Input sensitive op amp overdrive -Does one exist without using transistors?

[Electric_Death]

Just wondering if there's an op amp driven distortion that doesn't incorporate transistors but is input sensitive so you can go from clean to distorted with your guitar volume control as well as playing intensity when you set it to those "sweet spots". 

I've got some ideas of my own and while I'm a novice, I have what I call the Caveman's Workshop because I'm not trying to reinvent the wheel, just always looking for a different way to invent it. Seeing how others may have done it would be a great way to get started so I don't retread already covered ground. It also tends to give me ideas by understanding what they've done so I can come at things from a completely different perspective.

Thanks guys.

[aron]

Here's an old clip of the Shaka HV - uses one FET.

http://sounds.ampage.org/files/hv30.mp3

[jakenold]

Isn't that what the OCD does? AFAIK the clippers (in this case, FETS) are connected to a floating bias point, which in turn is connected to the input of the opamp. Very clever!

Kind regards, Jake

[brett]

Hi
the Proco Rat might have something for you, especially if you reduce the gain.  It's a single op-amp.
cheers

[the_random_hero]

If you lower the gain to a level where at a hard picked signal (say 120mV) is going to be just over the forward voltage of two diodes to ground (use LED's or germaniums and work out the appropriate signal level you should be hitting), as soon as you roll back the volume or pick softer you should get somewhere towards a clean signal.

[Johan]

if you make the first stage inverting, and place it first in your fx chain. turning down the volume pot creates a series resistance that interacts with the circuit, chainging the gainseting inside the box...
imagene a 10k input resistor and 1M in the feedbackloop. that gives you a gain of 100. turning down the volume on a strat A250k to half( audiopot, turned down to half givs 1/10 the resistance), you now increase the series resistance to 225k + 10k. chainging the gain to roughly 4
IIRC the Expandora does this in its first stage, but its been a while since I looked at that schematic...but..it only works if this box is the first thing in your chain, closest to the guitar...
j

[brett]

Yes.  You might have heard of a pedal called the Fuzz Face.  It uses the same effect with great results.  The FF input impedance is about 20 kohms.  An unfortunate side-effect is that the impedance losses are mainly in the high frequencies (due to the high inductance of pickups).  For a fuzz, that doesn't matter too much (or maybe helps), but for a distortion it might be a problem.  An input impedance resistor of 50k might be a good compromise.  (ie 50k/(225k+50k) = a fairly small number, while a 20k pickup loses less than 20k/(20k+50k) in impedance losses, which is also a smallish number).  I think you can see what happens if you change those 50k numbers to 10k or 100k.

Hmmmm..... maybe I need to vero some of this stuff....

[Electric_Death]

Thanks guys, that's all a lot to consider and aid me in my own designs.
What I hope to do is create an input sensitive distortion for a preamp and a mild overdrive to of course, overdrive it.

I heard the OCD drive recently on Ebay, seems to give the result I'm looking for but with tube. My goal is to do what Randall did with their Cube amps only completely op amp driven.

[Steben]

Yes.  You might have heard of a pedal called the Fuzz Face.  It uses the same effect with great results.  The FF input impedance is about 20 kohms.  An unfortunate side-effect is that the impedance losses are mainly in the high frequencies (due to the high inductance of pickups).  For a fuzz, that doesn't matter too much (or maybe helps), but for a distortion it might be a problem.  An input impedance resistor of 50k might be a good compromise.  (ie 50k/(225k+50k) = a fairly small number, while a 20k pickup loses less than 20k/(20k+50k) in impedance losses, which is also a smallish number).  I think you can see what happens if you change those 50k numbers to 10k or 100k.

Hmmmm..... maybe I need to vero some of this stuff....

exsqueeze me? 20k? I would say 2k for a germ.

[DougH]

No reason you can't do that with an op amp, or any kind of device really. Play with low Zin.

[Gus]

It is not that hard to do.

  Some people understand setting gain in stages for what they want and if you read thought this forum there is lots of information posted over the years. look for Marks  gruntbox, one of dargonflys opamp builds setting gain in TS type circuits dist + threads........................

[Electric_Death]

I've reached a decent level of success with this project. Gain stages working in harmony, 3 band EQ, hard clipping and I can go from clean to heavily distorted with my guitar's volume control.

I had to revive this thread because I'm dumbfounded this isn't common with solid states amps. The circuit is pretty straight forward yet 99% of solid state amps produced by commercial brands fail to incorporate this characteristic into their amps. Beyond the Roland Cubes, I don't know of any major company producing one.