How do I make an uncompressed overdrive

[samuelgin]

I am fairly new to effects creating as a whole and have a decent understanding of electronics and from my natural "I want to do this myself" mindset, I want to create my own overdrive. I am a big enthusiast when it comes to dynamics coming from the guitar, not what pedals are turned on, so the compression that most overdrives seem to have upsets me. The only one that I've been really content with is the fulltone OCD because I can leave it on and not have to worry about it. When i pluck lightly or roll off the volume its incredibly clean, but then I strum a chord and its so rich sounding. the schematic is pretty well hidden (i havent been able to find it anywhere) but I read somewhere that it gets its dynamic abilities from the diodes. according to http://www.pedalarea.com/ocd.htm, instead of going straight to ground as most diodes do, the ones in the OCD drive connect to a "floating bias point that is connected to the input of the gain stage". to me this says that the pedal has pretty low drive, but instead of compressing after it clips it allows it to cycle back through and add more gain to the signal each time, allowing it to build to such a high level of drive but clean up very well.

does anyone understand this and could help me make it work?
since I'm new to all this I'm not going by a from-scratch design. I'm experimenting with different transistors for different gains and different capacitors and resistors for tones to hopefully get what I'm trying to make. The schematic I've been going off of is the Trotsky Drive (http://beavisaudio.com/projects/TrotskyDrive/) from Beavis Audio because its more "beginner friendly" in its instruction and it only has 16 parts and instead of the site being a forum of specific questions that are no help if you dont know what any of it means yet. But now I'm here to ask specific questions and i guess move on to the "hard-food" of pedal designing/building

[Bill Mountain]

I am fairly new to effects creating as a whole and have a decent understanding of electronics and from my natural "I want to do this myself" mindset, I want to create my own overdrive. I am a big enthusiast when it comes to dynamics coming from the guitar, not what pedals are turned on, so the compression that most overdrives seem to have upsets me. The only one that I've been really content with is the fulltone OCD because I can leave it on and not have to worry about it. When i pluck lightly or roll off the volume its incredibly clean, but then I strum a chord and its so rich sounding. the schematic is pretty well hidden (i havent been able to find it anywhere) but I read somewhere that it gets its dynamic abilities from the diodes. according to http://www.pedalarea.com/ocd.htm, instead of going straight to ground as most diodes do, the ones in the OCD drive connect to a "floating bias point that is connected to the input of the gain stage". to me this says that the pedal has pretty low drive, but instead of compressing after it clips it allows it to cycle back through and add more gain to the signal each time, allowing it to build to such a high level of drive but clean up very well.

does anyone understand this and could help me make it work?
since I'm new to all this I'm not going by a from-scratch design. I'm experimenting with different transistors for different gains and different capacitors and resistors for tones to hopefully get what I'm trying to make. The schematic I've been going off of is the Trotsky Drive (http://beavisaudio.com/projects/TrotskyDrive/) from Beavis Audio because its more "beginner friendly" in its instruction and it only has 16 parts and instead of the site being a forum of specific questions that are no help if you dont know what any of it means yet. But now I'm here to ask specific questions and i guess move on to the "hard-food" of pedal designing/building

The OCD schematic is easy to find and it's not even an original Fulltone design.  Anyways...people seem to be quite happy with that pedal and I have seen many boutique versions so there must be something to the diode placement.  Personally when I hear "overdrive without compression" the first thing I think is no diodes.  Nothing compresses your signal as easily as diode clippers.  It doesn't mean it sounds bad but if you want less compression you may want to look at some of the Jfet circuits over at runoffgroove.com.

Good luck!

[dp]

Check Rory out. He was using a rangemaster on this.The volume knob swells are bone chillin.http://youtu.be/HKC8dPBXIw4

[slacker]

schematic is pretty well hidden (i havent been able to find it anywhere)

If you do a google image search for "OCD schematic" there's about five on the first page. They might not all be correct, but they have enough information in to show the structure of the the pedal and will allow you to build something you can then tweak to your taste.

but I read somewhere that it gets its dynamic abilities from the diodes. according to http://www.pedalarea.com/ocd.htm, instead of going straight to ground as most diodes do, the ones in the OCD drive connect to a "floating bias point that is connected to the input of the gain stage".

This is just wrong, the diodes are connected to a "bias point" but to audio signals this looks exactly the same as ground, it's just a different way of doing it, the end results are the same. Any magic in the pedal comes from a combination of the amount of gain in the first stage and the choice of diodes used to do the clipping.

Starting with something like the Trotsky Drive is a great idea, those simple little circuits can sound great.

Welcome aboard

[Mark Hammer]

1) ALL overdrives compress.  Every last stinking one of them.  The reason I can say that is because they generate harmonic content when they run out of headroom, and it is the additional harmonic content we seek from them.  The question is how much compression they introduce in producing the harmonic content they do.  Some introduce more than others.

