Audio Infuser 4700

"Not your grandpa's stereo"

In many ways, the brushed aluminum faceplate is what truly completes this project.  It's the type of finishing detail that I was after when I began this ordeal.  It was also the most challenging undertaking of the whole process, and an area which I previously had the least knowledge or experience in (i.e. none).  

When I began this project, however, I knew it would need a proper faceplate, so I was willing to do whatever it took.  The combination of wood and brushed aluminum is just too pinnacle to avoid. 


CAD Crafting

 I found an online service called, who will take your CAD file, waterjet the part, and mail it to you.  After carefully measuring every device and using manufacturer specs for the switch & knob cutouts, I drew up a design in AutoCAD.  I first had Big Blue Saw create a prototype out of acrylic so I could double check that my design and cutouts would work.  After doing some test fits and making much-needed adjustments, I sent away for the parts to be cut out of Aluminum 6061.  That was the easy part.




Many vintage Hi-Fi stereos featured etched faceplates.  That is, the lettering is actually engraved into the metal rather than printed onto the surface.  It creates a uniquely finished look, a detail I really wanted to capture.  After doing some research on how to accomplish this, I found a method called the "Saline Sulphate" etch.   It's a relatively safe and simple (compared to the more acidic and electrically charged alternatives) etching process that uses a liquid solution of salt and common chemical "copper sulphate" to effectively etch aluminum.


I began by laying out the typography and graphics in Illustrator.  The design is then inverted to a negative, flipped horizontally, and printed onto glossy photo paper using a laser printer .  The laser printer is crucial, because it uses a heat-activated polymer rather than ink, and glossy photo paper is best because it keeps the polymer from soaking into the paper fibers.


The idea is that you use an iron to transfer the design negative from the glossy paper onto the faceplate.  This covers the entire surface with black  except for the lettering, which is left exposed as bare metal.  After using tape to cover up the edges and all bare metal except for the lettering (because all bare metal will be eroded by the chemical), you dunk the panel into the saline sulphate solution.


After soaking for about 7 minutes depending on the potency of your mixture, the lettering will have been etched about 1mm into the panel, dissolved away by the saline sulphate.  Now just fill in the lettering with black paint and sand down the surface.  Since the lettering is engraved, when you sand the panel (making sure to sand against a straight edge to ensure perfectly uniform & parallel strokes), the abraded surface will be given a brushed texture while the engraved lettering remains black.

Below is a slideshow depicting the steps. 

I didn’t have any spare metal to test on, so I did only one test etch on the backside of a panel.  I soaked that for 30 minutes, which ended up being way too long.  I then tried 10 minutes on the first for-real production piece, which turned out usable, but ultimately ended up pulling the others out after just 7 minutes.  Guess my chemical solution was strong!



The faceplate is home to numerous components.  Most of them use either socket screws driven through the plate or panel-mount screws for the switches.  As for the speakers, since there are 4 mounting points for each, I didn't want so many screw heads visible.  My solution was to drill about 2mm into the rear of the faceplate and use J.B. Weld (a high strength resin epoxy) to secure threaded standoffs at each corner.  


Vibration of the speakers against the metal has not been an issue so far.  Perhaps the use of thread-lock prevented any chance of gap formation. 



The other major aspect of the faceplates would be the various switches.  It was important to me that all of the interactive knobs and toggles feel very solid and mechanical, so I used only the heaviest duty parts I could find.  The input and output knobs are 2 pole, 2 deck, 5 position rotary switches, allowing the left, right, and ground signals of the audio cables to be routed directly through the switch.  

 Most of the toggle switches are for power, so they're just your basic SPST (single pole, single throw) switches that interrupt current. The exception to that is the Picture Select switch which toggles the video input to the CRT between the Pi video feed and just static.  This switch is hooked to an Arduino that then drives a servo to physically toggle the original switch on the CRT, hidden inside.  More on that over on the Raspberry Pi process page, down under the Arduino section.

I purchased a bundle of assorted vintage knobs off of eBay, most of which appear to have originally come from vintage Peavey mixers or amplifiers. 

>> Continue on to read about  The CRT Visualizer

by Todd Kumpf   // //   //   @toddkay