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The ultimate DIY? A Stax SRM-T2!

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10 hours ago, 8street said:

What is the type of green resistors in batteries?

They are thin film 1206 .1% resistors.  Here are two examples: TNPV1206820KBEEN, TNPW120661K9BEEA.  The TNPV series is high voltage (700V). 

I had to break down resistors into two in some cases to get the proper voltage/power ratings.  I could have used thick film but they are much noisier.

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Wanted to add an HE90 jack but wanted to make sure I have the wiring correct from the front view. Not something anyone wants to take any chances on, but surprisingly little info out there on this (other than an old head-fi thread from 2010):

569a818982d54_HE90Jack.jpg.905b819e56132

Was planning on using the +500 supply for the bias and add another 5M resistor in series.

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It is scandalous how good this amp sounds.

An everyday amazement, thank you all once again for making it buildable.

:wub:

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A detail that could help other builders : 

I had to deal with a hard to adjust channel. It crackled sometimes, with blinking LEDs.

I was quite desperate, and started to read the batteries adjustment techniques again.

First method was Inu's, 10k at the middle and 2k full to R32 side, not good.

Then, I tried to adjust without using 2k, no crackling noise, but some random "whistles".

KG's silkscreen patern, all trimpots centered, not good either.

Finally Kerry's way cured the thing : 6.55V at 22k resistor using 10k pots while keeping 2k centered, then fine tune with 2k.

So keep in mind that each build and each channel is unique. Be creative, walk sideways.

Now the sound is scandalous, like I said...

 

PS : 95 % Takman even in the batteries, not even raised above the PCB

750 Ohm at k216 gates

2*5pF compensation caps

Matched 3675 at parallel spots

Balanced and noiseless Sylvania "Green Hornets" 6922

 

 

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So the only thing I need to adjust for is the 740V in the batteries? I've got one channel powered up where one battery's at 738 and the other is stuck at 781.

Edit: everything 740 everywhere :), but for some reason one of the tiny psu caps near the ref102 exploded. Major pain to replace, luckily I spotted/heard it before the electrolyte got anywhere. Didn't seem to affect the battery adjustment before I found it though.

Edited by nopants

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It is always nice when a problem is found, particularly if it is straightforward.  

When I built mine ages ago the damned thing blew up big time - sparks even - as a result of fake sand  Took out most of the heatsink mounted transistors in one fell swoop.

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Yeah I spent extra to secure real sand that was previously verified, big thanks to all the relevant parties there. Maybe I'll use the leftovers to shove a T2 CCS somewhere

 

Does anyone have a part number for the shaft and coupler to connect standard knobs to the rk50? Are there any real issues that require the use of a derlin shaft?

Edited by nopants

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the coupler needs to be .25 inch on one side and 8mm on the other side. you will have to take a standard coupler and drill out one side.

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In the process of taking off the faceplate I managed to shear one of the LEDs in one channel. After replacing it a few LEDs no longer lit up (D1, D4+D5), but a bigger concern is that I'm measuring about -500 and -440 for the -560 and -500 rails respectively. 

Any ideas? When I use the same umbilical for the other channel everything measures fine, -560 and -500.

PS I ran the amplifier for maybe a few minutes without good contact to the heatsink (in order to fit the faceplate back on), but nothing blew up.

Edited by nopants

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I was only using one umbilical at the time but that shouldn't matter right? Wary of firing up both channels simultaneously though

Also are you supposed to hear high-pitched whining noises while you adjust the batteries?

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the high pitched whining is not unusual, but there shouldn't be any once it is dialed in.

also, shouldn't be any downside to firing up both sides at the same time since you have fired up each side individually already.

hard to say what is going on after you sheared the led. Was it running when it happened? Also you shouldn't have to disengage the heatsinks to replace the front plate. As Marc pointed out, the lower negative voltages suggests current limiting, but it shouldn't be happening with only one side turned on.

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I had to loosen the heatsinks because when I didn't the face plate slipped and took out the aforementioned LED.

the led feeding the 6dj8 ccs still seems to test properly so I'll focus on the negative rail for now

so close yet so far

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-560V is just a -60V supply tied to the -500V rail as reference, so focus on something connected to the -500V rail. If the CCS LEDs aren't lighting up on the +250V and +500V rails something is wrong on that end, too, since the required forward voltage is not being dropped across them. If everything measures fine on the other channel, the PS should be fine so check the amp board.

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Everything lights up fine now, each channel powers up perfectly on its own. Replaced a few LEDs and reflowed some joints.

However, both channels powered up simultaneously seems to make the negative rails sag to -515, -450 and -215, I'm guessing that I should shift my attention now to the power supply. What sets the current limits, the 10m90s?

Also as a side note, has anyone else used Paul's Transformers for the T2? I'm getting some pretty obscene amounts of hum while testing things.

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