Booster or distortion from one circuit

[David]

Sorry if the title is confusing.  I have a new pedalboard and the stupid thing is getting crowded already!  I need to reclaim some space.  It seems to me that there is a lot in common between a simple op-amp booster and an op-amp distortion without diodes-to-ground:  It appears that boost vs. distortion is a function of the selected gain.

With that in mind, it would seem that one could make a selectable boost or distort by using a footswitch to change the total resistance in the feedback loop.  For example, a buffer has no resistance.  A booster would have what? Maybe a gain of 20?  So for a distorter, you kick in another resistor and crank up the gain.

Any thoughts?  Anyone?

[MartyMart]

Your problem is formost space right ?
Booster - 1590B size & leads I/O
OD/Dist - 1590B size & leads I/O
You could fit a boost/OD in a 1590BB with 2 x 3PDT switches and I/O jacks at the top yes ?
Would that solve the space problem ?

MM.

[Mark Hammer]

If your clipping threshold is high enough, then you can apply a fair amount of gain to the signal before clipping sets in.  So, if you had a 2+2 or 3+3 back-to-back diode combination for clipping, whether in the feedback loop of an op-amp, or between the op-amp output and ground, clipping would not set in until the signal exceeded either 1v (with 2+2) or about 1.5v (with 3+3).  Alternatively, a pair of back to back red leds gives a clipping threshold of around 1.6-1.7v.  Those are pretty hot signals.

A gain of x10 or even x20 would give only occasional momentary clipping under those circumstances (e.g., with hot humbuckers if you power chord), so what you'd have is essentially a clean booster.  Up the gain more to at least x50, and you start to get more noticeable and consistent clipping.  Since the gains needed to generate a more overdriven tone from those diode arrangements may well be much hotter than is useful for feeding other pedals downstream, it might be wise to use a switch of some sort (toggle can work) to pad down the output when gain is cranked so that the output volume control can be more useful over most of its rotation.

If you use a noninverting opamp configuration, like the DOD250, MXR Microamp, etc., then a DPDT toggle can serve to simultaneously add a fixed resistor in parallel with the ground leg of the op-amp (between the inverting input and ground) to increase gain, and "unshunt" a second fixed resistor in series with the input lug of the volume control to pad down the output.

[Gilles C]

You can also use a FET in parallel with a feedback resistor to lower the gain of the opamp.

Check DOD effects schematics for examples on how to do it. That's my favorite method.

Gilles

[seanm]

The Voodoo Labs Sparkle Drive can do both. It has a blend control that blends the clean and the O/D. If you turn the clean up all the way it is a clean boost.

The idea of putting an O/D and boost in the same box is good. The problem is physical, where do you put all the switches and knobs?

Mark's idea has also worked well for me. My "Mr Smooth" works that way but with no diode clipping. Two gain stages. When the first gain stage is low, no O/D. When the gain stage is cranked, it pushed the boost into O/D. You could add diodes just like Mark described.

[David]

Thanks for the replies, one and all!

I came up with a simpler plan.  What I do now is to set my distortion at slightly above unity gain for power chords.  If I need to bring it up, I kick in a boost.  I'll just do the same thing, but I'll build my booster and distortion into 1590A boxes, like Pushtone and Processaurus did to such great effect.  I'll also use 1/8" jacks, 9mm pots and a barrel plug power input.  Could be I could mount the PCB by extending it and putting the stomp switch through it.

Now the cool thing is, with so much commonality between the buffer, booster and distorter, a single PCB layout might be able to serve all three purposes.  I will try to create one and post it if I succeed.