# Fun with JFETs and servos



## jubal81

Chuck's thread on the Brownout OD finally motivated me to do some breadboarding with JEFTs and opamp servos for biasing.

So far, things are looking pretty neat. I put together a couple stages of the Wampler Black 65 for testing and sure enough, I'm able to swap in and out JFETs willy-nilly and have it bias up perfectly. I don't have any J201s, so I've been using 2SK170, 2Sk117 and the good old 2N5457. I've got it set up so I'm getting 5.8V on the drains with a 9V supply. When they begin to break up, the distortion looks identical on the scope, the only difference being that the higher |vp| transistors have less gain/more headroom.

As far as the Black 65 goes, I've read that they're set up with 4V on the drains and I've not been able to dial it that low with this setup and my JFETs. (Using a rail-rail JFET opamp). I've always done 5.8 or so on a 9V supply with my projects where top and bottom of the signal start distorting at the same time.

Anyway, here's the circuit snippet I've got worked up and some pics of scope readings with different JFETs.




2SK170:







2N5457:


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## jubal81

Spent a little more time playing around with this today. Unfortunately, an LM358 doesn't perform well. Think I'll look around for a cheaper JFET-input rail-to-rail opamp.

Also confirmed it's easy to raise or lower the value of the drain resistor to dial in the amount of gain while maintaining proper bias.

Another thing I'm noticing again is the clipping character of a JFET amplifier stage is different when the source is bypassed with a capacitor. With the cap, the clipping transition is rounder. Without the cap, you get a really sharp style of clipping. I think one interesting thing to explore with the Black 65 would be to use a 'lower gain' JFET on the clipping stage, but with a bypass cap to see if you get a rounder clipping with the same amount of gain. Or, with the servo bias, you might be able to switch from one style to another by switching the passives on the source of the JFET.

Schematic update: Using 18K and 24K resistors for the Vref into the servos. This is giving about 5.3V on the drains on a 9V supply.


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## fig

Thanks for sharing this.

Would a LM258 or LM324 work? I think I've got some. I may give this a try.


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## jubal81

fig said:


> Thanks for sharing this.
> 
> Would a LM258 or LM324 work? I think I've got some. I may give this a try.


Nope. those are pretty much the 358.

Only other rail-to-rail I've got on hand is a 2272 and it works fine (mosfet-based). It's through-hole, but they're $5 each these days. OUCH!

If something comes of this, I'd like to make it through-hole capable so you guys can build one if you want, and I might have found the ticket. The TL052 looks like a rail-to-rail version of the good ol' TL072 and it's only 84 cents for the dual opamp at Mouser. Can't believe I never knew about that one before now.


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## cooder

Very cool stuff indeed. I have been pondering building another Black 65 since the one I made on an etched MBP layout went astray staying on a friends board. Would be cool to get an updated version of this and very neat to dive deeper in these ideas. Cheers!


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## jubal81

cooder said:


> Very cool stuff indeed. I have been pondering building another Black 65 since the one I made on an etched MBP layout went astray staying on a friends board. Would be cool to get an updated version of this and very neat to dive deeper in these ideas. Cheers!


It's a good one. I was heavily into JFET ODs for years and the Black 65 is easily in the top 3 for me in that category.
If I build this out, I'm most likely going to leave off the bonus boost stage. There are a lot better boosts to hit it with, especially a good ol' Klone. Might look into emitter bypass caps for a low/high gain switch, though.


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## Feral Feline

Your post a couple above is the first I've ever heard of a TL052 ! 

Of course, I'm still but a padawan in the grand scale of things, and there's much I've yet to discover that's probably old hat to the sage JFETi knights that inhabit this forum.

Fascinating thread.


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## fig

Oh I see now. I missed the giant IC1_A/B 😖. Duh fig. I've got some TL062s, so I'll give it go with one of those...unless there's another glaring detail I'm missing? Apologies. I am applying the learn as I go technique, so patience is greatly appreciated.


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## jubal81

fig said:


> Oh I see now. I missed the giant IC1_A/B 😖. Duh fig. I've got some TL062s, so I'll give it go with one of those...unless there's another glaring detail I'm missing? Apologies. I am applying the learn as I go technique, so patience is greatly appreciated.


No sweat! I love this stuff.
The TL062 won't give the best results. I tried an 072 today and it couldn't keep up. Looks like the trick is a Fet-input, rail-to-rail opamp for this.


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## fig

TL052 it is. 
Thanks!


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## cooder

Awesome and I think leaving the boost off in the Black 65 is a good idea from my point as well. Maybe do the emitter bypass cap on a DPDT on/off/on for three different positions switching two jfets?


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## thesmokingman

op275 should fit your bill except for cost ...


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## danfrank

I was looking at the TL052 sheet and it says "minimum supply voltage: 10v". I guess a dual rail 9v would work or a charge pump to get 18v (15v regulated) also would work.


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## jubal81

danfrank said:


> I was looking at the TL052 sheet and it says "minimum supply voltage: 10v". I guess a dual rail 9v would work or a charge pump to get 18v (15v regulated) also would work.


