New Guitar Case Amp

Ever since I started making music again in ~2013, my main guitar amplifier has been the very case that the guitar travels in. I use this for all solo performances, as well as my duos with Werner Cee, Chris Abrahams, and most notably IMD with Axel Dörner.

The idea to use transducers to turn its shell into a speaker had intrigued me for a long time, however previous attempts had failed simply because the first cases I tried this with were made of ABS plastic which kinda sounds like a wet sack. So only after I came across a guitar case made of epoxy did this whole idea come to fruition.

I put in a total of 6 transducers to go with my 6-channel per-string processing that I’ve been employing for all my setups during the last 30 years. Their different locations also means various filtering effects as sound is panned across the outputs. I do compensate for this in software to a certain extend, however for the most part I embraced this natural behaviour.

One issue you run into when setting the whole shell into vibrations is that you have to make sure to dampen any mechanical buzz that can emanate from basically anything that’s connected to it, most notably the latches. So in the end I removed them all, but then kinda struggled to find a replacement to keep things shut tight.

A rubber band:

Sail-boat like posts

And this is what I use today – an ordinary suitcase belt…

Here’s some more pictures of the inside:

8-channel amplifier (50W Class-D)

6-in, 8-out sound card (XMOS dev. board)

Despite this mess, the guitar still fits in and can be safely transported.

In fact I installed a pannier handle and can lug my whole setup with my bike (at least to local gigs)

So why “new guitar case amp”? Well, lately the original build has been cracking at the seams – I had to replace broken transducers, fix cabling, and various mechanical issues. I also wanted to upgrade to a beefier power supply and type of transducer as well as a better soundcard. Mostly though, the whole effect processing should be embedded as originally intended, rather than always connecting a laptop externally. Moreover, as the new guitar will have a digital output, this needs to be reflected by the overall system design. So, as often, too many things to modify at once…

Sometimes then, it’s helpful when outside events force your hand a bit and you gotta move forward with a partial build. In this case that’s a concert I’m playing next Wednesday (Aug 26th) with Axel at Au Topsi Pohl, where I thought it would be a good idea to have a new blog post along with the newsletter invitation. If you read this in time and wanna come, make sure to rsvp on their website (because, you know, COVID-19 and limited seats and all)

Anyway, here’s some pictures of the current state:

New transducers

New soundcard

Cleaner wiring

But no embedding yet and no digital guitar link either. Stay tuned for updates on that!

Ratschen Reloaded

This project dates back to 1998, right after I had relocated to Berlin, when Jens Brand approached me about building an interface to control a series of geared motors. These motors were then connected to the axes of a number of ratchets, the general concept being to create very loud, computer-controlled sound, but without employing a speaker system.

© Jens Brand

Ratchets are used in Germany during the carnival season and even a single one can be brutally loud, even outdoors from a distance. So imagine 8 of these in a small room! If I remember correctly he once did a duo concert with a guitar player using a big Marshall stack, and that the latter was not audible anymore when the ratchets where running at full speed.

Back in 1998 there wasn’t a strong maker scene and no distributors like Adafruit or Sparkfun to provide all the control and driver modules that we can choose from now . So the interface was build from scratch, using a Microchip PIC16F84 and discrete MOSFET drivers. The controller didn’t even have sufficient PWM outputs, so it was programmed in Assembler with what I called “synchronous code”. What this meant was that I took note of the execution times of each block of code and added extra NOPs to match them to the PWM frequency. All while polling the serial input for MIDI messages.

Sadly there’s no picture of this initial build and in fact the reason why Jens approached me about making a new version was that the original device had been stolen :(

My go-to microcontroller in 2020 is the Teensy which is build around an ARM processor and programmed thru the Arduino IDE. I like its combination of small form factor, sufficient pin count, and processing power. There’s even a cool Audio Library.

To drive the motors I used some Pololu driver modules, and then all I had to do was to mount all these modules on perfboard and connect them with wires. Not a single discrete component.

The ratchets themselves were also worn down from decades of use, so they needed to be rebuild as well, which was done by our joint friend Paper Blattmacher.

Sadly, the premiere at Phill Niblock’s Experimental Intermedia had to be cancelled (*) due to COVID-19, but I hope we’ll soon have a chance to hear this instrument again.

* actually it was streamed, but that doesn’t convey the physical experience at all

Lots of sanding and polishing

Not much to write about the last 6 weeks – it’s been mostly sanding and polishing a lot. But wanted to share some pictures as it always feels quite rewarding when you’re done with the finest sandpaper and apply some wax.

The bottom and the sides still need to be done, though..

Next I plan to do a 3D scan of the top, so I can further deepen the back pockets without accidentally breaking thru the front. Also need to get started on the frets!

Welcome 2020

I’m making yet another attempt at trying to bring this website back to life. Will start by posting some pictures of the new guitar I’ve been building since early 2018.

I’ve set the publication dates to when I actually did the work, so if you prefer to read this chronologically you might wanna start down at the bottom of this page, or use the individual post links on the left.

