[Sim1]

Has anyone here tried to drive Stax amps from good preamp instead of doing any mods?
This way you can feed it with higher input than DACs/CDPs normally do. No need to increase gain or feedback loop. Based on my experience with 2 headamps (for stats and normal) it can really wake up the amp and push its performance it to the limits of output stage.

 

[K3cT]

I tried driving Lambdas with Gainclone before using SRD-7 Pro and even a measly SRM-1/MK2 beats that combo anytime. The lack of control is noticeable with the transformers.

 

[cascode]

Are Stax valve amplifiers self-biasing, or is occasional adjustment required? (and when replacing valves).  If so, is it an easy procedure - I've had valve speaker amps in the past that were tedious to bias.

 

[svyr]

>and when replacing valves

ummm, I'd say definitely required to re-bias after that (and there are posts about it http://www.head-fi.org/search.php?search=rebias+t1 ).

Not sure about regular re-biasing, I'll let someone else fill-in

 

[spritzer]

The main issue with the 727 is that it simply isn't linear and clearly coloring the sound.  This design would have made a lot more sense with much higher bias running through the output stage but as Stax are so keen to point out, that means more weight, more components and thus much higher cost.  The simplest solution is to just extend the feedback line all around the amp and it behaves like a high end amp should. 
 
Quote:

Spritzer, what about the gremlins?
Should I lower the gain again?
 

There were some noise issues that cropped up.  These are an issue with all of the Stax amps given the crappy PCB's they use so a change in the compensation caps would have been needed.  I never had the time to fully look into that though. 
 

 

[visualguy]

 
Quote:

The main issue with the 727 is that it simply isn't linear and clearly coloring the sound.

Thanks, but in what way does it color the sound?
 
I find that there is indeed a slight difference in sound between the 727 and KGSS (I don't have the 717) when driving the SR-007 mk1, but the difference is negligible. From my personal perspective, it's not something that's worth worrying about or that I would bother trying to "fix".
 
The SRM-007ta with the SR-007 is a different story... The flabby bass is a bothersome problem with that combination. On the other hand, I find that the 007ta drives the SR-507 fine, but the SR-507 is just not nearly as good as the SR-007.
 
It will be interesting to see if the 007ta can drive the SR-009 well.
 

 

[Knijsterbek]

 
Quote:

Are Stax valve amplifiers self-biasing, or is occasional adjustment required? (and when replacing valves).  If so, is it an easy procedure - I've had valve speaker amps in the past that were tedious to bias.

With 1 tube per channel, adjustment of the balance between the tube sections is needed. With 2 tubes per channel, adjustment of the balance between the tubes is needed (sections are paralleled). The measurements can be made on the outside of the amp in the Stax socket between the + and - lead of the channel. Adjust for 0V DC. This can be hard because the balance pot is a single turn type and even a tiny rotation makes a huge difference. Clever design indeed.
Also, DC offset has to be adjusted (offset pot). This is measured between the + lead and the ground clamp on the back of the amp.
An occasional check (can be done from the outside) wont hurt, valves tend to drift, especialy new ones.
Keep in mind that the power supply of these amps is not regulated, so fluctiations in mains voltage will effect your adjustments. For that reason, I feed these amps from a power regenerator which regenerates a true and clean sine of 230V (Europe) out of DC.
 
 
Quote:

The main issue with the 727 is that it simply isn't linear and clearly coloring the sound.  This design would have made a lot more sense with much higher bias running through the output stage but as Stax are so keen to point out, that means more weight, more components and thus much higher cost.  The simplest solution is to just extend the feedback line all around the amp and it behaves like a high end amp should. 
 

There were some noise issues that cropped up.  These are an issue with all of the Stax amps given the crappy PCB's they use so a change in the compensation caps would have been needed.  I never had the time to fully look into that though. 
 

I haven't noticed noise issues so far. Under what circumstances did (or will) they occur?
 
Am I putting anything at risk if I keep using the amp with the gain modded?
 
Which compensation caps do you mean (I'm not a tech expert, but i'm willing to investigate as I really like the sound of the modded amp)?
 
Does anyone have the schematic for the 727? That would help.
 
 

 

[spritzer]

No damage from using the amp with higher gain and the noise was always there but pretty low down. 
 