Good catch. Totally whiffed on that. I've already got a few coming, so we'll see if they start up with 9V. I think there's a pretty good chance.


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## fig

danfrank said:


> I was looking at the TL052 sheet and it says "minimum supply voltage: 10v". I guess a dual rail 9v would work or a charge pump to get 18v (15v regulated) also would work.


With the max @ 30vdc, I'll bet 18vdc is the sweet spot, even though I'd be swapping a few caps. I can dial in 10v to test it anyway.


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## fig

24-TL052s arrived today. I'll at least get started this evening, maybe finish it, if I can pay attention long enough. If anyone wants a couple of these, let me know.


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## jubal81

fig said:


> 24-TL052s arrived today. I'll at least get started this evening, maybe finish it, if I can pay attention long enough. If anyone wants a couple of these, let me know.


Wow! I don't think I even have a shipping notice yet on mine. Ordered from Digikey.

I'm also planning on trying a non-inverting servo bias on the JFET gates and possibly just a simple voltage divider on the gates. You can bias up a Black 65 with trimmers that way without changing the Drain or Source resistor values.

EDIT: Looks like my orders are on track to be here Friday.


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## fig

Yeah, Mouser knocked this one out. I don't normally use them but they had these and the MPSA14s that I needed for someone. Well played Mouser.


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## fig

I'll try the J201s and some 2N5485s. If I can get this probe working, I'll also try to get some waveforms posted. I think I've spent more time looking at replacement probes than scopes.


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## jubal81

fig said:


> I'll try the J201s and some 2N5485s. If I can get this probe working, I'll also try to get some waveforms posted. I think I've spent more time looking at replacement probes than scopes.


If you're going by the schematic above, use 18K for R8 and 24K for R9. That should give you about 5.3V on the drains with a 9V supply. Also, I used 2u2 for C3 ad C7 because I had those laying around. Those caps could probably drop to 100n without running into trouble.


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## fig

Cool. I didn't have a 25KΩ, and I have enough resistors to ward off the f'in Borg.


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## fig

Fired up first try. These are 2N5485s. No biasing done.













Oh, and say hello to my little friend.


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## jubal81

That fish is a whopper!

How the drain voltages looking?


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## fig

I kept seeing those over at Small Bear and one day they just jumped in the boat...er cart.

Q1 = 9.33vdc
Q2 = 7.77vdc


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## fig

Okay now @ 10.07vdc (sorry I forgot)
Q1 = 10.07vdc
Q2 = 8.47vdc


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## fig

ehh..grabbed 1KΩ instead of 10KΩ...derp, but that didn't change anything. Would it be R4 that needs to go down? (that's possibly wrong on 2 counts).  I have a FET bias calc on my phone but haven't messed with it much......okay at all.


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## jubal81

Supply voltage?


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## fig

I jiggled it a bit..

10.03vdc supply
Q1 D = 9.11
Q2 D = 8.26

 Thanks @jubal81  I'll check back tomorrow. Time to kick back.


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## jubal81

Hmm. What's the voltage at pins 3 & 5 of opamp? Junction of R8/R9?


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## jubal81

Also, looks like in the picture, if that's the pin 8/power side facing us, pin7 isn't connected to the JFET source - just the cap to pin 5.


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## Chuck D. Bones

jubal81 said:


> Chuck's thread on the Brownout OD finally motivated me to do some breadboarding with JEFTs and opamp servos for biasing.
> 
> So far, things are looking pretty neat. I put together a couple stages of the Wampler Black 65 for testing and sure enough, I'm able to swap in and out JFETs willy-nilly and have it bias up perfectly. I don't have any J201s, so I've been using 2SK170, 2Sk117 and the good old 2N5457. I've got it set up so I'm getting 5.8V on the drains with a 9V supply. When they begin to break up, the distortion looks identical on the scope, the only difference being that the higher |vp| transistors have less gain/more headroom.
> 
> As far as the Black 65 goes, I've read that they're set up with 4V on the drains and I've not been able to dial it that low with this setup and my JFETs. (Using a rail-rail JFET opamp). I've always done 5.8 or so on a 9V supply with my projects where top and bottom of the signal start distorting at the same time.
> 
> Anyway, here's the circuit snippet I've got worked up and some pics of scope readings with different JFETs.
> 
> View attachment 13991



Might be me, maybe I'm a little %$&^ed-up the head, but those integrating capacitors should be between the output and the -in terminals of the opamps.  Also try CA3130 if you want something that can swing to the rails.  R5 and R12 could be larger, much larger if need be.  Gives R6 and R14 more leverage.  The technical term is_ control authority_.


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## jubal81

Chuck D. Bones said:


> Might be me, maybe I'm a little %$&^ed-up the head, but those integrating capacitors should be between the output and the -in terminals of the opamps.  Also try CA3130 if you want something that can swing to the rails.  R5 and R12 could be larger, much larger if need be.  Gives R6 and R14 more leverage.  The technical term is_ control authority_.


