Stereo Class D Amplifiers

Factory bridged for monoblock operation

PRICE: $7,195 EACH


James L. Darby


They say that power corrupts and absolute power corrupts absolutely. If that is the case, the Spectron Musician III SE Mk 2 will absolutely corrupt your audiophile sensibilities in the best possible ways. The amp is rated at 600 watts per channel in stereo mode into an 8 ohm load. In monoblock mode, one would assume that the output power simply doubles to a monstrous 1,200 watts. But nay – one would assume wrong! According to the owner’s manual, in bridged mode one amplifier delivers TWO THOUSAND watts into 8 ohms. That’s right – 2 kilowatts into each speaker.




But the colossal numbers do not stop there. Listen to this from an email I received from Dr. Simon Thacher of the Spectron Design team:

“Less efficient loads will benefit from the fully balanced operation that triples the power and doubles the headroom to plus and minus 240 volts as well as 7000 watts peak power”.

Well, yes and no. Simon says 7000 watts PEAK power. That sounds like a lot of hype, and with most amps it absolutely would be because “peak power” can mean almost anything and it is far different than continuous power. In this case however, if what Simon says is true, there is some substance to that statistic. Any time you hear someone quote “peak power”, you first question should be “What is the duration of the peak”? Most amps can theoretically put out several times their rated output for a very short time – just a few milliseconds which is not really useful for even the shortest musical bursts. However, Simon says this about that:

“Spectron amplifiers deliver peak currents of 65 amps which allows the amplifier to deliver the full transient (burst of music) without "current clipping". Spectron not only delivers very high peak current, but also will hold that current up long enough to play loud passages. Specifically, Spectron will deliver this high current, 65 amps peak (with a staggering peak power of 3500 watts per channel) for 500 msec. On the other hand, most other amplifiers only deliver their rated peak current for sometimes a fraction of this time”.

Without getting too technical, 500 msec is long enough to reproduce the leading edges of very loud musical bursts, so the ability to reproduce 7,000 watts for 500 msec is important.

“Great”, you say. “Unless you are running sound in a stadium for the Grateful Dead, who in the heck would ever need that much power!?”

Good question. That’s why I posed it to Simon:

“The ability of amplifier to represent undistorted musical event is frequently mistaken, as its continuous power output (rms). However, its the headroom is a true measure of the amplifier's ability to reproduce large transients (e.g., the cannons in the 1812 Overture, or more often in piano music, or the rim strike of a snare drum often found in jazz). It's correct that power is the product of the voltage times the current. However, in reality an amplifier's voltage headroom is limited by the power supply voltage, which limits the peak voltage. Spectron's amplifiers are the highest voltage we know of in the audiophile world, using a plus and minus supply of 120V. By comparison most amplifiers use power supply voltages on the order of plus and minus 65V or even lower. (Additionally, most other amplifiers, particularly tube amplifiers, increase their distortions dramatically, sometimes immensely, at full output - making experience of listening loud passages highly unpleasant. Spectron's distortion level is almost does not increase with output!)

 What does the limitation in peak voltage mean? To illustrate, lets take the example of a CD player with typical output of 2.2V rms (i.e. peak  3V), and if you use XLR balanced output then its peak voltage is 6 volts. Lets for simplicity assume that you are using a passive preamp with gain set at this experiment to the unity i.e. 1.0, and finally, lets take a typical amplifier which has usually gain of 26 dB or x20. Thus, you must expect your amplifier to have output peak voltage: 6V x 1.0 x 20 = 120V. This means that when listening to the same music with most other amplifiers, particularly those solid state amps without output transformers, the signal would be "clipped". These amplifiers would be unable to deliver the transient voltage to the speaker. The effect is that the music loses some of its lifelike qualities i.e. signal is distorted and dynamics is compressed.
Above example represents situation when your preamplifier gain is unity i.e. with relatively high efficiency speakers. When your speakers (or music) demand your preamplifier to have gain more then 1.0 then even Spectron amplifiers might clip! Fortunately, Musician III SE is capable of operating both in stereo and mono (fully balanced) modes. While not necessarily an advantage for highly efficient speakers as described above, less efficient loads will benefit from the fully balanced operation that triples the power and doubles the headroom to plus and minus 240 volts as well as 7000 watts peak power. This results in effortless presentation, deeper bass and fast, powerful slam.
Regardless of the efficiency of the speakers, balanced mode of operation doubles the slew rate and bandwidth by virtue of the out of phase transmission. The other major advantage of this mode is dramatic suppression of both the intrinsic amplifier distortions as well as the noise and buzz, leading to a spectacular improvement in dynamics and resolution of detail in the music”.

