Moog ladder filter with JFET

[tca]

Hi,
 I've been thinking about building a Moog ladder filter... a JFET version of it. I haven't build it yet but the simulations look promising! Here is the schematic:



And the graphs of the voltages:



Any suggestions/comments how to improve the circuit?

Cheers.

P.S

There was an error in the schematic... but now it is a low pass filter!

[R.G.]

The need or lack thereof for matching in the facing devices in a ladder filter are murky. You may need to match pairs for the ladder. I'm fairly certain the bottom and top pairs need matching. And watch the step from simulator to reality. In most sims, same-part-number devices are identical to the limits of precision of the processor arithmetic, so unless there is some way to mismatch them, you get artificially good (and not obvious!) matching. Mother Nature is not so kind.

Using JFETs gets a little odd. The original worked on the principle of modifying the internal impedance of a BJT by changing its emitter current. JFETs function differently, albeit similarly. The problem I always had with the Moog Ladder was that it took so much voltage to make it work well with any size signal. I imagine that it gets worse with JFETs, as they tend to need even more voltage.

[tca]

Hi RG thanks for your reply (I've been reading some of your posts on a similar topic here on DIYstompboxes).

I do know that the devices should be matched but why do you say that using JFETS gets a little odd. You just have to modify the ladder's capacitor in order to fit the constraints of current that flows in the ladder and use the parameters of the devices that one is using.

"Mother Nature is not so kind" but I'm getting some nice results:



Blue = control voltage
Red =output voltage
Green = input voltage

[TELEFUNKON]

Have you simulated the ladder with LDRs yet?  
(It`s not a "Moog" any longer then, though).

[tca]

"ladder with LDRs"

Could you be more precise? Thanks.

[TELEFUNKON]

"ladder with LDRs"

Could you be more precise? Thanks.

You noticed the four cascaded lowpass stages from south to north? (yes: call it differential or push-pull. because it`s there twice).
Moog replaced the resistors with Transistors. I wondered if you tried simulating the resistors with Light Dependant Resistors (LDRs).

[tca]

ah, ok. Never tried...

[tca]

Here is the final version:



If someone would like to simulate the circuit the tsc file is this.

I'm using TINA-TI for the simulations.

Time for breadboard surfing!

Cheers.