Shocking!In my college days, I lived year-round in the small town where I went to school. As you might imagine, summers there were pretty boring because most of the students had left town. As a result, I often found creative ways to spend my time. During one of those summers, I got the idea to build my own electrostatic speaker from scratch. My roommate, Bill, found a schematic for one in an electronics book. It looked pretty simple, so I decided to try it. By this time, Bill was curious, so he also got involved. Our first attempt consisted of a large road sign for the back plate, aluminum foil for the diaphragm, and a window screen from our apartment for the front plate. I finally found a good use for my college textbooks as separators for the plates. The diaphragm was insulated from the outer plates by placing it inside a pillow case. We connected our "speaker" to an amplifier using a 70-volt audio transformer and a 475 vdc power supply. When we turned on the high voltage, the window screen pulled down about 1/4 inch and the diaphragm emitted a slight hissing sound. It produced audio, but not very loudly. To improve the efficiency, we tried a higher voltage. We replaced the 475 volt supply with the high voltage section from an old TV. The voltage was in the neighborhood of 13 kv. When we switched it on, the window screen was sucked down an inch and a half, and the diaphragm made a satisfyingly loud hissing sound and produced a considerable quantity of ozone. "All right!" we said to each other as we prepared to listen to our newly-improved speaker. Unfortunately, the efficiency was no better. We tried moving the plates closer together, but that produced arcing, so we went back to the 475 volt supply. Besides, the hiss was so loud at 13 kv that you could hardly hear the music. Our next attempt involved laminating three sheets of aluminum foil between waxed paper. It took several tries before I was able to find the correct number of waxed paper layers to prevent arcing. Once I had perfected my art, I decided it was time to build a REALLY BIG speaker. I was ironing together this six-foot monstrosity when some friends came over. At first they thought I was doing my laundry but soon realized I was up to no good. We hung the speaker from a hook in the living room and connected it to the stereo. It made a decent sounding tweeter, but we still had to pump quite a bit of power into it. As I was standing in front of the speaker admiring my work, I started to feel a little funny. After a few moments I had to move away because it seemed as though my guts were about to turn inside out. Bill checked the power meter on his stereo and discovered that a continuous 40 watts was being fed into the speaker. We knew the speaker wasn't THAT inefficient, so he tried to find out what was going on. After a moment, he discovered the cause of the problem. It turned out that Bill had been working on an SCA receiver and that it had somehow managed to inject a high level 67 kHz signal into the power amplifier, which was faithfully reproduced by my super tweeter. After all, an electrostatic speaker is essentially a high pass filter. Suddenly, I felt ill as I realized that my innards had been shaken by what was essentially the WORLD'S BIGGEST ULTRASONIC CLEANER!
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