Power Amplifiers

Spectron Musician III Mk2 Class D Stereo Power Amplifier

ARTICLE INDEX

Introduction

At one of the first CES conventions that I attended, I met a fellow who had a small booth against the rear wall. His name was John Ulrick. His product was something called a Class D power amplifier. I had no idea how it worked, because I had not heard of Class D. He explained some of the inner workings, but what most impressed me was how powerful it was, and yet, how relatively small its chassis was. He explained that, mainly, this was due to its amazing efficiency. Regular Class AB amplifiers are about 70% efficient, while Class D amplifiers are about 95% efficient. This means that if you are playing music that demands 200 watts per channel output using a stereo Class D power amplifier, the amplifier is only drawing 420 watts from the wall. At 70% efficiency, it would be drawing 570 watts, which might not mean very much to you in terms of expense. After all, your refrigerator draws more than that. But, a principle advantage of this amazing efficiency is that very little heat is produced. Nearly all the power is going to the speakers. Only 5% is wasted as heat. With Class AB, 30% of your AC power is going up the chimney, so to speak. With Class A operation, it is even worse, as it is only 30% efficient, with the rest being dissipated as heat. So, Class A amplifiers tend to be very hot during operation, and heat, of course, is an enemy of electrical circuits.

John Ulrick produced the first commercially available Class D power amplifier in 1974.

Specifications

  • Design: Class D (Switching) Stereo Power Amplifier
  • Power: 600 Watts RMS x 2 into 8 Ohms, 800 Watts RMS into 4 Ohms, 1,200 Watts RMS into 2 Ohms, 1,500 Watts RMS into 8 Ohms in Bridged Mode
  • MFR: 20 Hz - 20 kHz, ± 0.1 dB
  • THD+N: 0.03% at Full Output, 20 Hz - 20 kHz
  • Inputs: XLR and RCA
  • Input Impedance: 50 kOhms Balanced or Unbalanced
  • Outputs: Five-way Speaker Binding Posts
  • Power Draw at Idle: 40 Watts
  • Dimensions: 5.25" H x 17" W x 14" D
  • Weight: 52 Pounds
  • MSRP: $3,995 USA
  • Spectron

The Class D amplifier runs very cool by comparison, because of its high efficiency. But, its efficiency and small size are not the only things that make a Class D amplifier worth considering for your listening room. The technology has sound advantages too, and the Spectron in particular, is unique.


The Design

We have all heard about the use and misuse of negative feedback in amplifiers. This process feeds signal from the output back to the input, in the case of global negative feedback. The electrical phase of the feedback is inverted, and the difference between the input and the output is cancelled - not totally, but large enough to improve the quality of the output, because the difference between the input and output, besides the volume, is the presence of distortion. The circuit for negative feedback in a conventional amplifier usually consists of a resistor and capacitor. The resistor determines how much voltage (dBv) is fed back, and the capacitor determines the slope of the feedback, i.e., the relative amount of feedback in the low frequencies vs. the high frequencies. This is an analog process, and the time that it takes for the feedback to take place is about 2,000 nanoseconds, or 2 µs. Now, just think about this for a minute. A 20 kHz sine wave has a duration of 50 µs. So, because the negative feedback takes 2 µs to feed through to the output, the corrective effects of the feedback are 2 µs late, and extend 2 µs beyond the signal it is supposed to correct. The result is that we trade one type of distortion for another. While low order distortion is reduced, we end up with more higher order distortion. The bottom line with analog negative feedback is that it is a balancing act. What are you willing to give up in order to gain something else?

Shown below in the first diagram is a generic amplifier example illustrating conventional global analog negative feedback. I used a tube schematic because it is simpler to illustrate. By "Global" it is meant that the feedback goes from the output all the way back to the input stage. Local feedback refers to feedback between two stages or within a single stage. Notice how simple the feedback circuit is. Just a resistor and a capacitor in parallel (at the bottom of the circuit diagram). Electrons may travel at the speed of light, but only in a vacuum. When moving through an electronic circuit, they get delayed. Thus, the problem mentioned above.

The second diagram shows how Spectron utilizes negative feedback. It is accomplished by converting the audio to a PWM signal and using a digital amplifier (called the Forward Amplifier in the diagram). It performs its duties ten times faster (200 nanoseconds, or 0.2 µs) than conventional analog negative feedback circuits. Secondly, the Spectron includes a set of speaker cables that has a feedback circuit built in, so that the amplifier can sense the voltage actually being delivered to the speaker compared to what is being sent to the speaker, and correct for any error. Because the Spectron's digital handling of feedback is so fast, there is also less group delay. This means that each frequency in the music arrives at the speaker at the same time, so deep bass is in sync with violins, for example.

global-negative-feedback-schematic-with-red-lines

Because Class D is so efficient, it can deliver current into low impedances, so the Spectron can easily handle the low impedances found in planar speakers, such as electrostatics, where the impedance can dip to 0.1 ohm. There is also a large amount of headroom, again, due to its efficiency. The power supply is not wasting energy dissipating heat, so that energy can be called upon during extreme transients. There are 100 capacitors in the Spectron power supply, amounting to 33,000 µF of capacitance, and because there are so many capacitors, the output impedance of the power supply is extremely low.

