Pre/Power Amplifiers

Chord's Ultima preamp offers six levels of overall gain: '0.5', '1.0'... '2.5 and '3.0' amounting to 0dB, +5.6dB, +10.2dB, +12.3dB, +14.2dB and +15.6dB, balanced in/out, respectively. Maximum output level is 10.6V regardless of gain, from a 96ohm source impedance, while the S/N ratio is slightly below average at 83dB (re. 0dBV), a feature it shares with the Ultima power amp(s) with their 79dB A-wtd S/N (re. 0dBW). Incidentally, both amplifiers have a 'smooth' noise floor – decades of lab test data indicating a correlation between amplifiers that show a finite level of 'white' noise and those preferred on listening tests... The preamp's response, meanwhile, is flat to within ±0.1dB from 1Hz-100kHz with the EQ facility disabled, and ±5dB/10kHz and ±5dB/20Hz (all re. 0dB/1kHz) with the HF/LF EQ controls enabled and rotated to maximum/minimum.

The partnering Ultima power amp did not quite make its continuous 780W/1400W 8/4ohm power rating – 690W/1305W was nearer the mark at <1% THD – but the 0.2F of low-ESR reservoir capacitance and three ±110V PSUs operating in parallel delivered a stupendous 825W, 1595W, 3041W and 5617W into 8, 4, 2 and 1ohm loads under dynamic conditions [see Graph 1, below]. Into 8ohm loads, distortion is well managed at 0.001%/1W, 0.0012%/10W and 0.0018%/100W up to just 0.004%/600W (all at 1kHz). THD increases by a factor of ~10x at 20kHz, but remains lower overall than the preamp at 0dBV [see Graph 2]. Commensurate with an output impedance that increases from a low 0.016ohm/1kHz to 0.08ohm/20kHz and 0.9ohm/100kHz, the response rolls gently away at HF to –0.7dB/20kHz and –2.9dB/100kHz (10W/8ohm). PM