Specifications

Page 7
DESIGN PHILOSOPHY
Conventional amplifiers have serious problems
when forced to drive electrostatic loudspeakers.
Roger Sanders has developed the first amplifier
specifically designed to drive these unusual
speakers:
BACKGROUND
Electrostatic loudspeakers (ESLs) are very differ-
ent from conventional magnetic speakers and
place unusual and difficult demands on the way
amplifiers deliver power to them. A magnetic
speaker presents a resistive/inductive load to an
amplifier, while an ESL appears mostly as a
capacitor.
Resistors dissipate power as heat. So the voice
coils of magnetic speakers get hot as they use up
the current the amplifier sends to them. A
capacitor stores an amplifier's electrical energy
instead of dissipating it as heat. Therefore an
ESL doesn't actually “use” power like magnetic
speakers. ESLs are sometimes called “wattless”
speakers because of this. But their behavior is
highly reactive, which means that they send the
electrical current back to the amplifier when the
musical signal reverses polarity. Amplifiers tend
to be unstable with reactive loads.
A watt is a measurement of power. It is the
product of volts times amps. Volts is a measure-
ment of the pressure or “push” behind the elec-
trons flowing along a conductor. Amps (a short
form of “ampere”) is a measurement of the flow
of electrons along a conductor.
Amplifier power is measured in watts, which is
fine when working with magnetic speakers. But
an ESL doesn't operate on watts, it operates on
voltage. Therefore, an amplifier's wattage rating
can be very deceptive when evaluating its ability
to drive an ESL.
To take an extreme example, let's look at two
amplifiers, both rated at 100 watts. One has a 10
volt power supply that delivers 10 amps of
current. The other produces 100 volts at 1 amp.
Although both amplifiers generate 100 watts, the
one with the higher voltage will drive an ESL to
much louder levels than will the low voltage one.
Resistance in AC (alternating current) circuits is
called impedance, because it often varies with
frequency. The impedance of a magnetic speaker
will be essentially constant.
In a capacitor, the impedance is inversely
proportional to frequency. So an ESL will have a
high impedance at low frequencies, and a very
low impedance at high frequencies — typically
2Ω or lower.
PROBLEMS and SOLUTIONS
Current limitations
An amplifier must deliver more current as the
impedance of the speaker decreases. This
requires a larger power supply and output devic-
es that can pass large amounts of current. Such
parts are costly, so modestly-priced amplifiers
are only designed to drive relatively high imped-
ance loads — like 8Ω speakers. Better amps use
superior parts and can handle 4Ω loads.
But few of even the best amplifiers can handle
the very low impedance 2Ω loads of an ESL well.
Many otherwise fine amplifiers find themselves
unable to pass sufficient current through their
output stages to drive an ESL at high frequencies.
This is known as “current clipping”, and results
in poor high frequency performance.
The Electrostatic Amplifier (“ESL amplifier”)
solves this problem by using a massive output
stage. Each output transistor is capable of
delivering 250 watts — and there are eighteen of
these per channel. As a result, it can deliver a
staggering 135 amps of current with a combined
power rating of 4500 watts per channel!
The output impedance of an amplifier must be
lower than the impedance of the speaker, or
current clipping will result. With so many output
devices, the output impedance of the ESL
amplifier is nearly zero. Current clipping simply
is no longer an issue.