OBSIDIAN - Mosfet-ish for the noobs

[WGTP]

I guess I sort of have a Mosfet Fetish - Mosfetish for distortion.  They mysteriously lurk on the outskirts of the Distortion Box mainstream (Out here on the perimeter we is stoned immaculate).  Also note other distorted minds like Blackstone, ZVEX and Seymour Duncan are using Mosfets for distortions. 

Currently I have a modified Obsidian type circuit on the breadboard.  The input cap is small (1n - 2n) which reduces the bass thru the distortion, and then the BMP tone control at 33% restores the bass.  (This increases the ratio of distortion and sustain for the highs versus the lows and gives it a more ampish tone IMHO).  You can vary the mid-range from flat to deeply scooped by experimenting with the Duncan Tone Stack Simulator.   The EQ is very important and effects things like sustain and dynamics that you might not at first expect or understand.   

http://www.diystompboxes.com/analogalchemy/sch/obsidian.html

This is Joe Davidson's OBSIDIAN.  The mod's I'm using are:

1) Lower the input cap to 1n or 2n
2) Alter the tone stack to reduce the mid's the desired amount by adding a 10K resistor prior to the stack and:
    A) Change both resistors to 22K or for more scoop, 15K and 33K or even more, 8.2K and 47K.  Infinite possibilities
    B) Change the capacitors to 10n and 47n.
    C) Change the 100K pot to 50K (that is the best sized trim pot I had for the breadboard.
    D) If it is too bright, try adding a cap in parallel with the 3.9K resistor in the last stage.  Start with 4.7n and adjust to taste.

Remember Mosfets are sensitive to Static Electricity and will blow easily.

To me the Mosfets have an "air ripping" (ZVEX uses "Blizzardy") sound that is more like loud tube amp distortion than most.  More like a mini virtual raging marshal stack (RMS).  Let it Rip.   

[slacker]

Cool, I'll have to give it a try.
I was just playing about with a Box of Rock circuit on the breadboard the other day and mosfet distortion does sound good. Instead of the big muff tone control try a shredmaster contour or superfuzz style mid scoop on the end, very metal 

[suprleed]

I've been meaning to try one of these Davisson circuits for months now....Work has just been killing me lately and probably won't see daylight until next month.  Your mods sound very promising.  I haven't ventured into Mosfets yet, but this could be my first.

Rock on.

[John Lyons]

WGTP did you use a positive of neg ground version?
How is the "noise"?

I've been meaning to try some of Joe's Mosfet things for years...

John

[WGTP]

Everyone into distortions needs to get a bread board and try Joe's Vulcan and Blackfire as well.  Then you can mix and match stages.  Very cool unique stuff.   

http://www.diystompboxes.com/analogalchemy/

At first I tried a Vox tone stack by can get about the same EQ curve I was using with the modded BMP with fewer parts and only 1 knob.

I have tried it both with the N channel 2N7000, BS170 and P channel BS250.  As Joes says, the P channel is quieter.  Since I play at low volumes thru a dark amp, the noise isn't a problem, but some folks here have mentioned it.   

On Joe's earlier Obsidians that were N channel, he used the LED between the gate and the source for protection like ROG does.

A single stage is supposed to work well as a Booster.   

I'm wondering about adding an additional stage and increasing the resistance between stages to 100K or using voltage dividers like a tube amp.  Looks like I might need a larger bread board and a fire truck.   

[merc]

Do you have a pcb for the Obsidian?  I've looked everywhere, but I can't find any pcb layouts except for the "Obsidian T" or "Obsidian Transistor".. or are they the same thing?

[WGTP]

I think those are different.  The Obsidian is easy to build layed out like the schematic.  Stick the parts thru the holes a perfboard and use the leads to solder it all together.  Use sockets for the mosfets, input cap, and a bypass cap.  That way you can "tune" the bass to your taste.  Also remember that the mosfets are static sensitive.   

[merc]

So it's alright to use perfboard?  I had the impression it would be too messy and bulky.  If it works though, I would definitely rather use perfboard, since I have no experience with PCB's.  Now my only problem is what enclosure to use.  And the static sensitive part.  How would I avoid giving my fragile little mosfets a static shock?  And by the way.. by "tuning" you mean switching the caps and mosfets for different ones until I get the sound I like?  I'm completely new to this pedal making stuff, so I'm still learning the terminology.

[merc]

One more thing I forgot about:  I have heard from many places that the mosfet version of the Obsidian is too noisy.  Is this true for the schematic I'm using?

[WGTP]

Noise has been attributed to Mosfet distortions, which is why Joe used the P channel BS250's which are supposed to be inherently quieter.  Of course there is lots of gain there.  A tidy construction also helps.  Socketing the mosfets allows you to find the quietest ones to use.  Buy 10 and use the quietest 3.

Make sure you touch something grounded before you touch a mosfet.  That should discharge the static.  Another good reason to get 10.

I don't want to sound discouraging, but there are easier first projects.   

[merc]

Yeah, you're probably right about there being easier projects.  But I would rather make an overdrive pedal than the beginner project, which if I remember right is a booster of some sort.  True, I don't know what a booster does or what kind of sound it gives, but I've been craving an overdrive pedal for sometime now.  By the way, since you built all three versions, would you reccommend the BS170 or the BS250 version or the transistor version??

EDIT- Forgot to add the last question.
EDIT 2- Added on to the last question.

[WGTP]

The BS170/BS250 should sound about the same, but the BS250 is quieter.  You have to flip the electrolytic caps around and the battery clip, but that's the only differnece.  The transistor versions have a different sound, all good.  There are no crossing wires on the Obsidian or Vulcan, so that makes them easier to build.  Can you show me the schematicfor the transistor version your looking at?  There were some earlier versions that were fuzz face like, if that is what your after.   

[Renegadrian]

http://www.diystompboxes.com/analogalchemy/sch/obsidian.html

Looking at the schematic, it seems like this
http://geocities.com/electrictabs/dr.boogey.png

which obviously came from this
http://www.tubefreak.com/recto1.gif

[merc]

The first link there is the schematic for the BS250 version I was looking at.  For the transistor version, I was just going by a layout by 96ecss found at http://www.aronnelson.com/gallery/main.php/v/Daves-Layouts/Obsidian+T.gif.html

There are parts I don't understand on the layout though, and 96ecss didn't reply to my questions, so that version is looking doubtful.  I am now starting to realize what you meant when you said there are easier beginner projects... I'm taking a look at Mark Hammer's Chaos now but I don't know how easy that is either.  I've heard some good things about it though.  If both projects are too difficult, it looks like I might end up doing the beginner overdrive pedal with the complete tutorial.  I listened to some clips though, and I'm looking for something a little more distorted than that one.

[WGTP]

Although similar, the Obsidians pre-date the Jfet Sim's by several years IIRC.  They are simular in that they have multiple stages of FET's, but the Obsidians were not designed to be a SS "clone" of a Tube pre-amp.  Note that each stage is the same.  To me, it is more like 3 Mosfets Boosts in series.  The biasing on Mosfets is not as similar to tubes as Jfets, although you could do sim's with Mosfets.  Seems like I saw one somewhere.  3 stages of distortion makes all of these designs more Tube Amp like and sound superior to me than a single distortions stage maxed out.  

The Chaos is really cool and sent me off into using dual op amps for most of my experiments. 

Might check out the SMASH DRIVE.  It is supposed to sound good, be easy to build and has a number of variations to experiment with around here.   

[Renegadrian]

Here are some files I found in my archive...Hope that helps...

[ambulancevoice]

Joe D's Massacre

i bet the first stage alone would make an interesting boost/overdrive