Author Topic: Smoothie and other phasers: matching JFETs  (Read 405 times)

dmitrogen47

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Smoothie and other phasers: matching JFETs
« on: December 12, 2018, 02:35:21 AM »
Greetings everyone,

Could somebody please give some advice on matching JFETs for phaser circuits? I'm currently looking to build a Smoothie.
From what I understand, I need to match them by cutoff voltage.
I'll build myself a JFET matcher (saw a schematic somewhere on this forum, it used an op amp, half a dozen resistors and a capacitor or two), but how do I use it?
My plan is to visit my town's parts store and ask them to give me all the 2N5457s they have, and then spend a bit of time matching them.
Reason for this is that 2N5457s cost quite a bunch compared to other transistors, and I would like to avoid buying 50 of them just for one circuit that uses 2.

To be more specific, my questions are:

1. Is it possible to power the JFET matcher with a standard 9V battery?
2. Is the current draw low enough for battery to be a good power option?
3. Will powering the matcher with the battery give sufficiently consistent/accurate readings?
4. What kind of cutoff voltage reading should I expect? Will the regular DMM I use for other pedal building needs suffice?
5. How closely do the cutoff voltages must match? Does the number have to be exact, or just within some % tolerance?
6. I understand the JFETs need to be matched for phasers, but what happens if they are not? Does the circuit not work at all, or is the sound produced by it bad in any way?

Thanks in advance!

Soup39

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Re: Smoothie and other phasers: matching JFETs
« Reply #1 on: January 15, 2019, 12:57:14 PM »
I think I have a spare Jfet matcher etch board I could throw your way if that helps any.  The other option would be to buy a matched pair from guitarpcb or other sources.

alanp

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Re: Smoothie and other phasers: matching JFETs
« Reply #2 on: January 15, 2019, 10:10:50 PM »
6. I understand the JFETs need to be matched for phasers, but what happens if they are not? Does the circuit not work at all, or is the sound produced by it bad in any way?

The various allpass cells that cause phasing to happen will not move in sync, and the phasing will be quite disappointing.
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