The ultimate DIY? A Stax SRM-T2!

[sorenb]

@jokerman777
Based on your post's I'd recommend disassembling both angle brackets, making sure all holes are properly deburred, and all the tappings are clean, also clean all semi's. Make sure the screws can go into the tapped holes using only finger force -  if not re-tap the hole.
Re-assemble, using regular compound (the white stuff).

As for the buzz, you stated to have observed a flash in the region of the negative rails, and showed a burn'ed screw from the positive rail, showing that something was cookin here as well, and might have cause excessive current draw from the transformer causing it to buzz

[jokerman777]

7 minutes ago, Craig Sawyers said:

You also need to be careful on tightening torque. For correct assembly, the torque is surprisingly low. I bought a torque screwdriver for exactly this purpose.

What insulating bushes and washers have you used?

Have a look at  https://www.onsemi.com/pub/Collateral/AN1040-D.PDF

Basically overtightening can distort the semiconductor package and compromise thermal transfer, and that you have split the insulating bush suggests you are massively overtightening the fixing screw.

Craig  

As for torque I gotten a torque screwdriver and copied homework from one of the early post here and set it to 0.82Nm

5 minutes ago, sorenb said:

@jokerman777
Based on your post's I'd recommend disassembling both angle brackets, making sure all holes are properly deburred, and all the tappings are clean, also clean all semi's. Make sure the screws can go into the tapped holes using only finger force -  if not re-tap the hole.
Re-assemble, using regular compound (the white stuff).

As for the buzz, you stated to have observed a flash in the region of the negative rails, and showed a burn'ed screw from the positive rail, showing that something was cookin here as well, and might have cause excessive current draw from the transformer causing it to buzz

Sounds good, I will re-inspect all the mountings on the brackets I think I also do have a slight misalignment between the brackets and the semiconductor pins

[Craig Sawyers]

The recommended torque for the  AAVID shoulder washers is 5 to 6 inlb (0.565Nm to 0.678Nm) https://4donline.ihs.com/images/VipMasterIC/IC/AAVT/AAVTS01008/AAVTS01008-1.pdf?hkey=CECEF36DEECDED6468708AAF2E19C0C6

The TO220 specification for hole diameter is 0.139" to 0.161" - 3.53mm to 4.09mm. The spigot diameter on the 7721-10ppsg is 0.14" - so a potential clearance of 0.001" (25um) in diameter on the lower tolerance of the hole in the TO220 tab. Which is a push fit that needs careful alignment. But should be absolutely fine even in those circumstances.

But it could be worth reducing the torque on reassembly to about 5.5inlb (0.62Nm) to meet the AAVID requirements.

The torque could be with either dry threads or lubricated threads. Personally I'd go with lubricated threads, because any thread galling will really mess with transmitting torque to pressure between the semiconductor package and the heatsink.

 

[JoaMat]

On 3/11/2021 at 11:15 AM, JoaMat said:

Be aware. Recommended torque for 10PPSG is 0.565Nm to  0.678Nm. See attached Page from Aavid-Board-Level-Heatsinks-Catalog-2018-1507171.pdf for reference.

I use this setting

...and I don’t know how to convert Ncm from Nm 😢

Used above setting for a long time and haven't had any issues so far – knock on wood.

 Also, in JoaMat kitchen we don’t use any kind of thermal compound... , but we use 

[n_maher]

As far as I understand it the cm is just centimeters vs. meters so you metric folks should be well equipped to move decimal places accordingly.  There should be 100 Ncms per Nm.

[GeorgeP]

On 7/15/2023 at 11:38 PM, jokerman777 said:

Should signal ground meet chassis/earth ground in the PSU box?

I found that the center stand-off pad near the output terminals on the PSU board is connected to PSU board circuit ground and mounting a standoff there will make the chassis gnd to signal gnd connection, otherwise the two grounds will be separated in both boxes.

I found Kevin's old post in this thread mentioning "everything floats" so should I leave out this stand-off?

Kevin commented on how to ground the amp, at the end of this post. The picture doesn’t work any longer but there was a short wire going from the terminal block ground to the board stand-off hole. Make sure to remove some anodizing from the chassis so there is continuity among all the pieces, that particular standoff and the IEC. The chassis ground from the amp section would then go directly to the IEC ground tab. 

