The Stax Thread III

[ericj]

i would guess there is more to it than just changing the bias. Different stax socket and the amp topology is designed around higher voltage rails.

Not a fan of dropping resistors, maybe some zener diodes or similar but I don’t think there is much market for running old crusty estats on amps at these price points.

It depends a bit on the power supply and bias supply design, i guess. The SRM-1/Mk-2 admittedly has dual bias supplies. I haven't investigated the bias supplies of amps i haven't owned.

The SRM-300 appears to have a fairly robust 580v supply for the whole system and the bias supply is just a 10M 1-watt carbon composition resistor as ballast. Dr. Gilmore's schematic says 600v, but adjusting the trimpot doesn't get it all the way to 600v here, and it's already configured for 117v mains. I calculated that about 1M and 630k in a resistor divider will give me about 230v. It'll be literally 3 resistors and swap a socket in from my SRD-7/SB. Got the resistors in from mouser the other day. May get that done this weekend. It is admittedly key not to burn too much current in the divider. The values i picked for the SRM-300 will burn about 0.00037 amps off of the total power supply current by themselves.

The SRA-3S schematic says it has a 590v supply but also that it runs off of 100v. My SRA-3S has a bad transformer -- seems to be a common issue -- and I am waiting on an R-core from ali to replace it. It may turn out that adding pro bias to the SRA-3S is about the same as adding low-bias to the SRM-300. Said r-core has more than enough 6.3v for ECC99 heaters, so it might be amusing to do a maxed out sra-3s some day with upgraded tubes and a CCS. I'd have to design my own CCS board that can mount to a pretty flat heatsink though. This would be silly, with four small signal transistors in the driver circuit. But silly hasn't ever stopped me.

As for the amplifier part, that's all the same. Just don't turn it up so loud that you blow out your low-bias stats - but that's always been a concern, and many SR-X were destroyed by people who forgot they left their stereo turned up too high when they switched on their SRD-7.

 

[WILLJS]

Question - how much would you pay for a pair of SR-X Mk1 and SRD-6 fuctioning perfectly. some for sale in my area as negotiable.

 

[ludoo]

Question - how much would you pay for a pair of SR-X Mk1 and SRD-6 fuctioning perfectly. some for sale in my area as negotiable.

MK1 are quite rare, but risk of them being unbalanced is medium high. Can you give them a listen? I seem to remember mines had thermistors inside, which have probably gone bad, so plan on removing them. They might also take some time to charge initially.

I would pay about 100 euros for them in ok shape, something more if in great shape.

 

[icnaiwhimor1975]

So today I did a thing...

A couple weeks ago I spotted a very (very) cheap Gamma Pro on Italian ebay sold as damaged, and thinking that I would at least be able to salvage its cable, bought it on the spot. A few days later I had it in my hands, and while one of the headband forks had a broken pin and pads were all squashed, the drivers sounded perfect. One of the greatest bargains I ever had: 40 euros + shipping.

I already had a Gamma Pro in the past which I sold, I did not like its sound but breaking it down for parts seemed a pity as it's still an irreplaceable piece of Stax history. This one was in pretty bad shapethough, so I set down to work today and transplanted the drivers into an SR-Xmk3 with busted drivers I had around.

The conversion takes some time but it's pretty straightfoward. The hardest parts are separating drivers from the Gamma baffles, and adapting the original cable to the SR-X cups.

To separate the drivers I warmed up the adhesive film that glues them to the baffle: I masked the drivers with cardboard and put them upside down on my 3d printer bed at 80C for 10-15m. After that they came off very easily and I was also able to quickly scrape the original film.


To adapt the cable, I trimmed down the original strain relief then reused the SR-X clamp that keeps the wire in place.


The rest of the conversion is much like refurbishing an SR-Xmk3. Here they are with new pads on. It's too early to form impressions, but they seem to have slightly more body to the sound vs their normal bias counterparts.



Edit

Put the SR-X pro on my flat plate rig and took some measurements. Here they are in red vs two of my SR-Xmk3.



