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DIY Stompboxes => Building your own stompbox => Topic started by: petemoore on August 16, 2004, 11:41:53 PM

Title: Schematic Ex of Jfet used as V/R?
Post by: petemoore on August 16, 2004, 11:41:53 PM
I need a variable resistor that'll vary resistance on a cycle, like for the opamp wah {GEO].
 I just said Jfet because I saw they could be used for this purpose.-
 I need something that'll sweep resistance from about 1k to around 400k, at an adjustable sweep rate...and I don't want to build another wah case... :D
Title: Schematic Ex of Jfet used as V/R?
Post by: mikeb on August 16, 2004, 11:48:48 PM
There's a bunch of phasers that use a FET like this - phase 90 etc. I don't think you'll get a 400k sweep range though .... maybe a LED/LDR pair would be a better bet? A fast LDR would be necessary....

Mike
Title: Schematic Ex of Jfet used as V/R?
Post by: Rich G. on August 17, 2004, 04:54:14 PM
I was thinking the same thing-- that you won't be able to reach a high resistance like 400k by using a FET.  Better off using an LDR.

Reason being the linear ohmic region of a fet is pretty tiny at low current meaning you can't have high Vds and low Ids at the same time.  Se the yellow region of the chart here:
http://graffiti.virgin.net/ljmayes.mal/comp/vcr.htm
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