Jump to content
spritzer

The ultimate DIY? A Stax SRM-T2!

Recommended Posts

^ same as feedback cap

16 hours ago, G600 said:

Sometimes I hear some random cracks.

They come and go, and at some point the channel goes south.

Voltages are off, with something like 40 V between the two right batteries, and a very large offset at ouput.

It could be cured instantaneously by putting a Voltmeter between battery testpoint and +500V supply entry.

Tried to adjust the batteries in many ways, but it comes back anyway.

I have also uprated the compensation cap to 15 pF. The amp runs cooler afterwards, but the problems is still happening.

Have you or anyone else tried replacing the original style T2 batteries with the simplified LT1021 ones instead of all the LEDs? I don't have the schematic right now, maybe someone else does

Share this post


Link to post
Share on other sites

That works, but I don't think that is the source of that noise (I can't say this for sure).  When I was working on Al's way back when, the solve for me was to replace all the resistors in the batteries with Xicons.  I tried all of the other recommendations at the time with no success.  I still don't have a great explanation of why this solved the issue.

My new version of this amp uses the leds, fet and resistor to create the set voltage, same as the original, and does not have any issues.

Share this post


Link to post
Share on other sites

I've rebuilt a couple of batteries on original DIY T2 boards with LT1021 - works. Have looked for schematic without success but it is somewhere on my computer…

Anyhow, how do you like this one:

5924bd30db2af_T2-Battery1968.thumb.jpg.17e15cb3039c7e6eb28d4a3823490aca.jpg

LT1021 current set to 1.2 mA. Hard to find 2SA1968 and easy to find smd HN4C51. Fewer components then the original battery. Better? probably not.

  • Like 1

Share this post


Link to post
Share on other sites

Notice how it looks almost identical to the GRHV, which I think people find fairly stable :) Just making sure, is the 10m90s CCS going the right way? 

Edit: for the crazy people like Joachim, I tested some of my 2sa1486 awhile back and did find a decent number of them did measure above 750V breakdown. YMMV, beware, etc

Edited by mypasswordis

Share this post


Link to post
Share on other sites

Yeah, wish I had a big stock of a1968's.

It's not so surprising the the grhv looks similar :) 

The two pnp transistors along with the current mirror below form a very simple opamp of sorts.

Share this post


Link to post
Share on other sites
28 minutes ago, mypasswordis said:

 

... I tested some of my 2sa1486 awhile back and did find a decent number of them did measure above 750V breakdown...

Thanks a lot. Good to know I can save my small stock of 2sa1968 for more important missions.:)

Share this post


Link to post
Share on other sites

It has been a rainy day so I modified the batteries on one channel. Replaced the four 2sa1486 with two 2sa1968. Battery voltage, offset and balance just fine but sonically I got unpleasant hum. Now everything is back with a1486. Why change something that works well?

Edited by JoaMat

Share this post


Link to post
Share on other sites

So I've installed a 10pf bypass cap. At first the value for offset was way high (like 400V give or take) but after adjusting the batteries it fell back to within normal values. The T2 has been on for at least 5 hours now, and it hasn't misbehaved in that time. I've had to go back and check after hearing some puffs of noise which turned out to be in the recording itself.

At the risk of jinxing it, that seems to have worked. :)

Edited by s_r
  • Like 8

Share this post


Link to post
Share on other sites

Shouldn't have opened my mouth apparently, the T2 is acting up. I replaced the four K216s of the right channel with ones from my old board with resistors at their gates, also trying a 5pf bypass cap right now. Neither of these changes seems to have helped though.

It's been a complete gamble when I first turn it on as to whether the offset will be fine, -30V or -450V. Sometimes I'd measure around -30V offset, then as I try to use a scope to see if there's oscillation, the offset fixes itself and there's no oscillation. Other times I'd adjust the batteries, which fixes the offset, then as I'd turn the amp off to put back the top cover it'd regain its problems when turned back on. Regardless of the offset/oscillation I get an intermittent scratchy noise. Just maddening.

