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The ultimate DIY? A Stax SRM-T2!


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YESSS!!! first channel has a LIFT OFF!! All leds light up nicely afro.gif

(Replaced the other J79, that was only "slightly" dead, so it took my quite a long time to find the bugged part)

anyway - battery adjustable and fine at 740 - and all voltages measures fine.

However, i have oscillation problem:

There is a 860KHz (i think) oscillation around 14V peak-peak at each output (though in phase - so V-out-difference is only at 1,5 V peak-peak).

The V across R42 in battery

- minus-side: is adjustable through 2K pot, to 6,55V (like kerry). (A humming can be heard somewhere, while measureing)

- plus-side: is not adjustable through 2K pot (?!?), sits steady at 5,3V (No humming).

Oscillations sounds somehow like Craigs problem. (I do have 5pF caps across each 100K feedbackresistor ... gotta read/check, weather the resistor it has to be moved)

- Its the non-adjustability of the one side R42 that puzzles me ... undecided.gif

Edited by pedefede
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Have you replaced the o-rings on the rear side of the drivers on the HE60? If those are broken then yeah... they sound like crap.

Congrats on getting the amps up and running guys. :) I'm waiting on some parts and then I'll do further damage to the chassis since I can't just have the heaters to show the bloody thing is on. I was thinking about bottom mounted LED's under each chassis but I might just stick with a latching vandal switch on the front of the PSU. Assuming it fits...

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Well, oscillations killed pretty easily. I only had one C12/13 = 2,2pF (per channel), like Gilmore. So i added 1 more for the other side, and the oscillation died.

(It MIGHT have been caused by pickup noise etc, from all the test leads for the 8 multimeters (i borrowed from work). These were remove in the second test. )

I will go to sleep with such a happy smile on my face, that my wife will think I had sex elsewhere ....

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Please allow me to add a few pictures of my T2.


And these are a few headphones that I have tested with T2 so far.

What can I say? T2 has so much power. It loves big headphones. With T2, HE60 sounds like a child toy when compared to SR009, Omega or even my own DIY phones.

Nice. I love your collection of headphones. I'm very interested in your DIY phones.

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Congratulations to all those who are already enjoying their T2 and thank you very much Dr Gilmore and all users who have provided solutions for their contribution to this fantastic project.

At the moment I can not spend my free time this amplifier so I don't know how I could help to this project.

A few weeks ago I started buying parts for this amp and the KGSSHV. Last week I have sought ceramic screws to fix the sands to the heatsinks and I found a dealer of Singapore on Alibaba who sold me a few screws m3x12mm.


The screws look good but when I tried to mount a FET in a heatsink the screw was broken before I started pushing hard :-(. I have in my work a torque screwdriver, but the minimum adjust of this tool is 2 Nm for this reason I can not know the pair of screw breakage. I am looking for another torque screwdriver with a smaller adjust to calculate the pair of breakage.


I'm not sure if the problem of these screws is the quality of materials, the manufacturing process or are not suitable for the torque we need embarrassed.gif

-- o --


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Run a search in this thread for "PEEK".

These are the plastic screws that have been discussed and recommended previously.

M3 ceramic have a torque rating of 0.6 Nm, the PEEK plastic screw is half that.

Yeah, you are gonna need a new torque screwdriver.

As you have already learned the ceramic screws are very brittle.

Not what I would use for this application, where heating expansion of the clamped TO-220 could break the head off.

In general, ceramics perform better in a compression mode, not under expansion. (as a torqued screw would be)

Edited by livewire
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With the thread going all the way to the underside of the head certainly provides a weak spot for the head to separate from the thread.

You'd think they'd leave a few threads width of bare shaft for extra strength.

Edited by wink
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Keratherm film might be too thin.

I've just been flailing around trying to avoid using the 4171G ceramic insulators, since they aren't expected in to the various stockists until the end of July (nominally). I've been talking to Kevin privately thoughout this as I reasoned things through.

The result of that flailing just resulted in my spending a bit of money and stocking up the spares drawers.

The short conclusion is: THERE IS NO OPTION BUT TO USE THE 4171G!

I tried to use the Bergquist elastomer "alumina and beryllia replacement" with 6kV breakdown, type K10. I sourced insulating bushes with 2.54mm bush length so they went into the heatsink bracket and insulated the screw. Then I spotted the fallacy - there is still a breakdown path from the inside edge of the device mounting hole to the hole through the heatsink bracket - which is just the thickness of the washer away (0.15mm). That limits the potential to 0.15 x 3kV = 450V best case (air breaks down at 3kV/mm). Since the device tabs run up to 630V above ground this is a recipe for disaster. The nonsense is of course that Bergquist supply a TO220 bolt mounting stamping out of this material, which makes a joke out of its 6kV material specification.

The alumina washers that Kevin ended up specifying are 1.78mm thick, separating the device tab and heatsink by that amount. That should hold off 1.78 x 3kV = over 5kV!

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