15 years ago - Vulcan

[vigilante397]

Thanks, probably through-hole though, right? I need SMD

Yep. Well, there are a few traces that runs under some resistors/caps, so I don't know how doable it is to be converted to an smd layout. Anywaym if you want the eagle files, just let me know.

Yeah, converting layouts from through-hole to SMD doesn't usually work well, I'll just do it from scratch. I appreciate the offer though!

Edit: Wasn't too bad, also added a tone control and reverse polarity protection

[tubegeek]

I found this Vulcan clip and I think it's time to revisit this gem. I found a bag of parts with all the parts I need to rebuild it. I have no idea where the original pedal went. All of the sounds in the clip were just volume and pickup changes.


Check it out


I'm pretty sure this was a Vulcan with a tone control.

Any guesses which tone control you might have used? That's a pretty cool clip!

[aron]

At that time, it was probably the tone control from Doug's sweet thing. Which is tacked on to the Hot Fuzz Schematic:
https://www.diystompboxes.com/pedals/hfuzz1.jpg

[Fancy Lime]

Hi guys,

I just breadboarded the Vulcan again. Wow, what a fantastic circuit! I remember liking it when I first tried it but memory fades I suppose.

Some notes:
1. I played around with the R and C values in an attempt to optimize it. No such luck, Joe's original values are spot on for the best (or at least my favorite) sound. The only thing that turned out to be a big improvement, was to put a buffer in front of it. Makes interaction with the guitar volume more predictable. The sound itself is not affected by the buffer very much, it only gets a bit brighter and clearer on low gain settings, which is a plus in my book. I also added 2n7 caps in parallel with the collector resistors on the second and third stage to tame the highs a bit and put 100Ω resistors in series with the emitter bypass caps to control the gain and reduce noise.

2. What diodes you use makes a big difference. Higher voltage drops sound harsher, more "distortiony", lower voltage drops sound smoother, more "overdrivy". I settled on BAT42. Very smooth transition from clean to clipped, which to me is the essential quality for "amp-like" dis tortion. The other diodes do that too, but I find it most convincing with Schottkys.

3. Leaving out the third stage turns the Vulcan into a fantastic "base sound overdrive". The kind you want to have always on in front of a clean amp, or use it to dirty-boost a crunchy amp for fat lead sounds. I would use it the same way as a Red Llama or DOD250 or even a Klon. This thing cleans up with the guitar volume really well. I can definitely see me using this as my main overdrive / soft fuzz.

How is this thing not more popular? The Joe Davisson Diode Compression Stage is an absolutely fantastic building block for a wide variety of distortion tones. Try it today! Sure beats yet another Big Muff or Tube Screamer. I haven't tried slapping a BMP style tone control on the 3-stage version but I think that seems like a natural pairing.

Cheers,
Andy

[aron]

You beat me. I was thinking I would build it tomorrow.

[bluebunny]

I put this board together this morning:



Now I need to go drill an enclosure...

[soggybag]

I put this into Eagle and made a layout. Totally untested. I made the schematic put the parts into a logical order and played with the auto router until I got something that looked reasonable.

[soggybag]

Here is a completed board.

[Marcos - Munky]

Wow, that was fast!

[soggybag]

Hopefully I didn't make a mistake! I put the schematic into Eagle last night. I started on a PCB but it was a total mess. This morning I took a second look, ignored the traces and organized the parts in a logical arrangement, really I just followed the schematic. Then I started experimenting with the auto router.

I took a look at the results of the auto router results. If I could see an improvement that could be made by rearranging parts I'd ripup make the changes and let the auto router figure it again. After following this process through a dozen or more iterations I had something I thought was acceptable.

[soggybag]

Here's my Vulcan. I had a problem with the pad to output, it tore off the PCB, so injury rigged a jumper to the volume pot. It's working but I'm not sure it sounds quite right. It really loud. I had to make a couple part substitutions. I didn't have any 4m7 resistors so I used 3m9. I'm not sure how this would affect the bias here.

[rockola]

Can I ask a silly question? How does the signal get past each of the three diodes? I've been trying to figure out this for a week now and still don't get it.

[willienillie]

Only "part of half of it" does, distorted.  Think about AC through a half-wave rectifier, minus the Vf.

[Gus]

Can I ask a silly question? How does the signal get past each of the three diodes? I've been trying to figure out this for a week now and still don't get it.

