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:

Quantizer:
Nyquist's rule tells us that we must sample a signal at twice the highest signal frequency. Since our hearing goes to approximately 20 kHz, the minimum for music reproduction is 40,000 samples per second. Conventional CDs are sampled at 44 kHz, and the most advanced recording systems record at up to 192 kHz. Spectron's Class D amplifiers sample at 500 kHz samples per second, about 11 times oversampled. This highly oversampled signal minimizes aliased distortion.

Feedback:
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 speaker draws. 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's 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?s 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.

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, however, retains accurate group delay and preserves each instrument?s position in space.

Distortion:
The published measurements of many class D amplifiers reveal that while their THD is primarily second-harmonic in nature, there are also some higher-order harmonics present. This pattern of distortion is very similar to the pattern of distortion in poorly designed tube amplifiers. Thus, sonically, this euphonic coloration is confused by some with the warmth of tube amplifiers. In reality this deviation acts as both an annoyance and murky veil. In exchange for euphonic, overly rich harmonic texture (of absolutely artificial origin) the listener gets no transparency and a lower level of true detail. Additionally, the level of this distortion (as well as output impedance) increases with frequency, making the amplifier sound bright. Consequently, with increases in output impedance and the level of distortion, some class D amplifiers cannot control difficult impedance speakers at all. The distortion at high frequencies of the Signature Edition Musician III is exceptionally low, resulting in an amplifier that is not only not bright or muted, but also has a three-dimensional, transparent, and natural musical sound.

Stability:
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 complex impedance speakers. No matter what type of speaker, the Spectron will yield a nearly identical sonic signature.

Bandwidth:
Wider bandwidth translates into faster transients and greater detail, better preserving phase information. This results in a bigger soundstage, sharper image and more holographic representation. The potential drawback in wide bandwidth amplification is an increased level of distortion if the output filter is less than perfect. Spection avoids this by a sophisticated design of the output filter and feedback loop, which are mathematically optimized for maximum phase margin. Spectron's bandwidth is an exceptional 100 kHz, resulting in optimum square wave response.

Head room:
Spectron's Musicain 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 is common in high quality recordings to see voltages near 100V peak, with medium efficiency speakers. However, most amplifiers use power supply voltages on the order of plus and minus 65V. The Musician III's peak is 120 volts! In regard of the currecnt delivery, there are many well recognized regarded speakers whose impedance dips down, some even lower than 1 ohm. When a musical note is played at these frequencies where the impedance dips, the current demands skyrocket. When this happens with amplifiers that do not have large output current capability, they "current clip". These transients will be both attenuated and quite distorted. Spectron amplifiers deliver peak currents of 55 amps for about 330msec, which allows the amplifier to deliver the full transient (burst of music) without "current clipping" and/or "voltage clipping" as was mentioned above. On the other hand, most other amplifiers only deliver their rated peak current for sometimes a fraction of this time. This means that the Musician III Signature can drive everything from conventional speakers to complex loads such as ribbon speakers or electrostatics and represent effortlessly the fury of a symphonic crescendo, unlike any other amplifier!

Energy Storage Capacitors:
In the original Musician III amplifiers and Premiere amplifiers there were two 20,000uf energy storage capacitors, one for the +125V rail and a second one for the -125V rail. In the Musician III Signature Edition, the two large electrolytics have been replaced by one hundred 330uf capacitors. This lowers the power supply impedance by a factor of more than 10. The capacitors are arranged such that the left and right channels are now completely independent of each other. The result, along with other Signature Edition improvements is much improved midrange and high frequency fidelity and improved soundstage.

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 Signature Edition has two of these isolated supplies, one for the right channel and a second for the left channel. Having isolated supplies minimizes interaction between the two channels, as well as cross talk. Few other stereo amplifiers have isolated low level power supplies. These two +15V power supplies have input noise rejection filters and 2% regulators.

Hum and Signal:
Hum is usually measured with no signal, and it's related to the AC line. In all amplifiers, the hum increases with loudness. You can't hear this hum over the music, but when reduced, the music sounds cleaner. Spectron's high efficiency amplifier and sophisticated feedback minimizes this hum, resulting in cleaner sound quality.

Phase invert:
Phase reversals in original recordings sometimes leave the signal to the amplifier out of phase. Spectron's phase invert switch on the rear panel allows the user to correct this error. There are two phase invert switches on the rear panel that control 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.

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 automatically shorts out this resistor, eliminating the voltage drop across this resistor during loud bass passages. The result - more and tighter bass.

PROTECTION CIRCUITRY

Current limits: The Musician III has a 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 protection circuitry that 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 (which could damage your speakers), the protection circuit will latch the amplifier off. This protects the speaker from damage. 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. Spectron knows of no other amplifier with this protection.

SPECTRON REMOTE SENSE CABLES

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 amplifier. 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.

SUMMARY

In the above, Spectron has mentioned some of its many important design considerations and features. For an audiophile amplifier to achieve the highest quality music reproduction, each and every one of these components plays a very important role, and we believe must all be there.