# JFET Biasing - part 1



## cooder (Jan 3, 2021)

Right. If anything goes wrong in 2021 I can always go back to this to keep my mind occupied...
Thanks Chuck!


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## reubenreub (Jan 4, 2021)

Chuck, this is so incredibly helpful. I've been having a lot of fun playing with different ways to use jfets on my breadboard but this really helps make the biasing make more sense. I also found runoffgroove's fetzer valve article to be really informative as well for when you're trying to emulate a tube. 



			A closer look at the Fetzer Valve
		


Also, I think your chart has Rs listed for Rd. Just made it a little confusing at first glance.


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## Chuck D. Bones (Jan 4, 2021)

reubenreub said:


> Also, I think your chart has Rs listed for Rd. Just made it a little confusing at first glance.


Good catch!  It's been corrected.


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## Loxton (Jun 20, 2021)

Very helpful Chuck. As I'm having sound issues with the Six String Stinger. It sounds as tho the speakers have a blanket over them. I even made another one with the same results. I brought Jfets from Mouser. Do I need to bias them? If so how? Thanks


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## Chuck D. Bones (Jun 20, 2021)

You do not need to bias the JFETs in a Six String Stinger.

If your pedal is not working properly, and from your description that appears to be the case, then the thing to do is ask for help in the Troubleshooting Forum.  Be sure to do your due diligence first: 

Perform a thorough visual inspection.
Correct anything you find.
Make sure your board is clean.
Post good quality pix.
List all deviations from the build docs, especially mods and part substitutions.


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## Heisenberg (Jul 15, 2021)

Loxton said:


> Very helpful Chuck. As I'm having sound issues with the Six String Stinger. It sounds as tho the speakers have a blanket over them. I even made another one with the same results. I brought Jfets from Mouser. Do I need to bias them? If so how? Thanks


Hello Loxton. I'm having the exact same issue you described here. Very muffled sound, regardless of the settings. Did you find a solution for this problem? 
I bought it as a kit from Musikding and, for whatever reason, they sent pre-soldered SMD J201s. I fear the orientation might be wrong.




My board seems to be slightly different than the ones I've seen here on the forum, judging by the components distribution.

Thanks!


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## Loxton (Jul 16, 2021)

Heisenberg said:


> Hello Loxton. I'm having the exact same issue you described here. Very muffled sound, regardless of the settings. Did you find a solution for this problem?
> I bought it as a kit from Musikding and, for whatever reason, they sent pre-soldered SMD J201s. I fear the orientation might be wrong.


No I didn't get around to it. 
The T0-92 j201s aren't available now. so SMD is it. You can find them, but I don't believe they're worth it.


Heisenberg said:


> My board seems to be slightly different than the ones I've seen here on the forum, judging by the components distribution.


Your board is the newer version. with the added j201 SMD pad option on the board, behind the pre-soldered J201s. I have both boards with the same issue. so it's not a board issue.


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## benny_profane (Feb 22, 2022)

Hey @Chuck D. Bones, is part two still pending? A mu amp writeup would be a great resource.


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## Chuck D. Bones (Feb 22, 2022)

Yeah, I should do that.


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## fig (Feb 22, 2022)

That being the case, I'm going to review part one. I believe I might actually understand a portion of it _this_ time. TIA


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## benny_profane (Feb 22, 2022)

I was revisiting this to see what devices are suitable for source followers. This is a really helpful article. Thanks again @Chuck D. Bones


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## Chuck D. Bones (Feb 27, 2022)

I've been reading up on the mu-amp and the more general case, the SRPP circuit.  The math gets pretty messy.  I might just punt and run some LTSpice simulations to show you what does what.  Here's a preview...

The Dr. Robert output stages are mu-amps.





The Catalinbread SFT uses SRPP amps.  This is the 2nd stage.  The difference between a mu-amp and an SRPP amp is one resistor: R4, below.  Set R4 to zero and this becomes a mu-amp.





