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John Ulrick, the founder and engineer behind Spectron, has spent nearly twenty-five years pursuing digital amplification and his name is tightly woven with the history of our industry. The result of his quarter century of work is that he is now manufacturing arguably the most distinctive digital amplifiers in our industry. The Spectron class D digital amplifiers use the most comprehensive feedback network in the game. But what makes a digital amplifier so different from conventional linear power amplifiers?

Well, there are basically, regardless of whether we are discussing glass or silicon, only three other classes of operation used in audio applications today; class A, Class B, or some combination of them both, Classes AB or A/AB. Most audio amplifiers today use Class A, Class AB or Class A/AB, as pure class B is just not acceptable for such applications. I leave it to you to do further research should you see fit, as we do not have the space to go into them that deeply in the context of this review.

Some companies and industry specialists would have you believe that there is more than one type of Class D operation and would likely cite Class T, the name given to the Tripath circuit. In reality, the Tripath circuit and device, now licensed to the likes of Sony, Kenwood, Panasonic, Yamaha and Blaupunkt, to name just a few, is actually a Class D device as reluctantly confirmed by one of their applications engineers.

How Class D works may be better understood by taking a look at the Spectron Home Page. However, in John's own words,

"The input audio is converted to a pulse width signal called a modulated carrier, not unlike the AM or FM modulation associated with radio. Like radio, the audio signal is contained in the modulation of the carrier. This carrier frequency in Spectron amplifiers is 500 kHz, much greater than the highest audio frequency. This carrier is a square wave with a modulation index that varies from 0 to 1, or PWM (Pulse Width Modulation). The modulator drives a power section that converts this modulation index into a high voltage level, which in turn, can drive a speaker. This is done using two switches, actually high-speed transistors, switching ON and OFF at the 500 kHz carrier. These two switches raise the voltage level up to the required level to drive the speaker. The amount of time each of the two switches is ON or OFF is controlled by the modulation index. At this point, we have a high voltage (+125V) PWM signal to drive the speaker. We now need to pass this signal through a low pass filter in order to stop the carrier from passing on to the speaker but allowing only the audio to pass on to the speaker. This describes the typical open loop class-D amplifier which would be useable as is, but…with feedback we can make substantial improvements in terms of both measured and sonic performance."

Without belaboring the point, the most significant difference, both sonically and from an engineering standpoint, between the Tripath circuit and other digital amplifier designs and what John is doing at Spectron is feedback control. An amplifier feedback loop is a circuit that compares the output signal of the amplifier with the input signal and attempts to "correct" for any "differences" detected at the output, which is distortion. It attempts to eliminate distortion by introducing an inverted version of any signal noted at the output that is not present at the input back into the input in hopes of "canceling" that unwanted signal. There are usually three stages of feedback applied to an amplifier circuit, including the modulator, the power section and the output filter.

Obviously, this has to be done very rapidly or the process itself creates distortion rather than canceling any. With the Spectron Musician II, this feedback is achieved in two hundred nanoseconds, or .0000002 seconds! In the best conventional linear amplifiers, the time through the feedback loop is some two thousand nanoseconds, which is an order of magnitude (10 times) slower than the Spectron! Most Class D amplifiers include the first two stages in their feedback loops, but Spectron is the only Class D amplifier that includes the output filter stage as well, enclosing all three stages within their feedback control. Why is this important? John Ulrick explains further.

"The output filter passes the audio signal to the speaker and blocks the high frequency carrier. All filters have group delay errors. Group delay occurs when the various frequencies of an instrument arrive at a time out of alignment (i.e., delayed) when compared with the original recorded sound. Including the output filter in the feedback loop greatly minimizes these group delay errors. All of the harmonics of the music therefore appear at the output of the amplifier with the same time alignment in which they were recorded. Consequently, the four most important advantages of including the output filter in the feedback loop are 1) proper time alignment, 2) flatter frequency response, 3) lower distortion and 4) lower output impedance, all of which improve speaker damping. It is the resultant lower output impedance that minimizes the amplifier's interaction with the speaker. Interaction with the speaker manifests itself when an amplifier sounds better with one speaker than another."

