Outboard DACs

Unlike most DAC/headphone amplifiers where all inputs are 'digitised', the line input to EarMen's ST-Amp remains analogue to the ALPS volume control and to the TPA6120 op-amp based headphone amp. Similarly, the DAC's analogue output is also fed through this volume control and headphone amp, ensuring the ST-Amp remains true to 'purist' hi-fi principles!

EarMen rates the ST-Amp's single-ended headphone output at 500mW/32ohm even though, in practice, it achieves a far higher 1.57W/32ohm [black trace, inset Graph] and harbours sufficient current to support 359mW into a low 8ohm load [red trace]. The maximum 7.68V voltage output is sufficient to maintain 97mW into 600ohm [blue trace] but, as is also illustrated by the Graph, distortion is influenced by both output and loading. For example, an unloaded 0dBV output has a mere 0.0028-0.005% THD (20Hz-20kHz) but this increases to 0.0028-0.025% into 32ohm, the biggest uplift occurring at low frequencies. Otherwise residual noise is a very low –98.8dBV (11.5µV) – ensuring the ST-Amp is well suited to very sensitive ear-buds – while the A-wtd S/N ratio is a wide 102.7dB. The response is also super-flat, extending out to from 2Hz-100kHz (±0.05dB) from a very low 0.39-0.55ohm source impedance (a loss of just ~0.06dB into a 32ohm headphone load). PM