Are you buying this so far? Needless to say, any gobs of power will be meaningless if it does not translate to a musical presentation, and that we will address shortly. In the mean time, let’s take a look at some of the amp’s other features.


First, everything we have talked about so far assumes use of the amps in balanced mode, which is how they were set up for this review. There are standard RCA jacks for unbalanced operation if need be, but if you are going to spend $14,000 for these, why would you even think of running them in an inferior sounding configuration?

With that much juice on hand and at these prices, the Spectrons better have some serious circuit protection built in, and they do. There is an overheat circuit that shuts down the power section if temps reach 185 degrees . You would have to do something terribly wrong to reach that level because the Musician’s run very cool. Even when pushed hard, they never were more than slightly warm to the touch – a much welcome feature in South Florida. Air circulation is important though and the main intake vents are on the bottom on the chassis, so placing them on a rug is a big no-no. Exhaust is in the back, so they do need their space behind, too. Cooling is by convection I believe, there are no fans in the chassis.

There is also hefty fault protection if excessive DC or high frequency voltage appears at the outputs. This helps protect your speakers if there is problem with the amp. Should the load draw more than 65 amps (!), the amp will lower the limit to 12 amps until the problem is rectified.

There is also AC protection if the voltage to the amp is too high or too low. Isolated low level power supplies are employed which eliminates noise from AC lines in low level circuits.

What is not protected however is your speakers when the amps are operating normally. Don’t even think of running a brush over your turntable’s stylus with the volume turned up. If you do, you may end up spending days trying to clean up the particles of what was once your speakers.

Spectron recommends that you leave the amps turned on all the time because it takes a while for them to reach thermal stability and their optimum sound. Because I live in the lightening capital of the world, that is not always possible. When I had to turn them off, I found it takes a good 45 minutes to an hour for them to sound their best. They do sound a bit congested starting from cold, more so than the average amp. All amps do need some time to optimize. With no music going through them, they burn about 60 watts.

There is no shortage of Class D amps on the market and almost all of them use chips made by outside manufacturers. Not here. The chips are designed by John Ulrick who founded Spectron and is credited with designing the first Class D amp over three decades ago. Well here; I’ll let Simon tell you:

"Spectron does not buy its class D modules from a third party! Our designs are our own, on the cutting edge of the technology. Because we have complete control over our designs, we are constantly improving them. Moreover, Spectron's president and chief designer, John Ulrick, introduced the first commercially available class D amplifier into the audio world at CES 1974. At that time John was president and co-founder of Infinity Systems, famous for its pioneering development of the combination of the servo woofer with the electrostatic speaker. John has devoted the last 25 years to extending a "control system" approach to class D amplifiers, and the Musician III Signature Edition MK2 is his and the Spectron design team's latest achievement"

Appearance wise, the sculpted silver cabinets (available in black) are attractive in a rather high-tech way. There is a soft blue “power on” backlight behind the Spectron logo on the front. Thankfully, it does not light up a darkened room. A nice touch.

Many Class D amps are rather lightweight, however the Spectrons weigh in at 52 pounds each. There is nothing lightweight about them, even though they do not use the usual large can-type electrolytic capacitors. Rather, they use a bank of small capacitors to store energy for each channel.’