Shown below is a photo of the rear panel.

Besides the XLR and RCA input jacks (with selector), as well as the gold-plated speaker binding posts, there are several toggle switches. These allow for phase inversion. This can be used if, for example, your preamplifier inverts the phase. The phase inversion switch on the Spectron would be used, in that case, for re-inverting the phase so that the output is properly phased with respect to the music. However, the phase inversion toggle has another function. By using the included Y connector, you input the signal from one channel of your preamplifier's output into both inputs on the Spectron, and toggle the phase inversion on one channel. You connect the speaker cable to the + binding post on the left and right outputs of the Spectron. What you end up with is a 1,500 watt (at 8 ohms), fully balanced monoblock power amplifier.

Above the speaker binding posts is a special connector for the voltage sensing speaker cables (these are an optional purchase).

The inside of the chassis shows that it is almost entirely occupied by the power supply. Look at all those capacitors. There are only two output transistors for each channel, because that is all that is required. Note that this amplifier uses a conventional power supply, and it is the output stage that is switched.

spectron-musician-lll-mk2-amplifier-inside-chassis

The way Class D amplification works is that the input signal is modulated onto a carrier signal, and in the case of the Spectron, the carrier frequency is 500 kHz. The modulation is in the form of 1's and 0's, and the duration of the 1 or 0 determines the voltage of the music signal being modulated. This is called Pulse Width Modulation. This differs from the CD itself, which also has 1's and 0's, but the duration is the same for all of them. In fact, if there is any error in passing the 1's and 0's along the bitstream from a CD, this results in something we have all heard of, called "jitter".

The signal on the carrier in the Class D circuit is demodulated, resulting in the higher voltage (higher than the preamplifier) output that goes to the speaker. But, you still have that 500 kHz carrier in the amplifier circuit to deal with. You don't want it going out to the speakers too, so a low-pass filter is applied to remove it.


In Use

If you are the kind of audiophile who wants to kick butt with your hi-fi system, the amplifier is for you. I tested it with an OPPO BDP-83/NuForce universal Blu-ray player, Mysterè ca21 preamplifier, and Carver Mark IV ribbon speakers.

If anything tests an amplifier or speakers, it is pipe organ music, so I plopped this Bach SACD into the player and sat back . . . but not for long. I wasn't expecting the studs in the wall to start popping and creaking so soon. Turning the volume down a bit, I could enjoy the throbbing bass at a level that didn't give me blurry vision from my eyeballs vibrating. This amp has muscle . . . big time. I turned the volume up again, until it sounded like I was standing next to the pipes in Finland, where the organ is located. I didn't hear any evidence of clipping. Raw power does have its advantages. Now keep in mind this was just with one of the stereo amps. I moved the second review sample into place and wired both of them as monoblocks as described above. This proved too much for my circuit breakers, but I really did want to hear what a 1,500 watt power amp could do, so I put a few mono classic jazz LPs on my VPI turntable and ran the system in mono, with one monoblock powering one speaker. This gave me so much volume, I was able to play my drums along with the tracks and the playback was as loud as my actual drums in the room. If you plan to get two of these things and configure them as monoblocks, you will need at least a 30 amp circuit just for the amplifiers. Lastly, I was also not able to test the speaker connection feedback option, as the supplied cable length placed the power amplifiers too far from my preamp.

spectron-musician-lll-mk2-amplifier-music-bach

The Firebird is no butterfly when it comes to demanding performance from an amplifier or speakers. Clarity and detail abounded, with no hint of limitations. I had to go back to using the amplifier in stereo mode, rather than using two of them in monoblock mode, for this and the rest of my test recordings, as my AC just could not supply the power to deliver 1,500 watts per channel. Looks like I will be putting in another circuit. I hope the electrician can pull a 50 amp three-wire cable through the duct. If you a pair of these amplifiers to run as monoblocks, count on having a dedicated 30 amp circuit.

spectron-musician-lll-mk2-amplifier-music-firebird

This is a favorite disc of one of our other writers, and I can see (hear) why. It really is exquisite music, and the Spectron can play nice and soft just as well as it can blow out the windows.

spectron-musician-lll-mk2-amplifier-music-russian-nights

Although this is a 5.1 DVD-A, it also has the 2-track 24/96 version on the disc. Unfortunately, by this time, I had moved my drums against the rear wall, so all I could do is listen. The Spectron is just made for high rez recordings. Everything that is supposed to be there, is just there.

spectron-musician-lll-mk2-amplifier-music-fleetwood-mac

In summary, the sound of the Spectron was something in between a tube amplifier and a solid state amplifier. It is difficult to describe it any other way. It had the high frequency snap that a solid state amp has, but the fluidity of tubes. Very unique and quite enjoyable. But of course, this amp comes from the man who designed the first commercial hi-fi switching amplifier, a long time ago.


Conclusions

Out of all the Class D amplifiers I have ever listened to, the Spectron is the best. I suppose it should be, since its designer was the first engineer ever to build one for audio use. If you like power with finesses, be sure to listen to one . . . or two. I think you will like what you hear.