Share this post


Link to post
Share on other sites

Hate to suggest this - dry solder joint? Through-hole plating on one component flakey? Resistor with flakey end-cap?

I absolutely detest intermittents. Back in the day (late 70's) I was using a Tektronix 7912 transient digitizer to record fast laser pulses. This was in the days that you strapped a polaroid camera to the faceplate and photographed the trace. Except the readout of sensitivity and sweep speed would jump up and down just when you clicked the shutter. To cut a long story short, as part of the readout insertion circuity, the signal went through a plated hole from the top layer to an internal layer on a 6-layer board - and that was an intermittent contact - the plating had not taken to the internal trace. I flooded the damned thing with solder in the hope that the flux would help make enough of a solder bridge to cure the problem. It did - or enough to get me through the measurements with no further readout-jumping incident. That took quite a bit of detective work and quite some time, and Tek's excellent manual, to cure.

Share this post


Link to post
Share on other sites

Well I've reflowed a bunch of joints (probably more than was needed). Since then I haven't had problems with oscillation, excessive offset, etc. The sporadic noise seems to have gone as well, but of course just as I say that it'll come back to spite me. :rolleyes:

Anyway, thanks for the help. I think I can stop fucking with it and just use it to listen to music.

wxKzIFT.jpg

  • Like 13

Share this post


Link to post
Share on other sites

As I suspected, the sporadic noise hasn't actually gone for good. I've tried reflowing more joints just in case but that didn't do it. If it helps pinpoint things at all I've noticed that the back right EL34 has a tendency to physically ring. Even if the tubes are swapped around, the tube in that position starts to ring. I've tried putting in mullards, which didn't ring, so maybe the mesas are just more susceptible to ring as a result of whatever is going on.

Share this post


Link to post
Share on other sites

Transformers are on it way, but still waiting on chassis before heading into some "headaches".  Hopefully I can achieve it on time for Christmas with a lot of stuff going on, and to finally be enjoying the "legendary performances of Stax"

Edited by Whitigir
  • Like 2

Share this post


Link to post
Share on other sites

Kerry did a very, very nice thing (thanks again!) so I will be building one eventually as well now. Terrified to calculate the total cost of the thing, though...

  • Like 1

Share this post


Link to post
Share on other sites

I've been collecting T2 parts for nearly two years. Almost done with that phase. Hope to get to the build within the next year... just need to force myself to dive into it and be mentally prepared not emerge for a solid 2-3 weeks.

  • Like 1

Share this post


Link to post
Share on other sites
40 minutes ago, mypasswordis said:

Kerry did a very, very nice thing (thanks again!) so I will be building one eventually as well now. Terrified to calculate the total cost of the thing, though...

If you ever do calculate the cost (which you shouldn't) you'll realize I wasn't really being nice :indra:

  • Like 2

Share this post


Link to post
Share on other sites

Finally finished my one. Goes back to 2013 but i am a slow builder. Works like a charm, turned on and everything adjusted up as expected.

No noise i can detect and as i used heat sinks 4" high i do not have any heat problems. For those interested i used a smoked acrylic top

to the amp and put my led's on the top and fitted Kerry's 1" OLED display front center of the amp  under they acrylic and with a cool blue display sure looks sexy.

I am crap at taking pics so that is why there are none. Thanks to Kevin, Kerry and Birgir for another great project.

 

  • Like 4

Share this post


Link to post
Share on other sites
4 hours ago, iwik said:

 

I am crap at taking pics so that is why there are none. Thanks to Kevin, Kerry and Birgir for another great project.

 

Hmm, we still need the crappy pics..:)

  • Like 3

Share this post


Link to post
Share on other sites

Yeah, no pics = it didn't happen..........  :ph34r:

Share this post


Link to post
Share on other sites

Kevin, could you please upload the latest BOM (both amp and ps) of DIY T2?

Could not find it anywhere.

Share this post


Link to post
Share on other sites
1 hour ago, kevin gilmore said:

Thank you so much.

Curious, japanese fonts are used, who touched 

Thank you so much!

Edited by Helium

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×