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Quad
Roger Sanders: On amplifiers and ESL's

Roger Sanders wrote an extensive and well written article on amplifiers and ESL's. He kindly gave permission to reprint it here.

Electrostatic speakers are essentially capacitors. Magnetic speakers are essentially resistors. Unlike magnetic speakers that maintain a nearly constant impedance, the impedance of electrostatic speakers is inversely proportional to frequency. Therefore at high frequencies, they present a very low impedance load to an amplifier (often less than 2 ohms).

They also are highly reactive, which means that instead of converting the energy from the amplifier to heat in a voice-coil, they reflect it back to the amplifier's output stage. Compounding this problem is the fact that electrostatics have an out-of-phase drive characteristic whereby the current leads the voltage by 90 degrees. These characteristics tend to drive output transistors out of their safe operating area and can trigger protective circuitry or cause the output transistors to fail long before the amplifier has reached its rated power capability.

Although electrostatic speakers are voltage-driven, they still require large amounts of current at high frequencies due to the low-impedance load. Since wattage is a function of volts multiplied by amps, suitable amplifiers will be rated for high power.

Few transistor amplifier manufacturers design their amplifiers for driving capacitive loads. So you don't find specifications that tell you if the amplifier is suitable for electrostatics or not. But there are several clues that can guide you in the right direction:

The output stage must be extremely robust. This means that it will have lots of output transistors (like 10 or more per channel). This allows the nasty load to be divided across many output devices, reducing the load on each and greatly improving reliability.
The amplifier must develop relatively high voltages. Therefore it will be rated for high power since high-power amplifiers require higher voltage power supplies than low-power amplifiers.
Power MOSFET transistors do not suffer from the "second breakdown" phenomenon, therefore do not require protective circuitry. Since protective circuitry often misinterprets and triggers inappropriately on capacitive loads, the elimination of such circuitry is desirable. Note carefully that conventional bi-polar transistor amps can be designed without protective circuitry too and can perform better than MOSFETs in some ways (more linear with wider bandwidth and cooler operation). So MOSFET amplifiers are not essential, I just mention this as a way of being pretty sure that you won't have any protective circuitry involved.

Note that the apparent build-quality of the amp has nothing to do with this. For example, Ive seen expensive, 100-watt Levinson and Krell amplifiers fail on electrostatics while 300 watt Adcoms work just fine. So go for power above all else. Of course, if you can' find an amplifier that is specifically designed for electrostatics (like the InnerSound Electrostatic Amplifier), then that is likely to be an excellent choice.

Tube amplifiers are entirely different than transistor amps when driving electrostatic speakers. Their big problem is that their output impedance is too high, which severely limits their current capability and causes them to have very low power at the higher audio frequencies. Fortunately, there is not much energy in music at high frequencies, so even though they lose most of their power at high frequencies, they usually still sound OK.

Because tube amps have so much trouble delivering sufficient current, the best tube amp will have lots of power. Also, high-power tube amps tend to have lower output impedance than low-power tube amps. You will want to use the lowest impedance output transformer tap available - usually 4 ohm. OTL tube amps will have a particularly difficult time delivering a low enough impedance and their output tubes are easily damaged by electrostatic speakers. So they are best avoided.

In summary, power is the most important factor. After all, it really doesn't matter how well-designed your amplifier is, or what fancy components are used in it - if it is overloaded and clipping, it will not sound as good as a more powerful amp that is playing cleanly.

Great listening, Roger Sanders

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