The Conduit: A portable, Class D TPA3110 Audio Amplifier

Update: I made a major revision to The Conduit Amp with some improvements and a bit better fit and finish.

A few months ago I was at the hardware store looking for cheap enclosures for electronics projects. Some aluminum junction boxes for electrical conduit caught my eye, so I bought a couple. In parallel, I was interested in building a small, portable amp that could operate off 12v, which led me to buy some little, mono, TPA3110 modules for a few bucks each.

Surveying my box of parts a couple weeks ago, I noticed that the TPA3110 modules would fit nicely in the smaller of the two junction boxes I purchased, and I started tinkering with ways to assemble them into a finished product.


The first idea was to join the boards together with standoffs and slip them inside. I ordered some small terminal blocks for the electrical connections. When they came, though, and I tried assembling things as I planned, I realized I’d need to cut the standoffs down in order to fit a board to hold a potentiometer. I was feeling lazy, and didn’t want to deal with metal filings, so I looked for another way.

I decided to use pin-headers to mate the amp modules with small motherboard made from perfboard. For added mechanical strength, I cut the headers with more pins than needed, soldered the pin positions with through-holes on the amp board, and then glued the rest and trimmed them to the same length as the active pins.

Routing the power input and speaker outputs was kind of a nightmare. Rather than trying to plan it all out, I ended up working a couple of connections at once, trying to leave room for the other connections. It took quite a while. I was concerned about some of the routing and figured I’d probably end up doing a second version, so I soldiered on soldering my prototype.

Once I was done, I used my multimeter to check to make sure that there weren’t any short circuits on the motherboard. Fortunately, everything checked out.

Next I had to finish up the input connections and passive volume control. Rather than routing the audio input on the protoboard, I decided to take advantage of the shielding on the input to help keep the signal clean while passing the high-current power and output connections, and the inductors on the amp board. I connected it to the board with the volume-control board at the far end of the case.

After assembling the components on the volume control board I checked everything with a multimeter. Again, I was fortunate that I hadn’t ended up with any shorts. The volume control board was connected to the the “motherboard” with an excess of soldered pin headers for mechanical stability.

I covered the solder pads on the backs of the amp boards with kapton tape to keep them from shorting on the case. Then I made up a power cable and some speaker cables, screwed them in to the terminals on the motherboard and fed them out of the opening of the case.

Maneuvering the amp board into the case with all the cables attached took a bit more force than I was hoping for, a situation not helped by the fact that the position I chose for for the audio input connector interfered with part of the case casting, depriving me of a few extra mm at the opposite side of the case, and putting the volume-pot a bit off center.


In the end though, I got it to fit.

My plan is to power it off a Quick Charge 2 USB power bank set for 12v output. I have the powerbank, but I still need to make something to negotiate the 12v output, so I powered it off a 12v power brick to test it out.

It works! Even better, it sounds good! So, a second version is a luxury, rather than a necessity. At full volume, the output level with my phone as the audio source is maybe a little lower than I’d like for something intended for use outdoors. I’m not sure yet if that’s a limitation of the 12v supply voltage, or if I need to bump up the gain of the amplifier.

Now that I know it works, I still need to finish it up. I need to fit an extension to the volume potentiometer shaft and pick out a knob that looks good. I also need to put some thermal pads on the bottom of the amp modules to transfer heat to the case. Also I’ll probably find some thinner gauge speaker cable.

Parts Used

A Trip to the Museum of Communications

This morning, I woke up with an itch, an itch to see a switch.

Version 2

No, not that kind of switch, something bigger!

Battery Reserve Switch

Nah, that’s not a switch…

Panel Switch

That’s a switch! Well, part of one.

Museum of CommunicationsIt is part of a panel telephone switch, one of a number of operational telephone switches at The Herbert H Warrick Jr. Museum of Communications, a little known technological treasure trove in the Georgetown neighborhood of Seattle. The museum fills the top two floors of a Century Link central office building.

The museum’s collection includes all manner of equipment and memorabilia from over a century of telephone history. The presentation of the collection is a bit uneven. There are carefully dated and labeled exhibits of phones and other equipment, along with display cases stuffed with lineman’s tools, but to me, that’s all secondary.

The best part of the museum is that it houses multiple generations of telephone exchange switching equipment, the sort of stuff that used to fill small buildings and connect thousands of homes to the telephone network. Much of it of it is operational, and interconnected, and attended by a staff of volunteers, many of them technicians and engineers retired after long careers with Ma Bell and her successors. They answer questions, give tours, and maintain the equipment.

The automatic switching equipment spans almost a century. The oldest automatic switch is a panel switch that served the Rainier Valley. It was installed in the 1920s and served for over 50 years. Unlike that later automatic switches in the museum, which were made in a factory and then installed, the panel switch was assembled on site at the central office and moving it five decades required removing walls. They also have a #1 Crossbar Switch (#1XB) from the 1930s, and a #5 Crossbar (#5XB) from the 1950s. The panel and crossbar switches are all operational. Calls can be placed between lines serviced by the switches and you can hear the progress of the call set-up and tear-down sound across the racks as the various electromechanical parts do their thing. They also have a number of operational PBX systems, some old-time switchboards, some small Strowger step-by-step switches and a not yet operational #3ESS, a small variant of the first switches using transistorized logic for control.

The collection also includes inside plant, like power distribution equipment, outside plant, like cables, along with a variety of trunking and long-distance equipment, including equipment for carrying national network television broadcasts.

There is also test equipment spanning decades, and a nice cache of HAM radio equipment.

The museum was originally called the Vintage Telephone Equipment Museum when it was created in 1986 by Herbert H. Warrick Jr. Warrick was an engineering director at Pacific Northwest Bell, and started the museum with the company’s support to preserve generations of vintage telephone equipment that was being phased out in the transition to digital switching and transmission. More recently, it became affiliated with the Telecomunications History Group.

I’ve made many visits to the over the years, and each time, I learn something new. I recommend it to anyone with an affinity for technology, particularly communications and computing, but it should also interest anyone curious about industrial and economic history. I hope I’ve whetted your appetite.