They are claimed to be able to drive any speaker load down to 0.1 ohm. Including electrostats. One of the biggest problems in achieving quality sound is matching amps to speakers. I would bet that the majority of systems have amplifiers that do not properly interact with the system’s amplification. There are a lot of audiophiles who are unknowingly listening to excessive distortion and even clipping because their amps simply are not compatible with their speakers. Looking at amp specs and speaker specs do not assure a good match. With the Spectron Musician III’s, that issue is virtually eliminated.


Believe it or not, there are still some audiophiles who do not believe that components and speakers need time to break in. From what I am told by many audio companies, there are some reviewers who either think the same or are just too lazy to fully burn in products before serious evaluations begin. Each Spectron amp is burned in for a week before they are shipped. Why would a company go to that expense if there were no such thing? Even at that, the manual states that it will require “several weeks of 24/7 playing (in your system) at medium volume to reach its natural sound”.

Because Stereomojo wants to make sure that each product we review is performing at its optimum before we begin serious listening, I asked Simon to be more specific. I was told that 200 hours is on the low end, but a period of 400 to 500 hours is better. “True break-in takes more than 1000 hours.”, said Simon. Hmmm. If there are 168 hours in a week, it would take nearly 6 weeks of constant music playing at moderate volume to achieve that. Since we could only keep the amps for about 30 days, that was impossible. I started at about 300 hours, but that burn-in factor is something about which potential buyers need to be aware.

Recently, a friend of mine bought a new car – a Bentley Continental GT. This ultra-luxury vehicle weighs almost 3 tons yet has the power to be propelled to 60 mph in about 4.5 seconds. Now, this friend is really a good friend because he let me drive the thing that must cost north of 200 large. I will never forget the experience.

The V-12 engine was silent and silky smooth and provided the sensation of taking off in 747. Depressing the accelerator did not feel like a car at all, it just moved forward with no sense of internal combustion at all – it was if I was being firmly pushed by an Almighty breath with no sense of clatter, winding up or strain whatsoever. If you needed or wanted more power, it felt as if there was a limitless supply of it.

That is how I would describe listening to the Spectrons; quiet and luxurious with no sense of effort at any volume level.





If , like I, you are approaching geezerdom, you may recall this TV and print ad by Memorex that depicts a guy sitting in his chair and literally getting blown away by hurricane force sound while listening to his system. This might be what one would expect of ultra-powerful amplifiers such as these, but the exact opposite is the truth. The music does not assault you, it envelopes you with no perception of force. More than most other amps, the Spectrons allow you to feel the music (not the sound) as well as hear it. It is a sensation without being sensational. Like the Bentley, you don’t think to yourself, “Wow! This is powerful!”, you are not conscious of the power at all. You are conscious of the music in all its pristine glory without much of a perception of electronics at all; the music just is.

Years of reviewing various audio devices teaching you some things. For example, among other things, the Sanders 10b electrostats taught me the importance of speed and the role it plays in getting the sound of leading edges and transients right. Like the Bentley, the Spectrons excel at speed. Plucked and struck instruments were reproduced faithfully and never sounded slow and laborious.

Most class D amps can sound rather dry, especially on top. While I would not call the upper registers especially liquid, they were never strident, etched or powdery. Some might judge the sound to be a bit analytical, but I did not think so. Precise and accurate would be more fitting with a good amount of sparkle when called for. Highs were extended and not rolled off.

The midrange was similar with no excessive thickness. No one would call the mids “warm” or “romantic”; there was certainly no tube glow, neither was there the sometimes magical three-dimensionality that quality tube amps can impart. Those used to such characteristics might say the mids are slightly lean while those in the other camp would applaud the lack of tube coloration. Sounds as if the ‘Trons are close to the middle of the mids; something I would call neutral.

“Perfect Alibi” by Britain’s first lady of jazz vocals on Linn AKD 316 (see link to our review) was warm because the recording was cozy and toasty with her voice out front and well separated from the band. The instruments were rich in detail and easily located in the abundant soundstage. While each instrument was solidly placed with no congestion or graininess surrounding them, air, ambience and reverb trails were very good, but not excellent.

