X-Altra RIAA Phono EQ Preamp
The X-Altra MC/MM RIAA Phono EQ article Part 1 and Part 2 have been published in the February and March issues of audioXpress magazine.
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 of the X-Altra article
Supplementary information covering the BOM, assembly details and noise measurement methodology can be found on the audioXpress website here :-
Some builders have noticed one or two small discrepancies between the BOM in the link above and the PCB. The PCB component values and numbering takes precedence over the BOM above and is correct against the published circuit diagram. To help you, here is a BOM that was back generated from the PCB
You can buy a digital copies of the February and March issues for $9.50 each (£6 in the UK) here :-
PCB SETS ARE SOLD OUT. A NEW BATCH OF BOARDS WILL BE IN STOCK IN 2 WEEKS.
We hope to resume shipments in the first few days of March
PCB board sets have unfortunately had a cost increase and board sets will now cost cost $39.99 plus an additional $9.50 for postage (standard international 5-7 working days to USA or Europe) Total $49.49. Prices do NOT include any applicable taxes/duties for your country. Thank you for your understanding.
If you have any questions, contact me via the comments section below or email me at bonsai(at)hifisonix.com (replace the (at) with @)
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.
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’ aka transimpedance 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 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 with white silk screening (the aluminum 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.