2 JFET used as a variable POT with an LFO

[richon]

hi,

finally i bought a BREADBOARD  , and started test an idea I have:

first a senoidal LFO based on the LFO of the EA Tremolo, then changed the 1uF caps to 2,2uf and later to 4,7uF to have 0,6Hz - 4Hz LFO speed.

Then i want to "simulate" a Voltage divider (usually called "POT"  ) with 2 JFETs controled by this LFO....  to make a LFO controlled VOLUME control.

by using the DRAIN as input signal, SOURCE as output signal and GATE as the LFO input....   would the JFET change his value, not change the quality of sound, and act as a potentiometer in my idea?


the final idea is to use 2 or 3 of this LFO regulated "pots" to mix a few effect in a "crazy-hendrix-solo" way


did you get it?

[stm]

In Google search for "AN105 siliconix".  First result is a pdf document entitled "FETs as Voltage Controlled Resistors".  Very useful info on the theory behind and many example circuits.

Later you may want to try "AN101 siliconix", "AN102...", etc.  This is a series of quite interesting application notes on FETs.

[stm]

There is some more information here:
http://www.diystompboxes.com/smfforum/index.php?topic=39145.0

Also, it would be ideal if you can find the application note AN-129 "A linear multiple gain controlled amplifier" by National Semiconductor.  I don't seem to find it online though.

[Lurco]

http://headfonz.rutgers.edu/FET-gain.pdf

[PRR]

> controlled VOLUME control.

Note that you do NOT need a voltage divider to make a volume control. A divider is an excellent and low-loss connection for a pot between a low source impedance and a high load impedance. But most other resistive gizmos are not natural voltage dividers. Complexity more than doubles.

You can get a good range of volume with a 10K resistor for the top of the divider and your variable gizmo to ground. The in and out impedances vary with setting, and loss is never zero. But pre/post amplifiers can make these factors non-issues.

> "simulate" a Voltage divider ... with 2 JFETs

The usual stumbling-block is that a FET is a "resistor" only for SMALL voltages. Often less than 0.1V. You get a not-very-wide window between universal hiss and FET distortion.

Another issue is that the control voltage is Gate relative to the end of the channel (Source-Drain). In divider conection, the channel is bopping up and down with signal, different amounts at three nodes. It is possible to compensate this; but messy.

While it is more up-front work, and more power, LED photoresistors give higher level with less distortion and complete isolation between control and signal.

> http://headfonz.rutgers.edu/FET-gain.pdf

5 MegaByte PDF file. I wish it were better quality; I wish National would re-issue it with appropriate notes about obsolete parts and pointers to sexy new parts and design concepts.