FR is pretty much identical, as is impulse response (not shown above). FR is not all of course, and there are subtle differences when listening to them. But it's not a giant leap forward, their personality is a perfect match to their normal bias ancestors. If I were a cynic, I would take this as a great example of how we tend to over-rate whatever is very rare or very expensive, or both.

so this is mark 3 pro bias?

always wondered why they are so rare and expensive

 

[ericj]

Just finished installing a normal bias jack in my SRM-300. I'm getting about 210v with the resistor divider and that seems to be fine.

Sure is a tight spot to work in. Acetone took care of the red thread goo on the srm-300, the srd-7/sb turned out to not have any goop on the threads.

 

[honse]

are you making reference to this?

No. I'm talking about the audeze headband adapter. The long, thin prongs call for a stiffer material.

The spacer / earpad shouldn't require anything like that. Might want go for resin as it usually offers better accuracy. Denser print might be helpful with creating a seal.

 

[ludoo]

so this is mark 3 pro bias?

always wondered why they are so rare and expensive

Yes they are mk3 pro bias. They are rare because (according to various sources) only 100-150 were ever made by Stax, and cost follows scarcity.

While we're still discussing these, I spent the weekend listening to them and I am absolutely in love. Their FR (which I added to my previous post) is basically identical to the normal bias version, but they have better soundstage, placement and dynamics. It's not a night and day difference, but audible improvements on the mk3 normal bias which already have a great tuning, which is also markedly different from any other Stax (at least those I have or heard).

At this point in time they are my absolute favourites among all the headphones I ever had or listened to.

 

[catscratch]

The SR-X Mk3 Pro is a pretty special headphone. It's diffuse-field tuned and quite accurate to that curve so expect it to be bright, but with well recorded music it sounds pretty fantastic. Direct, focused, very lively and energetic, highly resolving, spotlit sound with surprisingly good dynamics and bass. I miss mine, sadly it was in a basement that got flooded and it's probably pretty damaged. I haven't gotten around to fixing it yet, but maybe someday, if at all possible.

I lost something like $20k in other electronics that day too. Of course, insurance said no. That's why they get the big bucks...

 

[honse]

This week I had one hell of a scare; and yes it's this topic related.

Last weekend I grabbed my L300; put them away for a bit as with the headband shenenigans I felt more inclined to use the pro classics.
Suddenly I noticed that the bass sounded bad. Very harsh, crunchy. Assumed that my mistreatment finally caught up to me. Because of work I couldn't look further into it.

Today I tried them again. Same thing.
This time I panicked a bit more; it wasn't just me looking for an issue.
Decided to plug my pro classics. Same thing. So at the very least it's not the earspeakers.

Quickly moved the rest of my chain and plugged it into the "spare energizer". Everything's fine.

 

[Svperstar]

This week I had one hell of a scare; and yes it's this topic related.

Quickly moved the rest of my chain and plugged it into the "spare energizer". Everything's fine.

So your SRM-252s died? Glad it didn't damage your hearing. Only get 2 ears.

 

[honse]

So your SRM-252s died?

Seems not.
Did swap some things around; Did get the same issue on my srm-xh. The only thing constant is the power supply.
Would be great if it's just that.

Glad it didn't damage your hearing. Only get 2 ears.

Wasn't even on my mind; but definitely glad.
And appreciated

 

[Rob80b]

So your SRM-252s died?

Looks like honse has a SRM-Xr....
Same form factor but quite different under the hood.

Xr..is it a DIYer?


252S

 

[ericj]

Looks like honse has a SRM-Xr....
Same form factor but quite different under the hood.

Xr..is it a DIYer?


252S

They don't look all that different to me, topologically speaking. But as with most long-running stax amp designs, there are multiple versions. The differences may be more about component availability than anything else.

 

[honse]

Xr..is it a DIYer?

No. Just a model before 252.

They don't look all that different to me, topologically speaking.

Oddly enough. My Xh looks completely different underneath. More like the 252.

From what I get there have been 3 different versions of that energizer. No clue what were the specific changes.

 

[oneguy]

I really wish I had a normal bias jack. Unfortunately I needed the second jack for the HE90. Kind of prevents me from going down the vintage stax rabbit hole.. but heh, that could be a blessing lol

Be glad. The hole is deep.