Bonsai’s X-Altra MC/MM Phono Preamp


Paid Member
2003-07-25 10:44 pm
Wow - beautiful build ZackPlonk! Thank you for your feedback!

masoste, indeed the rear panel is electrically connected to the rest of the housing via the fact that it is double sided THP PCB. I would connect the top and bottom plates to the rear panel with a wire connection. I have not tried to operate the board with a semi-screened housing so can't offer any pointers to you on hum pickup. In the Modushop housing it is very quiet. You must connect the rear plate near the turntable earth screw to the 0V of the main board near to one of the 0V jumpers towards the front of the board.

(Modushop should be able to ship a housing to you from Italy)
I finally got the right transformer installed and the voltages at the PSU look great for +10V, +25V, and +-15V. Pretty much dead-on.

However, once I installed all ICs, not as good luck. Negative voltages were spot on, but the positive voltages were off.

+5 volt test locations measured -.358v
+10 volt low noise measured -.697v
+10 volt test locations measured -1.744v and -.668v

IIRC, there may already have been prior posts with the same issue, so I'll go back through and locate them when I have a chance to determine if it's incorrect capacitors, resistors, bad solder joints, etc


Paid Member
2003-07-25 10:44 pm
Check that you do not have a cap the wrong way around - look at the voltages at U3 and U4 pin 6 - should be 5V and the voltage at the junction of R15 and R16 must be 10V. Check that C30 (1000uF 10V) is the right way around - if it is reversed, it will be very leaky and pull the voltage down. Same for C15 (220uF 6.3V).

Hope this helps

Here are the installed caps, which appear correct to me. I have v1.0 July 2020 pcb.

R15/R16 junction measures -.697 volts
Pin 6 of U3 and U4 is about -.358 volts.
Hello Greg, I ordered 10 pcs - you should be able to pull 2 sets of 2 that are resonably closely matched - the main thing is to avoid gross mismatches - eg one with a hFE of 50 and another with hFE = 200. Try to keep them to within 20% of each other.
Hi Greg. I'd agree. If you're placing an order it's worth getting a few extra for some rough matching. I didn't do this initially, but ended up with an unlucky combination on one channel. This meant a slightly frustrating wait for replacements to arrive.

Enjoy the build!
Things we're going along great and I really thought I would be listening to some vinyl by now, but I got hung up on the final checks.

  1. Measuring across hot and neutral with the switch on, my Fluke meter readings keep jumping around like crazy and I can't get an ohm reading
  2. Put my meter into diode check mode and I'm not getting any indication of lift at all. I did make sure I placed a probe at 0V and a point on the chassis.
Otherwise, it powers up just fine. Sigh, will dig around to see if I have a short somewhere.
I've reflowed the solder joints on the transformer and confirmed that I've confirmed that I flashed the correct pads for US voltage. One critical notes is that I couldn't get a Talema xformer but found that the Amgis was available. I purchased it w/o comparing datasheets, so I'll spend some time comparing the two.
I decided to be a riverboat gambler and with everything else measuring within spec, I hooked up the preamp. Nothing unexpected has magic smoke released, and the preamp sounds spectacular! I have various plans for today, but will spend more time listening over the week. I am quite, quite satisfied with the unit. Home run, Andrew!