[fiiom11pro]

Well, just wanted to show here what Dominik Stritt ("firschi") has done to my trusted SRM-T1 I had since quite a few years.

Had my STAX SRM-T1 serviced and modified using Constant Current Sources (CCS) instead of anode resistors and changed the original 6CG7/6FQ7 tubes to matched highvoltage and high-power ECC99 dual triode GOLD tubes from JJ Electronics, Slovakia.

The result is simply amazing, much clearer sound, no peaks clipping, much more available "sound voltage" - I'm really hooked on listening ...

STAX SRM-T1 before (btw. it still worked flawlessly in this condition):


STAX SRM-T1 after modification and service (looks like new! Cleaner sound, much more powerful):

(both images: Dominik Stritt)

how much did it cost you?

 

[KDS315]

Ask Stritt-Audio directly please! He offers three packages: 1) complete check and service (caps replacement, saftely resistors etc.), 2) CCS mod, 3) ECC99 mod. And a pckage deal consisting of all three.

 

[JimL11]

^ looks brand new!

I am considering buying the below upgraded SRM-T1 recommended by admedie (for ESP-95x and eventually a Lambda), can anyone comment on these modifications? The PIO coupling caps look pretty close to the tubes compared to stock.. they're labeled -40 to +85 deg C and towards the socket end so should be fine?

The main modification items are as follows ↓
① Circuit modification to the company's SRM-600 LTD (including changing the vacuum tube to Czech JJ ECC99 etc.)
② Smoothing capacitor large capacity (Philips BC 4)
③ Coupling capacitor upgrade (Denmark Jensen oil capacitor 2 this)
④ bias circuit capacitor upgrade (Sprague-made film capacitor 4)
⑤ large power resistor upgrade (Panasonic metal oxide film resistors 8)

https://page.auctions.yahoo.co.jp/jp/auction/d431573263 (buy now price is about 750 USD)

Thanks!

Of these mods:

1) changing the power resistors and vacuum tubes to ECC99 should make a minor improvement in current drive (about 10% based on the SRM-600LTD using 60 kilohm output resistors vs. 66k for the T1 and its variants.
2) Increasing the power supply caps should help a bit.
3) I don't understand the use of coupling capacitors at all since the T1 (and SRM-600) are direct coupled from input to output and do not need coupling capacitors. And anyway the outputs are balanced, which means if they are coupling to the outputs there should be 4 of them, not just 2. Unless they are being used as bypass caps across the power supply, in which case they may or may not make a difference in the sound, for a fairly significant amount of money.
4) The bias cap upgrade really shouldn't make any difference in sound, and in fact using larger caps is NOT beneficial, as the stock caps are big enough to provide a smooth bias voltage while limiting the amount of current that can be dumped into the diaphragm should things go wrong.
5) Finally, if you want to really boost the effective power output of the amp circuit, the use of CCS will more than double the effective output power since it removes the current wastage inherent in output plate resistors, as opposed to a mere 10% improvement by changing to lower value plate resistors per the stock 600LTD circuit. Of course I'm biased since I published the mod, but the objective results bear me out. You can combine the change in tube to ECC99 with the CCS mod, or stick with the original 6CG7. I haven't listened to the ECC99 in this circuit so have no opinion, although others have commented favorably on this.

 

[Mach3]

$750 for a modded T1 to become a 600. You can get 600 for a lot less then that, then add the CSS mod.

 

[JimL11]

$750 for a modded T1 to become a 600. You can get 600 for a lot less then that, then add the CSS mod.

Maybe in Australia, but the 600LTD's I have seen listed on eBay start at over $1000US, usually a few hundred dollars more.Remember they were only made for a couple years, the LTD stands for limited. The 006T, which uses the same tubes as the T1, often goes for around $700US.

 

[Moak]

$750 for a modded T1 to become a 600. You can get 600 for a lot less then that, then add the CSS mod.

What does the CSS mod do?
“Had my STAX SRM-T1 serviced and modified using Constant Current Sources (CCS) instead of anode resistors...”
Does it improve a lot and is it recommended? Or can I destroy my 600LTD with it?

 

[daytrader]

Or I’d suggest a used 007tll for way less than $1000 which results in a newer more updated version of the T1.

