The ultimate DIY? A Stax SRM-T2!

[simmconn]

57 minutes ago, audiostar said:

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.

That's a common misconception about differential signals from signal integrity point of view. No output driver works out of vacuum. They are all power and ground referenced. Even with transformer isolation, there is imbalance in the windings and capacitive coupling between the primary and secondary, from windings to the core. In a practical world nothing is ideal.

The AES diagram clearly shows how the current 'I' could create a noise voltage drop between the input shield point and the star ground point, which adds to the input signal in a common mode fashion. That's why there is a note in the end that says, 'as close as possible to each other'. How about at one single spot, can you get closer than that?

Personally I wouldn't want to inject any dirty stuff from the ground of the source to IN- of my amp. So if I find out an RCA female to XLR male adapter connecting pin 3 to RCA signal ground while keeping pin 1 floating, I'd want to modify it.

[Pars]

How about the XLR shield going to chassis, with pin 1 being only signal ground? I know that probably goes against Rane, but in my mind, if you don't want signal ground connected to PE/chassis ground, you should be able to do it. I know there are Neutrik XLRs that do not connect pin 1 to chassis, but allow for shield to chassis.

[audiostar]

11 hours ago, Pars said:

How about the XLR shield going to chassis, with pin 1 being only signal ground? I know that probably goes against Rane, but in my mind, if you don't want signal ground connected to PE/chassis ground, you should be able to do it. I know there are Neutrik XLRs that do not connect pin 1 to chassis, but allow for shield to chassis.

The shell and pin 1 are (mostly) joined in the XLR connector (cable side). Some leave the shell unconnected but then those cables can not be extended as the shield gets broken. You could use in the cable connector shell a 100pF cap between both as does Neutrik in their EMC connectors to satisfy kind of all conditions. Otherwise: get to know your cables and your devices.

Yes, with XLR jacks (chassis side) with separate pin 1 and ground connections you can keep things easily "configurable" with a jumper inside the unit.

 

 

Edited July 21, 2023 by audiostar

[audiostar]

13 hours ago, simmconn said:

That's a common misconception about differential signals from signal integrity point of view. No output driver works out of vacuum. They are all power and ground referenced. Even with transformer isolation, there is imbalance in the windings and capacitive coupling between the primary and secondary, from windings to the core. In a practical world nothing is ideal.

A differential signal doesn't need to reference ground at all. This is not a misconception.

There is also a difference between

1. a fully balanced device or source (4 channels for stereo), which I talk about and
2. a single ended source (2 channels referencing ground for stereo) with only a differential output added to that, which you probably refer to (transformer isolation?)

13 hours ago, simmconn said:

The AES diagram clearly shows how the current 'I' could create a noise voltage drop between the input shield point and the star ground point, which adds to the input signal in a common mode fashion. That's why there is a note in the end that says, 'as close as possible to each other'. How about at one single spot, can you get closer than that?

Of course you could go to the SP directly (a must with unshielded enclosures). Does it matter in a small metal box? Probably not much if everything else is right. But yes, you could do that.

13 hours ago, simmconn said:

Personally I wouldn't want to inject any dirty stuff from the ground of the source to IN- of my amp. So if I find out an RCA female to XLR male adapter connecting pin 3 to RCA signal ground while keeping pin 1 floating, I'd want to modify it.

Pin 1 is not left floating, where do you see this? It connects either to a metal shielding enclosure and goes then to SP or directly to SP in a wooden box. Period.
But where it doesn't connect to is directly to circuit ground.

 

Here is the pic for unshielded enclosures:

Edited July 21, 2023 by audiostar

[justin]

Reality is like this though

[JoaMat]

A wise person once told me - the only sure thing is that it is uncertain (google translated from Swedish).

[JoaMat]

On 4/15/2023 at 1:04 AM, JoaMat said:

DIY T2 amplifier with 300B tubes.

Amplifier is built on Kevin’s board. Only modification is decreased high voltages, +/-400V instead of +/-500V. 

Homemade 300B to EL34 adapters connected to four Traco 5V switched PS for filament power. 

I only replaced the EL34 with 300B along with the filament power supply. No other adjustments. It works – I’m in no harry to change back to EL34.

Have used the 300B tubes with DIY T2 for three months now. First two months with an amplifier with all original transistors and the last month with an all-modern transistors’ amplifier. 

Only issue so far is a failed small solid-state relay in filament supply. Otherwise, I find the DIY T2/300B pleasant.

[jokerman777]

I just went through a couple more rounds of PSU trouble shooting.

First time replaced 4 10m90s in the +500V/bias supply where short to bracket happened to one of them, I admit I didn't measure other components other than rectifier diodes, power on and 2 of the 10m90s exploded. (before explosion I was measuring bias voltage and it reads fine)

Took it apart and measured more things - D3 zener was dead to short, didn't know if it's the cause or the consequence of somethings else. I replaced all 10m90s again plus the dead zener, op27 and ref102, leaving the hard to source C3675 and FQP8N80C in question. Power on with no load again while monitoring power consumption - in-rush reaches 200W and didn't drop within a short period as I expect it to so I quickly shut it off, something smells funny too.

