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  • jfet substitutions

    Steve Ahola's blueguitar.org has a article on mosfets and jfets used in audio circuits [new/articles/other/hi_v_ss.pdf] that calls out a 2N5485 jfet in one example. How critical is this choice? I have a bunch of J111s, and other than a -3.0volt Vgs(off) instead of a -0.5volt Vgs(off), I can't see why I wouldn't just plug and play with it. Steve, ya listenin'?
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


  • #2
    How long is a piece of string? Critical? In some places it doesn't hardly matter what you use, in others you want to be closer to the original than not.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Cutoff is a range of values - JFETs are all over the place with regard to general spec, that's why so many circuits involve trimmers or recommend matching or some other selection process. The J111 is nearly always used as a switch, rather than in the audio path, though this doesn't mean that by careful choice you couldn't get good results as there can be some overlap between different device numbers. Depends on the circuit and how tightly it's designed around expected characteristics.

      Edit: bear in mind the Vgs for a J111 is -3 to -10v. Quite a wide range.
      Last edited by Mick Bailey; 07-15-2015, 12:27 PM.

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      • #4
        http://blueguitar.org/new/articles/other/hi_v_ss.pdf

        My apologies for not giving the full path to the article (above). The particular schematic is the second one, using the jfet on the bottom of a cascode circuit. I have seen multiple threads about how important it is to match jfets to bias conditions in some specific applications (phaser-type circuits come to mind) but as you point out, this is a bit different. If, in this circuit, the gate floats at some voltage above ground [Vgs(off) + Vr_cathode] then it probably doesn't matter much what Vgs(off) actually is in this application.

        I'm not sure, Enzo, but it sounds like you are advocating a 'try it and see' approach. I guess it couldn't hurt.
        If it still won't get loud enough, it's probably broken. - Steve Conner
        If the thing works, stop fixing it. - Enzo
        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

        Comment


        • #5
          The article suggests that you select the Source resistor Rs to set the output Voltage. A higher Vgs(off) will require a higher value resistor and that will reduce the gain. There are a couple of things you can do to bring the gain back, you have to be more creative with cathode bypassing. With some JFETs you can have problems with the bias drifting around when the circuit is overdriven.
          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
          REMEMBER: Everybody knows that smokin' ain't allowed in school !

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          • #6
            -3.0volt Vgs(off) instead of a -0.5volt Vgs(off)
            That's a HUGE difference.

            Not writing a full page on this, a 0.5Vp Fet is good for a preamp/gain stage.
            Not only by the biasing aspect but because it will have way higher voltage gain, think 20/30X, while a high (3 to 10 Vp) one will have a very poor one (think 3 to 5X ) so it's only really suitable as a switch.

            That's why J11x Fets cost 50 cents or less (30 cents?) while classic Audio ones (2N54xx) cost 2 to 6 bucks each

            Personally I buy 50/100 of the cheap ones (same cost as buying 5/10 of the "good" ones), test them, and separate in 3 groups by Vp :
            * < 1.8Vp: preamp duty
            * around 2.5Vp: general purpose ones, not for critical missions
            * > 4Vp: only suitable as switches

            Given Post prices, it pays to buy 50/100 cheap ones, test and label them, and already have them on hand for future projects, they end up costing way less that way.

            FWIW for phaser use any of them is fine (so use the "bad" ones first ) , as long as they are close to each other and the trimpot can reach the needed Vbias (no big deal)

            EDIT: what I call Vp(inchoff) is what you call Vgs(off) .

            Maybe my age shows

            Juan Manuel Fahey

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            • #7
              Originally posted by loudthud View Post
              The article suggests that you select the Source resistor Rs to set the output Voltage. A higher Vgs(off) will require a higher value resistor and that will reduce the gain. There are a couple of things you can do to bring the gain back, you have to be more creative with cathode bypassing. With some JFETs you can have problems with the bias drifting around when the circuit is overdriven.
              I don't I even want to hear what it sounds like overdriven.

              Originally posted by J M Fahey View Post
              That's a HUGE difference.

