The ultimate DIY? A Stax SRM-T2!

[GeorgeP]

That's good  news. And yes, current limiting is happening. It is the 30ohm on the -500 that does it - drop it to 25 or thereabouts.

On Paul's T2 traffos, someone else from that same buy is getting a lot of hum as well. My set is quiet though, so go figure.

[Craig Sawyers]

Well the hum thing is pretty disappointing.  Every transformer that Paul has wound for me is as silent as the grave - including the T2 ones.  When he was head designer for Avel Lindberg in the 1980's through to mid 1990s he designed transformers for Krell and umpteen other high end and pro audio companies.

If you are getting hum, seriously please take it up with Paul - he will be more than keen to sort it out.  I'll mail him and let him know that there is a problem in advance.

[nopants]

Thanks Craig, Paul is going through some health problems last I wrote so I was hesitant to email him again. I'll drop him a line soon though. I guess I should try to order a transformer or two for the carbon while I'm at it, given the circumstances.

[JoaMat]

Regarding transformers and humming.

Is it the transformer itself that hums? Or is it something else that hums due to the magnetics caused by the transformer?

[Craig Sawyers]

A DC offset on the mains can cause hum.  Usually caused by a local industry pulling power out on half cycles, so either the top or the bottom of the mains waveform is flattened (but not both, or not equally).  What happens is that the effective DC offset biasses the core of the transformer, and you hear harmonic distortion through magnetostriction in the core.

There is a good article, and method to remove it here http://sound.westhost.com/articles/xfmr-dc.htm

Basically two big electrolytics in opposed series, or back to back parallel, with diode clamps across them.  In series with the live power.  Worth a bash since some have silent transformers and others don't, which could well come down to local variation of mains quality.

[MLA]

There's a board for everything (almost)...

http://sjostromaudio.com/pages/index.php/hifi-projects/109-dct02-the-dc-trap-high-end-style?showall=1&limitstart=

http://sjostromaudio.net/shop/mains-circuits/11-dct02-dc-filter-for-toroidal-transformers.html

[JoaMat]

Thanks! Great have you guys around.

[Craig Sawyers]

1 hour ago, MLA said:

There's a board for everything (almost)...

http://sjostromaudio.com/pages/index.php/hifi-projects/109-dct02-the-dc-trap-high-end-style?showall=1&limitstart=

http://sjostromaudio.net/shop/mains-circuits/11-dct02-dc-filter-for-toroidal-transformers.html

I'd forgotten that there was a kit.  I've bought things from these guys before - and they are very cheap and exceptionally high quality.  Multilayer 4 oz copper is kind of standard. And they do the lowest noise variant of the Jung super-regulators.

[nopants]

George's suggestion to up the current limit worked like a charm, everything's up and operational. 

 

Sounds good, a bit of popping/clicking but nothing too noticeable. How should I go about tracking that down? For my personal use I don't know if I care enough yet, but I might try to fix it when I redo the internal wiring/etc.

Knob courtesy of insanity, I probably should have measured it before sticking it on

Edited March 6, 2016 by nopants

[GeorgeP]

Try the 6dj8 first regarding the clicks and the pops - that's where I would look.

[nopants]

I do have a spare quad somewhere, I'll dig those up.

I've been searching for some info on making the lsk389 adapters as 6dj8 subs as well, does anyone have any links? I have a bunch I bought from pete millett for no good reason.

[JoaMat]

Made those a couple of years ago.

All information you need can be found here.

[mwl168]

7 minutes ago, JoaMat said:

Made those a couple of years ago.

All information you need can be found here.

I have a dumb question. How do you supply the power needed? 

[JoaMat]

Only change to PSU was reducing +250V to +150V.

For amplifier I changed batteries from 740V to 640V. Also the 68V grid voltage was reduced to 33V, but that is taken care by the resistor on the small board with two 2sc3840.

Maybe one should build a modified T2 with lsk389 for input and EML 20B-V4 for output....

Edited March 7, 2016 by JoaMat

[s_r]

So after not doing anything with my T2 for a while, I'm considering buying a scope to try to track down the problem. Would this scope & HV probes be enough to find whatever is causing the noise?

[nopants]

I was about to say there's a cheaper one out there then I realized what the CAD - USD rate has become...my condolences

[JoaMat]

This is what it looked like with adapter boards replacing the small tubes - ECC88/6922/6DJ8.

[G600]

Anybody tried PCC88 / 7DJ8 tubes with the beast ?

Any reason not to ?

[GeorgeP]

aren't the voltages different from the 6dj8? Not something I would recommend or experiment with on a T2.

[spritzer]

Only the heaters are different so once that has been accounted for there is no difference. 

[G600]

I have installed a quad of Philips Herleen  ECC88.

Definitely sweet with xf2 EL34.

[peranders]

On 6 mars 2016 at 8:00 PM, Craig Sawyers said:

I'd forgotten that there was a kit.  I've bought things from these guys before - and they are very cheap and exceptionally high quality.  Multilayer 4 oz copper is kind of standard. And they do the lowest noise variant of the Jung super-regulators.

... not quite. The DCT02 has two layers and 105 um (3 oz.) copper. The small super regulator boards has 4 layers with 70+35+35+70 um. All high current tracks are doubled = 140 um if you add both of the sides. At the moment two designs have 4 layer boards.

[Craig Sawyers]

Nice to see you on head-case Per Anders - you are doing a fine thing for affordable DIY audio modules.

[JoaMat]

T2 with Emission Lab’s EML 20B-V4. Transformer to left is for filament 4 x 5.0V. Adapters are made by some help of a 3D printer, Teflon tube sockets, slauthered Neutrik XLR connectors, two pole male connectors. All held together with really good Loctite super glue.

[kevin gilmore]

great way to do this, however some heatshrink over the green things to avoid touching the terminals which are at -500v