The ultimate DIY? A Stax SRM-T2!

[MLA]

On 9/4/2022 at 6:54 PM, 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. (the pinout of the tta/ttc is different to the originals but they have not so far had to make any other changes).

It's Joamat who is the brains behind the substitutions, not me. They seem to work fine though. I don't have a scope at hand so can't verify performance on any detailed level, but I'm listening to a T2 with tta/ttc004 replacements right now (no other modifications) and it does sound very good. Have some problems with a buzzing HV trafo that needs sorting, but the amp boards themselves seem stable so far.  

[nopants]

is it just the pinout that's different? do the passives need any swaps?

[MLA]

Just different pinout; no passive swaps so far.

Will do some measurements in the coming weeks to check that all current sources have the right outputs, etc, but I don't expect any big surprises. JoaMat has run 1220/2690 replacement editions of his T2:s for a long time without issue as far as I know, and the tra/ttc004 pair seems quite on par with that  combo. 
 

Measured breakdown voltage in the units I have is around 210V (rated 160V), compared to about 230V for 79/216 (rated 200V) if I remember correctly from somewhere in this thread, so not quite the same but close.

[JoaMat]

Just move the two right legs on step to the left and swing the left leg all the way to the right…

On 10/23/2016 at 10:11 AM, JoaMat said:

One could say that the legs of BJT is shifted one step to the right. So I just bent the legs accordingly and put a pin header in the left hole and then mount the transistor with desired torque . As I do it the pin header is hold in place by the crossed leg. Solder and it’s done. Few pictures below. Hope they show how I did it.

 

 

 

 

I've modified one T2 and one Blue Hawaii this way. And it works fine.

 

[jamesmking]

So if my brain is not totally scrambled (which is possible its my first week back lecturing).

KSC2690 or TTC004 NPN replaces 2SK216 N channel. Base-> Gate, Emitter -> Source, Collector -> Drain

and 

Ksa1220 or TTA004 PNP replaces 2SJ79 P channel, Base-> Gate, Emitter -> Source, Collector -> Drain

which gives modern T2 circuit (green replacements original moved to the side) 

Edited September 28, 2022 by jamesmking

[JoaMat]

Thanks James, your schematics gives a clear picture how to switch to KSA1220 and KSC2690. Unfortunately, it seems that KSA1220 is out of business. Fortunately, TTA004 and TTC004 can replace KSA1220 respectively KSC2690.

 

[jamesmking]

1 hour ago, JoaMat said:

Thanks James, your schematics gives a clear picture how to switch to KSA1220 and KSC2690. Unfortunately, it seems that KSA1220 is out of business. Fortunately, TTA004 and TTC004 can replace KSA1220 respectively KSC2690.

 

*Sigh* why can't we have nice transistors?... I have updated the schematic to show both KSA/KSC and TTA/TTC series transistors.

I'm working on changing the gerbers so that leg twists will not be needed 🙂

One channel done, I think, needs checking...

 

Edited September 28, 2022 by jamesmking

[audiostar]

53 minutes ago, jamesmking said:

I'm working on changing the gerbers so that leg twists will not be needed 🙂

James, make the batteries exchangeable and go to daughter boards as well.
The board is too big and too expensive to justify itself for a transistor swap alone. 

Edited September 28, 2022 by audiostar

[jamesmking]

8 minutes ago, audiostar said:

James, make the batteries exchangeable and go to daughter boards as well.
The board is too big and too expensive to justify itself for a transistor swap alone. 

I completely agree that if you have T2 pcbs already its not worth getting new ones. I'm thinking gerbers with minimal changes for new builders who want a build and forget solution only using available transistors. Serial/habitual/continual modifiers can always edit the gerbers of my or others T2 pcbs.

[luvdunhill]

Like this?

 

[JoaMat]

Yes, very much.

[bbest]

I have 4 sets of T2 boards of morden jamesmking's design. If smb wanted just pm me.

[JoaMat]

Almost a year has elapsed since I reduced the 300V section to 150V. The amplifier has worked well, and I haven’t noticed any sonically differences. So, some time ago I decided to make the same change on my modified T2 and its original T2 PSU we have at our summer cottage. But unfortunately, it was not possible to bring the voltage down to 150V. 190 – 200V was the lowest I got. I’ve had a Golden Reference power supply for the T2 in mind for a long time. Now I had to do something the get the desired 150V section with the original T2 PSU…

Made a daughter board with GRHV regulator for the original T2 PSU. Works like a charm. 

[audiostar]

20 minutes ago, JoaMat said:

Made a daughter board with GRHV regulator for the original T2 PSU. Works like a charm. 

Congrats, JoaMat. 
Possible the post the schematics, maybe a gerber as well? Thanks!

[JoaMat]

Here a schematics.

Four connections to main board. The raw DC is taken before the two 10m90s current limiters. Better would be after the limiters but no free hole for it.

Works like a charm I wrote. Yes, charming it is when it works. When it suddenly stops working and give 13V instead of 150V – that is more annoying than charming. It has happened two times so far since my last post. Not sure what’s wrong but I change Q5 (dual small PNP) with a new piece and then it works for couple of hours. Now I’m just waiting for the next breakdown.

Suggestions what might cause the problem are welcome! 

