Occasionally, we get inquiries about the nature of the BLS (Beveridge Line Source) transducer and its drive electronics. This page is an informal description; please contact us if you need more detailed information.

Transducer

The BLS transducer has an unusual design, in several respects:

• Both the electrodes (i.e., stators) and the mylar diaphragm (1.e., membrane) are actively driven. (Most electrostatic speakers use a passive diaphragm.)

• The mylar diaphragm is coated with a thin layer of aluminum, making it very conductive. (Most electrostatic speakers use a high-resistance diaphragm.)

• The electrodes are composed of Epoxy, mixed with Carbon and Barium Titanate. A conductive coating distributes the charge across the back surface of each electrode. (Most electrostatic speakers use metal electrodes, coated by an insulating layer.)

The epoxy body of the BLS electrode acts like a "mesh" of (Carbon) resistors and (Barium Titanate) capacitors. The polarizing voltage is carried by the resistors; the signal is carried by the mesh, as a whole. Because the field at the front surface is very diffuse (i.e., no significant current is available anywhere), arcing and corona are eliminated as problems.

Note: Conventional (insulated metal) diaphragms do not work well in this transducer design. If the insulation breaks down at any point, arcing is quite likely to occur. This can create conductive paths in the insulation and even start fires!

In addition, the field intensity near the metal can be very high, causing the aluminum coating to be depleted on any mylar that comes nearby. This effect is particularly strong near the edges of the metal (e.g., around airholes).

The Electrodes "sandwich" the Mylar Diaphragm.

Drive Electronics

This (simplified) diagram depicts the transducer and drive electronics for a Beveridge Model 2. Two amplifiers are used, driven in opposite directions (by a phase splitter). This drives the electrodes in one direction and the diaphragm in the opposing direction. This achieves the same effect as a single, double-voltage amplifier, driving just the diaphragm.

The coupling resistors provide the transducer with a high-impedance source of DC polarizing voltage. The coupling capacitors provide the AC (audio) signal to the transducer, while providing DC isolation for the amplifiers and electrodes. Thus, the electrodes and the diaphragm tend to stay at the DC levels supplied by their polarizing voltage power supplies, but can be pulled away from these levels as needed by the audio signal.

Comparison

Because most ELS designs use the "constant charge" drive system, it may be useful to make a direct comparison. Here is a table which indicates what voltages the transducers experience during an audio cycle. Note that all values are nominal:

Drive System		Audio Input	Left Electrode	Mylar Diaphragm	Right Electrode

Constant Charge	+10 V	+1000 V	-1000 V	-1000 V
	0 V	0 V	-1000 V	0 V
	-10 V	-1000 V	-1000 V	+1000 V
Constant Voltage	+10 V	0 V	-1000 V	+2000 V
	0 V	-1000 V	0 V	+1000 V
	-10 V	-2000 V	+1000 V	0 V

That is, the constant charge system moves the electrodes further apart (electrically), leaving the diaphragm "balanced" between the electrodes as the signal varies. The BLS design, in contrast, moves the electrodes in one direction and the diaphragm in the other.