The other audio concept

This project is about a combination of piezo electric actuators and tube amplifiers for audiophile HiFi.

Analysing the flow of an audio signal many conversions in terms of energy and physical quantities are involved. Simply speaking, at the time of recording the air pressure at a given point (symbolising a virtual ear) is converted into a voltage signal in the microphone. After an analogue to digital conversion, the signal is represented by 16bit numbers on a digital media. Mapping the air pressure to digital numbers is the task of the recording institution. In order to reproduce the air pressure in front of your ears, the signal path is inverted, i.e. a digital to analogue conversion followed by an amplification and transformation into air pressure. This is the task of a HiFi system.

Even though there are many transfomators and gyrators involved, the conversion chain in a HiFi system should be short and simple. For example, The digital signal is converted in a more-less powerless signal in the first part of the chain. Afterwards this signal has to be amplified either in terms of voltage or in current in a middle part which is carried out by transistors or MOSFETs. The last part of the chain is the loudspeaker system with its inputs voltage and current, and the outputs pressure and velocity. If the loudspeaker principle is electromagnetic, the energy stored in the coil current is transformed into air pressure by this four pole. Therefore a current amplifier is needed in this case and MOSFETs seem to do a good job here as the following picture tries to show.

Another possibility is to use a tube amplifier in conjunction with electromagnetic loudspeakers. Tubes amplify the voltage and therfore a such solution requires a device transforming the voltage energy represented in voltage into a current in order to drive coils. This device is referred to as impedance adaptation as the impedance of a coil system has to be increased to meet the tube output stage. Nevertheless this solution seems not to be appropriate as it uses a voltage source (the tube amplifier) for a current consumer (the coil).

A more natural solution would be to use a high impedance voltage sink in combination with a tube amplifier. The loudspeaker has to convert the voltage energy in a pressure energy. In any case the natural impedance of the loudspeaker has to be already high enough to be directly driven by the tube amplifier as the following picture indicates.

David Pearce, Functional Materials Group, IRC in Materials, University of Birmingham says in his statement Towards a Piezoelectric Loudspeaker:

Over twenty years ago, the inventor of the TV remote control, Robert Adler, had a bright idea. He thought about how to drive a bass speaker with a piezoelectric. As you may be aware, piezoelectric materials do not normally allow for much in the way of movement, giving typically a maximum of 0.1% elongation. This is not a problem for high frequencies, i.e. for ultrasonic applications, where the air displacements are small, but for really low frequencies a lot more movement is needed. In order for a piezoelectric to move more air more compliant designs are needed which can mo