variable high resistance

logic level fets

NT

For a job the other week, I'd graphed the LED current versus resistance for a couple of cheap, Silonix, MSL 37V51 opto-resistors, (Farnell part 316-8785).

(rough values) LED current CdS resistance

regards john

That would work, then.

The circuit is as unsophisticated as you can imagine: it's just a variable load for an electromagnetic pickup in an electric bass. The issue is that, because of the high inductance and DC resistance of these pickups, their frequency response and overall tone (including other aspects such as the amount to which the drag of the magnetic field damps the strings' motion, etc.) depends strongly on the load they see.

Electric bassists (and guitarists) like to able to affect their "tone" in various ways via controls on the instrument. Most modern basses either feed the pickups into a passive tone network, or into an opamp-based buffer which then drives an active EQ section of some sort. Neither solution covers the range of desirable tones; some basses have switches to turn the buffering on or off, or some bassists simply use different instruments depending on what sound they want.

Anyway, I'd been wondering whether making the loading continuously variable, rather than just switched on or off, would be musically useful. But I wanted to be able to get the max impedance higher than what a commonly available pot would do - above 1M, the selection of pots gets small. Thus the question.

For the actual project at hand, it looks like there's not much sonic difference when the load goes higher than 1M, meaning I don't need an active solution at all. So it was more just a matter of curiousity - "if I needed this, how would I do it."

Much appreciated, -walter

Thanks, John. That's less current than I thought they would need.

But what's the response time like? ISTR the CdS photocells take a long time to recover to high resistances - I need something that will track pretty close to its final value within 0.1 seconds or less, preferably more like .05 seconds.

There are JFETs with guaranteed pinch-off less than 1.0V available. Just exactly what kind of circuit did you have in mind?- an attenuator?- a VGA?

time

Slow, yes indeed!. Step from 1k to 1M takes 400ms. regards john

How about a semi-digital method?

A1 and A2 are good quality buffering amplifiers. The circuits between X and Y are arranged so that it is non-inverting. When N = 0, the input looks like a 50K. When N is full scale, the input impedance is very high.

If you want a knob the user turns, replace the DAC with a pot the user adjusts.

That was the first thing I tried; but it turned into an oscillator (a 21Hz square wave, as it happened). But I had some other stuff going on in the circuit too, that undoubtedly aggravated the situation - if I need to get this working maybe I'll eliminate the extras and give it another try. Thanks!

I would consider what is called a transconductance amplifier. I do not know how well they do above about 50 kOhms equivalent though. Generalized immitance converters may be able to do this also. I see that you have some reason for not blindly settling on conductive plastic potentiometors.