Boulder 508 Phono Stage Lab Report

Lab Report

A 'one-size-fits-all' circuit configuration has its advantages but the MM and MC gain and loading chosen by Boulder will still not suit every choice of pick-up. In practice, while Boulder specifies its MM stage at +44dB gain, I measured +39dB (balanced in/balanced out) with an additional 26dB from the integral headamp bringing the MC option up to +65dB. This is equivalent to input sensitivities of 11.2mV and 562μV (0.56mV), respectively, which is a little low for the typical 5mV MM but fairly ideal for all but the very lowest output MCs. The 508's balanced output, meanwhile, will drive any line preamp you might care to choose – a full 17V is possible here!

Boulder's entirely active RIAA equalisation brings dividends, not least in the generous overload margins on offer. The MM input supports a very substantial 200mV input ceiling – a margin of +32dB for a standard 5mV MM (re. 1kHz/5cm/sec) – while the +65dB MC setting offers 11.3mV, or +27.1dB for standard 500μV MC pick-ups. Neither of these margins will be breached with even the highest modulated grooves, and allows the MC input to be used with high output types up to 1-1.5mV, further optimising its 71.9dB A-wtd S/N ratio (a generous 87.5dB is delivered via MM). The RIAA-eq'd distortion remains as low as 0.0005-0.0018% from 20Hz-20kHz [see Graph 2, below] which is vastly superior to Boulder's 0.01% specification and around 1000x lower than might practically be achieved by a modern pick-up. The RIAA eq itself is subtly tweaked by Boulder – the sub-10Hz high pass rumble filter is clear to see [Graph 1, below] but so is a bass/mid shelf, amounting to –1.4dB from 1kHz down to 20Hz. This is tied to a slight boost in the presence region. PM


RIAA-corrected frequency response from an extended 5Hz-100kHz at 0dBV via MM, balanced input/output (black = left; red = right channel)


Distortion versus extended frequency (5Hz-40kHz) via MM input re. 0dBV balanced output

Input loading (MM/MC) 47kohm / 100ohm
Input sensitivity (re. 0dBV balanced out) 11.2mV / 562µV
Input overload (re. 1% THD) 201mV / 11.3mV
Max. output (re. 1% THD) / Impedance 17.1V / 49ohm (balanced)
A-wtd S/N ratio (re. 0dBV balanced out) 87.5dB / 71.9dB
Frequency resp. (20Hz-20kHz/100kHz) –1.4dB to +0.11dB / –0.45dB
Distortion (20Hz-20kHz, re. 0dBV) 0.0005–0.008%
Power consumption 6W
Dimensions (WHD) / Weight 292x58x240mm / 5.3kg
Price £6150
Colorado, USA
Supplied by: Padood Ltd, Cambridge, UK
01223 653199