X-Altra RIAA Phono EQ Preamp
Update: The publish date for this article has unfortunately been pushed out to December or January 2021 due to Coronavirus.
The X-Altra MC/MM Phono EQ amp design is now complete, tested, documented and will be published in a US based audio journal. Some photos of the completed preamplifier are shown below.
The ‘X-Altra Phono EQ Preamp’ is an accurate, very low noise high performance phono equalizer that will accept moving coil and moving magnet inputs. A low noise mains power supply (3.5 Watts) is integrated into the unit, which is housed in a half width Modushop ‘Galaxy’ housing.
The MC features a ‘current injection’ input stage. This approach has become increasingly widespread over the last few years and in this design utilizes the highly regarded Leach configuration popularized by Richard Lee’s ‘Duraglit Special’ with a few added twists in the X-Altra implementation. A servo loop keeps the input at exactly zero volts in order to avoid any DC current flowing in the MC cartridge since this implementation is not battery powered.
The MC input stage uses 2 matched ultra-low rbb’ Zetex transistors to deliver <250 pico Volt per root Hz (1 kHz) input referred measured noise performance.
The preamp has DIP switches accessible from the rear of the unit that allow the gain (both MC and MM) and separately MM resistive loading to be adapted to suit a wide range of cartridges. Selection between MC and MM is by means of a push button switch located on the rear panel. The unit incorporates a ground lifter which can be bypassed by a switch located on the rear panel. There are three LED’s on the front panel – power, MC and MM.
A 40 dB/decade 20 Hz Sallen-Key rumble filter removes any infrasonic signals making for clean, crisp bass. The filter turnover can also be switched to 45 Hz to minimize the annoying ‘acoustic rumble’ found on some classical recordings.
A set of double sided, THP PCB’s (price TBA) will be available for both the main signal PCB and for the rear panel which saves the DIY’er from having to drill and file aluminium cut-outs and mounting holes. Both the internal component mount and the rear panel PCB’s are matt black, gold plated with white silk screening (the rear panel supplied with the Modushop housing can be discarded). Only three off 3mm diameter holes need to be drilled into the 10mm front plate for the indicator LED’s (power, MC and MM). A complete Mouser BOM is also available with a small number of components sourced from RS Components. The complete unit incl. the housing and PCB’s will cost about $350 to build.