Steampunk Plasma Speaker
The plasma speaker is a device which uses a small plasma discharge to create the acoustic waves. While the normal loudspeakers use a mechanical diaphragm to move the air , the plasma speaker heats the air with a high-frequency electric discharge and causes it to expand. Varying the amount of power fed to the discharge changes the rate of the expansion and produces the pressure waves - sound waves .
The high voltage powering the discharge is produced by a variation of the Tesla Coil called the bottom-fed Tesla Coil . In this type of Tesla Coil there is no primary winding . The power is fed to the resonant circuit by connecting one end of the coil to the high voltage , high frequency generator , usually build with a vacuum tube .
The electromagnetic field around the resonance coil is strong enough to light a neon tube .
The closed glass discharge chamber confines the discharge increasing efficiency of the speaker and reduces the production of the ozone . The discharge chamber is connected to the brass horn which improves the reproduction of the lower frequencies.
The output power of the Tesla Coil is controlled by the anode current of the HF oscillator tube and is set with the potentiometer on the front panel . The front panel also contains the ammeter , the power switch and the power indicator.
The videos of the speaker's operation and the two photograps of the speaker in the dark ( pictures taken with 4 second long exposure ) . The first video shows its operation with a MP3 player , the second one shows the close-up of the discharge chamber .
The high power , high frequency oscillator is based on the Ulrich Haumann design but instead of using the PL519 tube I used the GU50 tube . The GU50 is a 50W , Russian , military pentode intended for use in linear amplifiers . Thanks to its high plate voltage it is a good choice for a bottom-fed VTTC.
The oscillator is controlled by a grid modulation . The second-grid voltage is produced with a three tube voltage amplifier built with a double triode ECC88 and two 6Z22P pentodes. The double triode works as a differential amplifier , the first pentode is a voltage amplifier and the second one is a cathode follower. The use of the differential amplifier and the voltage feedback makes the gain setting easy and lowers the nonlinear distortions. The R10 potentiometer sets the offset voltage on the grid of the GU50 , and thus the power of the oscillator.
The oscillator and the amplifier are powered by a simple power supply :