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Ultra-LD Mk.3 200W Amplifier Module

Upgraded design has even lower distortion!

By Nicholas Vinen

The new and updated Ultra-LD Mk.3 is by far the best class-AB amplifier module design published anywhere. It has an astonishingly low total harmonic distortion plus noise (THD+N) figure of 0.004% at 20kHz for 100W into 8Ω (20Hz-80kHz measurement bandwidth) and less than 0.0006% THD+N at 1kHz and below.

The signal-to-noise ratio has also been slightly improved on the previous version (by 1dB) to -123dB with respect to 135W into an 8Ω load. The power output figures are unchanged with regards to the Mk.2 module.

All power measurements were made with a mains voltage of 230VAC, which is now common in Australia (although by no means universal). In locations with a higher mains voltage, slightly more output power is available. For example, if your mains voltage is normally 240VAC, you can expect about 8% more power output, eg, 145W into 8Ω.

The quiescent current accuracy, stability and thermal compensation have been dramatically improved compared to the Mk.2 and in fact are superior to any class-AB amplifier that we have tested. The new module has a trimpot so that the quiescent current can be set to the optimum (the Mk.2 was a bit hit and miss in this regard).

The new thermal compensation arrangement keeps the quiescent current well under control, even during and after sudden changes in dissipation. This contributes to the low distortion as it means that the output stage is always correctly biased.


High DC voltages (ie, ±55V) are present on this amplifier module when power is applied. In particular, note that there is 110V DC between the two supply rails. Do not touch the supply wiring (including the fuseholders) when the amplifier is operating, otherwise you could get a lethal shock.

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