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How to match FETs in a phaser?

8/5/1999 2:51 AM
How to match FETs in a phaser?
Rob ( asked:  
"I've decided to make some effects, including a phaser. I saw on RG's site a schem for matching the FETs, but it doesn't say how to use it. Can anyone explain what to measure, and why? What exactly is being matched?"
What's being matched is the voltage between gate and source for a specific drain/source resistance. The JFETs in a phaser are all fed identical gate-source voltages from the biasing arrangement and the LFO. The drain source resistance that is controlled by that voltage is what causes the phasing. If some FETs are full on (low resistance) while others are full off (high resistance) while others are in the moderate resistance region, the ones that are full on or off are not contributing to the phasing.  
The D-S resistances don't have to be identical, but they do have to be close enough that all stages contribute to the phasing. I have made up matched sets of four and eight with the matching fixture shown. These were subbed into an MXR P90 and did sound good there, so it works.  
Another gotcha in the 9V powered ones like the P90 is that there isn't much voltage there to drive the JFET gates, so the JFETs have to have a low threshold voltage. Some JFETs have threshold voltages as high as 5-9V, so they can never work properly in a 9V circuit. The original P90 devices were 2N5292, which were especially low Vgsth. 2N5484 and 2N5485 work if you select properly.  
I bought a batch of 100 to make up my sets and got a number of sets out of that. I still have some if you're interested.  
"Can anyone suggest a good/easy chorus circuit to make? I can't seem to find many around."
There aren't any that are both good and easy, near as I can tell. The MXR micro chorus is close. I keep intending to do a flanger/chorus PCB, just haven't had time.
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