Krell KMA-i800 Mono Amplifier

Differential Output

The KMA-i800 is a differential output amplifier. This means it contains two identical amplifiers operating out of phase to produce an output signal twice as large as a single amplifier. The main advantage of this configuration is cancelation of common-mode distortion and noise components. The KMA-i800 also incorporates special compensation to maintain very low output impedance at the low end of the frequency range. This produces high impact extended bass that is felt as much as it is heard.

iBias

iBias enables the KMA-i800 to operate in Class A up to its full rated power of 800 watts with absolute accuracy but without the heat produced by a traditional Class A design. It does this by using a novel circuit to unobtrusively monitor the current flowing through the output stage. This is part of a closed-loop controller that makes sure this current never drops below a preset level (Class A means that it never drops to zero). This is what makes the amp both efficient and accurate.

The efficiency comes from the controller adjusting the bias to maintain that low preset current level as the signal and speaker load changes. Traditional Class A amplifiers set it for the maximum level all the time so they’re always dissipating the maximum amount of heat. iBias is like having a variable size Class A amplifier. If, for example, the signal and speaker are only requiring 10 watts, it’s like having a 10 watt Class A amplifier, all the way up to 800 watts in the case of the KMA-i800.

The accuracy also comes from maintaining that low preset current level. It doesn’t matter what the signal is doing or how the speaker impedance changes with frequency, by maintaining that preset current level the amplifier is always operating in Class A. Other variable bias schemes monitor the input or output signal to adjust the bias but that doesn’t take into account varying speaker impedance. This means they will sometimes apply more bias current than necessary or drop out of Class A operation.

XD

XD is a technique for lowering the amplifier’s output impedance. This results in better control over the speaker drivers which improves musical dynamics, depth, and dimensionality. It is very dependent on maintaining thermal equilibrium in the output stage so that all the output transistors are contributing equally to the signal output. The KMA-i800 uses a massive aluminum device bar to mount the output transistors and also employs special temperature compensation across the device bar to limit temperature variations.

Sym-Max

Sym-Max is short for “Symmetry Maximization.” The circuitry in the KMA-i800 is differential, meaning that there is a positive half and a negative half. The transistors in this circuitry are complimentary, meaning opposite polarity but with equal characteristics. In practice they are not so equal which means the circuit is operating asymmetrically. Sym- Max makes small adjustments in the circuit parameters to remove this asymmetry. The result is that 2nd order harmonic distortion is almost completely eliminated. This dramatically opens up the sound stage with more detail and a better sense of pacing, making for a more realistic experience.

Power Consumption

The heart of any power amplifier is the power supply. The KMA-i800 has a massive power supply consisting of 5800VA of transformers and 188,000uF of filter capacitance. Delivering this power to the output circuit is a combination of gold-plated circuit boards and silver-plated solid copper buss bars with a very short physical path for extremely low impedance. This, along with the audio circuitry enables the KMA-i800 to double its power from 800 watts into 8 ohms to 1600 watts into 4 ohms and even 3200 watts into 2 ohms. It can drive 1 ohm speakers and will deliver a peak current of up to 56 amps.

Circuitry

The KMA-i800 uses Krell Current-Mode circuitry with advanced current mirroring that is balanced, differential, and direct-coupled. Each stage is independently tuned and matched to the previous stage for optimal operation. The output stage consists of 16 pairs of 200 watt audio power transistors and 8 pairs of audio driver transistors. This results in a frequency response greater than 100kHz for effortless reproduction of the audio band.

Control and Protection

The KMA-i800 is microprocessor controlled and has Ethernet connectivity and a 12VDC trigger input for easy connection to a home controller. The Ethernet connection allows access to the KMA-i800 web page for status information and control. The KMA-i800 is protected against high DC on the outputs, output short circuit conditions, and power brown outs. The protection operation is conveniently indicated on the front panel status lights and the web page.