The ultimate DIY? A Stax SRM-T2!

[GeorgeP]

56 minutes ago, Mach3 said:

Ok found out the reason why fuse blown. primary and secondary winding was wired incorrectly to the terminal block.

Grey and brown mixed around.

I need to replace the transformer, did a continuality test on P1 and it failed.

I tried to contact [email protected] at Toroidal Engineering Co.

But GeorgeP inform me it looks like he's close door since COVID.

Anyone got a spare one for sell?

@Kerry do you mind sharing what specs you're using with your transformer. Alternative route I get toroidy custom make one base on the correct current specs below. They made a custom one for my KGSSHV.

Kevin mention that the total of the primary winding should be 90watts, so would 0.5 amp each for P1 and P2 be enough.

P1 = 120V, (unsure of current rating)

P2 = 120V, (unsure of current rating)

S1 = 15V - 0 - 15V, 1 amp (Kevin provided these rating)

S2 = 6.3V,  5 amps (Kevin provided these rating)

S3 = 6.3V, 5 amps (Kevin provided these rating)

S4 = 6.3V, 2 amps (Kevin provided these rating)

S5 = 6.3V, 2 amps (Kevin provided these rating)

 

 

 

As discussed in PM, all three toroids are miswired. Someone has tinkered with the PSU and re-wired the toroids incorrectly. This amp was built in 2016 with Houlden's transformers, which are what I have in my T2. Toroidy may be the cheapest option for you given you are in Australia. Alternatively Primrose/Sumr can supply the required transformer. Hopefully your only issue the transformer.

[Mach3]

Thanks George,

As mention in PM seller stated it was working as the way I got it. Which makes sense because just swapping the brown and the grey still provided 120V with the jumper B on 1-4 & 3-6. It just doesn't give out 240V when I shorted out 3-4 because the two primary 120V wouldn't been in series.

[joehpj]

@kevin gilmore

Is there any benefit to use DC supply for the front end 6922?

The original design was AC and I was thinking if I can further decrease the AC induced noise by using DC supply.

If I am going to use DC supply, should I reference LT1083 to GND or using the 47R on amp board will be just fine?

Thanks!

[kevin gilmore]

dc filament power supplies normally cause way more trouble than they fix. and you definitely want that supply floating and leave in the 47 ohm resistors.

[Kerry]

On my prototype build, I had some hum issues.  I tried DC supplies on the front-end and they did not help.

What helped was how I routed the power / filament supply wires.

You should have no hum on the T2.  

[spritzer]

51 minutes ago, Kerry said:

You should have no hum on the T2.  

This just makes the original Stax T2 sad...   

That hum's like there is no tomorrow. 

[Craig Sawyers]

The unregulated power supply on the original T2 is its real Achille's heel. Other than thermal management that is.

One thing to watch with T2 transformers is that the heater windings to the pentodes float at 500V. So there needs to be adequate insulation barriers inside the heater transformer to cope with that, and adequate insulation in the umbilicals. With mine I ran the floating heater wires inside the umbilicals inside a glass fiber sheath.

[Rinat]

@kevin gilmore, what is the point in using two pairs of LEDs: D4-D5 and D8-D9 ?

Woudn't it be better to use one pair for both halves like it is done in the input stage ? 

It gives us mutual substraction of a LED's noise for example. 

Edited August 30, 2022 by Rinat

[kevin gilmore]

because that is the way the original did it. certainly possible to use one pair of led's.

[simmconn]

My take is that 2 LEDs in the string allows a higher emitter resistor value and therefore better local NFB. Sim shows that the effective impedance of the CCS roughly doubles with 2 LEDs vs 1. That come at a cost of reduced voltage compliance range and slightly increased power consumption. They are not a big deal for high voltage CCS, though.

[Rinat]

21 minutes ago, simmconn said:

My take is that 2 LEDs in the string allows a higher emitter resistor value and therefore better local NFB. Sim shows that the effective impedance of the CCS roughly doubles with 2 LEDs vs 1. That come at a cost of reduced voltage compliance range and slightly increased power consumption. They are not a big deal for high voltage CCS, though.

No no, I don't want to use one LED instead of two. 

I want to use the same pair of LEDs to feed both CCSs, connecting, for example, D8 and D9 to both Q11 and to Q6.

[Rinat]

Edited August 30, 2022 by Rinat

[simmconn]

Ah, okay. Sorry I misunderstood the question. It’s certainly possible, but usually the two CCS blobs are physically away from each other. IMHO given the other potential component mismatches, sharing the same LED string may not make a big difference in the current matching between the CCS.

