Originally published in US based audio magazine ‘audioXpress’ in February and March 2021.
You can buy a set of X-Altra MC/MM preamplifier boards for $49.50 + $15 for shipping over in the Shop.
The X-Altra MC/MM RIAA preamp is a state of the art design that brings new levels of performance to vinyl EQ featuring best in class measured thermal noise on both MM and MC inputs, tight RIAA EQ conformance and >30 dB overload margins.
Part 1 and Part 2 of the articles describing the design were published in the 2021 February and March issues of audioXpress magazine.
You can download a copy of both Part 1 and Part 2 from the audioXpress website here :-
Here is the BOM and other supporting supplementary data https://audioxpress.com/files/attachment/2737
Here is a link to the February issue overview and towards the bottom, a quick intro to the X-Altra MC/MM Phono Preamplifier :-
Here is a picture of the March issue of audioXPress with second part of the X-Altra article
The earlier BOM had the references out of sync with the PCB. Note if your ordered off the earlier BOM, the part quantities and values are still valid. The changes are: R40, R41, R55 and R56 are changed to 2.2k. If you have already built with the values printed in the audioXpress article (Part 2) circuit diagram, your preamp will still work as published.
All PCB’s shipped after 19th May 2021 have C9 and C25 orientation on the PCB corrected. The BOM in the audioXpress Supplementary Data zip file is updated and corrected as of 15th May 2021
If you have any questions, contact me via the comments section below or email me at bonsai(at)hifisonix.com (replace the (at) with @)
There is also a diyAudio thread about this preamplifier here :-
This document shows you how to wire the Filter T/Over switch, located on the rear panel, to the PCB :-
Here is a table that allows you to get an idea of how noisy a MC preamp is given the cartridge coil resistance and the amplifier input referred noise. Once you have got the number off the table, use the following formula to calculate the S/N Ratio: 20 log [(Rdg*144)/Cartoutput] where Rdg is the reading from the table and Cartoutput is the MC cartridge output in Volts.
For the X-Altra, the equivalent input thermal noise resistance is 1.4 Ohms, so its ABOVE the 2 Ohm row, but we will use the 2 Ohm row since it is close enough. The figures in RED show the expected X-Altra preamp S/N ratio for a 200uV output cartridge. The ‘A’ weighted figures will be about 4 dB better than those shown.
Some photos of the completed preamplifier are shown below.