Pre/Power Amplifiers

The ERCO may be equipped with ESS's ever-popular Sabre ES9028PRO DAC but its implementation here is at least partially bespoke, not least because it features custom digital filter coefficients uploaded into the chip. Not for Ferrum the raft of seven alternate linear phase, minimum phase, hybrid, apodising, brickwall, etc, offerings that are shipped with the ES9028 and ES9038PRO DACs by default. Ferrum is understandably coy about its own filter but I can reveal it's a slow roll-off minimum phase type offering a mere 11.9dB stopband rejection with 48kHz inputs traded for no pre-ringing/much-reduced post-ringing and, thus, reduced time domain distortion. Via its coaxial S/PDIF input – limited to 96kHz/24-bit – the ERCO's line response rolls away to –10dB/45kHz although the underlying response of the headphone preamp is flat to ±0.02dB (20Hz-20kHz), peaking slightly to +0.3dB/70kHz and +0.6dB/100kHz.

THD is fabulously low at a minimum of 0.00006% (via the XLR outs) over the top 20dB of its dynamic range through bass and midrange, but there's a hint of digital crush in the top 3dBFs at 20kHz, especially with 48kHz inputs [see blue trace, Graph 1, below]. While this is unlikely to be aggravated by music programme, the burst of distortion is not a feature of the ES9028PRO. Jitter suppression, meanwhile, is state-of-the-art, not only in the mere ~6psec correlated residual but also in the near total absence of clock phase noise [see Graph 2, below]. In the 'analogue realm', Ferrum has set its balanced line output at 2.6V, driven via a suitably low 22ohm source impedance, with a fine 109dB A-wtd S/N ratio and very wide 130dB stereo separation. This is all very solid, innovative stuff and certainly not a slavish copy of the ESS application note. PM