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The Headcase Stax thread


thrice

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So are you saying that the SRM-T1 is a better amp that just needs "a few" mods? >_>

The 6SN7GTA/B works really well on the T1, but it still distorts audibly (on transients) around the 12'o clock mark on O2s.

Of course it does. Lets see, running a 450 volt MAX tube on 700 volts.

Does no one ever bother to read the datasheets?

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The SRM-1 Mk2 (not the Mk1, very different design) is mostly identical to the SRM-313 except different parts used. This isn't a bad amp but comparing a stock unit to anything modern is madness since the PSU caps will be way past their sell by date.

As for the SRM-T1, it's a good circuit built on the cheap and with the wrong tubes. The ECC99's should solve some of the compression issues but the real "fix" is the KGST. 6S4A tubes with a 10M90 CCS which I should have up and running as soon as I place a new order with Mouser. I'm as you all know certifiable so I'm running it at +/-400V for added effect... :D

This did not give me any comparison info, caps in my SRM-1Mk2 are fresh, last owner had Stax replace them :P What I want to know how an amp like this would compare to Exstata, from specs, design point of view. The way I look at this after looking at design and specs, and correct me if I am wrong, amps like SRM-1Mk2 or T1 for that matter is in no way inferior to Exstata,.

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don't have the srm-1 schematics at hand at the moment, so i'll speak on the t1,t1s,006t,007t

all of which are the same identical design, and are grid drive with the tube as the output

stage. If you sub out the tubes with 6s4's things get much better. And if you add a current

source instead of the plate resistors, things get better yet.

exstata tube hybrid uses 6s4 in grounded grid as gain stage, and solid state output section

which is the same as the srm212,313,717. And i assume the srm323. 727 is definitely

different.

which is better in stock (unmodified form) , srm313 vs exstata solid state...

in my opinion, the srm313 is better.

clear as mud.

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This did not give me any comparison info, caps in my SRM-1Mk2 are fresh, last owner had Stax replace them :P What I want to know how an amp like this would compare to Exstata, from specs, design point of view. The way I look at this after looking at design and specs, and correct me if I am wrong, amps like SRM-1Mk2 or T1 for that matter is in no way inferior to Exstata,.

You would be correct as the exstata version which measures properly is very similar to the SRM-1Mk2 design. Now the PSU lacks regulation in the Stax amps but with fresh caps and a stable line voltage it shouldn't be a big deal. You certainly will get a lot cleaner, more transparent sound from the Stax amps.

Now speaking of the SRM-1Mk2, it appears I've just bought one and the tasty P.P. version at that. :) I see some mods in its future, adding balanced inputs and I might even try to add some regulation to the PSU...

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So what would you do to the psu? Just swapping the old caps for some fresh low-esr types as spritzer suggested or do have more mods in stock?

I've had some mods in mind myself, but without the schematic never got a place to start, so thanks for that - maybe I'll come up with something someday...

Edited by Cynric
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SRM-1 Mk2 schematic. This includes the Pro bias supply in case somebody wants to modify a non Pro unit... :)

Thanks Birgir, you are a total forum legend - my SRM Monitor has just started distorting in the left channel - slight volume drop also with 2 different phones, so now I can provide a circuit diagram for the tech.

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How about adding some simple regulator like a 317HV to the stock PSU? One could use shorter caps and place the whole thing above the stock PSU inside the box...

Thanks Birgir, you are a total forum legend - my SRM Monitor has just started distorting in the left channel - slight volume drop also with 2 different phones, so now I can provide a circuit diagram for the tech.

Happy to help and I'm pretty sure the SRM-Monitor is a SRM-1 Mk2 with the EQ added. Certainly appears to be the case with the SRA-14S as well but without great pics one can never be too sure.

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According to Stax SRM-1Mk2 is 370V (1KHz) with TDH 0.05% (100V 1KHz).

i measure with pabbi1 listening levels... :D

http://gilmore.chem.northwestern.edu/srm1mk2.pdf

even so we are pretty close.

-60db second harmonic is .1%

-85db third harmonic is better than .01%

i was quoting DC power supply voltages, not AC voltage swing.

How about adding some simple regulator like a 317HV to the stock PSU? One could use shorter caps and place the whole thing above the stock PSU inside the box

Ray samuels is the only moron i know that pushes 317HV's to those kind of voltages. Way in excess of their 150 volt ratings. (i think there is a 225 volt version now)

And besides which there is no high voltage version of the negative regulator.

Now what you could do is run the transformer on 120 vac, and set the jumpers for 100 vac, then use simple zener/fet regulators... That would work great.

And the transformer is going to get a bunch warmer.

Edited by kevin gilmore
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What I don't understand is why the eXStatA with it's "1% distortion" still drives my O2 Mk1 better than either of my previous SRM-1 Mk2 Pro ever could. The SRM-1 Mk2 Pro with O2 Mk1 or Mk2 was soft and muted with them, and had to be cranked to max yet still had poor dynamics and drive with either O2.

The T1 and GES drove the O2 Mk1 much better than the SRM-1 Mk2 Pro, but I still don't think they could match the SRM-717 with them. Those two still sounded soft around the edges with the 02, and needed more power, while the 717 could play loud, fast and dynamic with them (but still sounded detailed, spacious, warm and crisp at the same time). Not having compared them side by side, I still felt that my experience with the O2/eXStatA was similar to my short experience with the O2/717. I really liked the 717, but I also really like my hybrid eXStatA. I suppose I'll need to find a way to listen to someone else's 717 side by side with my eXStatA some day and see what I think then.

Aren't there different types of distortion, when one kind is more musical than the other? Is that the case with the eXStatA? Either way, I still find the eXStatA enjoyable to listen to.

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well there are definitely different kinds of distortion, mainly 2nd harmonic, or mainly 3rd harmonic.

and sound signatures are different too. push pull sounds a particular way, whether solid state

or tubes. same thing with the various kind of single ended things. There is also soft clipping vs

hard clipping. all the stax amps are really singled ended with constant current sources for the

solid state amps, and resistors for the tube amps. (the t2 is something else entirely)

the exstata output stage is a constant current source. you can add a current source to

the exstata gain stage and make it bunches better.

The vhv amp i posted a bit ago is the only example of a solid state push pull amp that

i know of that actually works. It depends on stacking transistors to meet the necessary

voltage requirements, and its performance suffers as a result.

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I recall you saying that you really liked the Exstata you heard...... :)

I did in fact like it when I heard it on two occasions at meets. Probably listened to it 1 hour or so on two or three separate occasions. Three different versions, the latest iteration my favorite, never heard the latest SS version. I was about as vocal as people were allowed to be about the previous versions not sounding right, and the current hybrid version was definitely better than the previous ones, as long as you didn't turn them up too loud. Anyways, all that to say then I built a SS version. Then I sold it.

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Okay, so I have this kind of weird problem. Sometimes, the sound goes out on my 717. It just disappears when I'm playing a game or listening to music. I can turn it off for a few minutes, and the sound returns after I turn it back on. Is it overheating? The top of the amp feels pretty hot when the sound goes out. Any explanation for this and how to prevent it from happening?

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re: 717, open it up and listen to it, when the sound goes away, look at the led's and see if all of them are lit.

If none of them are lit, then you are having a thermal cutout inside the transformer...

the srm1mk2 was made between 1985 and 1990. As such its more than 20 years old. More than enough

time for the electrolytics to dry up. So people who still have them and don't like the sound should probably

change out all the power caps. At this point the 717's are now 10 years old and probably need new caps too.

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