The “Shark Fin” Portable PA Project: Design Considerations

Only a few battery powered PA’s are available on the market today and most of them have design flaws or limitations. My goal is to create a design that uses digital amp technology (Class-D) that runs on standard batteries AA, C or D, or on a lead-acid car battery, that uses small speakers to give a full-range sound. The hope is to achieve relatively flat response from around 85 Hz to about 18 Khz. Who knows if it will work, but here’s the story of the first round of prototyping.

DESIGN GOALS

  1. Ergonomics. The PA should be able to be adjusted and moved easily. The design should allow for the most typical uses of a battery powered PA. The first most common use would be for buskers (street performers.) In these situations the performer usually is riding public transportation to and from the gig, so equipment must be easy to carry or move.

    • They are using the PA both as a monitor of themselves and as the “mains” for the audience to hear as well. Most often the amp is placed on the ground slightly behind the performer. I designed the cabinet to be more effective at being both a monitor and a main speaker by angling one speaker up at a 45° angle and angling the 2 mains speakers at a 75° angle.

    • The controls must be easily accessible when the performer is in the middle of a set of songs. If you have a guitar strapped on and you need to bend over to adjust the volume on a typical amp, the guitar slides off your shoulder. Thus the volume controls (at least) need to be at typical counter height of approximately 36 inches. The design solution for this was to make the controls mounted on a telescoping bar which also serves as a hand truck or wheeled luggage handle.
    • The ease of movement and stability of the cabinet was accomplished by adding in-line skate wheels on the bottom rear corner. The heaviest speaker is mounted in the lowest hole in the cabinet and all of the batteries and electronics are packed in the bottom of the unit. All of these items serve to keep the center of gravity very low, as does the sloped back design of the “Shark Fin.”
  2. Electronics. The PA system should be battery powered, either with 8 AA, C or D batteries or a single 12V lead-acid battery. The amplifiers that we found that have the efficiency that we needed were made by Tripath, a company specializing in what they call Class-T amps (which basically are just Class-D with a T instead). The model that sparked our choice was the TA2024C as used in the Sonic Impact Original Class T Amplifier.

    I chose two different speakers for the project: 2 Galaxy Audio S5N-8 5″ Neodymium Full Range Drivers at 8 Ohms and the Selenium 6W4P 6″ Woofer. The Galaxy Audio’s are the speakers used in the new versions of the Hot Spot mini-monitors. The idea behind these speakers is that they are incredibly light (made from Neodymium) and they concentrate on mids and highs without getting bogged down in low frequencies which are much harder to reproduce. This makes them great for vocal and guitar detail. The Selenium drivers are their to provide some low end for the audience especially. I will probably need to use a low pass filter to get this driver to be as efficient as possible. I may also use a high pass filter on the audience facing Galaxy Audio speaker.

The next installment of the blog will be the actual prototyping of the speaker cabinet. We will be using 1/2″ plywood for the shell and 2x stock ripped to size for the internal bracing. In a production version of the cabinet, ABS plastic would be a much better solution, but it’s not easy to manufacture plastics in your basement.

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