Posted by Al Sekela on March 16, 2003 at 14:12:35:
This should be of interest to SL owners, even though the evaluation is with Magnepan speakers.
16 March 2003
Field report: InnerSound ESL amplifier on Magnepan MG-20 speakers
Summary and conclusions
This amplifier may be the best under $3000 list price available for Magnepan true ribbon speakers, but it requires some case damping to reach its full performance potential. It has no obvious "transistoritis" in its sound. Resolution of individual performers is limited by the evaluation system configuration. It is capable of at least 100 dB peaks on MG-20 speakers without strain.
The gold-plated steel jumpers supplied by Magnepan to use with the outboard crossover boxes and to replace tweeter attenuation resistors are EVIL. If you have these speakers, by all means replace the jumpers.
Disclaimer
I am not financially interested in any audio company or dealership. The equipment is either owned or borrowed, was all purchased from dealers or manufacturers or constructed by the author, and there is no consideration for intended future purchase involved in the genesis or writing of this report.
Baseline system description
The listening room is 15 feet by 19 feet with 8-foot ceilings. Heavy drapes cover the window and glass door on one wall. Treatments comprise Sonex ceiling-wall corner triangles and a 2-foot by 4-foot Sonex panel on the wall behind the speakers. The carpeted plywood floor is raised and insulated underneath.
A Wadia 861 CD player with NBS Signature II power cord drives Atma-Sphere MA-1 monoblocks with 600 ohm balanced cable termination and series RELCAP Teflon capacitors to achieve 300 Hz first-order high pass filter (to mimic Magnepan's crossover), and a Bryston 4B (ST) stereo amp through single-ended cables terminated by the Bryston's stock 30 Kohms. The Atma-Sphere amps drive the mid/tweeter inputs of the Magnepan MG-20 speakers through Paul Speltz' ZERO autotransformers with 2:1 voltage ratio (4:1 impedance ratio, so speakers look like 16 ohms to amplifiers), while the Bryston drives the Maggie bass panels through the low pass filter sections of the Magnepan outboard crossover boxes. A REL Stentor II powered subwoofer takes its signal from the Bryston output terminals. Silver Audio cabling is used throughout and lifted from the carpet by ceramic tile bullnose pieces. Amplifiers and the REL have standard IEC power cords. The CD player and Atma-Sphere amps are supported by Lovan rack sections, while the solid-state amp has a simple wooden platform at carpet height and spiked to the subfloor. The CD player is on its own dedicated 20 ampere power circuit. The Atma-Sphere amps share a second dedicated 20 ampere power circuit. Both of these are wired with armored (BX) cable and have hospital-grade outlets on a single ganged metal box. The Bryston and REL are supplied by a standard convenience outlet.
Evaluation system
The Wadia CD player was connected to an InnerSound ESL amp with a 1-meter pair of Silver Audio Jetstream single-ended interconnects. The ESL amp drove the Magnepan outboard crossover boxes through a single 6-foot pair of Silver Audio Silver Symphony 48 speaker cables. A 3-foot pair of the same cables was used to connect the mid/tweeter outputs of the crossover boxes to the speakers, and a 2-foot pair was used to connect the crossover to the bass inputs. This odd arrangement results from reuse of the 3-foot pair that normally connects the Atma-Sphere amps to the ZERO autotransformers in the baseline system.
The Magnepan speakers are stock except for use of ceramic 3.0 amp fuses in place of the supplied 2.5 amp glass fuses in the tweeter path, ceramic instead of glass fuses in the midrange path, and silver jumpers made up of twisted bundles of Silver Audio scrap wire soldered with WBT solder. Temporary adapters comprising brass machine screws, nuts, and washers are used to hold spade connectors on the speaker cables firmly against the Magnepan banana ports (the signal path is directly from the spades to the port metal). The disconnected ZERO autoformers were left in place on top of the Magnepan outboard crossovers, as their mass and the Deflex sheets under them seem to provide some damping to the crossover case.
Background
Travis and I post frequently on the Planar Asylum. After I had made one too many clever insults of his taste for solid-state amplification, he offered to lend me his InnerSound amp if I would pay the shipping costs. This seemed a bargain, since either I would be impressed by it and learn something, or I could sneer authoritatively about a sand amp that gets a lot of favorable comments by users.