In both rounds I powered on all other supplies individually beforehand and output measured fine.

I think I'm going to take it apart again and measure more thoroughly including all the resistors and likely will try just replace all the semiconductors in that rail once I find the harder to get replacement parts. I know it's not the right way to test a device but I measured D-S resistances of the fqp8n80 in circuit and it gives me a stable ~6Mohm which is different from the same device measured in other rails, so something is funky around there.

more dummy questions:

Is passing forward bias test enough to say that zener diodes are fine?

As I was monitoring power consumption of the PSU with no load, each HV rail seems to consume about 10W when stabilized, is that more or less expected? my transformer still lightly buzzes.

 

[sorenb]

The PSU's I have at hand consumes ~25W unloaded

Exploding 10m90's indicate massive current draw ... have you checked for any carbonization in the vicinity of those?

[jokerman777]

6 hours ago, sorenb said:

The PSU's I have at hand consumes ~25W unloaded

Exploding 10m90's indicate massive current draw ... have you checked for any carbonization in the vicinity of those?

Thanks, it seems my PSU consume twice amount of power as yours.. powering up only all negative rails it's already 22W, adding +250V it becomes 33W so I expect when I get +500V/bias working it will be 40~50W

The exploded 10m90s were Q2 and Q3 (Q1 was unknown). I think the shorted D3 is the cause of Q3's death, as for Q2 I don't know exactly yet, possibly the FQP8N80C (Q4) is also dead from the first time failure due to shorting of Q1.

[sorenb]

Sounds like it's not only the +500V having a problem ...

Have you measured the drop across the "current setting" resistors for the various 10m90's? 
Summing up those per rail, might shed some light on the matter ...

[jamesmking]

RIP MY T2

ever had a day when everything goes wrong and you wished you had not got out of bed....

well I just wrecked my T2. Don't have the energy or will power to even troubleshoot it...

what happened.

I was slowly over time getting a DC offset on both channels, I would correct with the trim pot until I ran out of travel. My first thought was the valves were getting old. Yesterday I decided to open up on the amp case and check the virtual batteries and voltages.... Well the -560V rail was -600V... I spent most of yesterday looking for shorts, diode checking the transistors and doing the usual troube shooting. Since the psu was basically working but not regulating the -560V I also looked a live voltages... To cut a long story short I discovered partly by logic and partly by accident that even maxing out the trim pot only gave me -9.6V on the 20K resistor on the voltage set string. The voltage reference was correctly outputting 10V. I checked the trimmer and it would change in resistance over its travel. So my conclusion was with the two 540K resistors had drifted up in value and therefore reducing the voltage across the 20K resistor or else something was stealing voltage from the 20K resistor...

today I did more testing and desoldered one end of each of the 540K resistors and measured them... 580K and 700K... I remembered that I had replaced all the voltage set resistors with TE 0.1% 1/4W a while ago... I checked the spec sheet and they are rated at 250V.. too little for the job and I think this is what caused them to drift high. My guess is the 700K resistor started to drift high, got more voltage drop across it which naturally increased the drift. I replaced them with Xicon 1/2W 1% 350V and for the -560V rail immediately started regulating again.

job done back to listening right.... RIGHT....????

After some testing I connected the psu back to the T2 and the led on both channels close to the -260V terminal did no light. But with no load the -260V rail was rock solid. Connected to a  T2 the rail only provided +75V on top of the -560V rail rather than the +300V it was designed to give. I guessed the 2N3904 in the -260V rail had spontaneously decided to fail in the current limiter and so decided to replace it. The 2N3904 I desoldered measured on a peak dca75 as shorted so I was sure I had found the fixed the problem....

again testing without a load everything seemed fine.

So back to listening now... right... well...

Then I did something really stupid. I thought the psu was off and connected the high voltage cable from the psu to the amp (the low voltage cable with the heater supplies was not connected) there was immediately sparking noises and I quickly turned off the psu.

I tested the psu without load - all rails seemed good.

I connected the psu to the T2 and powered it up. All the voltage rails where at the correct levels on the T2 But I had massive ballance issues between the + and - halves effecting both channels. All 4  virtual batteries now only read about 550V, (all the leds light so that's something)...

SH*T SH*T SH*T DAMN DAMN

I guess next step is to test all the valves, take the amp apart and hope the 2SK216 and 2sj79 are ok.

I have one complete set of spare 2SK216 and 2sj79 and do have a spare set of amp pcbs so I am debating if it might be simpler just to build a new set of amp boards rather than try to troubleshoot and salvage what I have.

today I simply should not have got out of bed.

Edited August 2, 2023 by jamesmking

[jamesmking]

well all the el34s survived, right hand channel input 6922 is bad. Time to go to bed and cry.

 

[jokerman777]

6 hours ago, sorenb said:

Sounds like it's not only the +500V having a problem ...

Have you measured the drop across the "current setting" resistors for the various 10m90's? 
Summing up those per rail, might shed some light on the matter ...

