DOD-250 Overdrive

[guitarjack]

Please HELP!!!

Hi guys. I'm completely new to building pedals (or electronics for that matter!) so I decided to start with an easy one. I tried to build a dod-250 overdrive pedal because it has a relatively simple circuit, and it sounds okay from what I could tell from youtube. Here is a link to the schematics:
http://www.montagar.com/~patj/dodoverd.gif

As you can see, pretty simple. I spend a good 50$ so far on a bread board, wires and components, and I've been trying to get it to work, but I've been having a lot of difficulty. I tried following the instructions as closely as possible. Differences are:
Instead of 20 K resistors, I used 2 x 11 k in series.
Instead of 1 M resistors, I used 2 x 2.2 M in parallel.
Instead of 10 K resistors, I used 11 k.
I used a 520 K linear pot instead of a 500K logarithmic one.
Op Amp: I used a TL074CN instead of 741. The one I used is a quad, so I only used one of the opamps. http://www.futurlec.com/Datasheet/Linear/TL074CN.pdf
In stead of a 9V battery I have 6 x AA batteries.

When it's switched to "bypass" it works. However, when the circuit is switched to run through the opamp, the guitar signal isn't getting through. There is a bit of noise, but not much.

The voltage between by ground and the + terminal on the bread board is 9.3 V.

At the diodes: 0 V on both sides
Op Amp:
P4: 9.3 V
P8: It started at 100 mV and then it kept going down...
P9: It started at about 20 mV and it kept going down...
P10: 2.22 V
P11: 0 V

The only other thing I can think of that might be useful is that I'm doing this all on a bread board. Also, when I plug in the power source, it makes a little tic sound where the battery terminals make contact with the bread board.

Thanks!!!!!

[jrod]

Hey guitarjack-

Check out this breadboard diagram from Beavis Audio: http://www.beavisaudio.com/bboard/projects/bbp_DOD250Overdrive_Rev_1_1.pdf

Hopefully this will help you make sure that you have all the components plugged into the breadboard correctly.

Also, check out the project files here: http://www.diystompboxes.com/smfforum/index.php?board=33.0 . There should a lot of info there that should help.

That's all I got! Hope this helps!

[Mark Hammer]

That's, um, a lot of "instead ofs".  The only one I can see there that might be problematic is use of a quad op-amp with 3 unused sections just sitting there.  Everything else is completely within normal component tolerances.

Given that the more pins you have, the more ways there are to mis-assign them, your problem may lie in which pins you are using from the TL074.  Do you have a datasheet handy for cross-reference purposes?

[guitarjack]

Thanks a lot jrod! I'm going to try making it using the same diagram you posted. I'm just missing a few components, so I'll head out sometime in the next few days and pick those up.

I'll keep you posted!

And thanks MarkHammer, I tripple-checked the pins and they seemed to be okay. I used the datasheed that I posted above. Other than mixing up pins, is there anything wrong with having the extra opamps lying around?

[petemoore]

is there anything wrong with having the extra opamps lying around?
  I hesitate to say there would be, or wouldn't be, most use single opamp so there's probably not much data about using a quad.
  Instability of the unused amps may get into the assigned amplifier stage. Grounding the inputs, should keep the output held at a solid 0.0v, eliminating instability.
  Whether that is different that what a single OA uses is another matter I haven't read any test-results on.
  The LM741 is recommended for the 250 becuase it's a lousy enough opamp to add character [distortion] to the DIST+ type circuits. The performance characteristics of any 'cleaner than 741' type opamp changes the character of the distortion.

[Mark Hammer]

Other than mixing up pins, is there anything wrong with having the extra opamps lying around?

As petemoore notes, there is a risk of instability.  And even though there is no obvious audio connection between what you've built, and what's left over, they share the same power.

Think of it this way.  Imagine you had 4 different amp, all plugged in and turned on, and all sharing the same extension cord.   Now imagine that you had a pet rabbit like mine who had chewed through the cord (when it wasn't plugged in) and frayed the wires.  Any movement of the cord might introduce crackling that would be shared by all the amps, even though you aren't plugged into every single one.

I will ask for the assistance of others to recommend ideal ways of keeping the other op-amps stable, but I would imagine that a first step is to link the output and inverting pin together for each individual op-amp.  This will reduce their individual gain to 1x, which helps.

[jrod]

Thanks a lot jrod! I'm going to try making it using the same diagram you posted.

You bet! I found those diagrams to be very helpful. There are some other projects there that you might want to try!

Good luck!

[guitarjack]

You bet! I found those diagrams to be very helpful. There are some other projects there that you might want to try!
Good luck!

Come to think of it, do you recommend any other small starter projects that aren't too difficult? I'm on vacation for another 1.5 weeks, might as well keep myself busy!

I'm going to get the components tomorrow, so I might as well stock up.

[guitarjack]

I will ask for the assistance of others to recommend ideal ways of keeping the other op-amps stable, but I would imagine that a first step is to link the output and inverting pin together for each individual op-amp.  This will reduce their individual gain to 1x, which helps.

Interesting. Makes sense, I'll try that out tomorrow.

Also, what would it sound like if the op amp becomes unstable?

[jrod]

You bet! I found those diagrams to be very helpful. There are some other projects there that you might want to try!
Good luck!

Come to think of it, do you recommend any other small starter projects that aren't too difficult?

Most of the projects there are pretty easy. Here are all the breadbard projects on the Beavis Audio site: http://www.beavisaudio.com/bboard/projects/ You might want to check them all out and see which ones you are into.

But, I would highly recommend the Trotsky Drive! This is the Beavis version of the Electra distortion circuit and sounds great! It's also fun to try out different diode types and combos. You can eliminate the switch for breadboarding purposes. Just plug that 22nF cap in if you want to hear how it sounds, but it will sound great without it.

Have fun and I hope you enjoy your vacation!!!

[Joe Hart]

Come to think of it, do you recommend any other small starter projects that aren't too difficult? I'm on vacation for another 1.5 weeks, might as well keep myself busy!

Fuzz Face? You could even do a NPN Si one and make it easier on yourself. Although they do sound a bit different than the Ge ones.

A single transistor booster (LPB-1)?

Some sort of treble booster?

-Joe Hart