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Thread: Silly-simple JFET (guitar) buffer

  1. #1
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    Silly-simple JFET (guitar) buffer

    I have a Joyo compressor pedal that works fine with my acoustic-electric guitars, but produces audible treble loss when I plug in my electric guitars. The acoustic-electric guitars have onboard preamps and low output impedance; the electric guitars don't. Evidently, the input impedance of the Joyo isn't high enough to avoid the dreaded "tone suck".

    So I needed a buffer. I happened to have a few old MPF 102 JFETs sitting in the parts box. And I wanted to build the thing quickly, since I hoped to use it the next day, so I decided to try the simplest possible circuit (attached). Keep in mind, I was looking for a completely transparent, always-on, clean buffer - this isn't intended to produce any sort of overdrive or audible coloration to the sound.

    The price one pays for this super-simple circuit is that the bias point will vary considerably from one JFET to another, and that will affect how big a signal you can feed in before the buffer clips. (Clipping might not sound good - the circuit has 100% negative voltage feedback, and usually, lots of negative feedback equates to harsher clipping.)

    To see if this would be a problem, I bread-boarded the circuit; with a 10k source resistance and 9V DC power supply, all the MPF 102 JFETs I had settled down with the source somewhere between 1.41 and 1.72 volts. That means even the worst of these should be able to handle a guitar signal up to 2.8 volts peak-to-peak, which is comfortably big enough for most guitars and playing techniques.

    I built my final version on a little scrap of proto-board, using the JFET with the largest Vgs (1.72 volts). I added a red power indicator LED and (820 ohm) series resistor, which I forgot to draw in the LTSpice schematic. I power my pedals with a One-Spot wall-wart, so I did not include any kind of on-off switch or bypass capability in my little buffer. No controls of any sort, in fact.

    I thought about adding a footswitch and output level pot, so the pedal could double as a rhythm/lead level adjustment switch. But I'm using it in front of a compressor, which is the wrong place to try and change the signal level. So I decided not to bother with the pot or switch.

    I tested it out last night, and I can't hear any audible treble loss from any of my guitars now. I can't get it to clip with any of my guitars plugged in and strummed hard, either. Eureka, it works!

    I should stress that there is nothing magical or mojo-tistic about this circuit at all. Its designed from the perspective of a lazy audio engineer (though I am not in fact an engineer, I've played that role a few times). This circuit simply does its job, using a minimum of components, no fuss, no muss, no magic. If you need a simple, completely clean and transparent guitar buffer, this one might suit you.

    -Gnobuddy
    Attached Thumbnails Attached Thumbnails simple_jfet_buffer_001.png  

  2. #2
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    I build these FET buffers with DC blocking cap on the input and a second resistor from gate to V+. This resistor should be identical to R1 (in your diagram), but because they effectively appear in parallel need to be doubled in value to maintain the same input impedance as before. So where you have 1M I would use 2x2M. This improves the signal output symmetry between different FETs if they have wildly differing characteristics.

    This circuit is tolerant of increasing the source resistor considerably to reduce battery drain if it's (say) built into a guitar without adversely affecting performance.
    J M Fahey likes this.

  3. #3
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    Quote Originally Posted by Mick Bailey View Post
    I build these FET buffers with DC blocking cap on the input and a second resistor from gate to V+.
    Thanks for the comment! Oh, I know the "proper" way to do it. Yes, that would bias the gate to half the supply voltage, the source a bit higher, and give you near-maximum headroom for a given supply voltage. That is a bit closer to the carefully correct engineers way to build a JFET buffer.

    I built this circuit the way I did, precisely because I did not want to bother with an input coupling cap, or a second gate bias resistor, or additional power supply filtering. When you add that second gate resistor, half of any noise or ripple on the power supply appears at the gate - and at the output of the circuit. So now you have to break that upper resistor into two, and hang a filter cap off the junction, to make sure the DC bias voltage at the gate is clean. And because the input coupling cap now has DC voltage across it, you really should add yet another resistor to the input end of that cap, to keep it from popping loudly if you plug in a guitar with the JFET powered up.

    Supply noise is much less of an issue with the stupid-simple circuit I used. The JFET has considerable power supply noise rejection in source-follower mode, because small voltage fluctuations at the drain produce almost no change in source current.

    So the stupid-simple version I built gets rid of three resistors and two capacitors, compared to the "proper" way of doing things. As mentioned, the price paid for that is reduced input headroom, but it turns out not to be an issue. I ended up with an input headroom of roughly 3.4 volts, peak to peak, and that is far more than I can get out of any of my guitars, even by aggressively strumming big fat six-string cowboy chords. I tried! (Keep in mind the typical 12AX7 triode input stage has less input headroom than that!)

    BTW, I added some supply filtering anyway, to ensure stability at the end of a long 9V cable from the One-Spot power supply. I added a 470-ohm build-out resistor for the same reason.
    Quote Originally Posted by Mick Bailey View Post
    This circuit is tolerant of increasing the source resistor considerably to reduce battery drain if it's (say) built into a guitar without adversely affecting performance.
    Sounds good! In my case, I didn't have a high-efficiency LED lying around, so the LED power-on indicator draws 10 mA, and the JFET draws 0.17 mA! So my build of this circuit would actually benefit much more from a switch to a high-efficiency LED than a larger source resistor!

    -Gnobuddy

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