if that’s what BP and BN stand for.