Headphone amplifiers no longer form a single product category. With the burgeoning of interest in headphones has come not just a multiplication of but also a diversification in the devices dedicated to driving them. So we now have (category one) the classic minimalist headphone amp, with an analogue input, volume control and headphone output(s). Category two, appreciating that the headphone amplifier can double as a simple preamp, adds a line level analogue output and perhaps selectable inputs. Category three - into which the £700 UD-503 falls - is the DAC/ headphone amp hybrid, which allows direct connection to digital signal sources. Even here there are further subdivisions, such as between devices that accept PCM digital only and those like the UD-503 - a more feature-rich replacement for the previous UD-501 - which acknowledge the expanding catalogue of DSD downloads by also offering DSD compatibility. In the case of the UD-503 this stretches not just to conventional DSD64 (2.8224MHz) files but to DSD128 (5.6448MHz) and DSD256 (11.2896MHz) files also.
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