Pre/Power Amplifiers

BAT's flagship stereo amplifier is offered with a very sparse specification, nevertheless its core 500W/8ohm and 1kW/4ohm rating is well met in practice. Achieving 495W at 0.5% THD and 565W at 1% THD into 8ohm – one or two channels driven as the REX 500 is a dual-mono amp right back to the two transformers and AC inlets – the amp has sufficient stiffness in its supply to support 1.04kW/4ohm at 1% THD. Under dynamic conditions the REX 500 lifts its game to the tune of 595W, 1.165kW and 1.132kW into 8, 4 and 2ohm loads, respectively, at <1% THD before 'relaxing' back to 585W/1ohm at 2% THD (or 24.2A). At no point during these tests was BAT's electronic over-current protection triggered [see Graph 1, below].

BAT is not alone in claiming 'no global feedback' and while this does not preclude local/nested feedback to optimise the gain, bandwidth and linearity of individual stages, the REX 500 does exhibit some common 'low feedback' features such as moderate levels of distortion and a high-ish output impedance. THD increases with level from 0.04%/1W to 0.13%/10W, 0.23%/100W and 0.52% at the rated 500W/8ohm (all re. 1kHz) while also increasing with frequency to, for example, 0.1%/1W, 0.33%/10W and 1.65%/100W [all re. 20kHz, see Graph 2]. The output impedance increases from 0.32ohm to 0.52ohm across the 20Hz-20kHz audioband and to 2.65ohm/100kHz, the latter influencing its frequency response according to the HF impedance trend of the attached loudspeaker. Into a non-reactive 8ohm load, response is flat to within ±0.2dB (20Hz-20kHz) but dips to –0.8dB/20kHz into 2ohm. The 84.5dB A-wtd S/N ratio (re. 0dBW) is perfectly 'average' for the breed. PM