Transcript Slide 1
The Plasma Tweeter Colin Joye 115th NYC October 10-13 2003 Waves and Beams Plasma Science and Fusion Center Massachusetts Institute of Technology, Cambridge, MA 02139, USA http://ionAudio.net What is a “Plasma Tweeter?” Sound is generated by a high voltage corona discharge. An audio signal causes this flame to change size, creating sound waves! Magnat Tweeter Ionofane, Ionovac Acapella TW1 Positive DC Corona Try Helium gas for more efficient flame and wider frequency response. Improve system power efficiency by using a solid state high voltage oscillator •Invented around 1955 by S. Klein. •Sold in 1960s as “Ionovac.” The dome tweeter is too large for its small wavelengths, leading to the “beaming” effect. References: • Siegfried Klein’s patents: #2,768,246 • AudioXpress magazine, April 2003, Vol. 34, Issue 4, “Build a Plasma Tweeter” • F. W. Peek, Dielectric Phenomena in High Voltage Engineering, 3rd Ed., McGraw-Hill Book Company, 1929 • A. Von Engle, Ionized Gases, Clarendon Press, 1965 • http://www.plasmatweeter.de (Ulrich Haumann) • http://www.ionAudio.net (Colin Joye) Circuit Operation The Physics Flame Properties In positive corona, electrons are accelerated toward the electrode. In negative corona, ions are accelerated toward the electrode and bombard it, causing sputtering and secondary emission of electrons. Negative DC Corona Increase flame size to include midrange frequencies!! The flame has no directional preference: Omnidirectional Acoustic Point Source! History: Future Plans Improve overall system linearity The flame has no mass: Perfect Transient Response, Acoustic Impedance Match! •Later produced by other companies, Magnat, Acapella. In AC corona, the heavier ions form a region of space charge, causing much higher electric fields, sustaining corona. AC Corona Advantages, Concepts As noted by S. Klein, high voltage oscillation frequency below 3MHz produces hissing. This phenomenon is a direct result of ion motion. Helium, an inert, monoatomic gas, has higher electron velocities than air, leading to higher sound pressure levels. Basic circuit operation: 1. Some disturbance causes the current in the Tesla coil to shift quickly, generating a voltage. 2. This voltage is sensed by feedback and causes the 6KG6A tube to direct more current away from the coil. The feedback signal dies. 3. More current flows in the coil, triggering the feedback - An oscillator is born! The high voltage self-sustains a corona discharge. 4. The amount of current deflected by the 6KG6A tube is controlled by an audio signal, altering the flame height quickly. The Tesla Coil is chosen to have low capacitance, low losses and high Q. The feedback driven oscillator is necessary because the coil must operate at its selfresonant frequency. This resonant frequency drifts slightly as the flame size changes, and must be above 35MHz to avoid audible hiss.