Are JFET circuits naturally fuzzy?

[SpringbokUK]

So this is something that has been on my mind for a while and throughout my recent builds I didn't really challenge the thought but after building a few JFET overdrive circuits and building a JFET distortion circuit (the mu amp one to be exact) I've noticed no matter what it always sounds a little fuzzy. I've cut the bass, reduced the gain, tweaked the bias at different JFET stages and it just always has that grit.

If this is normal then fantastic I can carry on as I am and just accept that that's what jfets do. If not then I need to revise my circuits and see where I'm going wrong.

Thoughts?

Thanks
Liam

[EBK]

Maybe power supply noise/ripple modulating your signal?
Try a battery (assuming you haven't already), and see if the fuzziness goes away.

[amptramp]

JFET's can be made to operate in a linear region but the variation in Idss and pinchoff voltage is so extreme that if you build a circuit and put in a random JFET, you are likely operating near a non-linear region even if you design for nominal values.  If you are using a µ amp stage that is a proper SRPP (Shunt Regulated Push Pull) stage, it needs a minimum load because you have a totem pole structure and the if the current through the upper stage is different from the current in the lower stage, the resulting difference in current has to flow through the load.  If the variation in current multiplied by the load exceeds the headroom, you have a stage that can be overdriven.  Some people refer to the amp structure in a Rat as a µ amp but it is just a grounded-source amp with a constant-current source as a load.  This gives a high stage gain meaning it is easy to run out of headroom in both directions.

I have a Sencore SG165 signal generator with a JFET source follower stage where the pinchoff was about 8 volts.  This had put so much voltage across the 390 ohm source resistor that it had gone up to 2190 ohms.  I didn't change the JFET but I changed the resistor from 1/2 watt to 2 watt and it worked properly after that.  A lot of designers refuse to do anything with JFET's now because it is difficult to accommodate the variations.

[PRR]

The Yamaha electric piano and the Roland Jazz Chorus (FET) did not sound "fuzzy".

Thing is, it takes several Volts to bias a JFET (different for every part you grab) and 9V pedals don't have a lot more than that. So more often than not they strain.

[SpringbokUK]

Well I'll be honest, I'm not good with the technical stuff, still working up to that! However from what I can gather... IC is the way to go?

And does that mean that fets need more than 9v to bias properly?

Thanks for the replies!

[Steben]

"Fuzzy" is a very subjective.
Even a crancked tube amp can be "fuzzy", especially with certain (small) speakers.

[pinkjimiphoton]

personally, i despise the sound or overdriven jfets. not tubey at all,. but very fizzy fuzzy and i agree, no matter how you bias them, they never sound as good as discrete transistors for distortion. they DO sound ok in some circuits, but under distortion i've never cared for them.
mosfets are another story... but bog-standard jfets, to my ear, just ain't as hip, and every circuit i've built that uses them overdriven just doesn't work... for me.
i prefer them just used as clippers, but they're really lossy compared to normal diodes.

[Eb7+9]

jFET transistors, like Triodes and Pentodes (spell check insists on Ape Tide here)
rely on a reversed bias diode input for linearly operation

when operating in high dirt mode those diodes can start conducting
same as in tube overdrive ckts

the reversed biased diode in a JFET is an Si diode
and has sharper turn-on specs than the diode seen at the Grid of a tube

resulting in (typically) more fizz per pound or of signal

why JFET od pedals seem to sound best as direct recording gizmos
rather than in front of a amp box

poor s/n is another issue with JFET preamps
the Roland stuff ran at 30vdc and still had way poor noise specs

[jonny.reckless]

It's possible to get very nice amp like distortion from JFET circuits but you do generally need higher voltage than 9V, at least 18V in my experience. I designed a simple amp (Boba FET) that uses J113s in its preamp and has a really nice amp tone IMO.

Here is the original post including a video of its sound:
https://www.diystompboxes.com/smfforum/index.php?topic=118627.msg1104856#msg1104856

Here is the schematic:


For a pedal I'd probably use a NE555 based voltage doubler to get somewhere close to 18V to run the JFETs.

[amptramp]

^^^

This amp is just waiting for a 19 volt laptop battery and its charger.  NE555 voltage doublers are quite inefficient and add noise.

[MrStab]

hi Liam,

what kinda pickups are you using?