Hifisonix X-Altra RIAA Phono EQ Preamp

This project was published in two parts in the influential US-based audio magazine ‘audioXpress’ in February and March 2021 (links to the download page below). About sixty of these preamps have been built as of May 2023.

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 and tight RIAA EQ conformance. On moving coil inputs, the RTI noise contribution from the amplifier stage was measured at just 232pV/rt Hz (pico Volts per root Hertz), placing it amongst the very quietest MC phono preamplifiers available at any price, be they commercial or DIY offerings. Tight RIAA conformance extends from below 20 Hz to >50 kHz and over 20 Hz to 20 kHz remains within +-0.15 dB, while distortion remains below 50ppm up to 8V RMS output from the preamplifier (MM input) and better than 0.01% at 8V RMS output on MC inputs (50 ppm at 1V RMS output).  These performance figures come with class-leading input overload of >30 dB on both moving coil and moving magnet inputs. A novel input offset servo keeps the MC input at 0V +-25uV ensuring unwanted MC coil DC bias currents remain negligible and allowing direct coupling of the transducer to the amplifier, further contributing to the outstanding noise performance.

MC and MM transducers require very different amplifier characteristics if the designer is looking for ultimate performance.  For this reason, the X-Altra MC/MM separates the two input stages and uses amplifier topologies that offer the best performance in each case. The MC amplifier is configured as a current injection stage around a complementary pair of ultra-low noise bipolar transistors (ZTX851 and ZTX951).  This treats the source as a current transducer, rather than the more conventional approach where it is treated as a voltage source driving a 100-200 Ohm resistor load and is a major contributor to this preamplifier’s outstanding noise performance. In practical terms, compared to a very low noise JFET (en typically ~ 0.9nV/rt Hz) or high-performance op-amp-based MC head amp (for example an AD797), the X-Altra MC/MM is 3.8 x quieter (>11 dB), and 6 dB quieter than Douglas Self’s MC preamplifier (Small Signal Audio Design (2010), pages 213-218 with performance quoted as -141 dBu = 479pV/rt Hz).  Note that the figures quoted for the X-Altra MC/MM are all based on measurements of the full signal chain from the input right through to the output of the preamplifier. If the X-Altra MC/MM is quoted similarly to the Self-design, the spot noise performance is -147 dBu. Compared to other commercial products using discrete input stages, the X-Altra MC/MM is typically 6 dB quieter.

On the MM input, a dual ultra-low noise JFET (Linear Integrated Systems LSK389 or TI JFE2140), operating in parallel single-ended mode, is paired with a high-performance opamp to realise a single all-active equalisation gain stage with noise levels to within 0.3 dB of the theoretical limit, along with >30 dB overload performance.  The X-Altra MC/MM achieves better than -76 dB SNR (unweighted ref 5mV 1.3k + 500mH cartridge source). With the input shorted and ‘A’ weighted, the input-referred noise is -89 dB, ref 5mV. 

The design makes extensive use of SMD components, so the result is a very compact unit with small loop areas which in turn allows the unit’s power supply to be co-located on the amplifier board.  The mains peak noise floor of the finished preamplifier is 20 dB below the thermal noise floor of the amplifier with a standard MM cartridge (1.3k + 500mH) connected i.e. inaudible.

You can download a copy of both Part 1 and Part 2 from the audioXpress website here :-

Part 1    https://audioxpress.com/files/attachment/2735

Part 2   https://audioxpress.com/files/attachment/2736

The BOM and other supporting supplementary data is hosted by audioXpress here:  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 :-

https://audioxpress.com/news/the-audio-technologies-that-matter-more-than-ever-in-audioxpress-february-2021

Here is a picture of the March issue of audioXPress with second part of the X-Altra article

If you have any questions, or require technical support with your build, 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 :-

Bonsai’s X-Altra MC/MM Phono Preamp | diyAudio

Errata: Kindly note that in the circuit diagram in the published article C9 on the left channel (and C25 on the right channel which is not shown in the circuit) is the wrong way around. However, the PCB silkscreen is correct, so please follow the silk screen orientation for these two capacitors when assembling the board.

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 gotten 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.  It is clear from this table that the X-Altra MC/MM is in a league of its own in terms of thermal noise (and overload capability).

For the X-Altra, the measured 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, along with my Michel Gyrodec are shown below.