Utah Dynamic A-100 Speaker

BY JERRY WIELAND

Web Edition

As Jerry Wieland reminds us, a flea market find is sometimes more interesting than you imagined possible. A speaker with its own field coil supply could be unique. (Editor)

While searching for radios at a flea market, I saw the speaker in a wooden housing shown in Figure 1. From a distance, it didn't look very unusual. The first thing I noticed when I picked it up was the impressive weight. The grille cloth was not torn, so I couldn't see what was inside.

Then I noticed two cables exiting the back cover. One was an old cloth-covered cord with two wires and no pin tips. This, I assumed, was the audio connection. The other cable was terminated in an AC plug. There were two handwritten labels on the plug. One read: "115 VAC for magnet field." The other: "Speaker works as of March 28, 2000. Cone is good."

A small decal on the back cover, shown in Figure 2, identifies the manufacturer as the Utah Speaker Company of Salt Lake City, Utah.

I had never seen a stand-alone speaker with an AC plug before. Did it have an amplifier inside? Maybe that is why it is so heavy. I decided to do some haggling and buy it to satisfy my curiosity. If nothing else, the cabinet was in good shape, and maybe I would get a working speaker if I was lucky.

THE UTAH A-100 SPEAKER
INNER WORKINGS

When I got it home, I weighed it and found it was 20 pounds. After removing six screw holding the back cover, I removed the chassis, shown in Figure 3, from the cabinet. There was no amp, but there were two transformers, a multiplate component, and a decent looking speaker.

The line cord is connected to a large transformer, the output of which is connected to the assembly resembling a radiator, which turned out to be a full-wave bridge, copper-oxide rectifier. The speaker has a matching transformer mounted to it.

Figure 1. The Utah Dynamic A-100 speaker as found at a flea market.

The speaker diameter is about eight inches with a very rigid cone in good condition. It seems to have a clear coating of some sort, and it has a very flexible leather surround, as shown in Figure 4.

I drew the schematic shown in Figure 5 (see print version). The power transformer (T1) steps the line voltage down to about 14 VAC. After it was rectified by the bridge, I measured about 7 VDC which is applied to the speaker field coil. The field coil resistance is 12.5 ohms. A Sprague Midget capacitor is connected across the audio transformer (T2) primary.

I applied audio to the audio transformer primary and heard it in the speaker. When I applied AC to the power transformer, the speaker volume increased significantly with some 60 cycle hum noticeable at a low level.

Figure 2. The decal on the back cover of the speaker identifying the manufacturer as the Utah Speaker Company, Salt Lake City, Utah.

Figure 3. The chassis of the Utah speaker. The AC transformer is on the left and the bridge rectifier is on the right.

Figure 4. Very flexible leather suspension surrounds the speaker, which is about eight inches in diameter.

These markings are on the rear of the speaker:

Utah Dynamic A-100

110 volts 50-60 Cycles

A831

Has anyone else come across a speaker that has its own field coil power supply?

Jerry Wieland is an electrical engineer working in cellular phone development at Motorola. His interest in old radios began at age 12 when he started listening to shortwave broadcasts on a 1930s Silvertone console.