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Ultra-LD Mk.2 200W Power Amplifier Module

This new amplifier module supersedes both the Plastic Power module described in the April 1996 issue and the Ultra-LD module presented in the March 2000 issue. It produces high power at very low distortion. In fact, as far as we are aware, it is the lowest distortion class-AB amplifier that has ever been published.

By Leo Simpson & John Clarke

Specifications & Performance

    Output Power: 200 watts RMS into 4W; 135 watts RMS into 8W
    Frequency Response at 1W: -3dB at 4Hz, -1dB at 50kHz (see Fig.4)
    Input Sensitivity: 1.26V RMS for 135W into 8W
    Input Impedance: ~12kW
    Rated Harmonic Distortion: < .008% from 20Hz to 20kHz for 8W operation; typically < .001% (see Figs.5-8)
    Signal-to-Noise Ratio: 122dB unweighted with respect to 135W into 8W (22Hz to 22kHz)
    Damping Factor: <170 with respect to 8W at 100Hz; <50 at 10kHz
    Stability: Unconditional

Design Features

  • Very Low Distortion

  • No adjustment for quiescent
    current required

  • Double-sided PC board
    simplifies wiring

  • PC board topology cancels class-B induced magnetic

Click for larger image
Fig.1: the circuit employs the new ThermalTrak power transistors from On Semiconductor. These have an integral diode which is used to control the quiescent in the Class- output stage. The four diodes are shown separately on this circuit (ie, DQ12, DQ13, DQ14 & DQ15) for clarity but are actually integral with the output transistors which have five connecting leads instead of three. Note that the various voltages marked on the circuit will vary according to the supply rails.

The Ultra-LD Mk.2 Amplifier Module uses the new On Semiconductor ThermalTrak power transistors in a circuit which is largely based on our high-performance Class-A amplifier which was featured in SILICON CHIP during 2007. The ThermalTrak transistors are a new version of the premium MJL3281A & MJL1302A and have an integral diode for bias compensation. As a result, the circuit has no need for a quiescent current adjustment or a "Vbe multiplier" transistor.

This is also our first amplifier module to use a double-sided PC board. Ostensibly, there is no reason to use a double-sided board for a relatively simple circuit such as this, especially as our previous single-sided amplifier boards have had few links. In fact, we have used the double-sided design to refine and simplify the external wiring to the PC board which has been arranged to largely cancel the magnetic fields produced by the asymmetric currents drawn by each half of the class-B output stage. We provide more detail on this aspect later in this article.

Power output of the new module is on a par with the above-mentioned Plastic Power module and significantly more than the original Ultra-LD module. As well, it uses a considerably simpler power supply than the Ultra-LD module.

Power output is 135 watts RMS into an 8-ohm load and 200 watts into a 4-ohm load for a typical harmonic distortion of less than .001%. The new module also has slightly higher gain than the Ultra-LD module but still manages to produce an improved signal-to-noise ratio of -122dB (unweighted) with respect to 135W into 8W. This is extremely quiet.

A look at the accompanying performance panel and the performance graphs will show that this is a truly exceptional amplifier, bettered only by the Class-A amplifier described during 2007. In fact, some of the distortion figures we have obtained are so low, around .0007% for operation into 8-ohm loads, that we were amazed. We had expected this Class-AB amplifier to be better than anything we had published before – but not this good!

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