Pre/Power Amplifiers

As I noted in our Classy Class D sidebar the full bandwidth of the I15 is only realised via its digital inputs as the analogue line input is sampled at 48kHz and the response thus limited to ~20kHz. Otherwise the amplifier’s response scales with incoming sample rate, reaching –0.6dB/20kHz, –1.8dB/45kHz and –4.88dB/90kHz with 44.1/48kHz, 96kHz and 192kHz files, respectively. Volume is another matter however, for digital inputs are limited to a maximum safe ‘position’ of 55 out of 99. At volume ‘54’ a 0dBFs digital input delivers a maximum 64W at 0.017% THD and at ‘55’ the same peak level 0dBFs delivers 71W/8ohm, when the amplifier is in clipping. The analogue input yields finer control, revealing the I15’s true capacity of 72W/8ohm, 140W/4ohm and 72W, 140W, 255W and 161W under dynamic conditions into 8, 4, 2 and 1ohm loads [see Graph 1]. The fact that both continuous and dynamic outputs are identical, and almost precisely double from 8-to-4ohm, is a textbook result for a Class D amplifier with a very capable (switch-mode) power supply.

Distortion scales gently with output and with frequency via all inputs, from 0.00045%/20Hz to 0.003%/20kHz at 1W and 0.0045%/20Hz to 0.013%/20kHz at 10W/8ohm. The lowest distortion is achieved at –15dB to –25dB below full output [see Graph 2]. The A-wtd S/N ratio is as expected for the Hypex UCD102 module at 87.2dB (re. 0dBW) and slightly better at 89.3dB with digital inputs (re. 0dBFs/0dBW). The maximum A-wtd S/N ratio (re. 70W) is 106.7dB. Jitter is impressively low courtesy of the AKM AK4490 DACs at 175psec/48kHz and 160psec/96kHz (the Hypex modules are analogue in design and therefore cannot contribute to ‘jitter’, only intermodulation). PM

Continuous power (<1% THD, 8/4ohm): 72W / 140W
Dynamic power (<1% THD, 8/4/2/1ohm): 72W / 140W / 255W / 161W
Output impedance (20Hz–20kHz): 0.058–0.066ohm
Freq. resp. (20Hz–20kHz/90kHz): +0.0 to –0.12dB/–4.88dB
Distortion (20Hz-20kHz, 0dBFs/0dBW): 0.0012–0.0015%/0.0044-0.002%
A-wtd S/N ratio (re. 0dBFs/0dBW): 89.3dB (Dig) / 87.2dB (Analogue)
Digital Jitter (48kHz/96kHz @ 10W): 175psec / 160psec
Power consumption (idle/rated o/p): 19W / 145W (2W, standby)
Dimensions (WHD / Weight): 350x73x329mm / 6.4kg
Price: £1600