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The ultimate DIY? A Stax SRM-T2!

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Get a working T2 needs a little more than pure luck...........  Glad noise is solved.

I use 7721-3PPSG whenever insulation is needed. First I considered drilling but tried pressing the shoulder in place be means of a small vise. Pressing turned out to work all right . Set torque by feeling of wrist, but last year I’ve used a Swiss made torque screw driver with torque set to 0.8 nm. Steel screw is what I use.

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Awesome work Kerry! :jbl:

3PPSG requires a vise to press them through, or you need to drill out the tabs. 10PPSG will fit all the transistors used without modification or the use of a vise.

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I used 10PPSG, mostly for the long sleeve that extended significantly past the tab. I guess I lucked out in other ways, very glad I didn't have to do extra work to mount everything

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Thanks Kerry,

By the way  did you see my post in GB Digital Attenuator post. You probably missed it. If you have time woiuld you read it.

Les

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4 hours ago, Kerry said:

Noise issue solved on the right channel B)

I'd like to think I'm brilliant, but in this case purely dumb luck.  I changed out one of the K216s coming out of the top of one of the batteries.  I needed to do this because of the dumb part of my luck.  I do love this power supply :)

....

Was it just a faulty K216?

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It's possible.  DC was about right, but it could have been a bit leaky.  Might have been damaged by an earlier short in this channel.  I fixed it by blowing it up completely thus requiring a change.

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12 hours ago, iwik said:

im a bit curios re the Aavid    7721-3PPS shoulder washers.  About to finish my build and noted Craigs reference to these being the best to use with

Stainless screws and getting the required Torque of 1.1nm. I see from the Datasheet that it says recommended force be .5 to .7. So no one has had

any problems by cranking them up to 1.1 for T0220.

Thanks

les

I use http://www.onsemi.com/pub_link/Collateral/AN1040-D.PDF as my reference for power device mounting.  Table 1 shows a test torque of 8in-lb = 0.9Nm.  But figure 6 suggests 6lb-ft = 0.67Nm although the thermal resistance is rather flat from 3in-lb = 0.34Nm.  However, if you tighten to 0.34Nm it seems just too slack.  So I recompensate a tad and up to 1.1Nm for TO220 (9.7lb-in).  I haven't had any problems with my T2 since I built it - but there is nothing absolute about the torque I chose.  If you stick to the 0.5-0.7Nm it will be absolutely fine.  But follow the apps note's recommendation and use a Belville washer (a dished spring washer) to maintain tightness as temperature changes.

If you use plastic screws of any kind (I bought some PEEK ones from a list member in Japan) you can't get close to those torques - the threads strip out.  Nylon would be a disaster, polycarbonate strips as does PEEK.  As you can see I tried hard to not use something conducting!  But the long AAVID shoulder washers, steel hardware and spring washer works fine, and you can get the torque with confidence.

The TO126 transistors definitely need a lower torque - the surface area is smaller, and the important thing is contact pressure.  About 75% of the TO220 torque is about right.

Springs, either this http://www.pts-uk.com/Products/Washers_Disc_Springs or this http://www.pts-uk.com/Products/Washers_Locking_Washer_S_Type or this http://www.pts-uk.com/Products/Washers_Schnorr_Locking_Washer_S_Type would do the job.  I stocked up with these http://www.bellevillesprings.com/belleville-washers.html .  But any of those will do the job.

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You are right to use 4/40 UNC rather than metric, because the interface pressure depends on the thread pitch (it acts like a lever in this).  4/40 has a thread pitch of 0.635mm and M3 is 0.5mm.  The onsemi apps note torques all apply to UNC threads.

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I typically go with split washers. I used those on Al's build way back when and on most of my other builds.

For #4, I did not see spring washers at McMaster.com where I get all of my mechanical doodads. I might look around for another source in the US and give them a try.

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I typically go with split washers. I used those on Al's build way back when and on most of my other builds.

For #4, I did not see spring washers at McMaster.com where I get all of my mechanical doodads. I might look around for another source in the US and give them a try.

I would suggest Belleville washers.

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1 hour ago, luvdunhill said:

I would suggest Belleville washers.

Do you have a supplier for these in the US.

It's an easy enough thing to do, so I might as well.

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Just do a search on Google.  Lots of suppliers Stateside.  And the OnSemi note is from the US and says Belleville.  

And no JoaMat - I do not think that any compromise is advisable as being "good enough" for a T2.  Any break from best practice is not a good idea given the voltages and power dissipation.

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1 hour ago, luvdunhill said: I would suggest Belleville washers.

Do you have a supplier for these in the US.

It's an easy enough thing to do, so I might as well.

McMaster-Carr :)

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3 hours ago, Craig Sawyers said:

And no JoaMat - I do not think that any compromise is advisable as being "good enough" for a T2.  Any break from best practice is not a good idea given the voltages and power dissipation.

to quote Oscar Wilde:  "I have the simplest taste.  I am always satisfied with the best!" :)

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Craig I thought you were the one that mentioned that the torque doesn't matter as much when grease is applied? does this affect the to126 whose needs are lower?

I had a tough time with the peek screws historically so I just went with the steel hardware

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3 hours ago, nopants said:

Craig I thought you were the one that mentioned that the torque doesn't matter as much when grease is applied? does this affect the to126 whose needs are lower?

 

Well the thermal resistance seems to be pretty constant from lowish torque (at least from the graphs in the apps note).  The importance is that it stays put as temperature changes and polymer materials (like the AAVID insulation washer) creep with time under pressure and temperature.

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Did this last night, not finished yet. Modified T2 output section. All parts available at Mouser. SMD give space for bracket. Board size 150mm x 100mm. Darlington pair in the middle, one on heat sink and partner standing.

Bipolar T2 Output bracket - 3D Visualization.jpg

 

Below: draft of an input board.

Bipolar T2 Input bracket - 3D Visualization.jpg

Right/left channels will be mirrored.

Edited by JoaMat

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Careful of the feedback paths, particularly since it goes over two boards with I would guess wires to connect. The T2 is prone to oscillation which can be quite tricky to sort out. The thing you have going for you in this regard is the compactness with SM.

Why go to the trouble of mirroring?  If you have a layout that works, you might well run into problems with a mirrored version with different tracking (since active devices themselves do not mirror).

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This is my modified T2 from 2014.

IMG_0547.JPG

Later further modified with Cascaded CC a la JimL, replaced a bunch of to92 with small smd ants and replaced one battery with, an all SMD battery on a small daughter board. No oscillation, no hum or other issues. Works equally good as the three original T2s I’ve built. I frequently make layouts, mirrored or symmetrical and make the boards in our kitchen.

For me it’s a hobby and I’m really enjoying it.

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This build was the original inspiration for my mini build.

Thanks for always trying new things :)

I do have some oscillation in my build and I'm going to need to jump into that now.

Edited by Kerry

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