[Rinat]

29 minutes ago, simmconn said:

Ah, okay. Sorry I misunderstood the question. It’s certainly possible, but usually the two CCS blobs are physically away from each other. IMHO given the other potential component mismatches, sharing the same LED string may not make a big difference in the current matching between the CCS.

I am trying to make all the components to be in pairs 🙂 

I don't think that the difference will be big, especially due to matched LEDs. But.... why not ? I see a very minor advantages with no disadvantages.   

Oh... and I don't think about current matching, but only about the noise of the LEDs. 

Edited August 30, 2022 by Rinat

[Rinat]

Still waiting for last parts.... 🥱

And waiting for them to be delivered, I and my friend Dmitry decided to tests  a performance of the active battery at hight frequencies. 

The scheme of the stand has been given by @Kerry. Thanks!

We just slightly modified it. 

Safety note: we supplied the battery by a source completely decoupled from the mains.

That is why our generator is alive. Be careful. 

 

All the batteries are set to 740V. 

The first battery is the original one with 2SA1486 -> 2SA1413 and 2SK246 -> 2SK208 and 2SC3381 -> HN4C51J. The first two pairs are the same transistors in different cases. 

Results of the measurements: 

1. 130 Hz. Power off. 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514651.jpg

   Amplitude is about 200 mV 

2. 130 Hz. Power on. 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514652.jpg

   Amplitude is about 100 mV 

3. 1000 hz. Power off. 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514653.jpg

   Amplitude is about 200 mV 

4. 1000 hz. Power on. 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514654.jpg

   Amplitude is about 140 mV 

5. 10 kHz. Power off. 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514655.jpg

   Amplitude is about 90 mV

6. 10 kHz. Power on. 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514656.jpg

   Amplitude is about 100 mV 😨

The questions: Shouldn't there always be zero volts AC ? Shouldn't the battery always stay at 740V after powering on? 

It looks like the active battery in the T2 scheme suppresses low-frequency signal. 

It is a problem ?

We decided to test another version of the active battery.

Edited September 5, 2022 by Rinat
fix a typo

[Rinat]

The second version of AB was taken from this post. 

 

We changed R43-R44: 140k -> 150k, MPSA06 -> HN4C51J and FJPF2145 -> 2SC3675 selected with the best hfe. 

Why ?

1. We decided to slightly lower supply of LT1021 - it is still enough for normal work of this voltage source. 

2. HN4C51J has better hfe and also I prefer to use double transistor in one case in current mirrors. 

3. 2SC3675 has lower input capacitance and better hfe. 

STN9360 has much higher current gain and lower capacitance in comparison with 2SA1486/2SA1413 and we thought that performance of this battery should be much better. And so it turned out: 

1. 120 Hz, power off 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514659.jpg

   Amplitude is about 200 mV

2. 120 Hz, power on 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514660.jpg

   Amplitude is about 45 mV 

__________________________________________________________________________________________

3. 1000 Hz, power off

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514661.jpg

   Amplitude is about 200mV 

4. 1000 Hz, power on 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514662.jpg

   Amplitude is about 80 mV 

__________________________________________________________________________________________

5. 10kHz, power off 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514663.jpg

   Amplitude is about 75 mV

6. 10kHz, power on 

   https://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514664.jpg

   Amplitude is about 50 mV

__________________________________________________________________________________________ 

The new battery performs much better! 

But it is still is far from ideal. Why ?

At the next day Dmitry noted that capacitance of STN9360/2SA1486(Q17p/Q17) and really high resistance of the voltage divider (R33-R34) make a low-pass filter. Could it be the cause of problem ?

He added one 2200 pf teflon capacitor between the low end on R34p and the base of Q17p transistor. 

Results of the measurements are in the post below 

Edited September 5, 2022 by Rinat

[Rinat]

The new battery with added capacitor: 

1. 120 Hz, power off 

   http://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514671.jpg

  Amplitude is about 200 mV 

2. 120 Hz, power on 

   http://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514672.jpg

   Amplitude is about 20 mV (!)

__________________________________________________________________________________________

3. 1000 Hz, power off

   http://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514667.jpg

   Amplitude is about 200 mV 

4. 1000 Hz, power on 

   http://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514668.jpg

   Amplitude is about 10 mV or less (!!) 