 

On 7/15/2023 at 11:38 PM, jokerman777 said:

Edited July 19, 2023 by GeorgeP

[simmconn]

@jokerman777If the length of the spigot on the PPS washer gets close to, or is longer than the combined thickness of the TO220 tab and ceramic insulator, the hole on the L bracket must be large enough to let in the excess length. Otherwise the PPS washer would be crushed when you tighten the screw. Too bad you can’t do threaded holes on the L bracket for the same reason, and have to use the nut - split washer - flat washer combo which is a pain in the neck to work with when servicing the unit.

[kevin gilmore]

pic?

[GeorgeP]

Yup, that’s it. 

[audiostar]

6 hours ago, GeorgeP said:

Kevin commented on how to ground the amp, at the end of this post. The picture doesn’t work any longer but there was a short wire going from the terminal block ground to the board stand-off hole. Make sure to remove some anodizing from the chassis so there is continuity among all the pieces, that particular standoff and the IEC. The chassis ground from the amp section would then go directly to the IEC ground tab. 

Exactly as I wrote above. Meanwhile Kevin uses a 10R resistor between circuit ground and PE ground (like on newer GRLVs), same as Krell. Nelson Pass uses a thermistor and the IEC recommends a real ground break circuit consisting of a HF power cap, power resistor and a rectifier (inverse parallel coupled power diodes). The idea is that this network must survive longer that it takes the mains fuse to rupture and for the arc to extinguish. This is also described on Rod Elliott pages in more detail. 

The essence thing is to connect both PE and circuit grounds in the PSU box only at the star point and have both grounds run separately to the amp's chassis.

Regarding pin 1 grounding which in this context plays a role as well and was asked above, here is a pic. It goes to the amps chassis and then to the PSU chassis (remember separated chassis ground wire run) and then to PE and is (in a differential amp setup) used with metal chassis basically for shielding only. 
In order to satisfy the various environments and wildly used grounding schemes and avoid loops, there is a new series of expensive Neutrik EMC XLR connectors using a cap between the connector's shell and pin 1. Here is a good read up on this.

Edited July 19, 2023 by audiostar

[simmconn]

Ideally the PE connection doesn’t carry any supply current and is separate from the PSU return wire. However in the DIY T2 case, they are connected together in the PSU already. If you connect them again at the amp chassis XLR input (the amp PCB has a ground lug for that), it defeats the purpose of running them separately. Unlike connecting independently powered devices shown in your diagram, the T2 amp circuit is powered by the T2 PSU. So unless you have measured data to prove otherwise, I think using the PSU ground return wire(s) to the amp to double as PE should be sufficient. In my personal experience with DIY T2, the ground wiring is not a significant contributor to hum.

[audiostar]

1 hour ago, simmconn said:

Unlike connecting independently powered devices shown in your diagram, the T2 amp circuit is powered by the T2 PSU. So unless you have measured data to prove otherwise, I think using the PSU ground return wire(s) to the amp to double as PE should be sufficient. In my personal experience with DIY T2, the ground wiring is not a significant contributor to hum.

Yes, right, the pic above shows a source component and another component probably with integrated supplies.

With a separated PSU, chassis ground and circuit ground are carried separately between PSU and amp and they connect to each other only in the PSU's case. Doesn't matter if T2 or any other class A power amp like the XS300.

Edited July 19, 2023 by audiostar

[simmconn]

I’m just saying, connecting like you described may not have the best hum rejection, and if we need to connect the PE and signal ground again at the amp chassis XLR input for better hum rejection, it defeats the purpose of running two grounds separately. That’s why I was asking if you have any measurement data to support your ground wiring scheme.

[jokerman777]

Thanks for the all the explanations and the pic!

For grounding I wasn't talking about connecting PE and signal ground again in the amp box, ik they should only meet once whether through a breaker or not no matter how many boxes of an amp. I was trying to understand why it's better that they met in the PSU box instead of meeting in the amp box at signal ground, I don't have good reasoning for the later other than I did it that way in single box amp and in my imagination the former extended ground loop between source and amp by the length of umbilical. I will do the former with the advice given, just purely curious, and maybe it doesn't really matter all that much between the two 🙂

 

In terms of Kevin's pic, I saw a resistor lead coming out of one of the circuit ground terminal block, wasn't clear where it goes to but based on what George said I suppose it goes to rear standoff slot which connected to the chassis, then returned to IEC earth as well as pass to amp box as chassis ground. Idk if it's bc my board is a newer version or not - the pad on that standoff drillout hole is already connected to the on board circuit ground, seems like I just need to put the standoff back there and make sure it's also connected to the chassis I'm all good? rest will be what I already have here: IEC earth, chassis, magnetic shields, output to umbilical.

