Bonsai’s X-Altra MC/MM Phono Preamp

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#422
Part of my parts order has been with mouser for some months now, around five I think, and is only waiting on the 851 that was said to be available around mid April.
Unfortunately that has now blown out to an unbelievable date of around October! Will I have this built by next Christmas? Here's to hoping and crossing my fingers.
 
#424
Making some progress on mine, but can someone confirm if these caps are right. Build guide says the silk is wrong but everyone else seems to have them this way? Thanks
PXL_20230414_205511028.jpg
 
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#429
I've just checked my stock of boards and C9 and C25 are the correct way around on the silkscreen - Issue V1.0 boards July 2020 marked 'Final Production'. In both cases, the capacitor + must go to 0V - the circuit diagram published in audioXpress had these two caps the wrong way around - my error not theirs.
 
#432
Pendergast said:
Finished mine today, and I am pretty proud! I had an issue with the left channel not giving any sound, and then retraced the path and reflowed wherever there seemed to be a missing connection. I used pretty rigid and thick wire for the power supply, so some shaping was required. I had some cat5 cable, but also, some hook up wire from a previous project, so I used some connecting headers, sot it remains practical.

It sounds great! Barely any ground hum (pot turned to max!).

Thank you everyone for the pictures, and especially Bonsai for this awesome device: I heard stuff I never did from my previous tube preamp!

View attachment 1141545

View attachment 1141546

All I am missing are the 851 and 951...
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Great idea with the push on terminals. So much cleaner than my hand wiring and soldering.... I thought I had extra 851 and 951s, if I ever find them they are yours.
 
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#433
Thank you bobgroger for the compliment. I tend to buy a little more of everything so it is nice to use leftovers whenever possible!

And yes, still interested in those 851 and 951 if you can spare some!

This kit is really awesome. I am using an Ortofon black and it really sings well.
 
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#434
Andy,. That right input offset looks too high. I would as a matter of course replace both the capacitors connected to the bases of the input transistors on that channel (C24,C25). If they are leaky, you can get issues. Also, check the associated bias resistors R23 and R24. The other voltages are good.

Note, the servo will servo out that offset but it’s a bit high for comfort and may point to an underlying issue, that’s why we should try to resolve it.
 
#435
I am looking at the SOT223 versions of the 851/951 which look to be available. We might be able to fit those directly under the PCB. They are only slightly noisier (3 dB) than the originals and as far as I can tell this might be because of slightly higher base bonding resistance- that’s my guess. Anyway, I’ll report back in a few days.
 
#437
Yes I wasn't really comfortable with that offset either, I will replace the caps may have got a bad one. On the subject of those caps, is 220uF necessary? The reason I am asking is Nichicon has the KL Low leakage caps, but in that package 47uF is max. Also I noticed Mouser now has 851STZ in stock.
 
#438
I have not tried with 47uF, but would recommend you stick with the 220uF - you really want a low impedance at the transistor bases for noise reasons. The servo will deal with normal leakage levels in the caps, but 95mV is higher than I would have expected.

BTW I bought one of the plugs that measures power and I got 4.6W power draw on the mains side. So, I think what's going on there is these power meters are not accurate at low power draws, they are probably measuring IxV so its not real power draw (for example the peak current from charging the caps x voltage will be quite high) and finally, small transformers like the one used (even toroids) are inefficient compared to large ones. If you hunt around on the web you will see figures as low as 80%.

On 3.2VA and 15V AC secondaries the max current draw is 75mA peak per winding or about 50mA RMS. The total DC power draw is about 35-40mA per rail so well within the capability of the transformer, which also runs quite cool on my two builds.

🙂
 
#439
BTW, if you use a black housing, when you drill the holes out for the LED's (3.1mm diameter) you may still see the shiny aluminium around the edges. To fix this, I take a black permanent marker and run it round the inside of the hole and carefully around the top edge - hold the pen at 45 degrees. This does a fine job of covering the exposed aluminium.
 
#440
Bonsai said:
I have not tried with 47uF, but would recommend you stick with the 220uF - you really want a low impedance at the transistor bases for noise reasons. The servo will deal with normal leakage levels in the caps, but 95mV is higher than I would have expected.
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Ok, thanks for the feedback. So what your saying is the lowest ESR cap would be best in this position?

Bonsai said:
BTW I bought one of the plugs that measures power and I got 4.6W power draw on the mains side. So, I think what's going on there is these power meters are not accurate at low power draws, they are probably measuring IxV so its not real power draw (for example the peak current from charging the caps x voltage will be quite high) and finally, small transformers like the one used (even toroids) are inefficient compared to large ones. If you hunt around on the web you will see figures as low as 80%.
Click to expand...
Thanks for verifying this, that is the same draw I see as well.
 
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