The ultimate DIY? A Stax SRM-T2!

[kevin gilmore]

you assemble the tab transistors onto the angle brackets, add a couple of extra screws

then align everything and tighten, then solder.

The filament wires should be twisted in the umbilical, and between the bulkhead connector

and the solder points on the board.

[Craig Sawyers]

Two questions:

Would it be advisable to attach the heat sink brackets to the case before we solder the side fets to avoid any extra pressure on the fets or is this overkill?

Should we braid/twist in the umbilical?

Thanks.

Ah - those are interesting questions.

Because there is a little slop in the holes in the heatsinks, I loosely attached the heatsink bracket to the circuit board with a couple of transistors with washers etc. I then assembled the case with the brackets/pcb in place, and temporarily screwed the brackets to the heatsinks and snugged up the screws. I then tightened the transistors down to lock the bracket to the PCB in the correct place. Then off with one end panel and slide the PCB and attached brackets out. Now install all the transistors, tighten down and solder. You can then take the heatsinks off the case, smear them with heatsink grease, and bolt the brackets on and tighten down. You should be able to assemble the case without difficulty after this. And yes - it was a bit of a tricky process - but keep at it with two temporarily attached transistors per heatsink, until you are happy that there is no significant gaps between the brackets and heatsinks.

On the umbilicals, I twisted the heater wires and put them in a separate sleeve (I just happened to have some high temperature glass braid of the right diameter). Twisted so that the AC current would not couple to sensitive things like the bias, and in a sleeve because they are connected to -500V; possibly overkill, but I had the sleeving anyway, so what the hey.

[wink]

Originally Posted by

wink

TRANS DARL NPN 2KV TO-220 - STP03D200

Like I said - Digikey is showing zero stock.

I'm pretty sure they had some when I looked the site up.

Had the same experience at Mouser.

Looked one day for some transistors that had 5,000+ in stock.

The next day when I decided to get then with a few other bits, it was nil stock.

[pedefede]

Finally got my cabinet from the danish custom service

- It looks very cool, thanks Kevin

I allready had to buy the bunch of shinkohs from PCX, before the cabinet arrived (as I wanted to use the 20% november-discount). I had to do some tweaking, to source the right values. For instance - i bought 2W versions at some locations where

[luvdunhill]

Use parts that fit. With the voltage and heat in this amp, not to mention it's propensity to blow (historically speaking) if you are not careful and the unavailablity of the circuit boards, I'd save yourself the fustration and not try to shoehorn these resistors into place. You want an amp that will be a soild build for years to come that is troublefree.

Just my opinion though.

[Craig Sawyers]

Use parts that fit. With the voltage and heat in this amp, not to mention it's propensity to blow (historically speaking) if you are not careful and the unavailablity of the circuit boards, I'd save yourself the fustration and not try to shoehorn these resistors into place. You want an amp that will be a soild build for years to come that is troublefree.

Just my opinion though.

I'd second that.

[n_maher]

Use parts that fit. With the voltage and heat in this amp, not to mention it's propensity to blow (historically speaking) if you are not careful and the unavailablity of the circuit boards, I'd save yourself the fustration and not try to shoehorn these resistors into place. You want an amp that will be a soild build for years to come that is troublefree.

Just my opinion though.

100% concurrence. I would not half-ass anything involved with this project.

[luvdunhill]

100% concurrence. I would not half-ass anything involved with this project.

.. or double-ass anything, as the case might be here

[Fitz]

I suspect it would just be best to remove any ass from the equation period.

[luvdunhill]

I suspect it would just be best to remove any ass from the equation period.

I think we should reserve the right to keep "bad-ass", other than that...

[Fitz]

Yes, the badass exemption clause, but otherwise, no ass.

[guzziguy]

As the banner above the computer room in my grad school days said:

Don't attempt vast projects with half-vast solutions.

[pedefede]

At the risk of being totally flamed ... :chair:

I know that "solution" 1) and 2) are bad - as stated in my post. But I dont think, that my prefered solution no. 3) is NOT sooooo bad, "double-ass" compromising, vasting the project.

In 4 locations, I would use solutions, which are wellknown technics used in p2p projects in other tubeamps, that also show caracteristics of heat, highvoltage, etc. As an example - here is a picture from the audionote L3 phono preamp kit (from Mostly Audio - Building an Audio Note L3 Phono Stage Kit). Notice the twisted wires on the mica-caps:

http://www.mostlyaudio.com/_images/Builds/L3Phono/pb_2hour007.jpg

So at the cost of a little extra time, an extra solderjoint, and practically the same mechanical stability - i get to use a better sounding resistor, with higher voltage and heat ratings.

In the end, - this has to be a project of years to come. For me personally, that includes listening to the music - instead of thinking about that cheap magnetic resistor i put in the signal path.

[spritzer]

That A-N piece of kit is far from the last word in quality assembly. If you want to twist two components like that then solder them properly together...

