by Dick Olsher
Over the years, MartinLogan's forte has been electrostatic hybrid speaker design. Improvements in technology and transducer designer have culminated in the Summit X. The X in the model name suggests extreme performance that redefines what's possible in the hybrid ESL genre by coupling a Curvilinear Line Source featuring the XStat electrostatic transducer to an active bass system. Two 10" aluminum woofers and a bevy of electronics are packaged in a small bass module neatly tucked away on the backside.
There are no bass reflex ports to be seen anywhere. The enclosure is sealed and the frequency response is equalized and contoured to provide extension into the low 20s—lower in fact than that of many so-called subwoofers. The nominal crossover frequency is 270Hz, which means that the external power amp you connect only drives the electrostatic panels, while the built-in power amps take over below that frequency.
For the curious, here's a synopsis of the signal path flow based on information kindly provided by Devin Zell at MartinLogan. From the binding posts the signal proceeds in two directions. For the ESL transducer, there's a passive high-pass filter followed by a step-up transformer. For the bass, the signal is first stepped down to a preamplifier line level. The signal then passes through (not necessarily in exact order) low-pass and high-pass filters, EQ filters, and 25 and 50Hz EQ controls. The signal is then split and fed to custom filters to implement the controlled-dispersion PoweredForce bass. Bass signals are monitored to prevent amplifier clipping or overdrive conditions before being fed to the power amplifiers to drive the woofers. This electronic wizardry is designed by resident genius Joe Vojtko.
PoweredForce bass (as ML terms it) is an essential factor in the Summit's transducer integration. The basic idea is to make the woofer's dispersion pattern around the crossover point mimic the dipole radiator pattern of the CLS. It's well known that a dipole exhibits a figure-eight radiation pattern with little side radiation in the bass and a back wave that's 180-degrees out of phase with the front wave. Output cancellation occurs when the wavelength is large enough to wrap around the baffle. The Vojtko crossover allows the woofers to remain in phase below 100Hz and radiate omnidirectionally. Woofer phasing is shifted slowly between 100 and 160Hz so that the front and back woofer output becomes dipole-like above 160Hz. The end result is that in the overlap region between the transducers the wave launch blends cohesively, giving the impression of a single transducer at work.
Adapted and excerpted from The Absolute Sound's Illustrated History of High-End Audio, Volume One: Loudspeakers. Copyright © 2015-2016 The Absolute Sound. All rights reserved.