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 :-
Bonsai’s X-Altra MC/MM Phono Preamp | diyAudio
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.
Hope you are doing well.
I love your nx-amp and would definitely like to build this RIAA.
If you go for a group buy, just ping me, I’m in 🙂
I’ll mix it with the Doug Self pre from Linear audio #5 I just finished last month and which sounds great with the nx-amp (at least to my ears).
P.S. if I may, would you share / recommend a PCB supplier for small audio projects as I am trying to design some at the moment?
I’m still doing some work on the preamp. I have been approached to potentially have it printed in a magazine, so once development is completed, I will submit it to see if they want it. If not, it will go up here on the Hifisonix website.
I’ll keep everyone updated on the X-Altra page
For PCB manufacture, I use ‘PCB Cart’ who are based in China. I found the quality very good.
I was coming back randomly to your site and found your answer. I’m glad that a magazine decided to publish your work, although I guess that it will be hard to find here in France 😉
Your photos look great, I’ll come back here to read your updates.
Thanks for sharing your PCB manufacturer and one more time thank you for the nx-amp which I enjoy nearly every day.
congratulations on great design and making it to real product!
I like low noise designs, but I like simple solutions too.
In a real world of vinyl records I would say we can get S/N about 60dB(A).
If this is true, why just not use NE5534 instead of (unobtainable) FETs at front-end?
I am asking from practical point of view (I completely understand engineering challenge of building the best phono pre-amp). Do we need discrete solutions (for MM)? I mean good PSU and well designed RIAA should do, right?
The S/N improvement over an NE5534A with a MM cartridge connected approaches 5 dB which is not insignificant.
Certainly vinyl surface noise is much higher than the preamp stage.
The X-Altra MC/MM provides SOTA performance levels for very reasonable cost and provided a great design challenge. Similarly performing solid state vinyl preamps sell for more than $2000.
I read the articles in Audio Xpress, very good! You mentioned the headroom of 31.5dB across the audio band, although not clear in the text, not to me anyway, I think you mean for signals that are RIAA coded, not for siganls that appear after the cutting process like warped and eccentric records, cracks and plops and so on. This is relevant because most of the discussion about headroom concerns this type of signals.
Thank you Joost.
Yes, the headroom is for RIAA encoded signals. However, good overload recovery with RIAA signals also helps with non encoded signals like clicks (lot of HF content) and warp and arm resonance (LF). Stereophile has reviewed commercial phono preamps with only 6-9 dB O/load capability at HF, and one recent product was less than 3 dB. These are deeply flawed products in my view.