JFET Vulcan Experiment some hints needed :/

[KarenColumbo]

I fell in love with Joe Davisson's JFET vulcan and breadboarded a version which I slightly adapted to my liking.

The original (big fat thank you to Sir Davisson):



My humble take on it:



I omitted the gain pot, since I'm not planning to rob this haywire distortion of it's mojo.

There are some questions I can't seem to get solved:

1. The circuit, being very, very high gain, is quite noisy. Beside filtering the power supply and, when I box it, taking extremely good care by shielding cables where applicable etc.: is there something I can do in the circuit itself?

2. My version has quite a low output. How can I increase output volume a bit? I suspect the voltage divider (?) R11 & R12 to be the culprit, but R12 is on of the two bias resistors of Q3, isn't it? 

If you'd find a few seconds/minutes answering it would be, as always, very much appreciated!

[Ben Lyman]

I too love this thing. If you haven't already checked my thread, have a look. Joe even chimes in with some words of advice:
http://www.diystompboxes.com/smfforum/index.php?topic=113986.msg1056910#msg1056910

[KarenColumbo]

Ah, I stumbled upon this thread after searching the forum. Very revealing, thanks!

Hm ... how would you shield the diodes? Bit of copper foil, sort of a "roof"? or a tube around it?

I don't really wanna touch those res and cap values ... it sounds just right to me at the moment. But that's what BBs are for, isn't it?

[Ben Lyman]

I noticed you went with 100n on source of Q2 and Q3 in your schematic instead of 1uF on all 3 FETs. I think I recall most of the noise coming from Q2 and I only changed that one cap to 100n. It was still pretty noisy on the board but it was just fine after boxing it up.

I don't know about shielding the diodes, it never occurred to me. I suppose you could wrap something around them but I would probably put some shrink tube around them first to avoid accidentally shorting the anode to cathode.

[KarenColumbo]

Yeah, that's what I thought. As soon as I box this thing, most of the shielding problem should be better, won't it? I will try this on breadboard first: shrinktube them, then try to cover them. Should be fun, if probably naive at my end

I tried many combinations of baypass caps ... I noticed the most noise coming from Q3. Hm ... I still switched to 100n there. Mostly it affects how my 120 Euros HB lefty Strat evolves into an antenna, but it opens up the sound real nice. Couldn't go back to the 1uF elec after that

[KarenColumbo]

Got rid of the filter cap between Q1 and Q2, switched the 100n input cap for a 15n, output cap now 220n instead of 100n.
Q2 can be optionally bridged via toggle switch (maybe a stomp switch?) -> option 1: utter dispair, option 2: some hope.

[Fancy Lime]

Hey Andreas,

First of all, "Vulkanette" = excellent name for this! I'm so looking forward to the design. Where do you even attach pointy ears to a stompbox....

I have never built this one i particular but experimented a bit with similar designs. one thing that I noticed with respect to noise is, that the noise of JFET or MOSFET stages seems to be much higher at maximum gain than at (very) slightly less than maximum gain. the gain of the stages in your case is set effectively to max by the capacitors bypassing the source resistors. If you add a series resistor (100Ω to 1k, will need some experimenting) between the J201 source port and C2 (C5 and C7 for Q2 and Q3). You will loose a bit of gain but a lot of noise. Trade-offs, as always.

Also, adding a series 100Ω resistor to the power supply filtering (between 9V+ and C9) may help a lot depending on your power supply and won't cause much sag in a low current circuit like this.

I also always(!) add a simple RC low pass filter between the output coupling cap (C8) and the volume pot in high gain circuits to filter out anything above 5-7kHz. There is a lot of noise there and no signal. So 10k and 2n2 or 3n3 should be useful.

The low output is a bit strange. But trying to reduce R11 to 220k and seeing what happens can't hurt.

Cheers,
Andy

[KarenColumbo]

Hey Andreas,

First of all, "Vulkanette" = excellent name for this! I'm so looking forward to the design. Where do you even attach pointy ears to a stompbox....

I have never built this one i particular but experimented a bit with similar designs. one thing that I noticed with respect to noise is, that the noise of JFET or MOSFET stages seems to be much higher at maximum gain than at (very) slightly less than maximum gain. the gain of the stages in your case is set effectively to max by the capacitors bypassing the source resistors. If you add a series resistor (100Ω to 1k, will need some experimenting) between the J201 source port and C2 (C5 and C7 for Q2 and Q3). You will loose a bit of gain but a lot of noise. Trade-offs, as always.

Also, adding a series 100Ω resistor to the power supply filtering (between 9V+ and C9) may help a lot depending on your power supply and won't cause much sag in a low current circuit like this.

I also always(!) add a simple RC low pass filter between the output coupling cap (C8) and the volume pot in high gain circuits to filter out anything above 5-7kHz. There is a lot of noise there and no signal. So 10k and 2n2 or 3n3 should be useful.

The low output is a bit strange. But trying to reduce R11 to 220k and seeing what happens can't hurt.

Cheers,
Andy

Hey Andy,

Yeah - now that you mention the pointy ears ... NEVER thought of a kind of Lady Spock, just wanted to be diminutive to the original circuit Guess, I have to stay with "Vulcanette" instead of "ExistentiaDist" now

I put in those gain resistors at C2 (220R), C5 (680R) and C7 (220R). Hard to say what happened, 'cause I couldn't do an A/B comparison (no space left on BB), but it sounds "more even" now. Less fuzzy. Thanks mate!