 

[Moak]

Maybe in Australia, but the 600LTD's I have seen listed on eBay start at over $1000US, usually a few hundred dollars more.Remember they were only made for a couple years, the LTD stands for limited. The 006T, which uses the same tubes as the T1, often goes for around $700US.

If you “hold open your eyes” and have luck you could get the 600LTD for $850 in Europe.

 

[jamri]

Thanks JimL11 and everyone for the feedback. "coupling caps" may have been lost in translation from Japanese... They're the two large caps between the tubes in the photos... Maybe coupled because they are zip tied together?
Sounds like these mods are not well established, even if the builder is reputable. I think I'm going to play it safe and get a 353x for now (looks like the 600tA and 700tA are both no longer available). I can take advantage of the XLR inputs too

 

[Mach3]

I think COVID might be driving the prices up on the SRM-600. You are correct @JimL11
When I purchased mine earlier last year from yahoo Japan.
There was 2 that was listed for sale for like at least 4-5 month with no interest.

 

[JimL11]

What does the CSS mod do?
“Had my STAX SRM-T1 serviced and modified using Constant Current Sources (CCS) instead of anode resistors...”
Does it improve a lot and is it recommended? Or can I destroy my 600LTD with it?

The constant current load mod replaces the output plate resistors with a small circuit. Constant current means that the current through that circuit doesn't change with voltage changes - think of them as very high resistance resistors - over 100 megohms. With plate resistors, every time the signal voltage changes, the signal current in the resistors changes by Ohms law, V = I*R. What this means is that the majority of the signal current is burned up as heat in the plate resistor, leaving less than half of the signal current available to drive the headphones. The CCS mod basically eliminates that signal current wastage, more than doubling the amount of current available to drive the headphones (some of the signal current is used up in the feedback resistors). The sonic results are improved bass response and high frequency response, with less distortion. You can also measure the results in the form of increased peak voltages.

The Stax solid state amps all have CCS loads in their output stages, but their power supply voltages are very similar to the tube output amps, as is the amount of standing current in the output stages.. The result is that whereas the T1, 006T and 007T are specified at a maximum output of 340VRMS at 1 kHz, the solid state amps are specified as a maximum output of 450VRMS at 1 kHz. The difference is due, almost completely, to the difference between using a resistor and a constant current device as the load for the output devices.

 

[kevin gilmore]

3) Coupling capacitor upgrade

This is a complete waste of money. Those are not coupling caps, they are power supply caps and
the film caps that were in there is just fine.

 

[Moak]

The constant current load mod replaces the output plate resistors with a small circuit. Constant current means that the current through that circuit doesn't change with voltage changes - think of them as very high resistance resistors - over 100 megohms. With plate resistors, every time the signal voltage changes, the signal current in the resistors changes by Ohms law, V = I*R. What this means is that the majority of the signal current is burned up as heat in the plate resistor, leaving less than half of the signal current available to drive the headphones. The CCS mod basically eliminates that signal current wastage, more than doubling the amount of current available to drive the headphones (some of the signal current is used up in the feedback resistors). The sonic results are improved bass response and high frequency response, with less distortion. You can also measure the results in the form of increased peak voltages.

The Stax solid state amps all have CCS loads in their output stages, but their power supply voltages are very similar to the tube output amps, as is the amount of standing current in the output stages.. The result is that whereas the T1, 006T and 007T are specified at a maximum output of 340VRMS at 1 kHz, the solid state amps are specified as a maximum output of 450VRMS at 1 kHz. The difference is due, almost completely, to the difference between using a resistor and a constant current device as the load for the output devices.

Ah, ok, thank you.
Make it sense to do it with the 600LTD or is it dangerous?

 

[JimL11]

Ah, ok, thank you.
Make it sense to do it with the 600LTD or is it dangerous?

Sure, it makes sense. The current should be adjusted up a bit. to around 5.8 mA per current load to match the standing current from the plate resistors. It will void your warranty, but the unit is probably out of warranty at this point anyway. The basic procedure is the same as described in the AudioXpress article from July, 2017, back issues should be available from the AudioXpress website.

 

[Deolum]

What do you guys think about crinacles Stax rating? I think all of his headphone and IEM ratings are on point but seems like he tested all Stax, including the 009 and 007, on a 353x amp. May that explain the low ranking? They all got downranked because of their tuning though, not sure if another amp would fix that.