B&O Beomaster 2400-2 Lab Report

Lab Report

The 'baby brother' of B&O's Beomaster 4400 [HFN Jun '12] musters a very distinctive technical performance, beyond the difference in rated power specification of 30W/8ohm vs. 50W/8ohm (2400-2 vs. 4400). In practice our venerable test sample sustained a maximum continuous output of 2x32W/

8ohm and 2x56W/4ohm with sufficient reserve to accommodate transient peaks (1kHz/10msec/<1% THD) of 42W, 72W, 36W and 17W into 8ohm, 4ohm, 2ohm and 1ohm loads, respectively [see Graph 1, below]. The 4.3A current headroom is modest, suggesting the 2400-2 is best used with speakers rated at 6ohm and above, and of generous sensitivity. However, Graph 1 also reveals another unusual 'feature' of this amplifier not seen in our review of the 4400 – the 2400-2's output stage distortion actually falls with decreasing load impedance from 0.07%/8ohm to 0.045%/4ohm to 0.035%/2ohm (re. 1kHz/10W).

Otherwise, the 2400-2's performance is more predictable, with distortion increasing linearly with output from 0.02%/1W to 0.07%/10W and 0.15% at its rated 30W/8ohm, and with frequency to 0.1%/10kHz and 0.19%/20kHz [all at 10W/8ohm – see red trace, Graph 2 below]. The amplifier's output impedance is gratifyingly flat at 0.11-0.12ohm across the 20Hz-20kHz audio range and the response sensibly tailored between –3dB points at 10Hz-80kHz. Noise is low too at –79dBV – testament not only to the design of B&O's linear power supply but also the elegant circuit layout – and ensures an overall A-wtd S/N ratio of 84.7dB (re. 0dBW). This is a match for many modern amps even if age has caused some drift in the opto-coupled volume, contributing to a 1dB channel imbalance at the reference 0dBW setting. PM


Dynamic power output vs. distortion up to 1% THD into 8ohm (black trace), 4ohm (red), 2ohm (cyan) and 1ohm (green) speaker loads. Max current is 4.3A


Distortion versus frequency from 5Hz-40kHz (1W/8ohm, black) and 20Hz-20kHz (10W/8ohm, red)

Power output (<1% THD, 8/4ohm) 32W / 56W
Dynamic power (<1% THD, 8/4/2/1ohm) 42W / 72W / 36W / 17W
Output imp. (20Hz–20kHz/100kHz) 0.105–0.120ohm / 0.375ohm
Freq. resp. (20Hz–20kHz/100kHz, 0dBW) –0.7dB to –0.28dB/–4.2dB
Input sensitivity (for 0dBW/30W) 42mV / 245mV
A-wtd S/N ratio (re. 0dBW/30W) 84.7dB / 99.3dB
Distortion (20Hz-20kHz, 10W/8ohm) 0.065–0.185%
Power consumption (Idle/rated output) 21W / 121W
Dimensions (WHD) / Weight 620x60x250mm / 7.6kg