Differential Distortion Revisited

Differential Distortion PCBI designed a guitar distortion device in 1995. It was published in Popular Electronics magazine, August 1995 issue. I dubbed it “Differential Distortion”. With the advent of the web, the circuit was copied and added to many online compendiums of guitar circuits. I was very happy to see that, because I thought some people might benefit from some important features of the unit. Mainly, the extremely low battery drain, which might make it more practical to build into a guitar if desired. It’s also quite small, with a low component count.

Sometime around 2005, I had access to a full SMD workstation, and drawers of SMD resistors and capacitors. So the bug bit me again, and I decided to build the circuit using SMD parts. I ordered the transistors online, and had the boards made by a small run OEM manufacturer. The parts were insanely small – 0402 resistors look like grains of pepper in a jar. But I had a stereo microscope to help. I built a few of them, tested them. Then a big house move hit me, and the project languished in boxes for a few years.

In 2013, I went to an electronics mall called Chenghuangmiao in Chengdu, in the province of Sichuan in China. I have never seen so much electronics in all my life. It struck me that, not only there, but everywhere now, SMD parts have taken over from the old through-hole components. The new stuff is smaller, cheaper, and often has tighter tolerances. As I left the mall, I looked in a doorway and saw someone doing CAD layout. Looking around the store, I realized that they could build prototype boards. So it was back to the drawing board again.

I consulted my old plans, and went back to shop. But the low noise 2N5088 transistors were nowhere to be found. I realized that I would have to substitute another part. The sellers there are basically merchants, only a few have any deep engineering knowledge. So my attempts to explain my need for a “low noise” transistor were futile. One store assured me that, “all our transistors are low noise”, I think they believed I was impugning the quality of their wares. So I simply copied their lists of available parts, and went home to use the internet to look at data sheets. This led me to choose the excellent and common MMBT9014/MMBT9015 NPN/PNP types.

At the same time, I started reading opinions and experiences with the circuit that were posted on various forums. I realized that I had a few improvements of my own in mind. So a complete redesign was undertaken, with an eye to retaining all the worthy features. It has to be said, that the role for an analog device like this has been sidelined by digital audio and DSP emulation in the commercial market. Also, that as a consequence, nostalgia has led DIY builders in the direction of re-creating older designs, rather than looking in new directions. Still, some people had built it, and even reported the results of some modifications that they had dreamed up. Some implementations I didn’t agree with, but there were some good ideas to work with.

First, I made some important engineering changes. The single resistor emitter bias in the input transistor circuit of the original, was too unstable and sensitive to component and battery voltage changes. So I fixed that with an extra capacitor and resistor in the emitter circuit to lower the DC gain. The audio output level was too low, so I increased it. The input impedance was too small, so I increased it as much as I could. The 0402 parts were too small, so I used 1206’s. There is an advantage that the resistor values are readable. Next generation, I think I can step down to 0804’s and still be able to hand assemble with only soldering iron and tweezers.

Next I accomodated the mods. I added an inverting output to the differential pair. This allowed a kind of “distortion depth” control to be added. Or call it “fuzziness”, “bite”, “grunge” if you like. I also added a bias control circuit that allows the differential offset to be varied to change the harmonic tone and sustain. Some people thought the sound was too harsh, so I added an optional low pass filter on the output. The classic Fuzz Face that is a classic fuzz design, had an extremely low input impedance (stupidly low!). When guitar pickups are loaded this way, it cuts out the highs. However, the sources I was using claimed that it cut out the lows, so I added an adjustable high pass filter on the input to emulate that. It was implemented after the board design, so it is the only off board component option. That means that the area of input filtering needs a lot more experimentation.

At every step, I wrestled with the design goal of simplicity, with a lot of bang for every buck. So I didn’t allow myself to get too fancy. Most of all, I didn’t want to use any more battery current. Since I was mass producing one board, I tried to make it possible to change options just by changing the external wiring. For example, it can have the variable controls, or fixed settings, using only some jumper wires.

The board was laid out from a schematic and some sketches in about an hour, by a diffident but efficient young man, who was terribly late and had to be coaxed in by phone by the management. He did a good job, but didn’t fully understand my request to lay out thermal breaks on all the pads. With wave or oven soldering, the entire board is heated, so small components can have leads soldered to large areas of conductor. If you are hand soldering or doing repairs, this creates a nightmare as the conductor sucks up all the heat from the iron and refuses to get hot enough to make a proper joint. Or sometimes, the extra heat damages things. He did follow the instructions on ground connections, but didn’t realize that I wanted it on every connection. So a few of the components on the board were really pesky to place. I will be really demanding on this issue next time.

My order of 200 boards was completed in about a week. I found one mistake, my fault because I left the schematic at home and had to draw them the circuit from memory. I figured out that I could fix it with one 1/16 watt resistor, you can see it placed as a jumper on the top left of the board. Some boards had defects, but they had done a full inspection and marked them out. They even placed parts on two of them for me! The price was very reasonable, of course. It’s China. I remarked to my Chinese friend, that in my country a PCB company would look down their noses at me for even daring to ask to have such a job done. There are some mail order houses, but they are very pricey for a full build – mask, screen and assembly. There was no extra charge for the layout work!

I built about 80 boards. I don’t have them where I am now, but I plan to make some available as DIY kits. Originally, I thought I would supply the board and components separately, but as I mentioned, the SMD assembly is too difficult. I could do it next time with the board improvements. The circuit is simple enough that it can actually be built up on perfboard or whatever you like, as you like. I want to build a version that has the potentiometers and switches all on board, so it is a complete module that requires only battery and audio connections. The idea is to drill holes in the pickguard so it can sit in the control cavity of a guitar.

The first schematic I have posted is a shop drawing, so there are some things I need to tell you. The board is about 2.0 x 2.5 centimeters. The input is on the left, output on the right. The two potentiometers VR1 and VR2 are 10k linears. Switch S3 chooses clean or fuzz, S4 is the output low pass, S2 is the input high pass. You can substitute a 250k potentiometer for S2 if you want it variable. To eliminate the bias control, jumper T10 to T11. To eliminate the fuzz control, jumper T8 to either T6 or T7, whichever sound you prefer. To eliminate S3, just use T4 as the final output (T4-T5 jumper is not required). The dot marks on the potentiometers indicate the fully clockwise position – all the way to eleven as Spinal Tap says! 🙂

The second schematic shows the circuit design better. If you are interested in how the circuit works, or want to build your own, this is the one to study.

As a stomp box, it is possible to have the battery switched on from the jack, so it is automatically on whenever it’s in use. I didn’t show that, because it’s not how I do it. But it is possible. The people that copied my circuit included it in their version. Go have a look.

Here is the shop drawing, showing the off board wiring:
Here is the general schematic:
Here is the original 1995 schematic:

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