My baseline system has evolved over a decade and three different houses. The MG-20s were the culmination of my wife's and my search for speakers that presented music with a natural sense of space and timbre. I built my own clones of Atma-Sphere MA-1s with some, but not all, of the updates found in the present production versions. When I installed them with the MG-20s, it became clear they did not have enough power to drive or control the speakers to our satisfaction. I added the Bryston to drive the bass panels, but the baseline system still is under-powered from a typical audiophile perspective. We normally listen at levels well below 90 dB (Radio Shack SPL meter, C scale). Running the system at maximum output from the Wadia will achieve peaks of 90 dB and can cause the Bryston amp to clip, according to the indicator lights on its front panel. I would not recommend this system to anyone even remotely interested in powerful dynamics.
The system is highly revealing, and responds well to simple measures such as separate dedicated power circuits and 600 ohm loading of the balanced cables to the Atma-Sphere amps. An audition of Joe Cohen's Prana Wire interconnects and speaker cables provided the best sound I've heard with this system within its dynamic limitations.
Travis found that he likes the sound of the InnerSound amp with his MG-3.6R speakers best when the heatsinks became too hot to touch. He normally listens at high levels, and has blown his Maggie ribbon tweeters (even with stock fuses in place) by clipping his Atma-Sphere MA-1 amps. He has not blown fuses or tweeters with the InnerSound amp.
Procedure
When I first installed the InnerSound amp, I noticed the metallic ringing of the steel top plate that Travis had warned me about. I damped it with a bag of uncooked rice. There was still a tonal coloration to the sound, and massed strings sounded congealed. The tonal coloration was a result of ringing of the InnerSound amp's machined aluminum front panel. The congealed strings resulted from my use of the gold-plated steel jumpers supplied by Magnepan for single cable operation of their outboard crossover. I do not normally use these, as my bi-amp setup does not require them.
My wife noticed a reduction of clarity in the bass with the REL subwoofer engaged, so I disconnected it. The reduction in clarity was a result of acoustic feedback into the amp, since the REL is down-firing, and located a few inches from where the amp had to rest in order for the cables to reach the speakers. Damping the InnerSound amp's bottom panel removed most of the interaction.
I replaced the steel jumpers on the Magnepan crossovers with silver wire comprising 24 strands of what appears to be 30-gauge bare silver wire twisted together and soldered with WBT silver-bearing solder. The bundle is nearly the same diameter as the steel jumpers.
I improved the damping of the top plate of the InnerSound amp by resting a small piece of MDF on a Deflex sheet on it (taking care not to cover the ventilation holes near the front). I damped the front panel by draping another Deflex sheet over it, and held the sheet against the panel by shoving a 4.5-pound double-bag of raisins and a 5-pound bag of rat food against it. This had the unintended benefit of blocking the annoying purple light from the logo on the front panel.
Finally, I damped the amp's bottom plate by resting the amp on top of another Deflex sheet and a book of the same size, instead of the stock rubber feet, on the wooden amp stand.
Discussion
Many designers of audio equipment recognized the bad effects of case vibration a decade ago. Even the upgraded models of lower-cost mid-fi equipment these days tend to have extensive internal treatments to reduce the mechanical vibrations. My Bryston 4B (ST) is comparatively dead-sounding when rapped anywhere on the exterior, and does not respond to external damping treatments. I was surprised, therefore, at how clearly the InnerSound amp's case rings. I evaluated the changes in performance by damping the different parts accessible to me: since the amp is borrowed, I did not open it up to see if I could dampen the internal components.
The case is the limiter on performance of this amp. The front panel rings with a prominent note 35 cents from F below middle C towards F-sharp (Conn Strobotuner), or about 180 Hz. The first harmonic shows up almost as well as the fundamental on the strobe tuner. This panel rings like a cow bell when rapped with a knuckle. Damping the front panel removes a tonal prominence it gives to the F notes, and markedly increases the sound-stage depth resolution in general.
The power transformer is bolted to the center of the bottom plate, and the amp is supported by rubber feet in the corners. This makes the bottom plate subject to drum-head ringing at low frequencies, as the transformer is quite heavy. Resting the bottom plate directly on a Deflex sheet instead of the feet markedly improves the bass clarity. The amp was placed between the speakers, in their null zones, so I suspect placing the amp elsewhere would have made the bass muddiness worse.