I use 2W resistors quite often instead of the usual RN60 units and I just form the leads to make them fit. These are Kiwame/Koa SPR's so they are quite small but it can be done. I also think I used some 2W Riken's on my T2 and I had no problem fitting them...

[Craig Sawyers]

In the end, - this has to be a project of years to come. For me personally, that includes listening to the music - instead of thinking about that cheap magnetic resistor i put in the signal path.

I'm going to tread dangerously into the minefield of resistor sound, particularly relating to magnetic end caps. Cyril Bateman, a guy who donkey's years ago used to be a designer for a major capacitor company, produced a major series of articles in Electronics World on capacitor sound. He measured capacitor harmonic distortion by constructing a sub 1ppm distortion oscillator (<0.0001%) and associated notch filter followed by spectrum analyser.

Right at the end of the articles, he did some measurements on resistors and pots.

Apart from some very old (pre-laser) mechanically ground spiral cut carbon film resistors (0.00013%), he found that the harmonic distortion of modern, laser cut 1% metal film resistors was way below the noise floor of his measurement gear - ie <<0.00007% distortion. Regardless of magnetic or non-magnetic end caps. His only proviso is that a resistor is loaded at less than 25% of its rated power, and preferably significantly less (to reduce temperature rise above ambient).

On pots, the ones producing significant distortion were multi-turn cermet, at 0.00138% distortion. Alas, there is no commonly available option for these. The way to minimise distortion is to make sure that the minimum signal voltage occurs across such a trimmer, with the majority of the value made up from fixed resistors.

So if there is a percieved difference in sound quality between one resistor and another, it simply cannot be down to harmonic distortion.

[pedefede]

I use 2W resistors quite often instead of the usual RN60 units and I just form the leads to make them fit.

How excactly do you form them?

[spritzer]

Yeah and worrying about steel end caps is just nuts. I'm currently on my own resistor buying binge and cramming 2w carbon film resistors everywhere but that's because they are dirt cheap, about the same as RN60 Vishay's and can handle 500V.

How excactly do you form them?

Edited December 5, 2010 by spritzer

[pedefede]

Thanks for the picture, spitzer.

That A-N piece of kit is far from the last word in quality assembly. If you want to twist two components like that then solder them properly together...

Agreed - it can be done with MUCH more quality/stability, shorter wire etc.

But im talking about the principle ... and on the other picture:

ImageShack® - Online Photo and Video Hosting

you join/stack 2 components as well - soldering the leads together. Not very different from twisting.

I also think I used some 2W Riken's on my T2 and I had no problem fitting them...

I can fit them in the 1w / 3w resistor positions. But it is in the

[spritzer]

But im talking about the principle ... and on the other picture:

ImageShack® - Online Photo and Video Hosting

you join/stack 2 components as well - soldering the leads together. Not very different from twisting.

It's not the same since you are only seeing half the picture. The output transistors will be heatsinked and the resistors attached to the sink via thermal epoxy. That said, I also take care to solder them properly and dealing with a +/-16V Dynalo is very different compared to the +/-500V T2.

I can fit them in the 1w / 3w resistor positions. But it is in the

[luvdunhill]

On pots, the ones producing significant distortion were multi-turn cermet, at 0.00138% distortion. Alas, there is no commonly available option for these.

The best trimmers hands down are the Vishay 1280G. It's a bulk metal, PCB mount type and far superior sounding to cermet or plastic conductor (IMHO and all that):

http://www.google.com/url?sa=t&source=web&cd=3&ved=0CCsQFjAC&url=http%3A%2F%2Fwww.vishaypg.com%2Fdocs%2F63056%2F12801285.pdf&ei=FaL7TOzADYX7lwfq18WMBQ&usg=AFQjCNFeFMvx8XOkF-gFhxHMLpJlr2bPWg&sig2=Yz8nxy7IX3V9P3v5HkXncw

I worry about metal end caps. I'm okay with that foible.

Regardless of metal end caps, carbon film resistors will be an order of magnitude noisier than their metal film counterparts.

Edited December 5, 2010 by luvdunhill

[Pars]

Those are the ones that are ~$10 a pop? Pity they don't make them in the 3296W form factor...

[luvdunhill]

just want to check one things. The wire holes are .06" and the test points are 0.08"? Any other test points, other than the marked locations that make sense to add a terminal? ground perhaps?

[pedefede]

also a question about interpreting these (american?) Screw dimensions.

The BOM says: "#5-40/.25 hex cap"

Is that M5 thick / 40 mm long / 0,25 inches hexogonal hole?

[spritzer]

Ehhh no...

[Craig Sawyers]

also a question about interpreting these (american?) Screw dimensions.

The BOM says: "#5-40/.25 hex cap"

Is that M5 thick / 40 mm long / 0,25 inches hexogonal hole?

Um - nope. It means number 5 (an imperial size), 40 threads per inch, 0.25" thread length hexagonal cap head. Alas for those of us in metric countries, you need to source the right screws - the ones that go into tapped holes in the case have to absolutely right. Search the thread for European sources.