              Not writing a full page on this, a 0.5Vp Fet is good for a preamp/gain stage.
              Not only by the biasing aspect but because it will have way higher voltage gain, think 20/30X, while a high (3 to 10 Vp) one will have a very poor one (think 3 to 5X ) so it's only really suitable as a switch.
              All good points. I can do some 'pre-testing' and then breadboard my experimental rig. If I don't get the gain I want, I know where to go.
              If it still won't get loud enough, it's probably broken. - Steve Conner
              If the thing works, stop fixing it. - Enzo
              We need more chaos in music, in art... I'm here to make it. - Justin Thomas
              MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

              Comment


              • #8
                Originally posted by J M Fahey View Post
                I see you're garbed in your best alpaca-herding gear!
                If it still won't get loud enough, it's probably broken. - Steve Conner
                If the thing works, stop fixing it. - Enzo
                We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                Comment


                • #9
                  You can get pretty good deals on 2n5485s and J201s on e-bay (J201s seem to be a favorite for runofgroove's triode emulating circuits). A lot of them with free shipping.

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                  • #10
                    I'll widen my search, then. The few distributors I tried indicated the 2N5485s were obsolete!
                    If it still won't get loud enough, it's probably broken. - Steve Conner
                    If the thing works, stop fixing it. - Enzo
                    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                    Comment


                    • #11
                      A quick search yielded these two here...

                      Of course $1.19 each seems a bit more than most would want to pay.
                      2N5485 Central Semiconductor | Mouser

                      Tayda electronics is a Thailand based company and I have bought from them many many times. You will also find them selling right on FleaBay but I trust them more than most.
                      2N5485 JFET N Channel Transistor
                      When the going gets weird... The weird turn pro!

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                      • #12
                        I didn't think about checking Tayda. They're always out of J201s and that's what I'm usually looking for in the JFET dept. Yeah, love those guys. Also, if you're going to buy from Tayda, check out their Facebook page: https://www.facebook.com/TaydaElectronics

                        About every 3-4 weeks, they release a new coupon code that's good for a few days for a 15% discount. I generally start filling up my basket and wait for the coupon code to come out before submitting my order.

                        And just to sing their praises a bit, Tayda has really inexpensive shipping. All the items in your order come in little labeled ziploc bags. It's coming from Thailand, so it doesn't arrive overnight, but it's still reasonably quick, considering... I've ordered from them maybe 10-12 times and I've never been disappointed.

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                        • #13
                          I wasn't suggesting try it and see, though that can be fun, I just wanted to have the application in mind before recommendations were made. Channel switching or such has different needs from amplifying stages.
                          Education is what you're left with after you have forgotten what you have learned.

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                          • #14
                            Click image for larger version

Name:	jfet test rig.GIF
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                            Peer review, please: this is the test rig I put together ( +15/-15 bipolar power supply, 250k linear pot, led and resistor, breadboard). each of the j111 transistors I tried started to dim the LED at about -6.5v GS, fully off at almost -8v GS.

                            Since each of the 5 or so pseudo-randomly picked samples from my batch of 100 (still on the tape strip) all agreed, do I assume that this is a reasonable result? Do I need a better test method?
                            Attached Files
                            If it still won't get loud enough, it's probably broken. - Steve Conner
                            If the thing works, stop fixing it. - Enzo
                            We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                            MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                            Comment


                            • #15
                              Originally posted by J M Fahey View Post
                              Not writing a full page on this, a 0.5Vp Fet is good for a preamp/gain stage.
                              Not only by the biasing aspect but because it will have way higher voltage gain, think 20/30X, while a high (3 to 10 Vp) one will have a very poor one (think 3 to 5X ) so it's only really suitable as a switch.
                              This is not exactly true. Those high pinchoff switching JFETs make fine amplifiers if you bias them right. A long tail resistor in the source circuit will force the current to be whatever we want. A big capacitor will charge to whatever Gate bias is required. You can easily get a gain of 100 or more. You may need to actually limit the gain with unbypassed Source resistors.

                              Edit: JM won't believe this but I just looked at a PN4391 which is close to a J111. The transconductance at 20V and 1mA is 5000umho. That means in this circuit with a 100K load resistor the gain will be... you do the math but sit down first.
                              Attached Files
                              Last edited by loudthud; 07-16-2015, 03:59 AM.
                              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                              REMEMBER: Everybody knows that smokin' ain't allowed in school !

                              Comment

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