John adds,

"Not all class-D amplifiers are designed this way. For example, the Tripath amplifier chip uses feedback, but it doesn't measure the feedback signal from the output. Rather, it measures the feedback prior to the low pass filter. Therefore, errors caused by the low pass filter cannot be corrected. The Tripath chip also can't use overall feedback because the feedback signal would be too far out of time alignment to be useful. Because the reconstruction filter is a second order low pass filter made of a 20 micro Henry inductor between the amplifier and the speaker and a capacitor of about .2 micro Farads to ground, the speaker's frequency response will be affected by the amplifier and there is no feedback correction for this."

This more global approach to feedback control results in greatly reduced distortion. Moreover, since the output impedance is lower than with other digital amplifier circuits, which have additional filtering in series with the speaker, there are fewer reactive problems with speakers. Additionally, in terms of efficiency, Class D wins hands down, converting something on the order of 90% of the power it draws into work. This is very unlike linear amps, the best of which can only boast a near 50% efficiency. This means you will consume only about ⅓ as much power as a linear amplifier of similar output - and it never gets toasty hot! Those of you whose politics lean toward Green should appreciate this last benefit.

 
Setting the Stage

Very soon, you will be able to buy a module that will let you run a digital data stream from your CD player or transport directly to the Musician II. This will let you completely eliminate using any form of analog signal transfer between a line stage and the amp, and will permit the direct conversion of ones and zeros back to music right there at the amplifier. Though current standards favor the 24-bit word length and 192 kHz sampling rates, Spectron will support new standards as they become available.

The Musician II allows for both single ended and balanced operation, and I used both for my evaluations here. Though the default setting from the factory is for the single ended inputs, I found the balanced operation, when employing the Marsh Sound Design P2000b, to be the preferred method of operation. Switching between single ended or balanced operation is achieved by using a specially provided tool to change some internal DIP switches.

This evaluation was made with two very different preamplification systems. For single ended operation, I used the ridiculously over-achieving Channel Islands Audio VPC-1 passive volume control and the superb Monolithic Sound PA-1 with the HC-1 dual mono power supply. Both passive outputs were sent to the Source Components Electronic Harmonic Recover System, as I use a 6-meter interconnect to reach the power amp.

In general, the overall performance with the balanced output from the MSD P2000b was of significantly superior performance over its single ended output, and that was the pairing used for the bulk of this review. While the CIAudio VPC-1 was slightly more transparent than any other pairing, the resulting dynamic capability and blackness of background offered with the use of the MSD P2000b ultimately won me over. My testing incorporated full range dynamic loudspeakers (VSA VR-4 III SE/Buggtussel Lemniscus), minimonitors (Apogee Cassiopeia 6) and some giant capacitors, er, I mean Electrostatic Panels (modded Acoustat 2+2 Medallions).

 
The Sound of 1.34 Digital Horsepower Clapping

In broad strokes, this is simply one of the finest amplifiers I've yet heard. In many ways, in fact, it may be the best. It is so good, and in so many ways, that I have had to rethink all I once held true and sacred about amplifier design.

First of all, this is one powerful beastie! Rated at 500 Wpc, both channels represent an output of 1.34 horsepower! That, in and of itself, is not insignificant, as under normal home listening conditions, you will not find any application where you will force this amplifier into clipping, even with bass heavy source material or the most demanding of speaker loads. Though I don't typically play music significantly above 80-85 dB, even when pushed, the effortlessness and authority this amplifier exhibits on dynamic crescendos and deep powerful bass passages is significant.

Let's start with dynamics. While the Pass Labs Alephs were known to excel in the rendering of microdynamic variation, the Musician II leaves them cold and shallow in comparison. Both the range and subtlety it affords microdynamic events are simply superb and unprecedented in my environment. Listen to the explosive P sound when Mark Knopfler utters the word "pass" at 1:02 into "Fade to Black" from the Dire Straits album On Every Street [Warner Brothers 26680-2]. The entire room is pressurized by the sound of the air hitting the microphone.

Macro events have such weight and impact that you will often be caught off guard by their authority, no matter how often you've listened to or familiar you are with a particular recording. Chris Layton's drum snap near the beginning of the title cut from Stevie Ray Vaughn's Couldn't Stand The Weather [Epic 25940 - Absolute Analog Reissue] is sharply rendered in its attack: crisp and finely defined in both its physical space and in its material attack and subsequent decay. The corporeal assault presented throughout "L'Daddy" from James Newton Howard's James Newton Howard & Friends [Sheffield Lab 23] is breathtaking. I have been using these recordings since they were first released and I have never heard any of them recreated with such blinding transients or concussive, realistic impact

This amp manages to get to the heart of the musical message by accurately recreating the attack and decay of instruments, from strings and pianos to cymbals and tambourines. It has the ability to faithfully deliver the pace and rhythm of everything it is given, never blurring or homogenizing rhythmic events as some lesser amplifiers might.

