ESL Amp White Paper


Conventional amplifiers have serious problems when forced to drive electrostatic loudspeakers. Roger Sanders was the first to develop an amplifier specifically designed to drive these unusual speakers:


Electrostatic loudspeakers (ESLs) are very different from conventional magnetic speakers and place unusual and difficult demands on the way amplifiers deliver power to them.  A magnetic speaker presents a mostly resistive 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.  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 measurement of the pressure or "push" behind the electrons 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 1 volt power supply that delivers 100 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.  A resistor has essentially constant impedance, so the impedance of a magnetic speaker will be nearly constant, although there will some variation due to crossovers and resonances.

In a capacitor, the impedance is inversely proportional to frequency.  So an ESL will have a high impedance at low frequencies (perhaps several hundred), and a very low impedance at high frequencies -- typically around 2 Ohms.

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 (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.

If an amplifier is clipping, it really doesn't matter how well-built the amplifier is, or how impressive its design philosophy -- 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.  Modern speakers require several hundred watts/channel to loudly reproduce today's highly dynamic music without distortion.

ESLs have a legendary reputation of being able to produce effortless and crystal-clear sound, with magnificent resolution of subtle inner-detail.  A clipping amplifier will destroy these qualities.

Sanders Sound Systems’ ESL amplifier operates at high voltage (plus/minus 92 volts).  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.

Sanders Sound Systems's ESL amplifier has such a massive output section that it does not need any protective circuitry.  It can drive loads below 1 Ohm without 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."


Current limitations:

An amplifier must deliver more current as the impedance of the speaker decreases.  This requires a larger power supply and output devices that can pass large amounts of current.  Such parts are costly, so modestly-priced amplifiers are only designed to drive relatively high impedance loads -- like 8 Ohm speakers.  Better amps use superior parts and can handle 4 Ohm loads.

But few of even the best amplifiers can handle the very low, 2 Ohm impedance 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.  Tube amplifiers are particularly bad in this regard due to their relatively high, 4 Ohm output impedance.

The Sanders Sound Systems Electrostatic Amplifier ("ESL amp") 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 virtually zero.  Current clipping simply is no longer an issue.


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 a capacitor, the current leads the voltage by 90 degrees.  This out-of-phase power delivery drives transistors out of their safe operating area.  In conventional amplifiers, it can cause output transistor failure and/or the premature triggering of protective circuitry.  This can cause the amplifier to deliver only a small fraction of its rated power and cause harsh sound quality.  The ESL amp's output stage is so robust that it can drive out-of-phase loads with ease, and since it needs no protective circuitry, there is nothing to ruin the sound quality.

Sanders Sound Systems’ ESL amplifier can deliver more than 2000 volt-amps per channel into an ESL.  That means it will act like a conventional amp rated at more than 1000 watts per channel.

When driving magnetic speakers, the ESL amplifier will deliver over 300 watts/channel into an 8 Ohm load, and over 600 watts/channel into a 4 Ohm load.  Momentary output into a 2 Ohm load exceeds 1000 watts.

The ESL amp is available in a Monoblock version that will deliver 800 watts into an 8 Ohm load and 1200 watts into a 4 Ohm load.


Many modern amplifiers are extremely inefficient.  Enormous amounts of their power, in fact most of it, is wasted as heat.

Audiophiles who prefer to leave their amplifiers on continually have discovered that a large, inefficient power amplifier can add over $100 per month to their electric bill.  In a ten-year period, their amplifier could cost them $12,000 to operate!  To produce so much waste heat, some of these amplifiers even require special mains wiring.

At Sanders Sound Systems, we believe that the use of such amplifiers is absurd, unnecessary, and environmentally irresponsible.  Proponents of these amplifiers claim that inefficiency is necessary to keep distortion at very low levels.  Many years ago, this was true.  But with modern technology, it is possible to make amplifiers that are extremely efficient while still maintaining vanishingly low distortion levels.

Sanders Sound Systems’ ESL amplifier has un-measurable distortion levels (less than 0.01%) from 20 Hz to 20 kHz up to the onset of clipping.  It does this while dissipating only three watts at idle and actually idles cold to the touch.  It may be left on indefinitely without concern for electricity usage.


The weird phase angles and high reactivity of ESLs tends to cause conventional amplifiers to become unstable.  The Sanders Sound Systems ESL amplifier is specifically designed to be unaffected by reactive loads.

The result is an amplifier that is completely stable under all conditions -- even at turn-on where no muting circuit is needed or used.  There is no "pop" or "thump" at either turn-on or turn-off and the amplifier switches on instantly.

Size and Weight:

Many of today's best amplifiers are so large and heavy (over 100 pounds) that one person cannot lift them.  They cannot be placed on a shelf or in an equipment rack.  Some are so big that they are split in two parts a "mono-block" for each channel.  It often is difficult to find a place on the floor to put them.  Many spouses are less-than-pleased about having such large amplifiers cluttering their living space.

To produce large amounts of power for driving resistive loads in highly inefficient amplifiers, it is necessary to use outrageously large and heavy power supplies and huge heat sinks.  So it is not surprising that such amplifiers are immense.

Despite its vast output potential, Sanders Sound Systems's ESL amplifier weighs 52  pounds and is sized scarcely larger than a full-sized preamp.  Its dimensions are 17" wide x 5.5" tall x 16" deep(43cm x 14cm x 40.6cm).  The Monoblock version of the ESL amp is the same small size as the stereo amp, and even two of them will fit in most racks.

Sanders Sound Systems has achieved this remarkable compactness by using a chassis made mostly of lightweight machined aluminum, and using the power supply to drive speakers instead of having its power converted to waste heat.

Because the ESL amplifier generates so little heat, the heat-sink requirements are greatly reduced.  Additionally, high-efficiency heat-sinks are used that make it possible to further lessen the weight and size of the amplifier.  Nor is this compactness achieved at the cost of having a noisy cooling fan.  The amplifier is completely silent.