Switching amps, to me at least, still do not do this as well as good solid state or tubes. It used to be that digital, like CD’s, did not resolve those qualities well at all, but is changing rapidly as chips and processors gain speed and resolving power.But then early digital cameras produced images that looked pretty sad, too. Things in digi-land are getting much better fast.

Another thing I noticed after spending time with the Musicians were that they not only sounded musical at loud and moderate levels, they also were articulate and listenable at low levels. I cannot explain why, but it is true. More than once while playing at background levels, I was drawn in to the music to the point I had to sit down and listen. At least that’s what I told Linda when she saw me loafing one Saturday morning. “But I AM working, dear….”


Large scale orchestral recordings were especially good via the Spectrons. Here the extra headroom and power was obvious as tutti’s and long crescendo’s crashed and boomed. Again, the ear did not have to strain to identify individual horns, woodwinds and percussion solos or sections; they were all laid out perfectly in space, side to side and front to back in well defined layers.

You might think the bottom end would be particularly pronounced when propelled by such massive wattage. I never found myself thinking, “Holy molie! Listen to that bass!” But then I never found myself wondering what was wrong with the bass, either. Bass fiddles, tympani and the piano’s left hand were not overdone or unmusically boosted. Again, they were just there and never drew any attention to themselves for good or bad.

"Garden of Early Delights" is a very small-scale recording that features Pamela Thorby on recorder and Andrew Lawrence-King on harp. This is my favorite Early Music recording and it is again on Linn (CKD 291) in SACD and HDCD. Most such collections present performances that are so dry and academic that I soon find myself bored and uninvolved. Thorby’s verions are wonderfully engaging, lilting and sometimes even humourous, the antithesis of typical efforts. This is the recording you should try if, like me, you have found yourself put off by music of the 1400’s or so.

Both artists play different versions of their instruments throughout and it is easy and even fun to identify which is which. The ‘Trons allowed me to do just that. Suffice it to say that the MK2’s do small well, too.

The Halcro MC-20 is one of my reference amps. Like the Spectron, it too is Class D with chips designed in house. It puts out 400 wpc in stereo. It also claims a THD of <0.007% at 1kHz at all powers up to 400W into 4 ohms and < 0.03% at 7kHz at all powers up to 400W. The Spectron claims <.03% from 1W to 600W @ 8Ω ( in the listening range) and <.06% from 1W to 600W @ 8Ω ( > 22 kHz).

Check those decimal points; The Halcro states 0.03% while the ‘Tron is 0.30% suggesting the Halcro is about 10 times better. The Halcro further states an infinitesimal 0.007% at 1kHz at all powers up to 400W into 4 ohms. But the qualifiers are not the same. The Spectron’s spec is much more stringent and less limited with specs of 1W to 600 at EIGHT Ohms in the listening range – at just 1kHz or 7kHz like the Halcro One cannot compare apples to apples via those specs. { Which leads us to another Stereomojo…

This dirty little secret is there is no industry standard for published specs. Manufacturers are free to spin them any way they choose to make theirs look good. In this case, Halcro chose selective frequencies of 1kHz and 7kHz to highlight their numbers. The problem is, it would be pretty boring to listen to music that only encompassed two frequencies; you would be listening to two test tones! That, of course, is one of the many problems with test measurements in the first place. I have yet to see a measurement for the size of the soundstage or how an amp reproduces long reverb trails. They have not been invented yet and probably will never be.

But THD is a pretty standard spec, but it is then much more arduous (and honest) for a designer to spec an amp over all frequencies or at least, as Spectron has done here, “in the listening range”, though even that is not clearly defined. The <.06% from 1W to 600W @ 8Ω ( > 22 kHz) is a much more defined range and a much more real world one as well. In fact, it is more than real world spec because in most listening rooms, the amp will seldom if ever be called upon to product all 600 watts! Of course, that measurement results in the highest THD number and if you did not know better, might be a big minus.

