The tweeter module is a completely self-contained unit, accepting a line-level input from an RCA jack and amplifying the signal with a class-A amplifier. What is unusual about this tweeter is that it has no mechanical diaphragm. Instead, an intense radio-frequency electrical field ionizes the air between two electrodes to produce a distinctive, violet-tinged yellow flame in the quartz combustion chamber at the base of the horn. The RF field is modulated by the audio signal, causing the almost massless flame to expand and contract in what should be a perfectly spherical pistonic manner, producing sound that is then coupled to the outside world with the 9"-long brass horn. The upper limit of the ionic tweeter is indeterminate but is arranged by Acapella to be at least 40kHz. As well as the AC inlet and the RCA jack, the rear of the module carries three fuses and a rotary tweeter-level control. Internal adjustments are provided for the crossover frequency, control of the flame conditions, and the automatic turn-on voltage (set at the factory for 30mV at 1kHz).