Its recreation of the overall presentation is out of this world; by far the best to ever grace my listening room. The staging is simply spectacular, with the width, height and depth of the venue laid bare before you. Its control over specific instrumental locations and the delicate yet concise layering of those instruments throughout the soundstage lets it run the most incredible rein on imaging. Instrumental textures are woven so clearly and localized so accurately that you will be lost in the recreated expanse of music before you.

An area in which this amp simply outdistances any I've experienced is that of resolution. Subtle detail is flushed out with remarkable refinement. Very low-level events, including those normally obscured down near the noise floor, are developed effortlessly. Take the muted time-keeping foot tapping of drummer Chris Layton, again from Couldn't Stand the Weather. Early in the title track there are several breaks where the band repeatedly stops and restarts. During these pauses, Layton's subtle and hushed foot tapping is so readily apparent and clearly outlined in space that you can nearly guess what brand of shoe he is wearing!

Individual string plucks, strums or hammerings are presented with a singularity of voice that is so stark and real, you will begin to note missteps by the artists that were previously obscured. Listen to the individual and obviously circular brush strokes Jim Keltner applies to his snare and floor tom, or the slightly muted attack of the drumheads when he switches to mallets, from his improvisational romp on side 2 of The Sheffield Drum Record [Sheffield Lab 14]. Musical focus of this caliber has only been dreamed of in my system previously. The level of articulation, detail retrieval, focus and air the Spectron II brings to the table has only been hinted at in my room previously and I've only heard similar, not necessarily equal, performance in this area from amps costing tens of thousands of dollars.

Treble extension and quality is the best I can recall hearing from any amp, and especially from the other Class D entrants. In terms of both extension and grainlessness, it is completely unsurpassed in my experience. While I am not unenamored of the original Bel Canto eVo's overall performance, their slightly veiled and granular presentation of the highest frequencies left me wanting. The Musician II seems to go unfettered to the ionosphere, cleanly, clearly and precisely. It never got edgy, harsh or grainy unless the source material was known to be so. I cannot over-emphasize the value of this particular attribute because it offers a most alluring and magical quality. You never find yourself suffering listener fatigue at the hand of a "glaring" or "etched" treble response.

The timbral accuracy of this amp is simply spectacular. Its tonal balance is a study in seduction. In particular, the sound of gut, rosin and string has never been more realistically regenerated in my room by any amplifier; this synesthesia almost allows you to "see" rosin flying from the gut of a bow in particularly frantic passages.

The female voice, from Ricki Lee Jones's on her self titled debut Ricki Lee Jones [Warner Bros. 7599-27389-1 180 g. Vinyl Revival German Import] to Tori Amos' on her latest release Scarlet's Walk [Epic EK 86939] were magical treats. Ricki's wily charm comes through in abundance on tracks like "Easy Money" while the piano nearly comes to life on tracks like "On Saturday Afternoons in 1963." Tori, and her piano on many cuts, are so accessible you are drawn completely into the tapestry of the music.

Take the gentle stylish musings of Ivan Moravec, on Ivan Moravec Plays Beethoven [VAIA 1021], or Vladimir Horowitz, from The Last Recording [Sony SK 45818], or the rampant attack of David Helfgott, from David Helfgott plays Rachmaninov [RCA 74321-40378-2]. The piano is so intimately recreated, whether being gently brushed into a hushed whisper of a note, or being pounded into a cacophony of emotion, that you are drawn in to the performance and allowed to understand its message as never before.

The pleading elation that Jascha Heifetz wrings from his violin throughout The Supreme [RCA 74321-63470-2], or that Henryk Szeryng brings to Édouard Lalo's Symphony Espagnole [RCA LSC-2456 Classic Reissue], is both enchanting and completely captivating.

