Martin Logan Classic ESL 15A Active Electrostatic Speakers (Pair)

Martin Logan renaissance ESL 15A active electrostatic speakers sit above the 13A and 11A models and are the next best thing Martin Logan offer to the flagship Neolith speakers, at a quarter of the price. Obviously these are still very expensive speakers, but in comparison to many other offerings at or close to this level, they offer remarkable value for reference level performance – the best of both worlds with a superb imaging, excellent detail and incredibly well integrated bass response.

ESL 15A are very powerful, with dual 12” woofers and two 500 Watt Class D amplifiers. It also has a sophisticated DSP engine by Vojtko and Active Room Correction. The Airframe is made of aerospace grade aluminium, and its curvilinear shape gives the panel a wide soundstage.

PoweredForce bass from two 12” woofers.

One of the technologies developed by Martin Logan, and used in ESL 15A, is PoweredForce. This is part of the DSP engine and is used to prevent boominess from the rear-firing woofer. It uses smart phasing logic to prevent any sound from bouncing back into the room. The result should be accurate and realistic bass. The advanced crossover is also used to minimise reflected energy. There are actually two woofers, including a front-firing unit. Both are mounted in individual enclosed chambers.

MicroPerf stators

Another Martin Logan exclusive is the MicroPerf stator. An important part of any electrostatic speaker is the stator. In fact there are two of them, to either side of the diaphragm. The stators are charged to a high voltage and then modulated by the audio signal. Notably, the advanced MicroPerf technology results in a panel that has almost twice the radiating are of a standard panel. Finally, ClearSpar spacers are used resulting in an almost visually transparent panel.

Airframe panel mounting for isolation

ESL 15A is not only beautiful, but the aerospace grade aluminium Airframe, makes the panel sound more refined. The extra rigidity helps to dampen any vibration and resonance. It also minimised intermodulation distortion. The Airframe never obstructs the playing surface or interferes with the dipole sound. Finally, the curved shape helps with the width of the soundstage, ending the “beaming” effect. As a result, low level detail is enhanced.

The advantages of dipole sound

Unlike a conventional speaker, all electrostatic panels radiate sound in both directions. This is known as a dipole sound effect. Notably, the sound heard from a speaker is a combination of direct sound, from the driver, as well as any reflections from the walls and furniture. This information can be out of phase, as it arrives at the listener at different times. However, electrostatic speakers do not have any output from the sides. This minimises any reflections. The lack of output from the sides is due to the dipole nature of the panel. The front and back signals are antiphase so that any side signals are cancelled out.

Powerful Class D amplifiers

The ESL 15A is the most powerful active speaker in the range. It has two 500-watt Class D amplifiers. Class D amplifiers are almost 90% efficient. They also generate less heat and need less power, than linear amplifiers. Notably, Class D amplifiers are digital in nature. The output is made up of a series of pulses. This is known as pulse code modulation. The closer the pulses are together the higher the output. The pulses are then sent through a low pass filter, to recover the audio signal. Finally, the output transistors are usually fast switching MOSFETS.

Active room correction for perfect sound

All of the Martin Logan active speakers feature Active Room Correction or ARC. This technology uses software as well as a microphone to actively monitor the sound in the room. The walls and furniture in a room will affect the perceived sound, that is produced by a loudspeaker. Walls can cause reflections, while soft furnishings can absorb sound. This results in all kinds of frequency response anomalies, caused by phase shifts. 

To correct this, ESL 15A uses Active Room Correction. This uses a microphone to actively monitor the sound in the room. The actual sound is then compared with a stored set of ideal curves. The equalisation is then adjusted, in real time, to compensate for any frequency anomalies. The result is a clean and balanced sound with no peaks and troughs in the response that would otherwise be caused by your room – ensuring that you are hearing the speakers as they should be heard.