The amp is designed to be left on all the time, and uses a reported 6 watts of standby power. It is very mildly warmer than room temperature when idle. I investigated Travis' report of improved sound when the heat sinks are too hot to touch. I played selections from Kathleen Battle at low volume on the cold amp, put a Tower of Power disc on at full volume, left the room for half an hour so I would not condition my ears to the near-100 dB peaks, then played the Kathleen Battle selections again at the same reduced volume after confirming I could not touch the heat sinks comfortably. I could not detect a difference in sound or detail. I suspect Travis is simply partially demagnetizing his steel jumpers by running large signals through them to heat up the amp (he confimed to me that he is using the stock Magnepan jumpers).
My previous attempts to use the Bryston amp full-range on these speakers had always been disappointing, and I fully expected to hear similar colorations and lack of detail with the InnerSound. I was prejudiced against transistor amps for their cold, grainy sound. Some of that grain was due to the steel Magnepan jumpers, but the Bryston used full-range with the silver jumpers installed on the speakers is still unbearably bright, hard, and "transistory."
However, except for a slight change in timbre of the piano on Nojima Plays Liszt, neither my wife (a classically-trained harpist), my friends, nor I can detect any tonal problems with the InnerSound amp. The treble is clean, clear, and extended. The bass is more clear than that of the Bryston, even in the bi-amp setup (once the InnerSound amp's bottom plate is damped). The detail in this setup is still not as good as what I get with the bi-amp combo, but this may be due to the need to use the stock large series capacitors in the high-pass sections of the Magnepan outboard crossovers, or the single speaker cable pair. The dynamics are effortless, and the amp can play without apparent strain (there is no clipping indicator) at least 8 dB louder than the bi-amp combo.
The increased dynamic range this amp provides is exhilarating. Intimate jazz recordings like those produced by Mapleshade drew me in with this amp, so that I lost track of the analysis I intended to perform when I put them on. Patricia Barber's recordings became even more compelling. The richness of good Baroque recordings at lower levels was preserved: this amp would make an excellent summer-time power source for those with noisy air conditioners or a need to keep the house cool during the day so that it is tolerable in the evening. Dynamics versus detail: the choice becomes more difficult when an amp like this demonstrates excellent dynamic performance.
Magnepan MG-20 speakers present a uniform but inefficient load. It is difficult to find a power amp that combines tonal purity, detail, and sufficient power and current capacity to drive them to their full potential. I cannot make the categorical statement that no such amp exists, but I would not be surprised if it were proved to be true. The InnerSound ESL amp is not even close to the ultimate in dynamics, resolution, and tonal purity on these speakers in its stock form, but for those satisfied with 100 dB peaks (it might get louder but I am reluctant to find the clipping point with my speakers) and willing to experiment with external or internal damping of the case, it approaches the ideal combination and beats the Bryston 4B (ST) hands-down. With a list price of $2995, it is a no-brainer for anyone wanting to get close to ultimate performance within these limits.
I would be very interested in learning the results of a vertical bi-amp setup with the InnerSound ESL amp with appropriate case damping treatments, or with upgraded capacitors in the external Magnepan crossovers and bi-wire speaker cables.
Recordings for evaluation of audio equipment.
Classical
Danzon
Simon Bolivar Symphony Orchestra,
Keri-Lynn Wilson, conductor
Dorian xCD-90254
Exotic Dances from the Opera
Minnesota Orchestra
Eiji Oue, conductor
Reference Recordings RR-71CD
Ebony Concerto
John Bruce Yeh, clarinet
DePaul University Jazz & Wind Ensembles
Reference Recordings RR-55CD
Baroque Duet
Kathleen Battle and Wynton Marsalis
Orchestra of St. Luke's
John Nelson, conductor
Anthony Newman, Harpsichord continuo
Sony Classical SK 46672
Nojima Plays Liszt
Minoru Nojima on the Hamburg Steinway
Reference Recordings RR-25CD
Traditional
American Dreamer, Songs of Stephen Foster
Thomas Hampson, Jay Ungar, Molly Mason, and David Alpher
Angel D100285
Jazz
Canyon
Paul Winter
Living Music LD0006
Brasileiro
Sergio Mendes
Elektra 61315-2
Cachao Master Sessions Volume I
Israel Lopez
CineSon (epic) EK 64320
Modern Cool
Patricia Barber
premonition PREM-741-2
Big Sweet N' Blue
Norris Turney with his Quartet
Mapleshade 02632