13 hours ago, simmconn said:

@jokerman777If the length of the spigot on the PPS washer gets close to, or is longer than the combined thickness of the TO220 tab and ceramic insulator, the hole on the L bracket must be large enough to let in the excess length. Otherwise the PPS washer would be crushed when you tighten the screw. Too bad you can’t do threaded holes on the L bracket for the same reason, and have to use the nut - split washer - flat washer combo which is a pain in the neck to work with when servicing the unit.

Good point! Craig described the same to me in an earlier post that why I picked this insulation washer based on the length of its neck. I will double check when I re-mount everything 🙂

Edited July 20, 2023 by jokerman777

[JoaMat]

On 5/23/2023 at 12:10 PM, audiostar said:

Exactly because of their physical properties and being so thin, I wouldn‘t even think in using them in HV environments with naked tabs and inox bolts. Heck, I am not using them at all, especially if space permits it. 
 

Frst comments were already regarding those silicon insulators! Just no way one can tighten up the sands to specs at around 8-9 Nm with those without some damage .

 

21 hours ago, n_maher said:

As far as I understand it the cm is just centimeters vs. meters so you metric folks should be well equipped to move decimal places accordingly.  There should be 100 Ncms per Nm.

My torque setting of 82 Ncm equals to 0.82 Nm, or?

Upper post’s 8-9 Nm must equal to 800-900 Ncm

This link is very handy. https://www.convertunits.com/from/N-cm/to/N-m 😉

[justin]

20 hours ago, audiostar said:

Exactly as I wrote above. Meanwhile Kevin uses a 10R resistor between circuit ground and PE ground (like on newer GRLVs), same as Krell. Nelson Pass uses a thermistor and the IEC recommends a real ground break circuit consisting of a HF power cap, power resistor and a rectifier (inverse parallel coupled power diodes). The idea is that this network must survive longer that it takes the mains fuse to rupture and for the arc to extinguish. This is also described on Rod Elliott pages in more detail. 

The essence thing is to connect both PE and circuit grounds in the PSU box only at the star point and have both grounds run separately to the amp's chassis.

Regarding pin 1 grounding which in this context plays a role as well and was asked above, here is a pic. It goes to the amps chassis and then to the PSU chassis (remember separated chassis ground wire run) and then to PE and is (in a differential amp setup) used with metal chassis basically for shielding only. 
In order to satisfy the various environments and wildly used grounding schemes and avoid loops, there is a new series of expensive Neutrik EMC XLR connectors using a cap between the connector's shell and pin 1. Here is a good read up on this.

some of the quietest gear ends up accidentally built by people putting ground or chassis connections wherever they feel like it, even places they don't know about

btw almost all of the audiophile type gear I've ever seen is connecting Pin 1 directly to an audio ground plane. In some cases this is even bypassing the ground break circuit

[audiostar]

5 hours ago, JoaMat said:

My torque setting of 82 Ncm equals to 0.82 Nm, or?

Upper post’s 8-9 Nm must equal to 800-900 Ncm 😉

Yes, using 0.8Nm as you as well without issues. Looks like the above is missing a 0. 
But still a bit above the spec Craig posted. Probably +0.13Nm isn't a dealbreaker if everything else is right. 

[audiostar]

18 hours ago, simmconn said:

I’m just saying, connecting like you described may not have the best hum rejection, and if we need to connect the PE and signal ground again at the amp chassis XLR input for better hum rejection, it defeats the purpose of running two grounds separately. That’s why I was asking if you have any measurement data to support your ground wiring scheme.

I am not connecting PE (which is actually chassis ground at this point) to circuit ground in the amp's case.
pin 1 in the amp's case is tied to the chassis only (but not circuit ground) and goes then via the chassis wire to the PSU box and there it gets connected to PE and circuit ground only at the star point. This is described in one of AES documents, I have posted a link to somewhere earlier. 

