When to use matched JFET pairs?

[richard44]

I'm currently building some distortion circuits, and have a few question about when to use "matched pairs" of JFETs. I've read in various places about how using matched JFET pairs is beneficial. Usually one reads about these matched pairs in phaser circuits where the JFETs have to turn on and off together and you need them in a similiar range to do so.

First question -
One of the circuits I am building is the internet-known Dr Boogie circuit, where there are discreet gain stages in series. Would using matched pairs make any sense here? I bought and measured the Vgs of 80 J201s. They vary quite a bit. Should I use similarly rated ones, or is it possible to use ones that are quite dissimilar from each other in each separate stage? (FYI - this circuit includes trimpots on the drains of the JFets in each stage.)

Second question -
There are some distortion circuits, like the BSIAB2 and the Wampler Pinnacle, in which a mu-amp configuration is used - i.e., two JFETs together in a block configuration. Would using matched pairs make sense here within each block? If so, would value differences matter from matched pair to matched pair? I believe those circuits use different types of Jfets, i.e., a pair of J201s, then a pair of 2N5457s. So if I used a matched pair of J201s, their values might be very different than the matched pairs of the 2N5457s which have a different spec. I guess that's unavoidable. (Again FYI - these circuits tend NOT to have trimpots on the JFET pairs).

I hope these questions are clearly stated and someone can share some insights. Thanks.

[Ben N]

I've never heard of matching jfets in distortions, only in phase shifters, FWIW.

[richard44]

I've never heard of matching jfets in distortions, only in phase shifters, FWIW.

Well Ben, it seems you're the only one who had anything to say on the matter!
I landed up socketing the transistors so I could swap them out if need be. I put in 5 J201's which had measued close to each other, but I did not use my tightest groupings. The result was fantastic! The DR Boogie had a usuable gain all the way to 10. (Most videos I've seen of this pedal sound too fuzzy past 5). I'm not sure if it is the fact that I used a grouping of J201's, or it was which grouping I used (the range varied by a factor of 2 or more). If/when I have time, I might pull those JFETs and swap other groupings just to experiment.

[The G]

I'm currently building some distortion circuits, and have a few question about when to use "matched pairs" of JFETs. I've read in various places about how using matched JFET pairs is beneficial. Usually one reads about these matched pairs in phaser circuits where the JFETs have to turn on and off together and you need them in a similiar range to do so.

First question -
One of the circuits I am building is the internet-known Dr Boogie circuit, where there are discreet gain stages in series. Would using matched pairs make any sense here? I bought and measured the Vgs of 80 J201s. They vary quite a bit. Should I use similarly rated ones, or is it possible to use ones that are quite dissimilar from each other in each separate stage? (FYI - this circuit includes trimpots on the drains of the JFets in each stage.)

Second question -
There are some distortion circuits, like the BSIAB2 and the Wampler Pinnacle, in which a mu-amp configuration is used - i.e., two JFETs together in a block configuration. Would using matched pairs make sense here within each block? If so, would value differences matter from matched pair to matched pair? I believe those circuits use different types of Jfets, i.e., a pair of J201s, then a pair of 2N5457s. So if I used a matched pair of J201s, their values might be very different than the matched pairs of the 2N5457s which have a different spec. I guess that's unavoidable. (Again FYI - these circuits tend NOT to have trimpots on the JFET pairs).

I hope these questions are clearly stated and someone can share some insights. Thanks.

First question: in Dr. Boogie you don't have any perceptible benefit from matching JFETs, from a DIYer's point-of-view. If you had a business, it kinda make sense to match them and then use the same biasing circuits (not trimpots needed after you find the right bias network). And of course, the psychological benefit of knowing that your JFETs push the same way . Different transistors will lead to different bias circuits, different gains and different frequency characteristics. But there is no such thing as a standard Dr. Boogie, so every version is good to go. And if you want to use something like matched J201s instead of not matched J201s, you'll struggle to hear any difference at all, as long as the bias is right.

Second question: you might want to read R.G.'s Foolin' with FETs for some analysis. You'll also read there the reason why matching the transistors in mu-amps makes no difference.

Finally, a breadboard is a wonderful tool for these kind of experiments.

One reason for matching that crosses my mind would be to put the JFETs in parallel, the way Erno Borbely did it. Or to make a differential amplifier, although I don't know if it's any good for overdriving guitar signal. There are some articles about it on Linear System's literature page (look into Aplication Articles).

And you're right, FSB seems awfully quiet these days.

[richard44]

Thanks so much for the insights. The Dr Boogie was my first transistor build, so I am still digesting the transistor theory. Your response helps clarify things. (Previously I did a few builds with opamps and diodes, so I'm pretty clear on all that). Looking forward now to a muamp circuit. And yes, to the breadboard! Cheers.