More shaping

In 2019 I didn’t make much progress with the guitar at all, mostly because my day job kept me busy for much longer than planned. On top of that I moved both our home and my shop (into one location actually).

So it took until April 2020 that I sprung the CNC machine back to life and continued giving the guitar further shape. Here you can see it working on the neck-body joint:

To properly cut the neck contour I had to go beyond the 2.5D approach and go full 3D. In a move to use more open-source software I opted for FreeCAD (also switch to kicad for PCB design recently).

3D tools have a much steeper learning curve of course, so it took me a while to come up with this first design:

Here’s that section after some manual sanding:

While I started applying this 3D approach to the body as well, I realized that it was way too much work just to give all the edges a 5mm radius, so from there on out I went back to good old files and sand paper.

Here’s how that turned out for a section of the back:

I also applied the manual approach to shape the head-stock. Here’s a picture of its back:

With strings!

body shape

As you can see from my previous posts, the CNC machine was used for much more than originally intended. So as a next step it came naturally to also apply it to fine-tuning the body shape.

As CamBam isn’t really the best tool to draw Bezier curves, I resorted to good old Adobe Illustrator to draw the outline:

Although not trivial, this could then be exported as DXF and then imported into CamBam to base the toolpath on.

I had to order an extra long end mill tool to cut all the way down from 40mm height to ground (took a while too…)

This slope actually done manually:

The next design decision was not what I had planned (although one guitar maker had warned me when I bought the ziricote). The problem was that this slab of wood was darn heavy:

More than 50% above a standard Strat body:

So as I needed a lot of room for electronics anyway, I decided to cut pockets everywhere, turning this axe into a semi-acoustic (or maybe quarter-)

Much better now :)

headstock

As you might have guessed from the previous posts and pictures, the amount of wood I left for the headstock would not allow for the traditional placements of the tuners. In other words, it’s a so-called head-less design (technically there’s actually still a head – it’s just much shorter)

So this approach calls for two components:
First, you need tuning machines down at the bridge. This is relatively easy to source – in this case I went for ABM bridges with Graphtec pickups. Thanks to Peter Borowski from ABM, Berlin for helping with this.

The second part are the clamps to hold down the strings at the headstock. While there are purchase options for these too, most do not come as single pieces but as a block for all 6 strings. Which, again, doesn’t work with the wide string spacing I prefer.

So I decided to come up with my own design, where the plan was to work as much as possible with the wood itself, rather than attaching a full-metal solution.

Of course metal is needed in some parts, otherwise the wood would wear down over the years. So I designed an inlay where the string runs over a tiny metal bridge, with slots at each side for the metal clamp.

I milled this inlay out of a piece of brass that I had still lying around. This was the first time I used the CNC for cutting metal, so I was a bit nervous if it could do the job, but things turned out nicely without breaking anything:

And the pocket to match:

This was then extended with deeper holes for the clamps:

And holes from the back for screws to hold them down:

Tada, first string clamped down:

neck joint

I decided to practice the neck joint with a piece of maple I had left over, because I was afraid to get it wrong and then not being able to undo the damage.

I went for a slotted design to stabilise the alignment. Used three M6 Rampa wood screw inserts.

This seemed to work fine, so I applied it to the Ziricote body…

…as well as the neck:

And here’s neck and body joined for the first time:

neck

Here’s the neck with the truss-rod already in place. You can also see the fretboard behind it – I didn’t cut the slits for the frets myself, although with the CNC machine that would be possible of course. But back then when I ordered it, that wasn’t the plan

Mating fretboard and neck:

Not the most professional clamping job, but it worked out fine.

Milling the sides for the correct width:

The completed rough cut of the neck:

The headstock is still rather blocky, as I wasn’t sure what shape it should have, so I left more material on in order to have options.

Also note the round shape at the end where it meets the body. It follows the curve of the sound hole of a classical guitar. More on the neck-body joint in the next post

CNC

I bought this initially to
a) do quick PCB prototypes,
b) cut irregular holes into enclosures, and
c) for some very specific guitar jobs.

As it turned out I’ve used it progressively more for c), a little for b), and not at all yet for a).

The main guitar job I had in mind back then was to precisely plane the body and neck areas where they will be joined, as that’s something hard to get right with just manual tools.
But after playing more and more with this machine, I found myself applying it to many other tasks, while also improving my CAD/CAM skills.

The first step was to test the machine with the manual controls, so I made some exploratory cuts in regions that would be cut away anyway:

Next, I wrote a number of scripts in gcmc, i.e. one step above bare-bones g-code. This is used to cut a narrow trapezoid from the bridge to the end of the body to delineate the area where the strings would go.

Then, after this turned out nicely, I wanted to try more advanced stuff. So I bought a simple CAM application (CamBam) that would allow me to design basic 2D shapes that would then be milled to a certain depth (what is commonly called a 2.5D approach).

I applied this first to pockets for the bridge elements to ensure that they are perfectly aligned.

The software can calculate a tool path and then export this to g-code, which can be read by the CNC controller, in this case Mach3.

Here’s the machine in action:

And here the result:

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