Overdrive Sectrets...

[Alpha579]

Hey all,
Do any of you have any secret techniques (or not so secret) for designing a good overdrive? Or any eq tips before and after the od stage?
Thanks for any input,
Alpha

[Skreddy]

Okay; my philosophy on overdrive is this:

E.Q.: Cut bass before distortion to prevent flubberiness, cut highs after distortion to prevent harshness.

Cascade several *mild* stages with attenuation in between, rather than trying to get it all in one massive dose

E.g.; use a small input cap and add resistor between the stages of a fuzzface, and you get touch-sensitive overdrive/distortion instead of fuzz.

If a clean amplifier is to be used, a larger output capacitor can add a pleasing openness to the sound.  If a distorted amp is used, a smaller output cap will prevent the amp from getting flabby.

[Alpha579]

Thanks Skreddy, ill try that

[Alpha579]

Any one else got an idea?

[aron]

Did you check out the simple mods page off of my home page?

The "secrets" if any are in the archives of this site.

I'm not kidding.

[Alpha579]

Thanx Aron!

[Ansil]

six stages of a cmos inverter cd4049 ultra low gain on each stage.
two stages in paralle for spanky sound or a heavy bass sound

that tie into the other four inverters for gain and tone shaping..

[Mark Hammer]

Cascaded gain stages like Ansil suggests are wonderful.  Keeping the gains reasonable is important to maintaining some type of dynamic headroom - you would like for your sharply attacked notes to sound a little different than your lightly strummed ones.

Something I find crucial with cascaded stages is tone-shaping.  You would like for more lower order harmonics to be present for a more vocal quality.  If you don't roll off the high end in earlier gain stages, what you end up with is subsequent stages creating harmonics of upper harmonics.  The tonal result is ugly fizziness.

Of course, the question is what sort of strategy to employ in rolling off high end.  The way I figure it is this.  Each gain/overdrive stage is going to add harmonic content on top of what it already receives, so it makes sense to roll off more high end in earlier stages and "relax" the filtering in subsequent stages.  That way, the 2rd or 3rd stage receives lots of lower-order harmonics and much less higher order harmonic content.  If I was only using a single overdrive stage, I'd probably roll off high end around 6khz or so for a bit of smoothness, but if I was going to feed that stage to another gain stage in search of "drive", I would probably roll off high end somewhere in the 2khz-3khz zone for the 1st stage and only let the high end out on the last stage.

Finally, remember to factor in the amplifier.  Some of the very best overdrive sounds come from bringing the amp to the table.  Let the pedal provide some coloration to the signal, but make sure there is a nice hot signal coming from the pedal to the amp so that preamp, poweramp, transformer (if tube) and speaker distortion can become part of the equation.

[Transmogrifox]

some pre shaping eq 3dB cut-off frequencies I have liked:

560 - 600 Hz first-order high pass is good for a "smoother" over drive.

another thing I like is a 2nd order pre-eq stage band pass filter with lower 3dB cutoff at ~700 Hz and High cut-off at 2kHz, nominal pass-band gain (no more than 10), and a mosfet clipping stage following it.  The band-pass idea usually creates a tighter "chunkier" crunch sound.  

Post eq, I usually just reverse the pre-emphasis, and let the added harmonics fall in wherever they go...use a tone stack to manually toy with these.

So... in PSPICE, I like to make a pre-emphasis high-pass @ 600Hz or BP @ 700Hz to 2 kHz (ish), clipping amplifier stage, de-emphasis filter and design for a level frequency response from input to output.

[Alpha579]

K, so what ratio of potential divider would you use after say, a 18db gain stage?

[Transmogrifox]

Experiment. Make a mess on your breadboard with a whole bunch of pots.  Plug in your guitar and tweak the pots until each gain stage is properly attenuated to produce a tone to your taste.  When you get something you like, then measure the pots and and use resistors to match it.

I have found that driving a gain stage hard with a previous stage can quickly become "muddy" or fuzz.

Backing off too much can give you a bad signal to noise ratio, and make your pedal hum more than necessary.