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Overdrive w/Dynamics

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9/14/1998 4:01 PM

Overdrive w/Dynamics

I have been trying to figure out if using a compander in a Over Drive (OD) circuit will give you a dynamic OD. I would like an OD that responds to changes in picking dynamics with output level increases and reductions that track the input level. I was thinking a NE571 set up for compression, followed by a standard op amp diode clipping section, followed by a NE571 set up as an expander. Any Thoughts?
Thnx, CBS.

9/15/1998 1:18 PM

I'm afraid this setup might not work too well.
When guitar players refer to "dynamic overdrive", this usually means that the amount of clipping that occurs is easily controlled by the player's pick attack. Light picking results in a sound that is clean or nearly clean, and hard picking results in saturation and extra harmonics resulting from increased signal clipping.

Placing an expander after the clipping circuit does nothing to aid in touch-controlled clipping. Placing a compressor before the clipping circuit actually reduces dynamics and can actually make it harder to "clean up" the signal by backing off on picking attack (since the compressor will try to increase the signal gain to compensate for the lower input signal level).

It might still be interesting to go ahead and build the circuit you have in mind - it might yield some interesting effects - but don't be disappointed if it doesn't give you what you originally wanted.

9/15/1998 2:12 PM

When guitar players refer to "dynamic overdrive", this usually means that the amount of clipping that occurs is easily controlled by the player's pick attack. Light picking results in a sound that is clean or nearly clean, and hard picking results in saturation and extra harmonics resulting from increased signal clipping.

I've been experimenting with something that does this almost exactly. It's a modification to my clone pedal I made last year (tube-screamer based). I found that by using certain MOSFET devices as diodes, you get a progressive limiting effect. It compresses before it clips which allows you to go between a clean sounding signal to one that crunches nicely simply with pick dynamics. The reason for this has been discussed here before regarding how the MOSFETs conductance changes in proportion to the square-root of the current, similar to a tube.

It sounds dangerously tube-like. I think the compressive aspect gives it much more bottom end. Too much actually, I'm going to try messing with the cap/res combo on the clipper to be more TS9 like and see if that doesn't help out.

The MOSFET device I'm currently using (VN0300) I think is a little too 'soft' so I'm going to try some devices with more conductance to 'harden' the clip some more.

The best part is that I was able to implement the changes using a 100K pot with a DPDT push-pull switch so you can switch between the Tube-screamer sound and the new 'crunch' mode by pulling the volume knob out. The two sounds are quite different and it's like having two effects in one.

If I get the time to mark up a schematic and document the changes, I'll make them available to whoever wants to take a look.

--johng

9/16/1998 6:38 AM
jason	I'd like to see that schematic also if you get a chance to draw one up. Thanks, jason

9/23/1998 7:03 AM
cbs	John Greene John, Hows your schematic coming along? I'm interested in seeing it too. Thnx, CBS

9/23/1998 10:03 AM
John Greene	It's a work in process, you can see the current state here: http://www.cyberverse.com/~jandrox/images/scrmr+.ps --johng

9/15/1998 3:39 PM

Re: Overdrive w/Dynamics

I didn't mean the clipping will change with pick attack, I meant the r.m.s. value (output level) tracks the input r.m.s. value (input level). As I understand a compander circuit is suppose to comprees the dynamic envelope, then restore (expand) the dynamic envelope to approximate the input signal. Of course the expander might need to be set to a higher expansion rate than the compressor, due to the additional "compression" from the clipping process.

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