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