YOUR ESL QUESTIONS ANSWERED

Do nonlinearities exist in what is essentially a large capacitor? Do they treat small signals differently than large? They are fantastic with small detail signals, but loud sounds don't seem quite right. Am I being objective? (Asked by an anonymous reader of TAS)

Some ESL's may be large, but they are not necessarily large capacitors, in the sense of having high capacitance. The capacitance of an ESL panel is proportional to its surface area and inversely proportional to the distance between the stators. The capacitance presented to the amplifier is the capacitance of the panels multiplied by the square of the signal transformer's voltage step up ratio. These factors can be manipulated in various ways during the design process.

Neither capacitive loads in general nor ESL's in particular will inherently produce different results at different loudness levels. In fact, electrostatics are the only type of sonic transducer that has any hope of presenting an almost completely linear response. This is because it is the only architecture where the force is proportional to the voltage, and where the load is dominated by the air itself at all frequencies, although this is not necessarily every designer's primary goal in practice.

That said, there are several factors in less sophisticated designs that can lead to loud sounds not seeming quite right to the objective listener. We'll discuss two of the big ones:

Some electrostatic speakers have very high capacitance. This raises the potential for three types of undesirable amplifier interaction:

1) All else being equal, the amplifier load from an ESL increases with frequency. High capacitance ESL's will have a very low impedance at high frequencies. When an amplifier that can not drive a low impedance is asked to source a lot of current at high frequencies, the amplifier will limit its output at all frequencies, creating compression, clipping, or some other overload effect.

2) The feedback loops in some amplifiers will not remain stable when driving a large capacitive load. If such an amplifier happens to begin oscillating only when called on to deliver high power, this will cause problems only at high loudness levels. The oscillations may be ultrasonic and thus inaudible, but the indirect, audible effects can include compression, hissing, intermodulation, and chirping, among others. It is also possible to throw an amplifier into an

oscillating state called "motorboating", which sounds much like its namesake outboard engine. An amplifier may be harmed while oscillating, or just run warmer than usual.

3) Some switching (e.g., class D) amplifier designs are incompatible with ESL's. If a switching amplifier is designed without enough high frequency feedback, and no special considerations are made in the amplifier for driving ESL's or in the speaker for being driven by switching amplifiers, then the panel capacitance forms a tank with the amplifier's output filter inductance. This will resonate when excited by high frequency signal content, normally at frequencies somewhat but not far above the audio range. It can cause various sorts of distortion, and even arcing of the panels or damage to the amplifier, at sound levels well below the usual limit.

Secondly, if membrane resonances are not damped, then at high loudness levels, impulses/transients and frequencies that happen to match a resonant frequency can cause membranes to collide against stators. This can compress the sound for a very brief time after each event. In some designs, it can partially or completely discharge the bias from the membrane, resulting in a more lengthy decrease in the sound level. Of course, these resonances also degrade the transient response and color the sound.

There are also a few other matters that we consider important in our designs that contribute to linearity under all conditions, which we are keeping under our hats.

What makes JansZen's immune? Three things: They are designed with separately energized tweeter and midrange panel bays, which allows a higher impedance and lower capacitance than is otherwise possible, never dropping below four Ohms in the audio band. Our membrane material is chosen so the power goes mainly into moving air at all loudness levels. Also, our membrane resonances are critically damped. We believe we are the only ESL maker that combines all three attributes.