Series resistor was already in there - I have an 47R there: are 100 Ohms mandatory there?

I reduced R11 to 220k as you suggested - I don't hear much difference in volume, but it didn't become less, either, so I'll leave it in here. Guess it can't hurt

I tried your suggestion with the RC LPF - 2n2 seemed to steal some "brilliance", so I had a beer and took 1n8 (8.5-9 kHz). Sounds cool. Now all there's left is some hum - which is to be expected, especially when I'm near the amp with his 4x12 Mashall box




Can't wait to box this - but I have to wait for my Hammonds - those I purchased @ "The Big C" round the corner suck. Well - now I have time to look for some artwork. There must be some attractive female vulcan somewhere round the WWW.

Guess i have to name those boost switch positions "Reason" and "Emotion" now ...

[Fancy Lime]

Hi Andreas,

glad I could help a little.

47Ω for the series resistor together with the 100.1µF from C9 and C10 would have a corner frequency of 34 Hz, so that should work well enough for filtering 50Hz power-line hum.

The rest of the hum will probably go away when it's boxed.

Not to completely geek out and intrude on your design decisions but how about "pon farr" for the boost switch?

I think people would appreciate you posting the final schematic. I sure would.

Live long and prosper,
Andy

[KarenColumbo]

Thank you for your kind words And geek away, Sir, geek away. Will name the switch thus.
This is my final layout - it will be boxed this way. Sitting right at frigging ExpressSCH right now.

[Fancy Lime]

Thanks for the schematic! This looks really cool now, congrats! I hope I find some time soon to give this a try.

Rock on,
Andy

[Ben Lyman]

Awesome, I might have to breadboard your version sometime in the future.
Speaking of the future, I believe Spock was to wed T'Pring, his Vulcan bride in episode 34 "Amok Time" and he had to fight Capt. Kirk to the death!

[Kipper4]

Geekdome.

That's wild Ben.

Who won the fight? 'cause both featured long after I assume.

I too built the vulcan IIRC it's very high gain. Totally not the distortion I was looking for at the time....

You can take measures to combat the outside world, Ground wire // input attached at one end only.
I've still got some of the mosfets hanging around from doing the other design. P channel.

Some Jfet high gainers have a small value cap (pf) on the gate to ground.
Look up some other jfet stuff of the ilk. You're bound to find something.

I go look.

[Kipper4]

ROG is the winner.

http://runoffgroove.com/thor.png

[KarenColumbo]

Long time no hear, my mate Yeah, I BBd Thor - whoa Pretty damn load, that.
Me, I'm just searching for those extremely basic cranked things - Sir Davisson's designs suit me quite perfectly right now. But I'm evolving my taste
Just finished the PCB routing - man, what a drag this is, every friggin' time. No talent for this. But I'll etch it and box that thang - tomorrow eve latest.



Now I'll have some beer with my neighbours.

@Lime: My BSs just came in - weekend's task: your bass driver

@Ben: Perfect for the artwork. Hope I get it right with those waterslides - first time

[Fancy Lime]

Looking forward to your opinion on the Bass Preamp. Am I assuming correctly that you ordered some more BS250s and plan to also build Joe Davisson's Obsidian? I've been meaning to redraw his schematic for use with negative ground, there are some confusing schematics floating around the series of tube that we call the interwebs. And maybe I find the time to breadboard a version with easier biasing, an output buffer, mid control for the tone stack and so one. The usual polishing.

[KarenColumbo]

Looking forward to your opinion on the Bass Preamp. Am I assuming correctly that you ordered some more BS250s and plan to also build Joe Davisson's Obsidian? I've been meaning to redraw his schematic for use with negative ground, there are some confusing schematics floating around the series of tube that we call the interwebs. And maybe I find the time to breadboard a version with easier biasing, an output buffer, mid control for the tone stack and so one. The usual polishing.

I explicitely ordered those BS250ies for this eemu thing you posted. Looks like exactly whar I've been searching for some months - there's still some incomplete threads hanging in this forum to prove that :/ I'm quite inconsequent.

What I'm hearing in my head is this, but the other way round. Bass clean, from 1st E octave up driven. Your description hit it 100%. Can't wait.
https://www.youtube.com/watch?v=R2F_hGwD26g

BUT ... of course I'll be building those Sir Davisson things, I ordered those 5089ies he's having all over his designs. Will take a few days, though. But I just gotta BB those Blackfires and Obsidians.

There's a lot of reading I've had regarding noiseless biasing (JFETs, though) and workarounds ... I will build those designs and tweak til I drop No technical background, but motivation.

After all this distortion ... next thing will be a Chorus

[KarenColumbo]

Yeah, well, I'm not one for symmetry, but at least my trusty old etching thing works - except the bubble pump. but what the heck, it etched.
I won't have much fun cutting this into symmetry, though



IF this works as intended AND some of you wanna build it: Which format should I post the PCB files? I did it in Eagle - sufficient? Or shall I try and make a PDF?

[KarenColumbo]

Enclosure primed, step-drilled and fittings fitted. Not 100% in line, but what the heck As good as it gets. Still haven't decided where the "pon farr" switch will be located. I'll sleep over this.





Tomorrow: more priming, and then I think it will get an orange coat.



After this I'll try this waterslide decal thing, people freak out about.

[KarenColumbo]

Airbrushing is just not quite as easy as it looks. But it's coming along somehow. Tomorrow last coat and pics.