MY DIY Electrostats:
I've always been intrigued by electrostatic speakers but never could afford them. I was inspired by some guys on the DIY Audio Forum rolling their own electrostats and they convinced me to build a pair for myself. I bought Roger Sanders' book, "Electrostatic Loudspeaker Design Cookbook", and dived in.
After a great deal of study and deliberation, I devised a hybrid design using a 10” woofer in a transmission line (TL) alignment with a 1ft x 4ft flat-panel electrostat positioned above the woofer.
My initial concepts for a compact TL small enough to merge with the stat panel in a unitary enclosure/frame did poorly in modeling by GM at the DIY Forum and my choices narrowed to accepting a smaller but less capable sealed or ported box or a larger TL box. The challenge for the TL option was cramming all that volume into a reasonable footprint while also placing the woofers at best location to blend with the stats. My solution was extending the box volume upward, behind the stat panel. Of course, a flat box surface there would bounce the stats’ backwave back to the stat and degrade its sound—so, I formed the box (behind the stat) into a “V” shaped “beam-splitter” which diverts the backwave out the open sides of the speaker.
I chose the 10" Aurum Cantus MkII woofer for it’s nice construction and wide frequency response (20hz-1khz), as the woofer had to produce both bass and lower midrange in this design. I opted to cross the woofer over to the stat panel around 380 hz to mitigate the low frequency phase-cancellation or “suckout” which is characteristic of dipole electrostat panels The woofer is canted upward 6 degrees and mounted at the front of the 4ft3, 9ft long folded and tapered transmission line. The bass would be uncompromised in this design.
The heart of each stat panel is a whisper-thin (6-micron) Mylar diaphragm which is sandwiched between 18-gauge perforated steel stators and assembled with 3-M 1/16” UHB foam mounting tape. The stators are spray-coated with 12 mils of polyurethane paint for insulation. The diaphragm is coated with a proprietary high-resistance conductive coating to carry a 2.5KV DC bias charge supplied by a dedicated power module that plugs into a 120VAC wall outlet. The stators receive their high voltage AC music signals through an EI-core step-up transformer with 100:1 windings. All of these specialty items were purchased from Russ at Just Real Music. Overall speaker dimensions are 15" wide x 67 1/2" high x 21 1/2"depth.
The speakers are vertically bi-amped using a DBX model 223 active crossover with 24db slopes set for 380hz, feeding a pair of Carver TFM 25 power amps. In lieu of a dedicated shelving circuit, the stats’ dipole phase cancellation is compensated using my system’s Audio Control C101 EQ.
The Moment of Truth:
I fired these babies up on July 4, 2008 with [my Goddess] Diana Krall singing Nat King Cole's "You're looking at me". The illusion of Diana performing live in my living room was so scary-real I could almost smell her perfume during that first tune. Finally I have speakers worthy of Diana, who now sounds as good as she looks
The TL bass is full and tight and fast with less coloration than any speaker I’ve owned and blends surprisingly well with the electrostats. In the second tune, Diana singing “Peel Me a Grape”, the bassist does this wonderful riff with a first long note fretted upscale followed by two quick notes stepping down and down again and the Aurum Cantus woofers were right there all the way-- it was just yummy and affirmed my choice for the transmission line bass alignment.
The 4ft2 flat-panel stats are incredibly fast and detailed. Even at the lowest volume levels every nuance is heard. They also play to ear-splitting volumes with no loss of accuracy or hint of distress. They are, however, ultra-directional with a very narrow sweet spot (one person wide)—so, not a good choice for party speakers.
In their sweet spot, however, the 3D imaging is quite magical with exquisite detail and downright stunning speed. A friend described them as “remote
headphones”. A woman’s voice thru these stats takes my breath away and Diana's puts me on my knees.
Ironically, these speakers’ greatest asset is also their greatest flaw: I’ve concluded that flat-panel stats have such phenomenal imaging precisely because they beam sound like a laser; limiting late/reflected sounds reaching the listener. So, whether their beaming is a fault or a virtue depends on your perspective.
A pink-noise frequency response sweep with my Audio Control EQ in RTA mode revealed a roll off above 5 khz and I had to add 8db’s of boost in the 10k-20k band to correct it. I’ve since learned that the higher transformer step up ratios are needed for full range stats that have to play below 200 hz while the lower step-up ratios provide adequate bass output in hybrid applications while providing superior treble response. In retrospect, a 50:1 or 75:1 transformer would have been a better choice for my hybrid. At this time I am considering changing to tandem pairs of 50VA 230V/6V European power toroids wired backwards to give a 68:1 step-up ratio.
Interestingly, the sound emanating from the stats doesn’t seem to decrease with distance—it sounds about the same from 1 foot as it does from 20 feet. However, the balance between the woofer and stat outputs does change a bit with distance so precise balance only exists at the sweet spot. Moving out of the sweetspot in the horizontal plane, the highs roll off progressively and dramatically because the stats beam treble like a laser. Moving to another room, the sound decreases as expected but seems perfectly balanced and carries quite well.
Inside their sweet spot, no conventional speakers I’ve heard can match these for realistic imaging, detail and speed. Moving outside of their sweet spot, they are still clean and listenable but progressively unbalanced and less impressive.
I live alone so there's no competition for the best listening position. With the speakers precisely aimed and equidistant from my usual spot on the sofa, I’m always in the sweetspot, so their shortcomings outside of that zone are not an issue for me—and I do LOVE their sound.
If you can accept their narrow sweet spot, flat-panel electrostats provide a scary-real, almost magical listening experience.
Beam-splitter TL cabinets completed and ready for woofers & stat panels
Mylar diaphragm on MDF/bike tube tensioning jig. Mylar diaphragm was wrapped over jig and secured on backside with tape. Tensioning was accomplished by inflating the bike tube with a hand-pump. I used a fine tip marker pen to place reference marks 6" apart on the diaphragm, then applied tensioning until the distance between marks measured 6.060 to achieve 1.5% elongation. Holding my breath here because the Mylar was stretched very close to its breaking point.
Stator with tensioned diaphragm installed: 1/16 thick 3M foam mounting tape secures the diaphragm to the stator. At this time a proprietary high/resistance conductive coating was applied to the diaphragm to carry the 2.5kv bias voltage. The coating solution was obtained from Russ at Just Real music (available online).
Stator with copper foil charge ring. Charge ring contacts diaphragm on the mating stator and applies the 2.5kv DC bias voltage.
Completed speakers posing with the Jazzman