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Voltage limitations
ESLs are voltage operated devices. The higher an
amplifier's power supply voltage, the louder it
will be able to play an ESL (assuming it can also
deliver sufficient current).
Because high voltages are not needed for
magnetic speakers, and because high voltage
parts are expensive, conventional amplifiers
often lack sufficient voltage to drive ESLs to truly
loud levels.
When an amplifier runs out of voltage, it clips
(more correctly called “voltage clipping”). This
results in distortion and compressed dynamic
range. Depending on the amplifier and its
behavior when clipping, the music will take on a
wide variety of non-musical qualities. Note that if
an amplifier is clipping, it really doesn't matter
how well-built the amplifier is, or how
impressive its design philosophy is —it simply
won't sound as good as an amplifier that isn't
clipping. Therefore, the most important
amplifier specification is its power rating.
When observed on an oscilloscope, most
audiophiles are amazed at how often their
favorite amplifier is clipping when playing music
moderately loudly.
ESLs have a legendary reputation being able to
produce effortless and crystal-clear sound, with
magnificent resolution of subtle inner-detail. A
clipping amplifier will destroy these qualities.
The ESL amplifier operates at very high voltages.
The power supply can deliver 90 volts to an ESL.
This will drive most ESLs to “ear-bleeding”
levels with voltage to spare. The result is
electrostatic sound that retains its totally
effortless and clear qualities at any tolerable
listening level.
Protective circuitry
Most transistor amplifiers require protective
circuitry to prevent their output transistors from
being damaged when they attempt to drive low
impedance loads at high levels. In high quality
amplifiers, this circuitry switches the power on
and off to the output transistors very quickly.
This causes voids and voltage spikes to be added
to the sound which is one of the major causes of
the harsh sound often heard in overloaded solid-
state amplifiers. In fact, it is this and the
introduction of large amounts of odd-order
harmonic distortion from voltage clipping that is
the cause of the dreaded “transistor sound” —
not the use of transistors per se.
The ESL amplifier has such a massive output
section that it does not need any protective
circuitry. It can drive loads below 1Ωwithout
damaging its output transistors. Since it has so
much voltage and current capability that it
virtually never clips, it doesn't exhibit any
“transistor sound.”
Power
As previously mentioned, “power” in the usual
sense, does not apply to ESLs. But it is useful to
try to make comparisons to get an idea of what
can be expected for a purpose-built ESL
amplifier. Also, many of the features that make
the ESL amp so effective for ESLs also work
splendidly with magnetic speakers. So it is
worth rating an ESL amplifier using conventional
power measurements.
The term “volt-amps” is used instead of “watts”
when evaluating an amplifier's ability to drive
the capacitive load presented by an ESL. Volt-
amps is still the product of volts x amps (as is
watts) but the difference is that they are not
necessarily being delivered simultaneously.
Another way of saying this is that the voltage and
current are out-of-phase with each other.
When driving a resistor, the voltage and current
flow together (“in-phase”). In a capacitor, the
current leads the voltage by 90 .
The ESL amplifier can deliver more than 1000
volt-amps per channel into an ESL. That means it
will act like a conventional amp rated at 1000
watts per channel.
When driving magnetic speakers, the ESL
amplifier will deliver over 300 watts/channel
into an 8Ωload, and over 600 watts/channel into
a 4Ωload.
