Digital Power Amplifier Concepts

                                   courtesy of spectron digital amplifiers

With each generation, Spectron amplifiers receive rave reviews in professional hi-fi journals and more importantly from new Spectron owners, who frequently report, "With my new Spectron, I am hearing things in the music I never heard before. Also, It's the most musical amp I have ever heard. Why is that? Here's why:

A power amplifier is a voltage amplifier that has a fixed voltage gain of (usually) 20. The objective then is that the output voltage is 20 times the input voltage. This voltage must remain 20 times the input, no matter how much current the speakers draw. By definition, this is a voltage control system. Control systems are implemented with feedback loops. This feedback loop controls the output voltage for all load conditions. The speaker is the load. Feedback means that a fraction of the output is fed back and compared to the input. This difference is a correction signal. Spectron amplifiers are implemented with digital logic and power switches, the opposite of the circuitry used in a linear amp. This digital logic corrects for errors in the output about ten times faster than the linear circuits (transistors or tubes) in conventional linear amplifiers. Spectron amplifiers include the modulator, power section and output filter all in the feedback loop. Some class-D amplifiers use modules supplied by an outside vendor where the output section and filter are not included in the feedback loop. Thus there is no correction for errors caused by the speaker load. This results in the amplifier sounding different with various speakers. In other words, it is load dependent. Spectron amplifiers are not load dependent.

Group Delay:
It is important that all of the frequency components of the original recording retain their time alignment. These are the position cues to the listener. Group delay is a measurement of time alignment. If a drum strike is being reproduced and the low frequency has one position in time and the high frequency has another, the spatial position of the drum will be confusing to the listener. Spectron?s fast feedback loop with accurate group delay retains the stereophonic imaging, the ability to discern the relative position of instruments.

Head room:
The Spectron Musician III has a huge amount of headroom. This means that when the music hits loud passages, the Spectron will deliver undistorted power to the speaker. It will not run out of either voltage or current. The Musician III?s peak is 100 volts and 50 amps. This means that the Musician III can drive everything from conventional speakers to complex loads such as ribbon speakers or electrostatics. All of them with distortion-free signals.

Isolated low level power supply:
Some purist designers have used batteries to power the low level circuits. It's a good way to go, but the batteries must be replaced every few years. Spectron accomplishes an equivalent result by powering the low level circuits using a high frequency isolation transformer. This gives about 500 times more isolation than deriving the low level power from the conventional line transformer. This virtually eliminates the effect of noise and transients from the AC power line on the low level circuitry. In addition, the Musician III has two of these independent supplies, one for the right channel and a second for the left channel. Having independent supplies minimizes interaction between the two channels as well as cross talk. These two +15V power supplies have input noise rejection filters and 2% regulators.

Phase invert:
Improper phase in original recordings sometimes leave the signal to the amplifier out of phase. The phase invert allows for correction of this error. There are two switches on the rear panel which control inverting the phase of the left and right channels individually. For example, if a kick drum is recorded with a mic in front of it, the speaker should move out in response to pressure pulse from the drum. If out of phase, the speaker will suck in - some of the transient power will be lost. Inverting the phase will restore the transient, making the drum sound real.

The Musician III is stable to loads down to .1 Ohms. It will actually deliver 300 Watts into .1 Ohms. This means it will easily drive ribbon, electrostatic or any very low impedance speakers.

Inrush bypass:
Virtually all amplifiers have a special resistor in series with the AC line to limit the inrush current at turn on caused by charging the energy storage capacitors. Spectron does too, but after the capacitors are charged, a relay shorts out this resistor, eliminating the voltage drop across this resistor during loud bass passages. The result, more and tighter bass.


Current limits: The Musician III has a new full fold back current limit. This means that when the output is shorted or draws more than 50 amps, the current limiter reduces the maximum current to 12 amps. When the short is removed, the amplifier resumes normal operation automatically. Spectron provides modules to professional amplifier manufacturers where the amplifiers must be especially bullet proof. The Musician III meets this requirement.

Speaker protection: To protect your speakers, The Musician III has Spectron?s new protection circuit which detects excessive DC or high frequency signals, not associated with normal music. If excessive DC or a high frequency signal is detected at the output of either channel, the protection circuit will latch the amplifier. This protects the speaker the fault originated. The power is simply latched off and a red LED is illuminated on the rear panel. The reset button is also on the rear panel. If the reset button is pushed and the fault still exists, it will just reset again until the fault is removed.


Optional Remote Sense Cables: The Musician III has rear panel connectors and the required circuitry to accept Spectron?s optional Remote Sense Speaker Cables. The Remote Sense Cables put the speaker cable in the feedback loop of the amplifier, thus using the amplifier to correct for distortion of the cables. This means that the amplifier controls the voltage at the speaker, not at the output of the amplifiers. It's the signal at the speaker that is important, not at the amplifier?s output. So the Remote Sense Cables dramatically reduce the distortion inherent in the cables.

The output at any amplifier is distortion free, but due to the distortion from the cable there will be some distortion at the speaker end. With measuring equipment it can be demonstrated that the distortion at the speaker end is significantly less with the Remote Sense Cables.




Digital pulse width modulation switching amplifier.

Feedback loop 10 times faster than typical conventional amplifiers.

High head room, 100V peak and 50A peak.

No crossover distortion.

Drives the most difficult speakers, stable to .1 Ohms.

Foldback current limit. Automatic overload recovery.

Professional quality balanced line input.

Floating RCA input eliminates buzz caused by ground loops.

Phase invert. Rear panel selectable.

RCA and XLR input. Rear panel selectable.

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.



Technical Specifications






Watts per channel, both channels driven
550 Watts at 8 Ohms
800 Watts at 4 Ohms
1400 Watts at 2 Ohms

Peak current

50 Amps


.06% from 1 Watt to 600 Watts at 8 Ohms
(.03% in the listening range)


<180 V (22 Hz to 22 kHz)
-83 dB below one Watt (A measure of hiss)


110 dB

Frequency response

.1 dB 20 Hz to 20 kHz at 8 Ohms
-3 dB at .3 Hz and 50 kHz


60 kHz


26 dB (20 V/V)

Input impedance

RCA: 50 k?, XLR: 50 k?

Output impedance

.020 Ohms

Damping ratio



>92% (Amp module)

Line voltage

100, 120 or 240 VAC 50/60 Hz Switch selectable

Power draw, no signal

40 Watts


22 kg (50 lbs), Shipping: 25 kg (55 lbs)


431 mm wide x 133 mm high x 368 mm deep
(17?W x 5?H x 14?D)


US$4,995 MSRP


3 years