[audiostar]

2 hours ago, justin said:

some of the quietest gear ends up accidentally built by people putting ground or chassis connections wherever they feel like it, even places they don't know about

True. It may get audible in large installations and studios with equipment powered off different outlets on separate phases, different earthing and zero potentials, etc. 

Doesn't matter in a home with 2 or 3 boxes, all powered off the same wall outlet. 

[audiostar]

2 hours ago, justin said:

btw almost all of the audiophile type gear I've ever seen is connecting Pin 1 directly to an audio ground plane. In some cases this is even bypassing the ground break circuit

Also true, and they all do it wrong. Rane and AES talk about this. Benchmark does it right, they even have a paper about this. Neutrik introduced the EMC XLR connectors to cure this to some extent. 
Best of course would be to properly use and interconnect the various grounds. 

[audiostar]

11 hours ago, jokerman777 said:

In terms of Kevin's pic, I saw a resistor lead coming out of one of the circuit ground terminal block, wasn't clear where it goes to but based on what George said I suppose it goes to rear standoff slot which connected to the chassis, then returned to IEC earth as well as pass to amp box as chassis ground. Idk if it's bc my board is a newer version or not - the pad on that standoff drillout hole is already connected to the on board circuit ground, seems like I just need to put the standoff back there and make sure it's also connected to the chassis I'm all good? rest will be what I already have here: IEC earth, chassis, magnetic shields, output to umbilical.

Sounds good.
IEC earth = PE, protective earth

[simmconn]

36 minutes ago, audiostar said:

I am not connecting PE (which is actually chassis ground at this point) to circuit ground in the amp's case.
pin 1 in the amp's case is tied to the chassis only (but not circuit ground) and goes then via the chassis wire to the PSU box and there it gets connected to PE and circuit ground only at the star point. This is described in one of AES documents, I have posted a link to somewhere earlier. 

Hmm, I wonder what kind of noise and hum level would be when you connect like that. Unless your source is true balanced pro gear with transformer-isolated output, most DACs and preamps have ground-referenced output stage. In other word, the +/- are not using each other as the return path, but pin 1 in that XLR cable. Once you include the PE connection and the signal ground connection in your common-mode return path, noise and hum inevitably gets included in the signal coming to the amp. Remember any amp has a finite common-mode rejection.
However the theory says, I guess at the end of the day, whatever works best in my/your system matters the most.

[audiostar]

Yes, a source truly differential.
You could completely untie pin 1 on both ends and only use a drilled wire for hot/cold pin 2 and 3.

[justin]

31 minutes ago, simmconn said:

Hmm, I wonder what kind of noise and hum level would be when you connect like that. Unless your source is true balanced pro gear with transformer-isolated output, most DACs and preamps have ground-referenced output stage. In other word, the +/- are not using each other as the return path, but pin 1 in that XLR cable. Once you include the PE connection and the signal ground connection in your common-mode return path, noise and hum inevitably gets included in the signal coming to the amp. Remember any amp has a finite common-mode rejection.
However the theory says, I guess at the end of the day, whatever works best in my/your system matters the most.

I usually buy the XLR jacks with separate Pin 1 and chassis connection and then can test it with jumpers on the input board. Nelson Pass said he connects pin 1 to audio ground, though it was 15+ years ago he posted it on diyaudio. I think also people who are using XLR/RCA adapters are under the assumption Pin 1 is ground

I can connect Pin 1 to chassis only but unless something has changed in 15 years, my experience has been that customers report fewer issues when it's connected to audio ground.

Edited July 20, 2023 by justin

[audiostar]

A differential signal doesn't need circuit ground as return path and the XLR/RCA adapters connect cold pin 3 to RCA signal ground while the RCA jacks (chassis side) keep ground isolated from the chassis. So all good, in case of proper implementation of course. 

Here is a pic from the AES paper on grounding for a metal (shielding) enclosure. One box design is shown, but the idea should be clear. 

Edit: 
Look for Neutrik (chassis side) jacks with complete metal ring (B-series) or metal case (D/DL-series) for the best possible shielding/mating with a metal enclosure. A/AA-series use a plastic case and make a single point only connection (via the screw) to the metal enclosure. 

 

Edited July 20, 2023 by audiostar