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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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Which one is that again? If it was me that suggested that, then I know which one...

It has to do with the (lack of) stability with a base-biased transistor (emitter current isn't temperature stable). It would be even better to use a different design that isn't so dependent on transistor parameters (I.e. Beta) (collector-feedback perhaps?)

I think what happened with the original is that higher case temperatures than we see in the DIY version further inflated the gain of the transistor (this is a bit of a stretch, admittedly) which made the circuit work more consistently.

I think the result would be more stability, but I guess I am the few people that sees this part of the circuit as not being robust - likely because I seem to have the most problems.

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The darlington pairs of q32, q33 and q34, q35/q36 servo have emitter degeneration which dictates the gain and more or less stabilizes it with regard to temperature and base current. FJPF2145 has much lower beta compared to the original 2sc3675 but it's still adequate in this situation. I think the choice of heatsinking is mainly from deciding how much heat you're comfortable with. On the original DIY T2, Q35 and Q36 are heatsinked. It wouldn't hurt to heatsink Q32, too. Q33 Q34 don't need them.

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After reading Walt Jung’s article Audio Current Regulator I think there are reasons to believe that the original output tube CCS has better performance than the 10M90S CCS. According to the same article one can believe that cascoded 10M90S CCS is better than not cascoded.

 

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Don't know if a 10M90S cascoded with itself is better than a single 10M90S, but a 10M90S upper device cascoded with a DN2540 lower device measures about 1000 fold higher impedance compared with the 10M90S alone - > 160 megohms vs about 170 kilohms.  Whether that makes an audible difference in the T2, I don't know.  My wild ass guess is it makes more of a difference in a lower current design than in the T2, where the standing output current is already overkill.  Beyond a certain point, overkill just means the rubble bounces higher (always wanted to use that line :angel: ).

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Hi folks. Been awhile... deepak stoked the hunger for finally gathering some of the parts for the original boards that I have. I missed the boat with some parts that were being sold in the classifieds unfortunately, but I'm looking to buy parts from a guy who posted on head-fi.

 

I understand there's no way to guarantee the parts are genuine based on a picture... but was wondering if anyone would be willing to comment if these look genuine.

 

the guy says they were purchased from singapore in 2005

 

q7AXJ7k.jpg

 

pc9153E.jpg

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Will do, thanks for the advice. Is there any mention of what to look for in buying a meter in the thread or in other diy threads?  

 

Btw thanks for the pics you PM'd me early last year :) Haven't seen them until this week. I'm glad that it's living up to a decent purpose.

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I have 2 cases, 2 sets of original boards. Originally I planned on trading a kit to a builder to get mine built. 

 

It sounds like I should look to acquiring group buy boards or if you guys ever produce new boards please reach out :)

 

Also talking to GeorgeP it sounds like besides transistors there are resistors and capacitors that are becoming hard to score too :/ 

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