Boulder 866 Integrated Amplifier Lab Report

Lab Report

The balanced power amp architecture of the 866 is distinct from that of the 1160 [HFN Sep '21] although the former is no blushing violet when it comes to wrestling with the toughest speaker loads. Rated at 2x200W/8ohm, the 866 achieves rather closer to 2x220W/8ohm and 2x390W/4ohm with sufficient headroom to support 299W, 580W and a full 1072W into 8, 4 and 2ohm loads, respectively, under dynamic conditions. The 866 is protected above 981W/1ohm, or 31.3A [see Graph 1, below]. With the input trim set to '0' and the volume range '0-100', the 866's maximum gain is a sensible +30.9dB and the A-wtd S/N ratio an impressive 93.5dB (re. 0dBW). Distortion is fabulously low through bass and midrange frequencies (as low as 0.0001% at 10W/8ohm) but increases at HF to reach ~0.018-0.03%/20kHz over the initial 1-100W of its range. The 866's output impedance is also incredibly low at ~0.003ohm and its frequency response – flat to 1Hz – reaches out to an unvarying –0.18dB/20kHz and –3.1dB/100kHz regardless of speaker load.

As the 866 has no line output the digital module can only be tested via the power amp stage. With the volume set to '62' the 866 offers a 2.08V output from a 0dBFs digital input, its 94.7dB A-wtd S/N ratio necessarily impacted by the 'additional' power stage. Nevertheless, and despite the proximity of the huge PSU, noise is an inaudible hiss rather than a hum while rectifier switching contributes a mere 80psec of correlated jitter. Again, impressive. THD, meanwhile, falls from 0.006-0.05% at 0dBFs to 0.003-0.004% over the top 30dB of its digital range [re. 20Hz-20kHz, see Graph 2] while the linear phase digital filter offers a 99dB stopband rejection and –0.3dB/20kHz response. PM


Dynamic power output versus distortion into 8ohm (black trace), 4ohm (red), 2ohm (blue) and 1ohm (green) speaker loads. Max. current is 31.3A


Distortion versus 24-bit digital signal level over a 120dB range (1kHz, black; 20kHz, blue)

Power output (<1% THD, 8/4ohm) 220W / 390W
Dynamic power (<1% THD, 8/4/2/1ohm) 299W / 580W / 1.07kW / 981W
Output imp. (20Hz–20kHz/100kHz) 0.002-0.003ohm / 0.025ohm
Freq. resp. (20Hz–20kHz/100kHz) +0.01 to –0.2dB/–3.1dB
Digital jitter (48kHz / 96kHz) 80psec / 45psec
A-wtd S/N ratio (DAC/Amp) 94.7dB (0dBFs) / 93.5dB (0dBW)
Distortion (DAC, 0dBFs/Amp, 10W) 0.0055-0.051% / 0.0001-0.018%
Power consumption (Idle/Rated o/p) 87W / 650W (5W standby)
Dimensions (WHD) / Weight 440x190x390mm / 24.5kg
Price £13,499 (digital module, +£2500)

Boulder Amplifiers Inc
Louisville, CO, USA
Supplied by: Padood, Cambridge, UK
01223 653199