__________________________________________________________________________________________

5. 10 kHz, power off 

   http://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514673.jpg

   Amplitude is about 100 mV

6. 10kHz, power on

   http://ixbt.photo/photo/406668/17916TMfujOMyVu/Hgj7GE8yHX/1514674.jpg

   Amplitude is about zero (!!)

__________________________________________________________________________________________

Adding the bypass capacitor makes the active battery much closer to an ideal "voltage drop" element. I think that increasing the capacitance to 4000 or 5000 pf will decrease the measured voltage even in the case of low frequencies like 100hz.

[Rinat]

A short table with results: 

 

It seems that active battery without the capacitor increases amplitude of signal at the base of Q26 with frequency, especially the original battery. 

Has it been done intentionally? I just can't see a reason. This imperfection of AB probably decreases stability at HF and slightly lowers bass. 

Measurements have been made with Tektronix THS720 and P5102 voltage probe. 

I can't wait to measure these batteries in the amp itself

Edited September 4, 2022 by Rinat

[jamesmking]

2 hours ago, Rinat said:

A short table with results: 

 

It seems that active battery without the capacitor increases amplitude of signal at the base of Q26 with frequency, especially the original battery. 

Has it been done intentionally? I just can't see a reason. This imperfection of AB probably decreases stability at HF and slightly lower bass. 

Measurements have been made with Tektronix THS720 and P5102 voltage probe. 

I can't wait to measure these batteries in the amp itself

 

Nice work, its always good to see progress and new ideas for the T2.

Just to clarify. I did not design the mostly modern T2 or its battery. I simply created a series of posts on my build and posted my modified gerbers and updated and clarified schematics. I have been trying to collate the information scattered over years and years of posts into one or two long posts to provide a detailed starting point for new builders. I would not want to take credit away from the people who actually did the hard work of designing and testing.

Your battery looks promising. If I could suggest, PTFE caps of sufficient voltage are quite large and not easily available new. I think it would be possible to find a place on the amp pcb for 1KV film caps e.g. wima. I would also be interested in the effect on the existing mostly modern T2 battery if just the cap is fitted - since such a modification would be very cost effective for existing mostly modern T2s. If the cap mod is shown to be stable and reliable I would be happy to update my existing post to include it in the schematic as a option and update the gerbers to make a place for adding the cap. 

regards

James

P.S independently of your effort MLA and I think Joamat have been working on a variant of the mostly modern T2 amp board that replaces the unavailable new 79 and 216 transistors with tta004 and ttc004, which if they prove stable, could result in a T2 only using current production components. (the pinout of the tta/ttc is different to the originals but they have not so far had to make any other changes).

Edited September 4, 2022 by jamesmking

[Rinat]

1 hour ago, jamesmking said:

Nice work, its always good to see progress and new ideas for the T2.

Thank you! The idea is also not mine, but my friend's. 

1 hour ago, jamesmking said:

Just to clarify. I did not design the mostly modern T2 battery. I simply created a series of posts on my build and posted my modified gerbers and updated and clarified schematics.

Oh, I didn't know this. Anyway this is great work, that made my labor much easier. Thanks! 

Now I am building 2 amps: 

1. The original T2 with as less changes as possible.

2. The possibly better version of T2. In this version all the upgrades would be made not to use modern parts where it is possible but only to improve the amp. For example STN9360 is better in many places than old and rare 2SA1486 and vice-versa for the FJPF2145 and 2SC3675 pair. 

And this amp is based on the scheme that you posted here. Thank you very much for that. And of course thanks to all the authors of ideas! 

1 hour ago, jamesmking said:

Your battery looks promising. If I could suggest, PTFE caps of sufficient voltage are quite large and not easily available new. I think it would be possible to find a place on the amp pcb for 1KV film caps e.g. wima. I would also be interested in the effect on the existing mostly modern T2 battery if just the cap is fitted - since such a modification would be very cost effective for existing mostly modern T2s. If the cap mod is shown to be stable and reliable I would be happy to update my existing post to include it in the schematic as a option and update the gerbers to make a place for adding the cap.

Dmitry used the PTFE cap only because we have only 0.001 uF(too small) and 0.047 uF(too much IMHO) PP caps, and to make a funny photo where the cap's size is comparable with the whole board 🙂 .  

I am sure that 3000-4000 pF x 1000V mica or polypropylene caps would be also good here. 

They are cheap and small enough even to upgrade the original T2 amps using daughter boards. For the old-style battery I would increase its value even more due to higher input capacitance of 2SA1486.

