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The ultimate DIY? A Stax SRM-T2!


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  • 4 weeks later...


Thoughts please on the following question:

what is the max voltage allowed between the +- outputs to ground? If this voltage is allowed to go greater then

what damage is being done to the Headphones long term?

I had read somewhere that 30v was suggested.

The reason i ask this is that my T2 has been  going well, no noises and sound was excellent.

So i decided to measure this voltage, it was 50v hot and 100 cold and i asked myself why as i had checked it a few days before and it  was under 6v.

Decide to look further and found the -560v was at -620v. Measured the series fet (FQP8N80C)  and it was sc.

Havent replaced it yet but assume this the reason for the high voltage measured on the outputs.

With this fet shorted the sound is fine and would not have found this had i not decided to measure.

Worrying as this can happen and you will not be aware of it.

So this is the reason i am asking about any long term damage by having such a high voltage present.

Any comments appreciated as i am about  to pass this on to a non tech person. 

One should also ask how often is it necessary to check this.



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Ad long as the +/- legs of each channel are somewhat balanced it should not hurt the headphones.

That being said, I'm not sure how the negative rail going further negative would push the outputs to +50 or +100.

I'm assuming your -500v rail is now unregulated at about -560v and the other rails are all good.  Is that right?

After you get the PS back in order be sure to check out the amp as well.

Good luck and keep us posted.

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Hi Kerry,

Thanks for coming back. I think both + and - to ground were the same.

All positive rails were fine. All negative rails were up by the same % about 10. With the amp connected.

When i found this i did not bother to check anything else with the amp disconnected. Just ordered the fets.

Cant try it at the moment as i removed the transistor. Replacement wont arrive for a week.

I have some FQPF8n80c FET's which i understand have a PD of 59w as against the originals which have a PD of 179w.

Can i try them in there just to see if my Output return to normal? Don't expect to drive the amp with a signal.


UPDATE: Have replaced the FQP8n80c with the FQPF8n80c and the voltages are back to ~10v and diff is 50mv.

       I remembered Kevin saying that he was able to destroy the -500v Fet and suggested replaceing the diode with a

       30v zener connected between gate and source. Well i had one of Kevins earlier boards which showed 1n4007.

       Lo and behold somehow i only managed to change the +ve side to zeners and not the negative. So i guess that 

       this fet just gave up. So hopefully all is well again.

        Is it safe to leave the FQPF8n80c in as i see it has only a Pd of 59w as against the other at 178w

Edited by iwik
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When I hooked my AMB Sigma 11 PSU into the same outlet as my T2 and has it turned on.  Somehow my T2 and SR009 has noises (hums) on the right channel.... I pulled the Sigma 11 away out of the socket and my T2 is noise-free again ? Anyone has any clues that can please share ?

Never mind, I found out that it is actually the device I am using as a source that is giving out noises rather than the psu or t2 *

Edited by Whitigir
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Just finished testing a new version of the battery with the LT1021-7 reference.  From what I can measure, it seems quieter on the scope (mostly above 25KHz) than the LED version. 


It's nice to be able to test without risking the whole amp :) 

I'll probably start using the LT1021-7 in the GRHV supply as well since it needs less current to operate.  I'm currently feeding it about .94mA with the base of the transistor it connects to pulling about 70uA from that as well.

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  • 1 month later...

Thanks, I've already start thinking angle grinder FTW to butcher my nice completed T2 PCB.




Just kidding, I would do it on CNC if that happened. Kevin, so how do I wire, simply each PS board to each channel (=half T2 board)? I'm afraid I will encounter heavy ground loops.

Edited by Helium
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  • 1 month later...
  • 1 month later...

This is my modified T2 that I’ve in our summer cottage.


She has been kind to me for a long time. But lately there has been some funny (not so funny) disturbing sound. So I brought her home for surgery. Didn’t know where to start but when checking the small tube holders some of the pin holders were kind of very sloppy. Anyhow, I swapped small tubes and then she has played excellent - no disturbing noise. Next step will be to desolder tube holders and check/adjust their pins. Tube holders are Teflon type with golden pins with Chinese quality. Need to be checked and adjusted.

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So the annoying popping came back on left channel with the sloppy small tube holder. This time I took a screwdriver and attacked one of battery trimmers… and a complete silent came from heaven.

Measured battery voltages – meter set to no decimal - they all read 640 volts (reduced all high voltages by 100 V). . Haven’t adjusted them for a long time.

If you are lucky the T2 can be very reliable. 1156590752_Skrmklipp.thumb.JPG.0aab6c560ebc093bc6d1ae5a2d963c48.JPG

Edited by JoaMat
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I found that pulling the 10K pot in the batteries really helps if you’re having issues.  I replace it with a 4.7K resistor in my case.

I’ve also had good luck with the 7v reference in the batteries. 

Glad to hear that all is well now :) 

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Thank you, I think it’s a very good idea to change the 10K trimmers. The latest T2 I built has some unwanted noise during warm up that hopefully could be cured by resistors. But if it doesn’t get worse I’ll stick with trimmer. I promised myself not to change/modify this one and keep it as an original DIY T2.


The above modified shrinked T2 has 10V reference and it was the 2K trimmer that needed a light squeeze. The 2K could probably also be replaced by an appropriate resistor. 40 ohms change gives a change of a volt or so (if my calculation is correct).


By the way – your 7V reference is that on daughter board or on the main board?

Edited by JoaMat
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