If you redraw the circuit a little it will make more sense
from the base point the diodes "down" (vertical)
Connect the two resistors at the bottom side of the diodes
Transistors have a VBE drop
Si diodes have a voltage drop
Note the DC voltages at the bases and each end of the diodes

[rankot]

Hi guys,

I just breadboarded the Vulcan again. Wow, what a fantastic circuit! I remember liking it when I first tried it but memory fades I suppose.

Some notes:
1. I played around with the R and C values in an attempt to optimize it. No such luck, Joe's original values are spot on for the best (or at least my favorite) sound. The only thing that turned out to be a big improvement, was to put a buffer in front of it. Makes interaction with the guitar volume more predictable. The sound itself is not affected by the buffer very much, it only gets a bit brighter and clearer on low gain settings, which is a plus in my book. I also added 2n7 caps in parallel with the collector resistors on the second and third stage to tame the highs a bit and put 100Ω resistors in series with the emitter bypass caps to control the gain and reduce noise.

2. What diodes you use makes a big difference. Higher voltage drops sound harsher, more "distortiony", lower voltage drops sound smoother, more "overdrivy". I settled on BAT42. Very smooth transition from clean to clipped, which to me is the essential quality for "amp-like" dis tortion. The other diodes do that too, but I find it most convincing with Schottkys.

3. Leaving out the third stage turns the Vulcan into a fantastic "base sound overdrive". The kind you want to have always on in front of a clean amp, or use it to dirty-boost a crunchy amp for fat lead sounds. I would use it the same way as a Red Llama or DOD250 or even a Klon. This thing cleans up with the guitar volume really well. I can definitely see me using this as my main overdrive / soft fuzz.

How is this thing not more popular? The Joe Davisson Diode Compression Stage is an absolutely fantastic building block for a wide variety of distortion tones. Try it today! Sure beats yet another Big Muff or Tube Screamer. I haven't tried slapping a BMP style tone control on the 3-stage version but I think that seems like a natural pairing.

Cheers,
Andy

Getting ready for the first run, used BAT 43 as Andy proposed. Really curious how it will sound!

[anotherjim]

Can I ask a silly question? How does the signal get past each of the three diodes? I've been trying to figure out this for a week now and still don't get it.

The diodes are normally on. Cathode voltage is at the bias minus the diode drop. Signal will get through until it's positive enough to take the cathode voltage too high compared to the anode at which point the diode starts to block further increase.
This is actually a diode "gate". If you take the resistor pulling down the cathode to a higher voltage than 0v and if that voltage was the + supply, the "gate" will be closed and no signal could get in because the diode will then be very much reverse biased. This was the basis of key gating in old organs letting a single key contact switch multiple footages - before cheap signal diodes, they had to have separate contacts for the footages.

[Mark Hammer]

That is VERY interesting.  Not that you didn't try hard, but there is an enormous amount packed into a few short sentences that may not be fully comprehensible to a great many.  For the broader audience here, could you elaborate a wee bit on what you said?  Not a criticism, just a desire to polish up what might be a hidden gem.

[rankot]

Just finished the test - this thing is a killer! So nice, it goes to the enclosure first thing in the morning! <3

[anotherjim]

That is VERY interesting.  Not that you didn't try hard, but there is an enormous amount packed into a few short sentences that may not be fully comprehensible to a great many.  For the broader audience here, could you elaborate a wee bit on what you said?  Not a criticism, just a desire to polish up what might be a hidden gem.

If you're asking me Mark, there isn't many examples to be found to explain, although the "Analog Switch" in this article shows the principle again...
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/diode-switching-circuits/
So long as the diode in the signal path has sufficient DC forward bias and the signal swing is not large enough to change that, the diode is a conductor. If diode forward bias is removed and it become reverse biased unless the signal swing is big enough to forward bias it - but while reverse biased, the signal won't get through.

...as to hidden gem, the problem for simple diode gating is that the control means a DC level shift, which could be heard as a click or thump and that the signal itself changes the diode threshold (this last is exploited in the Vulcan). This is not a problem in an organ since it acts on large square waves which it cannot distort and tone filtering happens later. In fact, it can also be a VCA when you only have square waves to control.

Nice diode VCA can be made, but as far as I know, they had to be balanced/differential so the DC shift gets cancelled out. Something about that here...
https://sound-au.com/articles/vca-techniques.html#s9

A thing is, the articles out there are very often working from a hi-fi perspective. Any distortion/artefacts might actually be interesting to us.
Thinking of another discussion about BJT control...
https://sound-au.com/articles/vca-techniques.html#s8

[Mark Hammer]

Perfed one up this evening.  Nice work, Joe!