These circuits are counter-intuitive.  If you want to read up on them, Valve Wizard has a great article here. He's writing about mu-amps built with triode valves, but the principle is the same. Valves (vacuum tubes, glass-FETs) are very similar to JFETs except they will give you a shock or burn you fingers if you're not careful. 😧


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## claudio (Apr 11, 2022)

Heisenberg said:


> Hello Loxton. I'm having the exact same issue you described here. Very muffled sound, regardless of the settings. Did you find a solution for this problem?
> I bought it as a kit from Musikding and, for whatever reason, they sent pre-soldered SMD J201s. I fear the orientation might be wrong.
> View attachment 13700
> 
> ...


did you find out if it's the right orientation?


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## claudio (Apr 11, 2022)

Loxton said:


> Very helpful Chuck. As I'm having sound issues with the Six String Stinger. It sounds as tho the speakers have a blanket over them. I even made another one with the same results. I brought Jfets from Mouser. Do I need to bias them? If so how? Thanks


Did you find out what was your issue? I'm about to build one.


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## Loxton (Apr 27, 2022)

claudio said:


> Did you find out what was your issue? I'm about to build one.


No, they're in the draw. I've yet to look into these.


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## HamishR (Apr 30, 2022)

That was amazing, Chuck. Really clearly explained, and I was with you right up to Intro. The fault is entirely mine, I know!


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## lrgaraujo (May 10, 2022)

Hey Chuck, would using jfets in parallel (like it's done in the Wampler '57) affect biasing?


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## Chuck D. Bones (May 10, 2022)

Yes.  Paralleling FETs increases the transconductance (gain) which increases the DC drain current.  The 1K resistor (R17, I have the same sch) regulates the DC drain current, but it will still be higher than if we have one FET.  It's important the the FETs are well-matched for Vp, otherwise one FET will hog most of the current.


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## lrgaraujo (May 10, 2022)

Nice, thanks!


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## droneshotfpv (May 12, 2022)

I literally just saw this after just posting a related question.. lol my forum search function / capability must not work very well... lol


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## lrgaraujo (Jul 5, 2022)

Hey Chuck, here I come again to bother you with a question. In the following muamp stage (last stage of the softii), can I replace Q6 with a different jfet, since gain comes mostly from Q5 (from what I've read)? My stupid self has forgotten to check if I had enough transistors, and I'm way too cheap to pay shipping for a single fet.

Thanks!


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## Chuck D. Bones (Jul 5, 2022)

They don't need to be matched.  What do you have for Q6?  You'd like something with a Vp similar to Q5.


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## lrgaraujo (Jul 5, 2022)

Thanks, that's great to know! Looking at the datasheets, I will probably socket and try some 2sk193, 2sk246 and 2sk304


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## Chuck D. Bones (Jul 5, 2022)

You'll know you have a decent match when Q5-D is between +5V and +5.5V.


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## lrgaraujo (Jul 5, 2022)

Chuck D. Bones said:


> You'll know you have a decent match when Q5-D is between +5V and +5.5V.


Awesome! Is it gonna be approximately the same for the other mu-amp pairs?


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## Chuck D. Bones (Jul 5, 2022)

No, because the source resistors (R16 & R24 in the circuit above) are all different ratios and you are not matching the transistors for Idss & Vp.  I can only guess whether Catalinbread screened & sorted their JFETs.


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## lrgaraujo (Jul 5, 2022)

Man, I love how jfets sound, but they definitely are too much for my head (you, sir, do make it more understandable, and I thank you)


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## lrgaraujo (Aug 13, 2022)

Hello Chuck, it's me once again demonstrating my absolute lack of knowledge.

I have a bunch of 2sk304 that measure quite similarly to J201 in terms of Vp (between -0.6V and -0.8V) and Idss (close to 0.8mA for a lot of them), which I have been using in place of J201 in some breadboard projects with good success soundwise (they seem to bias ok as well).