2) The general strategy for maintaining as much dynamic range as possible whilst still getting the additional harmonic content, is to a) keep a fairly high clipping threshold so that there is a bigger range of input signal amplitudes between the noise floor and the point where clipping sets in, and b) distributing clipping across multiple stages so that each stage is not imposed on so much as to exhaust its headroom and produce compression.

3) Many people will tout the virtues of "asymmetrical clipping" in their descriptions, goals, or ad copy.  In truth, the clipping produce by diodes is rarely asymmetrical in the purist sense (i.e., all one set of harmonics and none of the other).  I could make a pedal with 2 LEDs in one direction and 4 in the other for "asymmetrical clipping" and I probably wouldn't hear any clipping at all; it's all in the relation to signal amplitude and power supply, not JUST the diodes.  But what differential dfiode clipping of different half-cycles WILL do is raise the clipping threshold enough for one half cycle that you get the additional dynamics, while keep the threshold lower for the other half cycle so that you still get to hear a reasonable amount of clipping.  That is, i fact, what a lot of folks like about the varios pedals that aim for asymmetry, like the Boss SD-1, or many of the Fulltone pedals, or even the Interfax Harmonic Percolator.

[jafo]

1) ALL overdrives compress.  Every last stinking one of them.  The reason I can say that is because they generate harmonic content when they run out of headroom, and it is the additional harmonic content we seek from them.  The question is how much compression they introduce in producing the harmonic content they do.  Some introduce more than others.

Unless you use several frequency multipliers (each set to a different harmonic) and mix to taste. No clue how to do it in a stompbox, but it's how additive synthesis works -- not to mention pipe and tonewheel organs.

You could also use FM synthesis, with the signal modulating itself. (Again, no idea how to do it in hardware.)

[earthtonesaudio]

There are ways.  One could detect the dry signal amplitude, then clip, then adjust the post-clipped amplitude based on the input amplitude.  Compander ICs can do this.

The main problem is semantics.  Overdrive only occurs because of compression, etc.

[Gus]

Search here and the web for "Bixonic Expandora" study the circuit for ideas
Some good threads about it

Another thing to do is use an EQ between the guitar and amp to increase and/or decrease frequencies, you might want to keep the EQ from clipping.

[WGTP]

IIRC the common Boss DS-1 has the diodes connected that way also.  LED's produce less compression and distortion in the typical op amp distortion than the silicon or GE diodes because of the higher clipping threshold.  I usually find the standard 2 silicon diodes too compressed for my taste.  Overdrives using discrete components like Mu/Srpp amps, multiple Mosfet or Jfet (boost) stages can be even more dynamic if not driven too hard. 

[Mark Hammer]

There are ways.  One could detect the dry signal amplitude, then clip, then adjust the post-clipped amplitude based on the input amplitude.  Compander ICs can do this.

The main problem is semantics.  Overdrive only occurs because of compression, etc.

And that's roughly what the EHX Graphic Fuzz does.  It taps the input and derives an envelope signal before any clipping is applied, and then applies that to an OTA (CA 3094 in this instance) located after the clipping stage.  This restores the original input dynamics that were effectively removed by the clipping.  It's not technically companding, since there is no compression, only expansion.

At the same time, I don't think I would be too  far off in suggesting that use of techiques like this may well work better on paper than in practice.  It takes a lot to produce an envelope follower that tracks the moment-to-moment dynamics of a signal well.  I suspect this is really more for making a hard-struck chord sound as loud as it ought to, and is not the level of envelope-following that would track the finger dynamics of fast picking.

[Jazznoise]

1) ALL overdrives compress.  Every last stinking one of them.  The reason I can say that is because they generate harmonic content when they run out of headroom, and it is the additional harmonic content we seek from them.  The question is how much compression they introduce in producing the harmonic content they do.  Some introduce more than others.

Unless you use several frequency multipliers (each set to a different harmonic) and mix to taste. No clue how to do it in a stompbox, but it's how additive synthesis works -- not to mention pipe and tonewheel organs.

You could also use FM synthesis, with the signal modulating itself. (Again, no idea how to do it in hardware.)

FM is a method to generate harmonic content..but it probably wouldn't sound like a fuzzbox! Since modulating Phase and Frequency are essentialy the same thing the most simple way to do it would be to use the signal to modulate an all pass filter at an audio rate. Maybe even force it to modulate against itself! However the harmonic complexity would be relative to amplitude and in order to maintain any sort of repeatable results I imagine companding would be required. Otherwise those Bessel Functions will just get wacky, yo!

An expander is probably the most simple solution besides doing a psuedo New York compression thing and just mixing heavily distorted guitar with the clean signal and filtering the 2 to taste.