Whisper Amp Strap Clip

Back in 2015, there was a kickstarter for an all-analogue, tough-as-nails practice amp for guitar and bass. Intrigued, I followed the project. The project was successfully funded and I ordered my unit once the web site went live. It is a great little gadget! I ordered the kit, as it didn’t have that many parts and I needed to improve my soldering skills. I spent an evening assembling it and have used it mostly at night when the kids are sleeping. Another great use is taking it along with me to guitar shops so I can compare other guitars with the same amp.

My only complaint with the Whisper Amp was that I had to find a place to put it. Putting it in my pocket was a little awkward with the guitar cable protruding from the top. So I got to thinking about a way to attach it to my guitar strap. I wanted it to be relatively easy to attach and remove, so I knew Velcro would be involved somehow. As I often do, I turned to OpenSCAD to put my thoughts together. My first attempt was a solid band with a loop for a nylon strap. I would use 2 of these and fasten nylon through the loops. The problem with this design was that there wasn’t a good way to keep the bands secured to the body of the amp.

Initial design

I quickly devised a second design and sewed together some straps with Velcro fasteners. This worked a little better, but it had the same problem. I needed a different approach.

The second prototype was easy to attach and remove. A little too easy to remove, though.

The final design solved the attachment problem by securing a nylon strap between 2 “caps” on either end of the amp. This proved to be much more secure, but required a lot more design time to get the proper shape of the caps and the placement of the cutouts for the connectors. To get the curves on the edges of the caps, I resorted to tracing the shape of the amp on paper and calculating the radius of the rounded edge. Primitive, but it worked! To achieve this effect I used the minkowski sum function in OpenSCAD. It’s a really useful tool for nicely rounded edges. for the strap loops, I planned to use 1″ nylon webbing, so I made the loops accommodate one layer.

The bottom cap
The top cap with cutouts for the headphone and 1/4″ plugs

In a rare flash of maker inspiration, I got the measurements correct the first time. Well, on the rounded corners, anyway. I think I had to reprint due to the plug cutouts being a few mm off. I wasn’t sure how to finish the edges of the nylon. A quick search seemed to indicate that heating the frayed edges would melt them and nicely seal them off. I waved the cut ends over a gas flame on the stove, being careful not to hold it there very long. My caution paid off, as the ends melted quickly, finishing up very nicely. I sewed in a few pieces of Velcro and got it all assembled

Ready for action

Interesting note about using a 4-segement plug (TRRS). The output jack is designed for 3-segment (TRS) plugs, so you may need to slightly adjust the position of a TRRS plug to get stereo output.

And, finally, here’s a photo of the amp attached to a guitar strap while playing

I attach the amp near the end pin of the guitar

Overall, I met my objective of making an easy-to-attach clip for the amp, which means it’ll be falling off of tables and other places a lot less. If you’re interested in making one, or just want to see how I designed it, I’ve put the STL files on thingiverse!