Another Interesting OD: Schorched Earth

[Transmogrifox]

I just last night got an OD design working to my satisfaction.  It's a kind of a fun sensitive pedal to play with.  You can get it to break up a bit when you dig in.  I think it does a bit better than the average standard OD, and has a little more gain available for tight crunch...and I thought some of you may be interested.

Here's the schematic:

http://www.geocities.com/transmogrifox/SchorchedEarth

Make sure you copy and paste this link to another browser as geocities generally doesn't let it fly the first time

I ended up getting the BMP tone topography to work well after all. I just had to design it with a cross-over frequency that was not immediately intuitive.  I must say, however, replacing the tone control I put in it with a standard Fender amp 3-band tone controll will make it a heck of a lot more flexible.  I did the 1-knobber because I designed it for a friend as a graduation present and he wants something simple to use so he can just plug and play without having to mess with knobs.

Audio samples will probably be another week or so because I just started transferring the rest of the circuit to perf board, however, let me know if any of you breadboard it or anything. I thought it was a similar concept to Blackstone Appliance's OD after listening to his audio samples. This Scorched Earth distortion is voiced slightly differently and has a bit more grit to it. The "Fizzle" switch and LED clipping is purely optional. This just makes it sound a bit more like a "Distortion" pedal instead of an "overdrive".

One of the greatest assets to this circuit is that it's guitar volume knob sensitive. I can max the distortion control on the pedal and roll my guitar back to about 2 or 3 and get almost completely clean output, then roll the knob up to max and have a raging crunch. YOu can get fuzz out of this if you roll your tone all the way back and crank the pedal's distortion.  The simplicity of distortion control was another way to help simplify it for the buddy I designed it for.

[Jason Stout]

looks cool, What MOSFETS did you use?

[puretube]

RESPECT!  

[Alpha579]

uh, says geocites site not found...

[Marcos - Munky]

Copy the link and paste in another window.

[Eric H]

I think there's a missing component at q2

-Eric

[Transmogrifox]

I think there's a missing component at q2

-Eric

Actually, the schematic is correct.  This is part of a current mirror, and R4 defines the current through the differential pair. I know Q2 may look a little weird because it's not a common use of transistors in discrete component designs, as most commercial FX designs seldom use a current mirror.  It IS important that q1 and q2 are the same transistor type and relatively well matched.  I used 2N5089, and the difference from component to component on these is probably not anything to worry about.  YOu may also use 2N4124, or the MPSA18 or even the old 3904 or 2N2222.  5089's are low noise, so it's that much noise that is not added.

looks cool, What MOSFETS did you use?

2N7000
I think anything with a threshold voltage of less than 2V could be used just fine.  I'm sure there are lower noise MOSFETs out there than 2N7000.  I would like to know what they are if anybody has any suggestions for lower noise FETs (without re-designing with JFETs...though that may be kinda fun, I'll have to try it).

[Eric H]

Actually, the schematic is correct.  This is part of a current mirror,

:oops:
Sometimes it pays to take two looks before posting.



-Eric

[Paul Marossy]

It appears as though you have created a circuit that does something silmilar to an OTA (like a CA3080), but with a pair of NPN transistors? Anyhow, looks interesting. I am looking forward to hearing a soundclip.  

I had an idea for an OTA based distortion a while ago, but I never actually gave it try...

[puretube]

I wouldn`t compare it to an OTA,
rather call it a "long tail" diff.-amp.

[Paul Marossy]

Ahhh, OK.

[Transmogrifox]

I have actually tried modulating that 20k resistor on the current mirror.  Yes, it does work like an OTA.  Makes a kind of a fund "fuzz swell" using my proximity detector thing I made for my proxxi-wah.  I hooked it in series with the wah filter and modulated both simultaneously and that's when stuff started to sound more like Zachary Vex was in town.

[Paul Marossy]

Oh, so it does?!? I only thought it might be similar to an OTA because they use current mirrors, too. I still am interested in hearing a soundclip.

[puretube]

free information block:

[Peter Snowberg]

Puretube said it well and I feel the same way.

RESPECT!  

Nice work!

Take care,
-Peter

[Transmogrifox]

Puretube said it well and I feel the same way.  

puretube wrote:
RESPECT!  


Nice work!

Thank you both for the compliments  :oops:  ...er-->  

I went hardcore on it this weekend and got it in a box and functional, now it only needs some knobs on the pot shaft and I need to get an SPST switch for the "fizzle" LED/diode option (right now I just have the two wires sticking out the hole in the side of the box).  I wanted to minimize cost because I'm giving it to a friend, so I used a cheap RatShack push-button switch for a stomp-switch to activate the relay switching, but I quickly realized yesterday that this would be a dirty trick on the guy.  I broke the switch after 10 minutes of playing (stomping on and off about 5 or 6 times) and had to replace it. Waste of $2. I think I should get a good quality durable switch for this.

The moral of the story is, now that it's working, I should be on schedule to get sound clips up this week.

So...any of you have some cool ideas for exterior decoration?  I was thinking "Scorched Earth" for a name...maybe I should take the components out of the box and light it on fire for a while or something :twisted:

EDIT:  

I completed it yesterday and it looks beautiful.  All of it is together: knobs, switches, pots, artwork.  Pics and soundclips will be coming soon, hopefully.  I used a red sharpie to color the box, clear coat, red sharpie, clear coat, black sharpie labels, clear coat, fix label mistakes, clear coat, dry, clear coat again.  The sharpie turns out looking REALLY nice.  I just need to see now how durable this clear coat stuff is.

[Transmogrifox]

bump :lol:

[Jay Doyle]

Trans, that is great work with discrete devices.

As I am sure you know, you just learned a lot about designing opamps.

To jack the gain, try a current source load on the diffamp. You only need to use one, the other will match the current automatically.

Again, great work.

Jay Doyle

[Transmogrifox]

Audio samples are now available.  I got them this afternoon.

http://www.geocities.com/transmogrifox/ScorchedEarthPage

copy and paste the link to another browser.

The Audio links are at the bottom of the page with some explanation.

Thank you all for your comments and compliments.

Jay:  Thanks for you comments...and yes, I have thought of trying active loading.  For this design, it was kept passive so that the Drain Voltages would be close so the diode clipping would work properly.