Should I be worrying about other datasheet characteristics (such as transconductance) or do Vp and Idss describe everything we need well enough?


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## Chuck D. Bones (Aug 13, 2022)

Vp and Idss tell us everything we need to know about operating a JFET in an audio circuit.  If they bias correctly and sound good, then they are viable subs.  Note that some pedal circuits are very finnicky (Fairfield Unpleasant Surprise, for example) and require a much lower Vp to work correctly.

Look at the 2nd equation above. Transconductance is determined solely by Vp, Idss and the circuit's bias point (Vgs or Id).  The datasheet transconductance value is measured at an extreme condition (Vgs = 0) which does not normally occur in a pedal circuit.


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## lrgaraujo (Aug 13, 2022)

Cool, thanks, Chuck! That's what I had gathered from the equations, but the datasheets confused me. 

Too much information can be dangerous for the uninitiated


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## Chuck D. Bones (Aug 13, 2022)

lrgaraujo said:


> Too much information can be dangerous for the uninitiated



Indeed.
TMI can be confusing even to the experienced designer.

Something else to keep in mind about the datasheets: they mostly list performance limits.  You will _rarely _find a part that has a parameter right at limit, and you will _never _find a part with all of the parameters at the limit.


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## bowanderror (Aug 14, 2022)

Chuck D. Bones said:


> Last example.  This is the 3rd stage in the M800 OD.
> 
> View attachment 8904
> Q3 makes the gain in the 3rd stage. Q4 is a buffer that keeps the tone stack from loading Q3.  Q4's gate voltage is the same as Q3's drain voltage (because they're connected).  Lucky us!  We can use TRIM3 to dial-in Q4's bias point.  Vp for the J201 is -0.3V to -1.5V.  We can set TRIM3 to give us something around 5V at Q4's source, which puts Q3's drain around 4V.  Then we tweak by ear from there.


After first spotting it in the preamp section of the Low Tide, I've been seeing this direct-coupled JFET-buffer setup a lot lately (Beetronics Fat Bee, JC-120 Preamp, JCM800 Emulator). Most of those circuits use it before a tone stack or into a power amp, and the Low Tide uses it to split signal to the LPG, BBD, & blend circuits. You mentioned the buffer prevents loading down Q3, and I'd guess you can get a pretty low output impedance from the buffer as well. Are there any other advantages to this arrangement? Will there be much of a difference between using a JFET vs. a BJT for the buffer?

The preamp section from the Low Tide is pretty simple, but I really love the way it sounds:




In LTSpice, it looks like the JFET drain is biased right around 4.5V:



I think main characteristics I like about this preamp is that it's super sensitive & pleasantly noisy, with only a little bit of what you might call overdrive. The gain is ~10dB, or ~16dB with the Boost switch on, and there doesn't seem to be too much filtering going on. Would I be correct in assuming the sound is primarily due to the use of a high gm (low Vp/high Idss) JFET?

I tried running through your math above for this circuit, does this look about correct?:


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## Chuck D. Bones (Aug 14, 2022)

That preamp was designed to provide a high-impedance input, low-impedance output (to drive other parts of the Low Tide), clean, low-noise gain and a flat freq response.  In other words, it should be transparent.  If you're hearing overdrive then you're setting the gain too high for the pickup output.  But hey, if you like it that way then go for it.  This preamp should not be noisy.  Are you sure you're not hearing BBD noise?  I would not call 746μS "high gm." The preamp gain is determined by Q1's gm, R3, R4 & R52 (Low Tide sch ref).

Q2 could be a JFET or a BJT, it makes little difference in this circuit. 

C4 does nothing in your sim because it's in parallel with an ideal voltage source (V2).  Same goes for R1 & C1.


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## steviejr92 (Aug 29, 2022)

ive read this about 20 times aleady throughout this last week and i have to say i still dont fully understand it all, but this morning i felt i made more progress than any other day and im starting to understand much more than what i did even a day ago. i cant stress enough how valuable this is!


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