The X-Altra Mini II has been discontinued as a complete project. The individual PCBs are still available over in the shop however and are great line stages and power supplies. The X-Altra Mini II is a superb preamplifier and it sits at the heart of my system, but it was complex with lots of inter-module wiring and I think that put some builders off. There is a new preamplifier coming – tentatively called the X-Altra Mini III – that places everything on one PCB (PSU, headphone amplifier, phono amp, line stage etc). A single ribbon cable connects the main analog board to the controller board. It offers exactly the same functionality as the original design but is easier and much cheaper to build – stay tuned for updates!
The rest of this post discusses the listening tests I conducted on the original X-Altra Mini II back in 2022.
X-Altra Mini II Listening Tests
I conducted listening tests using an Oppo BD103 CD player and an external phono source (X-Altra MC/MM Phono EQ Preamp published in audioXpress Feb and March 2021) for all three vinyl recording assessments and on one of the vinyls, I also listened to the X-Altra Mini II internal MC/MM EQ preamp board. The discrete line stage is open with a fantastically smooth sonic signature that is quite different from the AD797 commercial preamp, despite both preamplifiers measuring well under 10ppm distortion and with measured hum and noise at or below -120 dBV ie inaudible. The measured X-Altra Mini II spot noise floor is c. -140 dBV and so it is exceedingly quiet with no discernible hiss from the speaker tweeters. The -3dB bandwidth of the Model 1501 is c. 200 kHz while the X-Altra Mini II is 130 kHz. I have to point out that my hearing is not that of a young person – but HF is just one part of the overall listening experience with things like imaging, bass extension, midrange articulation etc playing equally, if not more, of an important role.
I used my kx2-Amp (class A 15W RMS, 28-Watt peak class A ) and Dali Oberon 5, B&W 703 and KEF LS50s’ to do the listening evaluation and the smooth, open sonic signature is consistent across all three speakers. The X-Altra Mini II brought the venue and the artists into my listening room unlike any of my previous system setups. The imaging is absolutely outstanding on the X-Altra Mini II – one of my go to test CD’s for this is Fourplay’s eponymous CD that features fantastic left to right width and back to front sound stage depth. This of course has a lot to do with the speakers and the recording, but if the electronics are subpar in any way, they will not support the illusion that the sound is not coming from the speakers themselves – you really want to be able to close your eyes and not be able to point to a speaker source but rather individual instruments laid out left to right and front to back. The top end is smooth and silky with no hint of sibilance. This is the first discrete JFET line level design I have done, and I think the JFET front end plays an important part. I also listened extensively to Chick Coreas’ (RIP) ‘Super Trio’ CD recorded in 2005 by Chet Himes and Bernie Kirsh. This is a superb live recording with great bass extension and a wonderful soundstage and lots of excitement and sonic fireworks on display which was faithfully conveyed by the X-Altra Mini II.
For the vinyl assessment, I listened to three recordings – ‘Ella Fitzgerald Sings the Irving Berlin Songbook‘, and two Chick Corea recordings from the 1970’s – ‘Mad Hatter’ and ‘The Leprechaun‘. Out of the three, the Ella Fitzgerald recording is the best (I’ve written about it here). On the Ella Fitzgerald recording, the X-Altra Mini II conveyed the sense of weight in the lower registers with an effortless, open sound in the mid-range and top octaves, especially noticeable on brass. Images were precisely placed on the sound stage, extending well beyond the edges of the speakers and a long way back behind them. On this particular recording, I also listened using the X-Altra Mini II MC/MM preamp board and whilst not quite in the same league as the stand-alone X-Altra MC/MM preamp, the overall sound was nevertheless superb and still quiet for an opamp based MC preamplifier. The Chick Corea recordings don’t really lend themselves to trying to assess sound and stage tonal quality in the same way the Ella Fitzgerald recoding does. They however are exciting recordings with a lot of propulsive bass, ethereal vocals (courtesy Gale Moran) and drum kit fireworks from the inimitable Steve Gadd. The preamp conveyed the excitement of the recording with great pace, attack and timing.
A few words on the headphone amplifier are in order. The X-Altra Mini II uses the HPA-1 class A headphone amp and my iteration has the OPA1642 dual JFET opamp (although you can also use an LM4562 or NE5532 as discussed in the HPA-1 write-up) driving a MJE15032/33 push-pull output stage biased at 90mA into class A (180mA peak class A current), so it will deliver 1.75 W in class A into a 32 Ohm pair of headphones. I use 12 year old Audio Technica ATH AD900 ‘Air’ headphones I bought in Japan which are not particularly bassy but have the best imaging, midrange and top end I’ve heard bar the Stax tube driven electrostatics – they really are quite special in my view. Through the phones, the preamp retains its open, smooth sound, again without any hint of harshness or sibilance in the top registers. Since the headphone amplifier output impedance is very low, the bass is crisp and goes much deeper than is the case with large diaphragm, open back headphones driven from high impedance resistive dividers. The dedicated, low impedance output headphone amp makes a huge positive impact on the sound.
The addition of two unbalanced (one main and one auxiliary) outputs in addition to a pair of balanced outputs means that with this preamp, I can now have both my power amps (kx2 and Model 1721 240-Watt power amplifiers) along with the sub bass permanently hooked up without recourse to cable swapping or special split cables – so it’s really convenient. The kx2 drives the Dali Oberons and the KEF LS50’s, and the Model 1721 the B&W 703 with the sub bass switched on with the KEF LS50’s.
The picture below is of the ‘Classic’ version of the X-Altra Mini II.
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