The "Chafer Drive"

Background
I was talking to my brother-in-law last December and mentioned that I was hoping to re-ignite my electronics hobby, mainly with the intention of building myself some guitar effect pedals.

A month later, my brother and sister-in-law kindly gave me an Amazon gift card for my birthday (always appreciated), so I had the idea that come his birthday, I would repay the gift with a boutique, hand-made effect pedal. And so the story of the "Chafer Drive" begins.

I thought a simple overdrive (mild distortion) with a single control knob fitted into the smallest possible enclosure would be appropriate, and it would be named the "Chafer Drive" because that, coincidentally, is his surname. The name has a triple meaning though, since a Chafer is a variety of beetle, and there is a party-game called a Beetle Drive, so we have Chafer - Beetle - Over - Drive, get it? Never mind!

I though I'd also document the whole build process too, so he (any anyone else who might be remotely interested) could see exactly what went into the creation of a DIY stompbox.

Choosing the circuit
The I circuit chose to build was a version of Joe Davisson's "Easy Drive" which is frankly a great sounding overdrive that I had breadboarded in the past, but never got around to making more permanent. I did make a couple of changes to the circuit in the link - mainly the addition of a 1K Gain potentiometer, and I also substituted the transistor for a 2N3904 because, I had a bunch of 2N3904's on hand, and secondly it would result in less gain a which was inline with the sound I was after.

Drawing the circuit and layout
I like to build it on stripboard, so I first drew up the schematic in CADSoft Eagle and made a strip board layout. The board layout I made was 9 strips x 9 holes with just one track cut (you can see that on row 6) and no jumpers. Nice!

Stripboard Layout Of Circuit

Next up, breadboarding the circuit.

Breadboarding

It's usually a good idea to breadboard the circuit first to make sure it works. This involves selecting all of the components and connecting them together on a breadboard. Here is a picture of the circuit built on the breadboard. It's a big mess of wires!

Circuit on the breadboard

To make all the external connections such as DC power, input/output jacks and foot switch easier to test, I have a test rig (essentially a pre-wired box) that I built some time back. This contains all of the off-board components such as the DC power jack, the input and output jacks and the foot-switch. I really recommend making one of these for anyone thinking about making they own effects as it makes the testing part a whole lot easier. The rig can just about be seen on the left in the picture below, and the whole thing is then plugged into to my Marshall Micro Amp.

Breadboarded circuit and test-rig

The good news for me was that it worked great once I wired it correctly, so I was ready to move to the next step. At this point, I decided to take a short break from the electronics, and instead prepare the enclosure.

The enclosure (aka a box)

The box I wanted to use was a Hammond 1590A (actually a Chinese clone of the real thing in this case) which  is about the smallest box DIY stompbox builders tend to use, and it's a real challenge to fit the all the hardware required into an enclosure this size. Do do it, successfully you need to be very accurate with the placement of components and with the drilling.

I found a nice template on the MadBean pedals forum, printed it, cut it out and taped it to the enclosure.

Enclosure and drilling template

Next I used a centre punch to mark the centre of each hole I was intending to drill and used a small diameter drill to make a pilot hole. For the larger holes I used a step drill bit (which I thoroughly recommend to DIY pedal makers).

Here is a picture of the enclosure about to have a pilot hole drilled.

Drilling a pilot hole

It looks like an almost impossible task to fit all the hardware inside the drilled case (and there is still the circuit board, wiring and an LED to go in too.

How do we get all that in there?

But a dry fitting shows that it is indeed possible

The hardware dry-fitted

Back to the circuit

Next the components are taken off the breadboard an soldered onto the stripboard. First I cut a piece of stripboard from a larger piece using a hack saw and sanded the rough edges. The dust is pretty nasty stuff, so you need to be careful not to get any in your lungs! Then I cleaned the strip board to remove any grease and oxidisation so there would be a good surface for the solder to cling to. I use scotchbrite pads for this, but fine sandpaper is ok too.

Then the low-profile parts like resistors and diodes are soldered in place, followed by the small capacitors then the large electrolytic capacitors and finally the transistor. 

Here is a picture of the first few components placed on the board. The board is only slightly larger than a postage stamp!

Resistors and diodes in place

Finally we have everything soldered to the board including the wires (not connected to the external hardware yet) and we are ready to test once again.

Testing part II

Well guess what? I got it all wired up to the test rig again and naturally it didn't work.

Wired and connected to the test-rig

Firstly, I noticed I had actually soldered the transistor one row out of place, so I de-soldered that and re-soldered it in the correct place, and...

Still didn't work.

So I checked some voltages, checked my grounds etc. and I was stumped. Everything seemed to be in the right place. Then I realised I had made a really stupid mistake. I had missed the one track-cut on row 6 to the left of the transistors emitter. I made this cut, et voila, success, it worked like a dream. All that remains is to box everything up.

Wiring up the enclosure

Many DIYers dislike this step and I'm no exceptions, doing all the off-board wiring is a right royal pain-in-the-ar*e, but it has to be done. I took my time and did it in stages.

First the ON/OFF status LED and current limiting resistor. You can just about see the 3mm Red LED to the right of the switch in the picture. The resistor is attached to limit the current and prevent the LED from burning out. Note that I've tried to make a good mechanical contact between the leg of the restore and the wire that will run to the 9V input. This is good practice to do prior to soldering. The negative leg of the LED (cathode) has already been soldered to the 3PDT switch (bottom right)

LED and current limiting resistor

The joints are soldered, then the exposed parts are then wrapped in heat shrink so they don't short-out on the metal enclosure. Excuse the blackened heat-shrink, it's because I used candle as the heat source, it's not burnt, just dirt from the smoky flame.

Heat-shrunk to prevent shorting

Finally, after a few hours fiddling about trying to get everything to fit together nicely, all the soldering is done. Just a couple of hiccups where wires broke off and I had to re-solder them, and a bad connection on the switch which cause the LED to not operate correctly, but they were relatively easily resolved, and 

All wired up and ready to go

Then I tried to put the bottom on the case, and the bloody screws that had been supplied were way too long, so I spent the best part of an hour filing those down to size. Grrrr!

The finished item

I have to say, I'm pretty darned pleased with the result. I bought some clear labels for my DYMO label printer, that just happen to be almost the exact size of a 1590A box, so I was able to knock up some simple artwork for the pedal and print it using the DYMO software. 

I put 25mm knob (ooh, err missus) on for the gain control and it was done.

Here are the final pictures. First the left...

Output side

Then the right...

Input side

And finally, from the top (with the LED on)...

Overhead shot

Soundclips

So how does it sound? well I like it, but judge for yourself.

Here is a quick sample on soundcloud. I didn't have enough time to do it real justice unfortunately. Recorded directly into Garageband on my Mac with Peavey Ampkit Link, no other effects etc. Guitar is SX Thinline and it was on the bridge pickup. Name that tune!