DCS Overdrive

In 2013, I bought a Chapman Stick, which is an electric instrument with stereo output. I decided early on that I was going to build my own stomp boxes. One reason for that is that I anticipate to need two copies of each stomp box. I would therefore implement stereo stomp boxes. This is my first attempt at building such a stomp box. Actually, It is my first attemt at building a stop box at all for an electric instrument. Since it is a stomp box for a stereo instrument, it has two sides, each with one copy of the circuit described below. Those two sides are labeled A for the base side and B for the melody side. They are almost identical. The only difference is in the setting of the filter.

Experiments

I tried out many constructions and tried many versions of each of them before I settled for the final solution described here. The two photos here are of my experimental setup. A rather big box with breadboards. On its front panel, there are a few switches and several different potentiometers.

Yes, I know! This looks chaotic. I am not sure in what state of development this is.

Schematics

The final circuit is given below. Click on it to see a larger version.



The leftmost OP-amp is there to provide a high input impedance. Then, there are two signal paths. The lower path is the distorsion with filtering, and the upper path provides the possibility mix in the original signal with the distorted one. The lower signal path starts with the OP-amp with two diodes in its feedback network, which provides the distorsion. The next OP-amp and its surrounding components form a second order LP filter with adjustable cut-off frequency. The next with its surrounding components is a settable amplifier. Apart from that, the circuit should be self-explanatory.

The six controls do the following:

  • Sens(itivity): This sets an initial amplification.
  • Dist(orsion): This sets the amount of distorsion. Essentially, it determines the amplification of the linear-ish part of the distorting amplifier.
  • Char(acter): This determines a resistance in series with one of the diodes. With this control, you can go from significantly asymmetric distorsion to completely symmetric distorsion. Totally symmetric distorsion produces mainly odd harmonics, resulting in a raw or dirty distorsion. By making the distorsion asymmetric, even harmonics are added to the signal, which is perceived as more smooth or harmonic sound.
  • Tone: This sets the cut-off frequency of a second-order lowpass filter that affects the distorted signal. It can be set from 70 Hz to 19 kHz (about 8 octaves) on side A, and from 140 Hz to 19 kHz (about 7 octaves) on side B.
  • Vol(ume): This sets the volume of the distorted signal.
  • Mixin: This allows you to mix in the original signal with the distorted one, both positively and negatively. You can set the amplification from −3 to 3 in linear scale, which means that the signal can be amplified from −∞ to +9 dB. None of the other controls affect this part of the signal.

Circuit Board

The circuit is built on a veroboard with parallel copper stripes. On the component side, I used a permanent red pen to mark where components and wires should be.

The soldering side with copper stripes cut where needed.

Populated board.

Component Placement

The component placement is given below, shown from the component side. The crosses correspond to places where the copper stripe should be cut on the solder side. Click on it to see a larger version.



The Potentiometer Board

I mounted the potentiometers on a separate board, that was eventually placed a few millimeters under the surface of the box. This means that there will not be any nuts holding the potentiometers on the surface of the box. The result is that the knobs can be pushed down all the way to the box surface.

The rear side of the potentiometer board. Yes, I know. The angles are not all the same.

Design

Click on it to see a larger version.

The Box

The box is a 4S6500 from Mammoth Electronics. First, I printed the design on ordinary paper and taped it on the box to be able to drill the holes at their inteded locations.

The inside of the box.

When the box was drilled, I printed the design on a so called water slide decal, and applied it to the box. Then several layers of varnish to protect the design.

Search on Youtube for instructions on how to use water slide decals. You can find several tens of instructions.

The box, with twelve thin layers of varnish.

It shines!

The Two Cards

The circuit board and the potentiometer card are screwed together in a sandwitch construction.

Assembly

Finally everything is assembled.

Result

The result is rather neat.

Lessons Learned

As with every project, some things could have been done differently. So, here are some lessons learned from this project.

  • Think twice before you order OP-amps. I used O74, which are four OP-amps in one 14 pin package. Decent quality OP-amps. Perfectly fine for audio. But! They want at least 10V of supply voltage, and I intended to use one 9V battery as power supply. So, I ended up using two 9V batteries.
  • Do not use relays in battery powered constructions. As mentioned above, I used two 9V batteries in series, and I connected the point between them to ground. As with professional stomp boxes, I used one input jack to connect one battery to the circuit. However, that cannot be used to connect the other battery. So, I used a relay to do that. Bad idea! When the batteries are almost empty, that relay started to oscillate, resulting in unbearable noise in the loudspeakers. Just never, ever, do that again!

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