The male voice is presented wonderfully as well. The three distinct voices of David Crosby, Stephen Stills and Graham Nash are reproduced with chilling body and power. Listen to cuts like "Daylight Again" and "Find The Cost Of Freedom" from the 1991 Atlantic four-disc compilation Crosby, Stills & Nash [Atlantic 782319-2]. The robust, charismatic voice of Stevie Ray Vaughn, which is all too often overlooked in favor of his obvious guitar mastery, is astonishingly emotive on cuts like "Tin Pan Alley" and "The Things (That) I Used To Do" from the previously mentioned Couldn't Stand The Weather. The Musician II does such a superb job with pianos, violins and voices, some of the hardest "voices" to get right, that I found myself listening for much longer periods of time that I had allotted.

I want to briefly touch on the use of aftermarket AC cables. While I find most amplifiers can benefit greatly from the application of aftermarket power cables, the Spectron Musician II was the least affected by such cable changes of any other amplifier I've yet auditioned. While AC cable changes did reveal slight changes in the amplifiers resultant performance, the changes were so inconsequential that I gave up pursuing that route as a necessary method of maximizing the sonic output of the amp. I settled on the Harmonic Technology Fantasy AC-10 with Furutech ends and let it go at that.

 
Extending The Feedback Loop

One of the truly unique features of this product is the potential to use Spectron's own unique four conductor cable, the Remote Sense Cables, which connect to the back of the Musician via a set of high performance Neutrik connections.

These special cables ($595 a set up to 5 meters) literally extend the feedback loop of the amplifier all the way to the speakers! While two conductors carry the audio signal, the other pair sense the actual voltage at the speaker end of the cable, allowing compensation for the otherwise unavoidable loss due to wire inductance and capacitance, as well as the speakers own complex interaction with the audio signal. To my knowledge, this is the most advanced and comprehensive approach to amplifier feedback control ever attempted in a consumer product.

At the $595 asking price, the Spectron Remote Sense Cable sounded better than all the other speaker cables I had on hand, save for the Harmonic Technology Magic One biwires. In my listening, the extension at both frequency extremes was slightly better with the HT Magic's, while the lower mids seemed just the slightest bit more liquid with the Remote Sense Cables in place. I wonder how fair a comparison that is as the Magic One biwires sell for nearly six times the Remote Sense Cables asking price.

My point here is that at $595, the Remote Sense Cables are simply superb! I have never heard the level of frequency continuity, resolution and faithful timbre they afford from any cable in this price range. It is a shame that they will only work with Spectron amps!

 
Thirty-Eight Pounds Of Musical Magic

Using the latest realization of John Ulrick's near quarter century dream has brought me substantially closer to the magic of the music I love so dearly. This is the second milestone product I've had the pleasure of reviewing this year, the first being the VSA VR-4 III SE. The Spectron Musician II is a remarkable achievement, truly pushing the state of the art.

Is this amp perfect? Not likely: nothing is. Yet it comes as close to my idea of perfection as I've yet encountered. In terms of power, dynamics, attack, control, presentation, grace, resolve, involvement and voicing, I don't know of a single amplifier, at any price, that would embarrass the Spectron Musician II. It has more successfully achieved that delicate balance between brute force and artistic expression than any amplifier I've had the pleasure of hearing. And at $3500, that makes it possibly the best value in amplification available today. Run, don't walk, to your nearest Spectron dealer and hear what you've been missing. John, you can't have this one back!

Power: 600 Wpc at 4 ohms/500 Wpc at 8 ohms/200 Wpc at 16 ohms
Output current: 40 amps peak (burst)
Output impedance: 0.03 ohm @ 1 KHz Damping factor: 260 @ 8 ohm
Distortion, (1 KHz THD): <0.06% @ 500 watts into 8 ohms
Frequency Response: ± .1 dB 20 to 20,000 Hz
Noise: 83 dBW (<200 uV) or -110 dB below 500 watts
Input: XLR balanced, 25K? impedance/RCA input: 50K? impedance
Warranty: 3 Years
Size: 17" W × 5" H × 14" D (431 mm W × 133 mm H × 368 mm D)
Weight: 38 lbs (18 Kg)
Price: $3,495 analog inputs/$3,995 analog and direct digital inputs when available

Spectron
9334 Osso Avenue Unit E
Chatsworth, CA 91311
U.S.A.

Telephone: 818-727-7603
Fax: 818-999-5616
Email: marketing@spectronaudio.com
Website: www.spectronav.com

Copyright 2004 OS&M. All Rights Reserved.