Doing a booster to distortion mod as described in the "Mod" section...

[Steel Curtain]

Hello everyone!

       My name is Rob and I'm new to this forum and to pedal building all together. As of right now, I'm trying to learn as much as I can about the schematics and what does what. I've been reading the info on the main site here almost everyday and I'm gradually picking up on the concepts and theory, if that's the right word. Soon I will order up a kit and build my first project.

        Anyways, I was reading about a mod on the main site here that describes how to convert a booster schematic to a distortion schematic.. Here it is...






My question is, if I put this setup on the output like this instruction suggests, would the volume control that once controlled the boost now control the distortion instead? I figure this is likely because it would be increasing or decreasing the signal strength into the diodes. I'm thinking that I would have to insert a volume control after this mod to control the overall output, is this correct? Or am I totally misunderstanding this and the volume control still controls the volume and there is a constant level of distortion?

So yeah, basically what I'm asking is, if I do this mod, what happens when I turn the pot on my booster?

This is likely a noob question, but I've got to learn sometime! Thanks everyone.

-Rob G-

[Electron Tornado]

My question is, if I put this setup on the output like this instruction suggests, would the volume control that once controlled the boost now control the distortion instead?  -Rob G-

Take a look at the schematic of the MXR Distortion + here:  http://www.diystompboxes.com/smfforum/index.php?topic=90241.0

See where the diodes are placed - after the output capacitor on pin 6 of the op amp, and before the volume control. The volume control will not have any effect on how much the diodes are clipping.

Here is another kind of circuit, which is basically a booster with clipping diodes:
http://www.beavisaudio.com/projects/TrotskyDrive/   

You can see the diodes are also placed after the output cap coming from the collector of the transistor. Again, the volume control does not have any effect on how much the diodes clip.


Hope this helps.

Welcome to a great hobby!

[amptramp]

Welcome to the board!

If you add the diodes to the output of a booster, any change in gain ahead of the diodes sets the volume up to the point where the diodes begin to conduct.  Beyond that, the signal strength only increases because of the small internal resistance in the diode and the larger amount of time with the waveform above the clipping level.  So yes, the existing level control would become a distortion control.  A volume control after the diodes would control only the level and would have little effect on distortion.  Note that the series resistance of the diode should be lower than the output impedance of the amplifier stage that is driving it because the change from high to low resistance at the diode breakover voltage is what divides the voltage and causes the change in waveform.

You may want to reduce gain at high frequencies since the clipping generates higher harmonics, so most distortions that work with antiparallel diodes to ground add a capacitor to ground to limit the high frequencies.  Note that if you use identical diodes going up and down, it tends to favour odd-order harmonics - third, fifth, seventh, etc. and the ultimate result would be a square wave that sounds more like a clarinet or a Lowrey organ once you have dialed in enough signal boost ahead of the diodes.  If the diodes are not identical or you have two in one direction and one in the other, you will get even harmonics as well.  You will see similar designs in this forum where the designer has used a switch for symmetrical / asymmetrical clipping.

There is a section of this forum that has schematics of some of the common pedals and there are links to other sites that have schematics as well.  Examine these and figure out what the designer intended.  Start with the fuzz / distortion / overdrive devices and then you can move on to the more advanced items.

[Steel Curtain]

Take a look at the schematic of the MXR Distortion + here:  http://www.diystompboxes.com/smfforum/index.php?topic=90241.0

See where the diodes are placed - after the output capacitor on pin 6 of the op amp, and before the volume control. The volume control will not have any effect on how much the diodes are clipping.

Here is another kind of circuit, which is basically a booster with clipping diodes:
http://www.beavisaudio.com/projects/TrotskyDrive/   

You can see the diodes are also placed after the output cap coming from the collector of the transistor. Again, the volume control does not have any effect on how much the diodes clip.


Hope this helps.

Welcome to a great hobby!

Thank you ET.

       Actually, I have been comparing the project I am proposing to the DOD 250 schematic, which I understand to be almost exactly the same as the MXR. I'm even figuring that I would just copy the clipping arrangement all together, including the volume control on the very end.. The booster circuit that I plan to use has a 1.5uf capacitor before the proposed clipping section and the DOD 250, at least according to the schematic that I'm looking at, has a 22uf followed by a 10k resistor and then the diodes. The Trotsky drive schematic that you linked me to has a 100nf cap there and no resistor. I'm unsure what difference this makes in the circuit or which arrangement I should use. Can someone tell me what differences the cap value would make?

Thanks again.

[Steel Curtain]

Welcome to the board!

If you add the diodes to the output of a booster, any change in gain ahead of the diodes sets the volume up to the point where the diodes begin to conduct.  Beyond that, the signal strength only increases because of the small internal resistance in the diode and the larger amount of time with the waveform above the clipping level.  So yes, the existing level control would become a distortion control.  A volume control after the diodes would control only the level and would have little effect on distortion.  Note that the series resistance of the diode should be lower than the output impedance of the amplifier stage that is driving it because the change from high to low resistance at the diode breakover voltage is what divides the voltage and causes the change in waveform.

You may want to reduce gain at high frequencies since the clipping generates higher harmonics, so most distortions that work with antiparallel diodes to ground add a capacitor to ground to limit the high frequencies.  Note that if you use identical diodes going up and down, it tends to favour odd-order harmonics - third, fifth, seventh, etc. and the ultimate result would be a square wave that sounds more like a clarinet or a Lowrey organ once you have dialed in enough signal boost ahead of the diodes.  If the diodes are not identical or you have two in one direction and one in the other, you will get even harmonics as well.  You will see similar designs in this forum where the designer has used a switch for symmetrical / asymmetrical clipping.

There is a section of this forum that has schematics of some of the common pedals and there are links to other sites that have schematics as well.  Examine these and figure out what the designer intended.  Start with the fuzz / distortion / overdrive devices and then you can move on to the more advanced items.

Thank you AmpTramp!

     Very informative! I'm also shocked that I understand it, I must actually be learning something.

The DOD 250 schematic that I'm studying has that cap after the diodes to smooth out the highs like you suggested. I'm sure I'll be experimenting with it's value. I'd prefer the asymmetrical clipping, but I understand that putting two diodes in succession to ground after a single diode looking the other direction reduces the overall distortion due to the totaled values of the two diodes together, Right? I'm hoping for a healthy amount of distortion with this pedal. I'm not sure how much signal strength I will have before the diodes with a single transistor boost section. If it's not enough power, I'm not sure how to strengthen it. Can I just change some values? Or would another transistor be necessary? I've got more reading to do!

This is fun.  

[Electron Tornado]

 Can someone tell me what differences the cap value would make?

The DOD 250 and Distortion + are very similar, and make an easy project for a first time builder.

Capacitors - when you see a capacitor think "filter". A simple way to think about it is that a larger value will let more low frequencies through, and a smaller one will allow more highs through. As for what you will like - well, here's one of the coolest parts of DIY - you can take the time to experiment, try different values, and find out what sounds best to you. Do the same thing with the clipping diodes. Try different diodes, and different combinations.

Don't worry about the amount of distortion with the DOD 250, or something like the Electra or Trotsky, there's plenty there. I think most of the questions you have will be answered when you build it. Which brings up another point - I highly recommend buying a breadboard so you can build the circuit and experiment without having to solder (it's also faster when you want to try different things). You can order one from one of the online stores or Radio Shack has them as well.