Bonsai’s X-Altra MC/MM Phono Preamp

Shoot, Andrew. I took the emitter readings for Q1-Q4. Doh! Here are the COLLECTOR readings:

Q1 & Q4 = -1.520V
Q2 & Q3 = 1.509V

Not 2 V readings, but very consistent. I appreciate your input before I move forward. I will touch up my solder joints with some flux to ensure everything has a good contact. This is my first project with "no-clean" solder and it doesn't seem to wet the contacts as easily as normal solder.
 
My X-Altra build was finished a couple of months or so ago and I’ve been testing this when time has allowed over what ended up being a rather busy summer. Many thanks to Bonsai for all the excellent design work and for enthusiastically supporting the community during the building stages. This was a very enjoyable build and my confidence with SMD soldering is now substantially higher. I think I actually prefer SMD to through hole now, at least for the bigger packages! The use of a pcb for the back panel was a wonderful idea and made a huge difference to a reluctant metalworker like me. It saved hours of time and the end product looks much better than I could have achieved by hand.

The X-Altra sounds excellent to me and is a noticeable improvement on my previous MM preamp. I actually needed to readjust my tonearm’s VTA, as I could now hear that it wasn’t correct, but this was masked by the previous amp. I’m very happy with it. It’s a very nice companion to my Ovation NX - another of Bonsai’s creations! A few pictures are below.
 

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Bonsai

Member
Paid Member
2003-07-25 10:44 pm
Europe
www.hifisonix.com
Last edited:
Seems the nx-Amp is still going strong - I think about 130 have been built so far.

I'm not surprised it's still popular, it's a great sounding amp!

A couple of things that I noticed during the X-Altra build:

* Like a previous poster, I also found that I had a larger than expected DC offset (~110 mV) on one channel of the MC amp. I needed to replace Q1 and Q2 to bring this down to 35 mV, which is a little high, but the servo seems to handle it fine. Is it worth matching the ZTX851/951 pairs in upcoming builds to keep the offset to manageable levels? I didn’t do this initially, but did roughly match when Q1 and Q2 were replaced.

* The rumble filter switch that I went with (RS 320-972) switches the contacts in the opposite direction to the way that I think must be intended (based on the instructions on the hifisonix site). This means that the filter would be engaged when the switch is up (looking from the back), rather than down, if wired as per Bonsai’s diagram. I needed to turn this particular switch 180 o to get the action to match the silkscreen on the back panel – something to watch out for if you’re currently building.

* The user instructions document needs a small update regarding the system gain in the table on page 3 so that the gain available matches that available with 2k2 resistors for R54-56 and R39-41 (i.e. +6 dB for one dip swich engaged and +9.5 dB for both). The left/right labels are wrong for switches 9-12 as well I think – RHS = 9 + 10 and LHS is 11 + 12.
 

Bonsai

Member
Paid Member
2003-07-25 10:44 pm
Europe
www.hifisonix.com
If you are getting high offsets off the bat, please check the orientation of C9 and C25 on your board. On the earlier boards (which you have) these two caps were oriented incorrectly wrt polarity on the silk screen. The cap +ve must go to 0V.

This does cause high offset due to capa leakage, although as you mention, the servo takes care of it, but its not a good idea to leave it like that.

Thanks for the feedback on the other points - I will come back a bit later this week.;
 
Hi Bonsai. I did indeed have the caps installed the wrong way round - this was before the board revision and info in this thread. However, I spotted the polarity issue when troubleshooting the high offset. Replacing these caps, with the correct orientation, didn't significantly reduce the offset in my case though. It was only by replacing Q1 and Q2 that I could bring this down to something reasonable. Cheers. John
 
I finally finished this project a couple of nights ago and am still listening to my favorite records. So far, I am really pleased with this preamp. I don't have a super high-end comparison but the main thought that bubbles up in my mind is that the music sounds clean and accurate. The soundstage has depth and is an area I continue to evaluate.

This replaces my Elekit TU-8500DX with a few upgrades and Telefunken 12AU7 tubes that have a rich sound with a soundstage that can place vocals/instruments 1-2 feet in front of my speakers and 2-3 feet behind.

My system is a Sony record player (Ortofon Bronze MM) --> X-Altra Phono --> Elekit TU-8600S --> Zu Dirty Weekend Omen speakers

The record player is the weak link, as the plinth transmits sound to the cartridge. Bonsai's X-Altra phono preamp obviously has stronger bass than my Elekit 8500 since certain recordings have activated that plinth resonance, causing a strange bass drone. This never happened before.

The more I listen to this new preamp, the more I fall in love with the accuracy and small details that stand out in different ways. Thanks for your input along the way, Andrew! Very nice project!
 

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It's definitely worthy of adding some screenprint or etching to the front panel to identify the led's.

I'm going to pull out more albums with piano. I was listening to a Julien Baker album earlier and was surprised the piano's timbre sounded closer to a live piano than before. It was clearly an upright--piano has always sounded muted on my system and I have blamed recording techniques. I'll continue enjoying my music collection and will post updates. Cheers!:cheers:
 
I got totally sucked into this fascinating design. I'm kind of a tube person and I was trying to learn more about solid state pre-amps and various approaches on treating MC cartridges. I stumbled across Bonsai's MC stage survey and then across the X-Altra and then it dawned on me that I must absolutely build and understand (in no particular order) this thing.
PCB's are on their way and I was able to secure most of the parts between Mouser and RS (which took some fiddling as stuff is not available under one designation but may is under a slightly different one) - some remaining bits (mains transformer) had to be sourced elsewhere but I think I'm good to go...
Thanks for publishing your awesome work, Bonsai!
 
Hey ZackPlonk, I had to purchase 10 and have 8 remaining. The bad news is that I live in the USA.

If you cannot find a good source, send me a PM with your address and I'll figure out shipping to Germany. No upcharge on them--the shipping will probably be prohibitive, but I figured I'd offer.

I also have a spare transformer, IEC inlet, switches, and 8 x Wurth 100uF caps (Mouser P/N 710-860010473007).