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The ultimate DIY? A Stax SRM-T2!


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11 hours ago, G600 said:

jokerman777, could you remind me/us which PCB version/origin you use?

@G600 I got my boards from GeorgeP and it should be whatever is the latest version of Kevin's design with original parts (as opposed to "modern version")


Haven't got back to this thread for a while but I made some progress over the holiday time identifying the problem with a lot of help from simmconn as well as finally using a scope.

-Balance: turns out by changing the permutation of my preamp tube locations in certain case I can get both channels balanced, likely some of my 6dj8 has better matched sections than others which were kicking the servo out of working range.

-Offset: Replacing R73 with 10k pot I was able to trim out the remaining offset to near zero no problem.

-Oscillation: quite possibly my last bit of problem, that I was observing low frequency (60-80Hz) sawtooth waves at all outputs. left-channel has seemingly matched waveforms on O+ and O- with hundreds of millivolts peak-to peak and right-channel has unmatched waveforms with one measures 1.42V pk-pk and the other much less and also reversed pattern along time axis. Slightly trimming the battery voltages doesn't seem to affect this behavior.

When I got time again I will inspect things around the balance servo as well as breaking the loop to see if I can find out where it comes from. 🙂

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  • 2 weeks later...
On 1/8/2011 at 1:50 AM, Craig Sawyers said:

(1) The power supply mod developed by Kevin and Inu to make it stable

Can anyone please point this mod for me?

I can't find it reflected in the (latest?) schematic or PS PCB.

R14, R29, R41 and R53 resistors are all 2.2k, or am I looking at wrong locations?

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Yep, I myself had to scroll this thread again as Im in the process of building one.

I think the mod is referred to this post:

But I dont see what resistors were XXohm value in the original design and became 33kohm in the updated design as Im building the latest rev of PS PCB (speaking of original DIY T2).

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  • 1 month later...

Fixed this amp a while back and been running it day to day ever since. So far everything works and seems to be stable, I finally get to enjoy the music coming out of it 🙂

It's definitely quite a thing building and troubleshooting it in the past 8 months or so, major lesson learned is that I could've gotten a scope on day 1 if I could turn back time.

Many thanks to Kevin for providing such a design - the major heavy lifting of bringing back a great amp. Thanks to GeorgeP for supplying the hardware with all the building advices, or I would never have considered building one.

And a special thank to Simmconn, who spent a lot of extra time not only helped me with diagnosing the issues, but also taught me a lot of the nuts and bolts about proper troubleshooting practices in general as well as demystifying the T2 circuit to me through patiently explaining.

Of course there are a few more to mention, thanks to the folks out here who kindly helped me. It takes 10+ headcase veterans to help a newbie like me building a working T2 😄


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5 hours ago, Pars said:

Cool! What was the problem/fix for the oscillation you mentioned in your Jan. 8th post?

😃Was trying not to dump more flat-earth theories of T2 to the thread as I already did a bunch, but here are what (I think) my problems were:

  • I didn't find the cause of the oscillations appeared at the output, but they are killed by upping the feedback caps from 5pf to 10pf.
  • Some "AC components" I measured around output plate CCS might actually be oscillations excited by me sticking my handheld meter probes across floating nodes - I recall when I measure referencing ground it was different but better, but in the end when I probe with oscilloscope I see nothing there.
  • When I trim both channels in place, turns out the 10M90S in -500V rail reached the boundary of current limiting so it's trying to sag and gives a sawtooth wave on that output rail voltage, hence appeared at the amp output. Lowering the current limiting resistors fixed it.
  • When it comes to getting preamp tubes for T2, probably the most important and minimum requirement for the circuit to work is to have well-balanced triode sections for each individual tubes. Some permutations of the quad JJ tubes I got will give too much imbalance that is outside the correction range of the servo. Also I find my left channel having more tolerance on input tube mismatch for the servo to correct than the right channel - suppose the better matched the rest of the circuit is, the more tolerance on input tube mismatch you will get.



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