Thanks for the suggestion yeah I will probe the voltage drop across them next time I probe the resistors. maybe the line voltage plays a role here too? My trafo primaries are spec'ed at 115V and I believe my mains is above 120V on average, also need to check out how accurate that power measurement thingy is that I borrowed from my friend.

[jamesmking]

parts stuffing completed on two new mostly modern T2 amp boards. Now waiting for the ultrasonic cleaner to warm up. Then drying, mounting on L bracket and heatsinks... and then *gulp* the stressful part. power on and testing... *fingers crossed* I have no more spare NOS parts, only one 10V voltage reference left, no spare valve sockets etc... so this had better work.

 

[JoaMat]

Great! When the RIP MY T2 was announced, I said to myself - James will have a new T2 ready in 10 to 14 days… 

Now only three days has elapsed. No rush, James.

[jamesmking]

1 hour ago, JoaMat said:

Great! When the RIP MY T2 was announced, I said to myself - James will have a new T2 ready in 10 to 14 days… 

Now only three days has elapsed. No rush, James.

I moved flat 3 weeks ago (downsized - long story) and the T2 is literally the only stax amp I have on hand here and is my only source of sound! (The mini T2s and blue hawaiis are in storage. Megatron still has no case for the psu). So its a priority project.

This is exactly why I have a policy of either building two or each amp or keeping enough spares in stock to build a second one.

(The reason I originally did not build a second mostly modern T2 was I wanted to build a fully modern T2 using alternatives for the 2SJ79 and 2SK216 but I had not got around to checking the pcb layout and getting pcbs spun).

PCBs are currently drying...

Edited August 5, 2023 by jamesmking

[jokerman777]

4 hours ago, jamesmking said:

parts stuffing completed on two new mostly modern T2 amp boards. Now waiting for the ultrasonic cleaner to warm up. Then drying, mounting on L bracket and heatsinks... and then *gulp* the stressful part. power on and testing... *fingers crossed* I have no more spare NOS parts, only one 10V voltage reference left, no spare valve sockets etc... so this had better work.

 

I had previously considered ultrasonic cleaner for the boards but curious would it mess up with the thermal paste applied? 

Anyway good luck and hope everything will work out fine! 🙂

[jamesmking]

32 minutes ago, jokerman777 said:

I had previously considered ultrasonic cleaner for the boards but curious would it mess up with the thermal paste applied? 

Anyway good luck and hope everything will work out fine! 🙂

i clean before applying paste. I also don't want to contaminate the cleaning solution. The aim is to get rid of the flux residue, which can be acidic and potentially can breakdown and conduct at the sort of voltages we have. 

Edited August 5, 2023 by jamesmking

[audiostar]

Left with two unused Amphenol PT02A-14-15P connectors (T2, PSU-chassis-side), anyone in need let me know. They were recently bought from Mouser. 

Edited August 5, 2023 by audiostar

[jamesmking]

It lives! the son of the mostly modern (RIP) T2 was powered up for the first time this morning (not in a case and with cheap valves) and looks good.

All virtual batteries within 0.6V of 740 volts with no adjustments to the trimmers.

DC offset after warm up <4V without any adjustments to the trimmers.

DC ballance <0.03V 

power consumption as expected.

I did have a bit of a scare. When powering on with my variac for the first time the led next to the EL34 was very dim compared to the others and all the leds where less bright than my original build. Then I remembered that I used different leds in this build and they are in general less bright. I don't think I can be bothered to remove these leds and replace them with the same as the RIP T2.

Latest update in the rip T2 troubleshooting, diode testing and comparing the silicon to the son of T2 it looks like one of the J79s is dead... which is a pain since I have no replacements. This dead J79 is on the same channel as the failed 6922. The op-amps and voltage references tested fine.

This does not explain why the other channel also had a massive ballance issue between + and - outputs but at least its some progress.

Edited August 8, 2023 by jamesmking

[JoaMat]

Run out of k216/j79? Go TTA004B and TTC004B. To my knowledge half a dozen T2 with “modern” components including TTx004B have been built in Sweden and owners/users seem to like them.

[jamesmking]

1 hour ago, JoaMat said:

Run out of k216/j79? Go TTA004B and TTC004B. To my knowledge half a dozen T2 with “modern” components including TTx004B have been built in Sweden and owners/users seem to like them.

I'm thinking about that. I didn't realise there were so many around. I guess I could cut the legs short and run high voltage silicone wire to the appropriate through holes....

Edited August 8, 2023 by jamesmking

[jokerman777]

Hard to find FQP8N80C replacement now as it's experiencing some long lead time.

I bought off the 3pcs stock of FQPF8N80C from bdent, and also have ~10pcs of STP8NK80ZFP I bought originally as backup plan, both comes with isolated package and has lower overall power consumption rating.

On the surface for now I only need one to fix the +500V/bias supply first, I'm inclined to just use STP8NK80ZFP as I have a slight OCD of if replacing one works I want to replace the other 3 too....

I had success using these in the KGSSHV PSU, or is there good reasons that I shall stick with the Fairchild FQP device?

[justin]

what about the 90C version which is 900v instead of 800v