Wouldn’t it be nice if every audio journal in the world got together and came up with a set of specs that companies would have to adhere to if they wanted their products reviewed, mentioned or even advertised. Not likely to happen…

The moral is, be aware (and beware) of how specs are presented in ads and brochures. Don’t take them at face value and if they are important to you, dig a little deeper and look a little closer. Or read us. We’ll try to do it for you…

Spectron’s noise specs are similar; <180 µV (22 Hz to 22 kHz) -83 dB below one Watt (A measure of hiss). See that 22 to 22,000 range? That’s pretty quiet. Kudos to Spectron for better quality specs!

I will just state flatly that the two Spectron monoblocks sound better than the single chassis stereo Halcro.; not by a wide margin, but in most every area. The Halcro goes for about $5,000, so there should be a difference. I wish I could have had the opportunity to listen to one of the Spectron’s in stereo mode, but there was little time and the amps had been set up for mono at the factory.


FACT A: If you have speakers that are legitimately very efficient with high sensitivity (those specs are dodgy as well), you do not need these amps. Or probably even one of them. Examples are single-horn types or full-range models like those by Coincident which have good sensitivity and benign, amp friendly impendence curves.

FACT B: If you have 100 or 200 wpc amp (specs are tricky here, too!) driving rather inefficient speakers with impendence curves that range all over the place and you listen at moderately loud to loud levels to music other than harpsichord solos in a largish room, you are probably listening to large amount of distortion and/or clipping.
The size of the speaker does not matter. Some small speakers (like Brit monitors) are notoriously hard to drive.

If the later is the case, you might be an excellent candidate for the Spectron Musician III SE Mk 2. They will drive most anything to staggering levels in a large room in balanced monoblock mode and do it with reference amp quality. They have top notch protection circuits and run extremely cool – warm climates take special note.

If you are not sure you need the amount of power provided by the monoblocks, you might want to try a single amp in stereo. Either way, the Spectron Musician III SE Mk 2 is good enough and powerful enough that you could well find yourself with the last amp you ever need to buy.

The Spectron’s are warrantied for 3 years.








Digital pulse width modulation switching amplifier.
Feedback loop 10 times faster than typical conventional amplifiers.
Staggering headroom of 3500 volt-amps over 500 msec.
No crossover distortion.
Drives the most difficult speakers, stable to .1 Ohms.
Foldback current limit. Automatic overload recovery.
Professional quality balanced line input.
Phase invert. Rear panel selectable.
Accepts either XLR or RCA input: True balanced line.
In-rush resistor bypass. More and tighter bass.
Isolated low level power supplies. Eliminates noise from AC line in low level circuits.
Speaker protection. Turns amp off for excessive DC or high frequency.
High efficiency for minimal power consumption.
Small size.


Technical Specifications


Watts per channel, both channels driven
600 Watts at 8 Ohms
800 Watts at 4 Ohms
1400 Watts at 2 Ohms
<.03% from 1W to 600W @ 8Ω ( in the listening range)
<.06% from 1W to 600W @ 8Ω ( > 22 kHz)
<180 µV (22 Hz to 22 kHz)
-83 dB below one Watt (A measure of hiss)
115 dB
Frequency response
±.1 dB 20 Hz to 20 kHz at 8 Ohms
100 kHz
Power Supply Voltage
+ 120 VDC
Input impedance
RCA: 50 kOhms, XLR: 50 kOhms
Output impedance
@ 1kHz .018 Ohms
Output impedance
@ 20kHz .095 Ohms
Damping ratio
26db (20v/v)
>92% (Amp module)
Line voltage
100, 120 or 240 VAC 50/60 Hz Switch selectable
Power draw, no signal
40 Watts
52 lbs, Shipping: 64 lbs or 28 kg
431 mm wide x 133 mm high x 368 mm deep
(17"W x 5¼"H x 14"D)
US $7,195 MSRP
3 years
Silver or Black (Anodized)





Back to other reviews