Pre/Power Amplifiers

How much gain (amplification factor) is sufficient? In more than a few of our amplifier lab reports I have puzzled over the high >40dB gain on offer – a throwback to the pre-CD era when line-level sources offered rather less than today's 2V output. Bricasti recognises this and so the M20 preamp offers just 2.8dB (x1.4) gain while the fully balanced/bridged M25 is engineered for +26.9dB (x22.1), the combined +29.7dB gain requiring a perfectly reasonable 93mV input to achieve a 0dBW (1W/8ohm) output (or 1.13V for the rated 150W/8ohm output). The main 'feature' of the M20 is its R2R ladder volume control operating in 1dB steps over a 90dB range. This is combined with a 'Filter' facility that, in reality, is not a filter at all but a channel balance trim that operates over finer ±0.5dB steps. But the M20 is a very linear preamp, offering a >12V output from a 28ohm (balanced) source impedance with a wide 100dB A-wtd S/N ratio. THD is vanishingly low at <0.0003%, 20Hz-20kHz [black trace, Graph 2] and the response flat to within ±0.1dB limits from 2Hz-100kHz.

The partnering M25 power amp makes a mockery of its 150W/8ohm and 300W/4ohm spec. by delivering a substantial 2x304W/8ohm and 2x585W/4ohm on the bench, although the dynamic outputs of 320W and 615W into 8 and 4ohm, followed by a reduction to 458W and 279W into 2 and 1ohm loads, indicates some conservative current-limiting at work [Graph 1]. THD climbs very gently with level from ~0.0003% at 1W to 0.0007%/10W and 0.01% at the rated 150W/8ohm (all 1kHz) and more generally with frequency from 0.0002%/100Hz to 0.0007%/1kHz and 0.03%/20kHz [red trace, Graph 2]. Noise is very low and the A-wtd S/N ratio wide at 99dB (re. 0dBW). PM