has anyone tried 4066 as variable resistors?

[brett]

Hi
to control the resistance of a CD4049 or CD4069, you simply need to vary the gate over a small range around the "ON" voltage, which is usually close to Vdd/2.  For a 9.X volt supply (fresh battery), a 10k/(10k+/- a few ohms) voltage divider should work well.
A while ago I designed a compressor/limiter (sounds similar to the one described by Mark Hammer, above).  I was a 386 power op-amp that has a MOSFET to ground in front of it (drain to op-amp input, source to ground).  The gate was connected to the output via an envelope detector which turned the MOSFET on slightly.  The envelope detector was (in order from the output) a rectifier (1 diode), and a cap (22uF) and bleed resistor (?470k) between the control line and ground.  It worked quite well.  It biases it self so that the MOSFET is acting as a variable resistor.  Too much input and output, and the resistance of the MOSFET goes down, bleeding the signal to ground.  When the signal gets bled beyond a certain amount, the output falls and the MOSFET resistance rises and the signal is bled less.

Here's an old thread that discusses it.  Unfortunately the link is dead.  It'll see if I can find it somewhere over the next few days.
http://www.diystompboxes.com/smfforum/index.php?topic=41768.0

cheers

[brett]

Hi again

here's the original MOSFET-as-a-resistor compressor schematic (thanks to puretube for sending me this copy  ):



Note that there are many improvements that you could make to this.  It is intended as a starting point.
In the context of this discussion, I'm fairly sure that you could use 1 MOSFET or inverter (e.g. from a CD4049) as an input buffer, another as the amp (in place of the LM386), another as the variable resistor (in place of the MOSFET), and another as an output buffer.

cheers

[synthmonger]

I spent a good 10 minutes tonight using some 4066 switches as my VCOVR's in a phaser circuit. I gotta say, I wasn't that big of a fan of phasers until now! It sounds very smooth and worked splendidly well. It's still in it's early stages but I do plan on making a modular version and maybe a simple one for a stomp box. Schematic to follow when I get my other work done 


I'll get around to testing the PWM method versus mine. I do use a PWM circuit for my aliaser pedal with some fun results. Adds a lil' extra timbre!

[dschwartz]

Thanks all!!
i think i now understand a lot more. In fact, i´m designing my own PWM (as simple as simple can), and planning to design a 4049 phaser

[Mark Hammer]

I spent a good 10 minutes tonight using some 4066 switches as my VCOVR's in a phaser circuit. I gotta say, I wasn't that big of a fan of phasers until now! It sounds very smooth and worked splendidly well. It's still in it's early stages but I do plan on making a modular version and maybe a simple one for a stomp box. Schematic to follow when I get my other work done 

I'll get around to testing the PWM method versus mine. I do use a PWM circuit for my aliaser pedal with some fun results. Adds a lil' extra timbre!

I look forward to it.  IMHO, one of the properties that makes a phaser pleasing is a "graceful turnaround".  That is, how it negotiates the extremes of the sweep.  That factor is the principle reason one strives to match JFETs in a phaser: so that all control elements are changing their resistance at the same rate/time.  With switched-resistor arrangements, the matching is much easier to achieve.

[dschwartz]

that 386 compressor looks nice.. why did you use a 386 and not an opamp like TL072?

[brett]

Why a 386?
Because someone was raving about them at the time (??Charlie aka Moosa??), and they are a single supply device.  Today, I would go for a TL071 set up for high input impedance (probably non-inverting, with a 1 M bias resistor, and gain of around 100 (?).  Then 0.05Vp-p = potentially 5Vp-p, but half that (2.5Vp-p) would be just starting to turn on the MOSFET and divert the input.  So any signal over 0.05V would be compressed).  Compressing when the MOSFET resistance falls below 1 Mohm is a much more respectible figure than the 50k set by the input resistance of the 386.

Actually, with a couple of (or three) feedback diodes, you could have a combined distortion unit and compressor in one.  Maybe I'll think about that a bit more......

Thanks for the interest in this simple concept.