Bricasti M20/M25 Pre/Power Amplifier Lab Report

Lab Report

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


Dynamic power output versus distortion into 8ohm (black trace), 4ohm (red), 2ohm (blue) and 1ohm (green) speaker loads. Max. current is 16.7A


THD vs. frequency for M20 preamp (0dBV output, black) vs. M25 power amp (10W/8ohm, red)

Continuous power (<1% THD, 8/4ohm) 304W / 585W
Dynamic power (<1% THD, 8/4/2/1ohm) 320W / 615W / 458W / 279W
Output imp. (20Hz–20kHz, pre/power) 28ohm / 0.002–0.009ohm
Freq. resp. (20Hz–100kHz, pre/power) +0.0 to –0.13dB / +0.0 to –1.2dB
Input sensitivity (for 0dBV/0dBW) 725mV (pre) / 129mV (power)
A-wtd S/N ratio (re. 0dBV/0dBW) 99.5dB (pre) / 98.9dB (power)
Distortion (20Hz-20kHz, 1V/10W) 0.0001–0.0003%/0.0002–0.016%
Power consumption (idle/rated o/p) 135W/490W (27W, preamp)
Dimensions (WHD, Pre/Power) 425x115x320/425x260x457mm
Prices £13,500/£19,500 (pre/power)

Bricasti Design Ltd
Supplied by: SCV Distribution, Hertfordshire
03301 222500