Headphones

Apogee's claim for a 'zero-ohm' 6.35mm headphone output is adjusted in its detailed specifications for a more realistic 0.5ohm, although this, and the 150-1600mohm (20Hz-20kHz) achieved in practice, is very impressive indeed. Thus the BOOM's properly buffered output will remain resilient to the low impedance swings of some headphones, maintaining a predictable 'system' frequency response. In practice, again, the BOOM's response is governed by the 44.1kHz-to-192kHz sample rate chosen on the 'System Settings' panel of the Apogee Control 2 software. Be aware that this setting may be over-ridden by a default (usually 48kHz) in your computer's System Properties. Otherwise, the response reaches out from 4Hz-42kHz (±1dB), with a –3dB limit of 65kHz, when the internal ADC is set to 192kHz sampling. Of course, Apogee's Symphony ECS Channel Strip software allows this flat and extended response to be crafted into any 'shape' you might desire...

With Input 2 and +4dBu selected, and the Control 2 software volume/gain rotary set to maximum, the BOOM offers a full +16.3dB gain, which is sufficient to raise a 0dBV output from a 153mV input. The maximum achievable output, and A-wtd S/N ratio, will almost certainly be moderated by your choice of USB hub, but driven via a high-quality desktop Windows 10 PC, the BOOM offered a full 4.5V (unloaded) with 33mW/600ohm and a useful 339mW/32ohm [see Graph 1, below] – the latter rather higher than Apogee's 24mW/30ohm spec. Apogee also offers THD+N figures for its ADC, DAC and headphone amp sections although these look lower than the 0.03%/1kHz, increasing to 0.08%/20Hz, measured here. THD also increases with headphone loading to 0.02-0.85% (20Hz-20kHz) into 32ohm [Graph 2]. PM