Friday, 27 July 2012

JFET Matcher

This should be a useful little circuit for a few people and will allow you to match JFETs VGS (or more precisely, the voltage at which RDS is 10K) when required in some phasers etc.  Based on the information posted by RG here which should explain how everything works. 

Compact version with onboard sockets suitable for both DSG and DGS JFETs.



This version with separately mounted sockets for JFETs.

38 comments:

  1. yessssss! thank you. I'm going to build this later on tonight.

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    1. Bear in mind when you build it, it's worth setting up a second set of sockets for DGS pin layouts. You can just daisy chain from the sockets shown so then you should be able to test the vast majority of different JFETs. My idea was to use a plastic box to house this and have the two sets of sockets mounted on the outside of the box and then maybe have banana sockets mounted to one side for you to connect up your multimeter. Then everything is kept neat and tidy and you have external access to everything you need.

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  2. verified, works great, thanks!

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  3. It's 4AM and i can also say that this is verified. I added two rows and put a socket in for the transistor. I plan to use this just on the breadboard for now, i used two wires with a loop bent in the end to place my dmm test leads in. When i test the circuit with no transistor, i get a reading of +3.8V, i was just wondering if that sounds accurate to you. i had to use a 33uF capacitor because i can't find my 47uF's. Here is a pic i took with my cell phone, it's not the best quality. http://i59.photobucket.com/albums/g304/jokersmile32/jfettest.jpg

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    1. Yes that sounds fine and the 33uF cap is fine too, RG said you could use anything from 10u to 47u.

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    2. I've added a version with onboard sockets above for people like you who want to keep everying together on the board. I've also included extra sockets so both DSG and DGS JFETs can be measured.

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  4. Hey Mark, could I use LM741 instead TL071?

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  5. Hi Mark
    Any chance of explaining how to use this tester to me as a newbie...what i set my meter to, what im looking for etc...I looked on the suggested page from the post up a few places...but it dont actually tell me how to go about testing the components, well if it does, it lost me pretty early doors to be honest..on the greatly improved matcher layout it says for vp measurement..you have to " Negate " reading to obtain actual Vp, what does that mean ? sorry to be a complete knob but im trying to learn and need some of your experience please

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  6. No probs..made it up..tested my jfets as best i could ...paired up values that were identical or v close..so i guess ive worked it out myself :0)

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  7. This may sound a little bit stupid, but how do you know which pins you connect? I see what I need to put in which place, but what do I need to solder?

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    1. This comment has been removed by the author.

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    2. Each horizontal line has its pins connected together right?

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    3. They're links, so only the top and bottom where the blue dots are.

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    4. And what are those red squares with red dots inside them?

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    5. Track cuts. This may be useful for you:
      http://tagboardeffects.blogspot.com/2012/04/vero-build-guide.html

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    6. So they're just breaks in connections. I have a board when every hole is separated, so I dont need to drill and cut anything, but I need to solder all those horizontal connections + jumpers of course etc.

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    7. Yes, that's perfboard not stripboard. All these layouts are based on stripboard because I think it's simpler to break the tracks than create them. Having said that, some things work very well with perf because you can take advantage of vertical as well as horizontal lines if you're creating your own tracks. But I stick to stripboard just to keep a common form.

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    8. I cant get it to work, my DMM shows voltage of 0.02 or smth... im so resigned, I already tried 2 jfet matchers and cant any of them to work...

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    9. What do you have your DMM set to?

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    10. Its set OK, it shows 9V on battery or power supply.

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    11. It also shows 4.5V on every resistor and 9V between 4 and 7 pin of TL072.

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  9. 4.5V voltage on resistors is shown when my 2N5952 is in, without it there's 4.5V on only one resistor. Can you help me?

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    1. I've inserted JRC 386 and it shows around 5.25V when there is no transistor, but when I plug 2N5952 it shows on all off them the voltage of 4.10V, while the max for them is 3.5V.

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    2. OK hold on, I just spotted that I soldered capacitor in wrong direction. Clever, eh?

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    3. Still nothing. 0.02V on DMM...

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    4. I don't think you can use a JRC386 for this... that's an audio amp. You want a single channel op amp like a TL071.

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    5. it doesnt work with TL072 either.

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  10. it doesnt work with TL072 either.

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    1. Tl072 is a dual opamp, you need a single opamp. As a general guide single opamps usually end with a 1 (tl071 etc) dual opamps end 2 (tl072 etc) and quad opamps end 4 (tl074 etch)

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    2. Nooooooo waaaaaaay, I spend like 6 hours today working on this matcher... Thanks for your reply, gonna check it tomorrow when I get my hands on some single channel opamps :)

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    3. Or you could always build the improved JFET matcher:

      http://tagboardeffects.blogspot.co.uk/2012/07/greatly-improved-jfet-matcher.html

      which doesn't use any opamps.

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    4. I have enough of soldering for a year :D I have the JFET Matcher built so I just gonna put right opamp in the socket tomorrow and match the fets :)

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    5. It works, thanks a lot for help guys!

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  11. I built this and it works great. One question though. I don't have a power switch on mine, so when I insert and remove the JFETs, the power is always on. Is this bad for the transistors, to do that when the power is live? Should I add a little switch to kill the power for the insertion/removal process? Thanks!

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  12. is this also good as a tester? on what i have to set my dmm, an what does it show, if the transistor is right oder defekt.

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