Power Amplifiers

Spectron Musician III Mk2 Class D Stereo Power Amplifier



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.


  • 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.