The more I think about it, the more I think the whole idea is a little %$&^ed-up in the head. Feels like I'm on Mouser shopping for the right $200K supercar someone can leave parked in their garage just in case their 1987 Chevy Corsica needs a jump start. 

In practicality, I think it could be an option if you're Brian Wampler and building 10,000 of these and the extra $ on parts costs less than having someone manually sort or bias JFETs. For DIY, you can get trimmer pots at Tayda for 17 cents and it takes just a few seconds to set and forget a bias voltage on the JFET gate.

I'm gonna keep tinkering with it when my parts arrive, though, just because it's an interesting puzzle to solve and could be useful for something down the road. I see what you mean on R5 and R12. Always seemed kinda wonky to me, but I was going with that diagram from Elliot Sound. I'll also experiment with those caps. I forgot those on the first go and the system wasn't working.

BTW, you think a TLC085 would be an option? 4 channels for $4.


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## fig

I'll check those issues and correct. I'll leave it on the BB, and I have this thread on watch. Thanks!!


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## Chuck D. Bones

I don't think that auto-bias will totally eliminate the need to screen JFETs for production builds.  When a JFET has a 6:1 Vp range (ex. 2N5457), there is no way around having to select devices that fit into a narrower range.  There is an economy of scale when screening parts.  If someone buys 10,000 JFETs, they can pay a couple $K to have them all run thru an automatic tester and sorted, either by the manufacturer, distributor or a 3rd party testing house.  It's very common in the electronics manufacturing industry.

The mu amp circuit _mostly _eliminates the need to screen JFETs for Vp and Idss because the top FET sets the drain voltage and the bottom FET sets the drain current. FETs at the extreme ends of spec range can still cause problems, but no trimmers or select-in-test resistors are required.

My opinion: auto-biasing JFETs is the tail wagging the dog because JFETs don't drift.  Set the bias once by selecting a resistor in test and you're done.  Still, there is value in experimentation so don't let my opinions dissuade you.

To quote Friedrich Nietzsche:
_“It is hard enough to remember my opinions, without also remembering my reasons for them!”_


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## Chuck D. Bones

jubal81 said:


> BTW, you think a TLC085 would be an option? 4 channels for $4.



Those will work.  If you need four channels, then it's cheaper than buying four CA3130's.  For that circuit, we want an opamp that meets these criteria:

very low bias input current -> FET input
output can swing close to the -rail


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## jubal81

So here's what I'm talking about for biasing at the JFET gates. Of course I just altered the drain resistors on my build and it sounds great. This is a solution for keeping the drain and source resistors stock.


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## fig

So, adding this to each gate...cool.


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## Chuck D. Bones

jubal81 said:


> So here's what I'm talking about for biasing at the JFET gates. Of course I just altered the drain resistors on my build and it sounds great. This is a solution for keeping the drain and source resistors stock.
> 
> View attachment 14499


Q: What happens if you need to bias the gates more negative?

A: The source resistor gets bigger, which consumes headroom.

I think you're better off making the source resistor a trimpot and bypassing that as req'd.


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## jubal81

Chuck D. Bones said:


> Q: What happens if you need to bias the gates more negative?
> 
> A: The source resistor gets bigger, which consumes headroom.
> 
> I think you're better off making the source resistor a trimpot and bypassing that as req'd.


Sweet! That sure simplifies things. I've always been able to gate bias, so I guess I hadn't considered that problem.


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## Chuck D. Bones

I feel like we've come full-circle.


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## fig

no guys, my head is definitely still circling...


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## jubal81

So the order came in and I've done some more experimenting.

1. The TL052 doesn't work in the application at 9V. Damnit
2. You can't use half a dual opamp for a servo and the other for audio. Noise with my opamps on hand
3. J201s I got will not bias correctly with a 5K6 drain resistor. Had to go to 15K. Wampler must've had a really big batch of some specific oddballs for this design.
4. Other JFETs I've got all worked fine, with pretty consistent drain voltages. PSA: Don't forget this is a small sample size.
5. JFET clipping looks the same on the scope across part numbers and sound pretty much the same. I think with some tinkering, you could make this work and sound pretty much the same with a lot of different JFETs.

Long story short, I can get this to work pretty well with good results, but it's not a solution that can completely eliminate having to manually make sure everything is working properly - exactly how Chuck characterized it. (Thanks, bud!)

It's a cool concept and I don't regret working on it, but I think I've taken this as far as I need to right now. The thing is a pretty good OD on the breadboard, but I've got others I'd recommend before I'd recommend this one.


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## fig

Thanks to both of you!



jubal81 said:


> You can't use half a dual opamp for a servo and the other for audio.


This is what rules out a charge pump?


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## jubal81

fig said:


> Thanks to both of you!
> 
> 
> This is what rules out a charge pump?


Not really. I wouldn't want to use a charge pump in this case because it adds more circuitry and cost. Plus, it already has plenty of headroom at 9V.


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