The schematic can be found on the other site and there are some mica caps scattered around the amp to stop stuff like this. 
 
Quote:

 

Thanks, but in what way does it color the sound?
 
I find that there is indeed a slight difference in sound between the 727 and KGSS (I don't have the 717) when driving the SR-007 mk1, but the difference is negligible. From my personal perspective, it's not something that's worth worrying about or that I would bother trying to "fix".
 
The SRM-007ta with the SR-007 is a different story... The flabby bass is a bothersome problem with that combination. On the other hand, I find that the 007ta drives the SR-507 fine, but the SR-507 is just not nearly as good as the SR-007.
 
It will be interesting to see if the 007ta can drive the SR-009 well.
 

The sound is thicker, bass is exaggerated etc.  I'm surprised that you find so very little difference between the 727 and a KGSS as that it is quite obvious to me.  Once it was modded then it was much closer and I  was a bit tempted to sell my KGSS for the 727 since it is smaller. 

 

[visualguy]

Quote:

The sound is thicker, bass is exaggerated etc.  I'm surprised that you find so very little difference between the 727 and a KGSS as that it is quite obvious to me.  Once it was modded then it was much closer and I  was a bit tempted to sell my KGSS for the 727 since it is smaller. 

Yes - I did notice what you're describing. However, it's noticeable only on some music, and even then it's not something that jumps at me - it's pretty subtle (unlike the case with the 007t).
 

 

[Knijsterbek]

Quote:

No damage from using the amp with higher gain and the noise was always there but pretty low down. 
 
The schematic can be found on the other site and there are some mica caps scattered around the amp to stop stuff like this. 
 

Thanks. I will keep listening for noise.
 
 
Quote:

Yes - I did notice what you're describing. However, it's noticeable only on some music, and even then it's not something that jumps at me - it's pretty subtle (unlike the case with the 007t).
 

 
Although I don't have a KGSS to compare and I cannot switch back to the unmodded 727, I did experience quite an improvement in sound quality after the mod. Much more control. Especially in the bass/upper bass this was clear to me in just a few seconds.
Hard to describe in English as a non-native speaker, but it sounded a bit "overblown" to me befor the mod.
 

 

[Robot Metal]

I have a question: Sound quality-wise, is the KGSS a good match for headphones that aren't the Omega2? The ESP950 and Lambda phones mainly. I'll admit that I kinda remember reading somewhere years ago that the ESP950 wasn't a good match, but I can't seem to find that post anymore so I don't know for sure. (I'm curious to hear the ESP950.)

 

[visualguy]

Quote:

Although I don't have a KGSS to compare and I cannot switch back to the unmodded 727, I did experience quite an improvement in sound quality after the mod. Much more control. Especially in the bass/upper bass this was clear to me in just a few seconds.
Hard to describe in English as a non-native speaker, but it sounded a bit "overblown" to me befor the mod.
 

I know what you mean, and I agree with you that the 727 doesn't do the bass completely right - it sounds slightly bloated. I haven't noticed any problems with the mids and highs. The bass problem, although noticeable in some music, doesn't bother me enough to perform the mod - it's really minor to my ears.
 

 

[Gradofan2]

Quote:

There were some noise issues that cropped up.  These are an issue with all of the Stax amps given the crappy PCB's they use so a change in the compensation caps would have been needed.  I never had the time to fully look into that though. 
 

When you say "noise issues that cropped up," do you mean... the noise is noticeable in all Stax amps, or just after the mods are completed?
 
What's the nature of the noise? 
 
I didn't notice a lot of noise in the 007t II I had - though there might have been some at high POT levels with the 507's - just not at listening levels.
 
 

 

[Don Quichotte]

There is noise in my 313 if not grounded, otherwise it is extremely quiet. I can turn the volume knob to the max (without signal of course) and I can't hear anything at all.

 

[Gradofan2]

Quote:

There is noise in my 313 if not grounded, otherwise it is extremely quiet. I can turn the volume knob to the max (without signal of course) and I can't hear anything at all.

You mean grounded via a dedicated ground wire connected to the ground terminal on the back panel? 
 
If so, how should that ground wire be connected (to what)?
 
I guess this would work... if, the electrical wiring ground is wired correctly:
 
http://www.diyaudio.com/forums/solid-state/141478-ground-vintage-amp.html