But of course, before suggesting it to anyone I need to check this update in my amp or ask someone to check it in his T2. Just in case, I've attached the updated scheme of AB with the cap 🙂. 

1 hour ago, jamesmking said:

P.S independently of your effort MLA and I think Joamat have been working on a variant of the mostly modern T2 amp board that replaces the unavailable new 79 and 216 transistors with tta004 and ttc004, which if they prove stable, could result in a T2 only using current production components.

This sounds nice and I would like to contribute to this project 🙂 . 

But I thought that Joamat collected all his thought and updates in mini-T2. Am I wrong ? 

Edited September 4, 2022 by Rinat

[jamesmking]

1 hour ago, Rinat said:

 

But I thought that Joamat collected all his thought and updates in mini-T2. Am I wrong ? 

 

I would not want to speak for Joamat, but the impression I got was that the mini T2 was an attempt to simplify the complex design and make it as small as possible by going surface mount whenever possible. 

Ther is not a lot of space around Q17n but I think a 13mm by 4mm 1KV 2200pF (10mm lead spacing) film cap like a wima MKP1O112203C00JSSD will fit and there are film caps of the same size and rating ranging from 1000pF to 6800pF.

 

The area around Q17p is more problematic due to an extra track which results in one tracking having to be rerouted. Channel 1:

channel 2:

 

 

 

 

Edited September 4, 2022 by jamesmking

[Rinat]

1 hour ago, jamesmking said:

The area around Q17p is more problematic due to an extra track which results in one tracking having to be rerouted.

🤔 

Hmmm, did not think about tracing, because am doing AB as a separate small board. 

 

[simmconn]

The so-called 'active battery' is a shunt regulator IMHO. I would recommend that you simulate any circuit changes for performance (shunt impedance) and stability. The original circuit does not seem to have enough phase margin and can use some compensation. My calculation shows either adding a 1.2pf cap across the two 820k feedback resistors, or increasing the shunt capacitance to 0.1uF would help. They each have their pros and cons. The calculated capacitance is so small that parasitics can throw the circuit in either way and make it 'touchy'.

Using 2.2nF compensation cap would significantly increase the loop gain in the mid- to high-audio frequency range and reduce the output impedance as you have observed, but also make the stability worse.

Since the active battery is part of the signal chain, I would rather shoot for flat impedance across the audio range and as small phase shift (group delay) as possible. I'm not sure what your test gig looks like. It would be important to test it with a source impedance similar to the real circuit. My sim tells me that the original circuit when properly compensated can be pretty flat within 20KHz and has -3dB point over 200KHz.

On the other hand, the LT1021 due to its high-ish operating current requirement, does not necessarily bring much gain to the overall performance of the active battery compared to the LED strings.

Edited September 5, 2022 by simmconn
Minor corrections

[Rinat]

3 hours ago, simmconn said:

I'm not sure what your test gig looks like.

[Rinat]

3 hours ago, simmconn said:

The so-called 'active battery' is a shunt regulator IMHO. I would recommend that you simulate any circuit changes for performance (shunt impedance) and stability.

Yea and they often use such a capacitor to decrease output impedance at high frequencies if a voltage divider uses high resistance and it forms RC chain with control transistor's capacitance: C3 in this scheme for example. 

3 hours ago, simmconn said:

Using 2.2nF compensation cap would significantly increase the loop gain in the mid- to high-audio frequency range and reduce the output impedance as you have observed, but also make the stability worse.

Since the active battery is part of the signal chain, I would rather shoot for flat impedance across the audio range and as small phase shift (group delay) as possible. I'm not sure what your test gig looks like.

1. I thought about the stability, but we don't see any instability or generation during the measurements. 

2. Currently the impedance of the AB is not flat at all. In my measurements it increases almost twice from 100 HZ to 1 kHZ. 

But I understand your point, thanks. I will try to simulate or even measure a phase shift too. 

3. I will make a simulation after insuring that all the component in my library has proper models including parasitic capacitances. Do you have spice models for transistors?

But the final check is always a measurement 🙂 . I will probably also change the test circuit by changing the R5 resistor with 2SK216 transistor and modulating it by AC signal.  

4 hours ago, simmconn said:

On the other hand, the LT1021 due to its high-ish operating current requirement, does not necessarily bring much gain to the overall performance of the active battery compared to the LED strings.

Why does the operating current matters here ? We always feed the battery by the same 5mA current.