[Kerry]

The golden reference supplies work very well in the T2.  I've been using it for a while now with no issues.  You do need to take the raw DC after the 10m90s current limiters though.  I'm using a single IXTP08N100D to limit the current for each supply.  You should be able to get away with a single 10m90s for your 150V supply if you adjust the set resistor accordingly. The current draw there should be around 50mA.

I've added a protection diode (1N4007) with the cathode connected to the base of Q6 and the anode connected to 0V.  I also got rid of D3.

Here's my T2 supply (I've been meaning to post this for a while) - all the high voltages are Golden Reference supplies (even the -60V using a negative version of the GRHV79xxx).  I've very pleased with how it performs

 

Good luck!

Edited December 12, 2022 by Kerry

[JoaMat]

Thanks Kerry.

 I just had to wait four hours for it break down this, the third, time. Same procedure as last time – replaced HN4A51J. Also reflowed all solder points – hot air – hope this will help...

[Kerry]

Hope it works for you now. If not I’ve got some other ideas. 

[simmconn]

@JoaMat Did you measure the removed part and see which transistor and which junction failed? If it takes a while to fail, chances are the power dissipation is over spec. Something else could be leaking that causes excessive current through the part. An IR camera would be handy in troubleshooting problems like this. The STN0214 may have a different Cob than the original part KSC5026, and the closed loop gain of the regulator is also different (20x vs 40x), the compensation capacitor values may need to be adjusted to maintain stability.

[Kerry]

I use PBHV2160Z instead of STN0214. You can also try STR1550. 

I also changed C4 to 1000pF. 

[simmconn]

1 hour ago, Kerry said:

I use PBHV2160Z instead of STN0214. You can also try STR1550. 

I also changed C4 to 1000pF. 

PBHV2160Z is nice due to its high HFE and low Cob. I'll be using PBHV2160Z and PBHV3160Z in my upcoming GRHV build with off-board e-caps. From simulations using those transistors I learned that C5 needs to be increased to 15pf to 22pf for best phase margin, and 47n is still the preferred value for C4 that provides a low and consistent output impedance across the audio frequency range. Another interesting find is that the circuit needs about 100uF output capacitance to be stable, and increasing the value of that cap has a diminishing return in the performance, unless the cap has really low ESR (such as film caps). In case the cap's ESR is high, some additional compensation can be added to ensure stability (hundreds of pf across R7). The above findings are for the original GRHV with 400V output. YMMV if the output voltage changes.

[JoaMat]

Thanks a lot for all your inputs, Kerry and simmconn.

Now it seems I’m back on track (knock on wood). Since “reflowing” the board and a new HN4A51J the PSU has been working for almost 24 hours with several on/offs. Voltage is stable, decreases some tenths of a volt from cold and than bang on the intended 150V. 

After some struggling, I managed to measure the three faulty HN4A51J. All Q5:B are unknown components on the tester while Q5:A are PNP with hfe of 4 or 5?.

Now the angels are singing on my modified T2 with EML tubes.

Edited December 13, 2022 by JoaMat

[JoaMat]

On 12/12/2022 at 6:30 PM, Kerry said:

Here's my T2 supply (I've been meaning to post this for a while) - all the high voltages are Golden Reference supplies (even the -60V using a negative version of the GRHV79xxx).  I've very pleased with how it performs

Thanks for posted picture Kerry. Your astonishing builds are very inspiring.

In the original T2 PSU the 60-volt section is regulated by LT783 which I think is good enough. But inspired by Kerry I did this little fellow (the small copper clad with test leads) to replace the LT783 regulator.

Connected to main board via three connections. Works like a charm – knock on wood.

[audiostar]

Just stumbled upon this "best class-A amplifier" called "T2", ever heard of? 

Looks like it was officially renamed to something else... 

[jamesmking]

Here the latest T2 psus, I have modified the layout to provide more space around the rather hot R11 3W resistor and added vent holes to the pcb. The pcbs, transformers etc still fit inside a 400mm deep case without issue.

I noticed that the R11 equivalent resistor  on the + and - 500V rails runs considerably hotter than the equivalent resistor on the other rails (which also run quite hot).... doing some quick maths, R11 is approximately 1000ohms * the value of the zener serial train /1.818 on all the psus rails except for the +and -500V... Using this formula gives 300Kohms for the + and - 500V rails but in the original gerbers I worked from only have 240K there. This results in higher current flow through the resistor and more power dissipation and hence the higher temperature. 

EDIT: thank you Joamat for suggesting the current through R11 should be 1.1mA or more. The R11 values here on all rails are based around 1.4mA with values available in the PR03 Vishay range or 3W resistors. Resulting in lower power dissipation on all rails and especially the +-500V

The gerbers with the vents have been used to make functioning psus and can be considered tested.

This pcb is 220mm wide by 137mm deep and contains the -260V, -560V and +250V rails

t2 kgsshv 250v -260v -560v corrected.zip

----------------------------------------------------------------------------------------------------------------

This pcb is 220mm wide by 148.5mm deep and contains the low voltage rails, delay and the +-500V rails with the adjusted 3W resistor

 

t2 kgsshv +-500v, lv, bias and delay corrected.zip

 

Edited February 6, 2023 by jamesmking