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The ultimate DIY? A Stax SRM-T2!


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Finished...   Back plates before I mounted them...

I finally got the last plate machined.  I had to replace the spindle on my CNC and also made a mistake on the CAD so had to redo this plate, but it's looking great now.  

Been busy at work, but managed to pull a new version of the T2 amp board together. I've added separate modules for the active batteries, balance servo, pre-CCS feeding the output CCS and the opto

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So I've gotten to the point in my build where I can fire up the amp itself. Unfortunately some LEDs are unlit and the batteries don't want to change their voltage (+250V/-559V) with the trimpots. Specifically D1, D6/D7, D10/D11 and D24 are unlit. Aside from those the LEDs outside of the batteries are dim. The PSU voltages measure dead on. Any obvious culprits I should check?

If the adjustment of the batteries does not fix the problem, take a look around the J79 MOSFETs there should be around 50V between the drain and the source.

The above is only valid only if the batteries are correctly adjusted

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I'm curious about current flow around the batteries.  I wanted to ask what the voltages are across R1, R2, R3, R42, R58 & R59.  Sounds like some dead transistors/LEDs for the current sources/sinks + something in the batteries as well.

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Good news: George's suggestion worked. Things just sort of fell into place once the 22k resistor was adjusted to ~6.55V. Then all the batteries could adjust to +200/-541V. Offset and balance are also nice and low.

Dc5t0g2.jpg

Unfortunately now that I've actually plugged in a pair of headphones, I'm hearing a constant noise. Doesn't change at any position of the volume pot either.

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Well after fiddling with the trimpots for a while the noise doesn't want to disappear. I'd run out of adjustment range (as in the LEDs would turn off) in either direction before any change in the noise happened. Any ideas?

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Nope, not while the batteries are within proper range. Gonna try cleaning the pins of the input 6922s in case that cures it. If it doesn't I'll probably look at the grounding in the PSU, since I do hear a faint hum (which I think might be 60Hz) among the noise.

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Grounding is very important for noise. You should make sure there is continuity between the two cases, and you will need to scrape off some of the anodizing to make sure it doesn't insulate where you don't want it to. If you have continuity, then probably not grounding.

Was also wondering how close your resistors are to the board, especially the resistors in the batteries.

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Well I've managed to solve the hum at least (noise still remains). There's continuity between the two chassis, between pin K & IEC ground, and between circuit ground & IEC ground. Essentially I've done the grounding the same way you have.

The resistors are all lifted off the board by a few mm. I'll take a picture later that shows that height.

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Ahhh saw the batteries.  I bet that is the noise you are hearing, throw some Xicon's in there and it should work just fine.  

Birgir, my T2 (Andy's second unit) is basically noise free.  

Low noise maybe but not Carbon levels of utter dead silence.  Just can't be done with that T2 circuit. 

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Ahhh saw the batteries.  I bet that is the noise you are hearing, throw some Xicon's in there and it should work just fine.  

Low noise maybe but not Carbon levels of utter dead silence.  Just can't be done with that T2 circuit. 

I think Inu used Takman Rey (metal film) without any issues, but I do seem to recall that he made a number of other modifications to deal with the noise... I have some 820k and 390k that I can send his way to see if that is the issue.

I will compare the Carbon to the T2 in November. The T2 does have a quad of input tubes that need to be as silent as possible - I suspect that everyone may have a different noise floor based on that alone.

Well after fiddling with the trimpots for a while the noise doesn't want to disappear. I'd run out of adjustment range (as in the LEDs would turn off) in either direction before any change in the noise happened. Any ideas?

Did the noise change at all when adjusting the trimmers (all 4)? Noise is the same on both sides? 

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The 390k/820k resistors are takman metal films (REYs). Would be slightly annoying if they turned out to be the culprit, given their price. Anyway here's a pic showing their height off the board.

9IJh63g.jpg

I get continuity between the ground screw of the amp and the backplate screws of the PSU. Not between the two sets of backplate screws though.

The noise didn't seem to change at all when adjusting the trimpots. It's also close to identical on both channels, but not quite.

If it helps the input tubes are JAN Philips 'low noise' 6922s.

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Yea, that noise is similar. But it's less crackly/more like white noise on mine. Guess the next step is to try swapping the takmans for xicons.

For what it's worth, even despite the noise I can tell this amp will be something truly special. B)

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Darn, I'm in the latter stages of my build and used Takman for the battery 390k/820k. And as things turned out (and a little help from my friends :) ) got Xicon 390k/820k. It would be easier to pull now rather than later, should I swap them? Or just give them a shot? 

 

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