X-Altra Mini II Line Preamplifier

X-Altra Mini II Line Preamplifier

Coming Soon: A remote control board for the X-Altra Mini II

The board will optionally replace the current manual select control board and will require a hole for the IR sensor with a smoked Plexiglas window located to the right of the existing indicator LED’s. The remote control board has a H-bridge output to drive a motorised ALPS RK27 or RK16 potentiometer (+-5V at 100 mA) and is short circuit proof. Further, a mute LED is located at the top of the select control dial and will illuminate the gap between the dial and the front plate RED when mute is active. The select is implemented with a rotary encoder. I am arranging with Modushop to have the front panels supplied with the Plexiglas IR sensor window pre-installed should you decide to go for the remote control option. Additionally, there is a 12V trigger output (40mA max) with current limiting. The boards will be supplied with some of the components installed (for testing purposes my end) and the microcontroller (ARM 32 bit LPC1114) pre-programed and mounted.

The remote control is a standard Apple TV remote. These cost £19.99 in the UK and they are available world-wide. By going down the Apple TV Remote route, I don’t have to worry about trying to supply remotes – you just order one from your local Apple store. They are slick, beautifully made and a perfect match for the ‘Mini II’. Pricing on the remote board and the upgraded housing from Modushop will follow once I have a few more things sorted out. Stay tuned for further updates!

Introduction

This is my new all analogue line level preamplifier. I built the first ‘X-Altra Mini‘ in 2009 while living in Japan. Since it really was a bare bones LM4562 preamp, I called it a ‘Mini’ to reflect the fact that it did not do much else other than select an input, amplify it by 14 dB and present it to the output. There was a very basic class AB headphone amp included. Two other line level preamps came along in the interim – the ‘SCA-1‘ a PGA2320 based design (2010 also while I lived in Japan) and then the ‘Symphony‘ in 2013/4 while living in Taiwan – both since scrapped off along with the original X-Altra ‘Mini’. In 2016 I did a commercial preamp, the Model 1501, featuring remote control, phono input, balanced output and up to six inputs. This preamp used and AD797 and a 10k level control Alps RK27 to achieve very low noise and low distortion. Fast forward onto 2021/22 and my fifth line level preamp has arrived, the X-Altra Mini II. I’ve stuck with the ‘Mini’ moniker to reflect the fact that all the modules are small and compact, making extensive use of SMD. This preamp is different to all the others because the boards, housing etc are all available for DIY constructors who want to build their own. The measured performance is very good and at the same level as any well engineered high end commercial unit.

Here is a PDF presentation overviewing the X-Altra Mini II PCB’s and mechanical layout (updated 24th July 2022):-

PCB Price list (see presentation above for explanation of configurations) ALL PRICES ARE IN US$. kindly email me if you want to purchase PCB’s at bonsai(at)hifisonix.com.

Here is the BOM (Excel) for all the modules (updated August 28 2022)

This is the drilling template for the 3mm aluminium base board that comes with the Modushop housing kit. Print it to actual size on A3 and then mark just ONE of the corner holes for each of the PCB’s then use the actual PCB’s to mark off the mounting hole locations. The diameter of all the holes in the base plate is 3.5mm. Don’t forget to deburr the holes.

You can buy a complete housing kit from Modushop with a milled and screened front plate plus base board for €200. The kit part number is XALTRAMINI

Here’s a short (18 minute) YouTube Video explaining the X-Altra Mini II assembly

Initial Listening Tests

Initial 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) have shown the discrete line stage to be open with a smooth mellifluous sonic signature that is quite different from the AD797 commercial preamp, despite both preamplifiers measuring well under 10ppm distortion and with hum and noise at or below -120 dBV ie inaudible. The X-Altra Mini II spot noise floor is c. -140 dBV and so it is exceedingly quiet with no dicernable hiss from the speaker tweeters. The -3dB bandwidth of the Model 1501 is c. 200 kHz while the X-Altra Mini II is set to 130 kHz – I don’t think this alone can explain the difference, but may just be a one of a number of contributing factors. 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’s, B&W 703 and KEF LS50‘s to do the listening evaluation and the smooth, open sonic signature is consistent across all three speakers. 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 discombobulated from the speaker enclosures. The top end end is ultra 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 plays a part. More listening notes to follow in the coming days.

PCB’s are available – see below. As always, I am on hand to help you get your build up and running.