[Jimbtoo]

Hello! I am a new member, just getting back into headphones a bit, and well, you guys are the masters of these issues!

I am the original owner of the 1975 Stax system in the topic title. It has been little used in decades - just sitting in a plastic bag in a cabinet. I need to get back into headphones for a variety of reasons and thought I should at least start with what I used to love so much. This system was my first, truly high-end component, back in 1975. So I fired it up and it sounds limp compared to memories. I have read about rebuilding 'energizers', and the SRD-7, in particular. I noted that the nominal bias is 230V for these, non-Pro, units. Mine measures about 100V from the main bias pin to any of the electrode drive pins. Not good, right? Assuming this is in fact low, there is some failure in the bias circuit. I've also read some great notes about replacement parts for these original components and I am game for replacing/upgrading any or all of them.

I have also just purchased NEW Focal Clear and Utopia headphones (it's complicated). I will be keeping ONE headphone system at the end of all of this. I am evaluating a new Benchmark HPA4 driving the headphones (the alternative is the Benchmark DAC3 HGC, directly). I've not yet tried the Stax on the HPA4, but I suspect it might have sufficient drive range (at least 6W) IF the Stax system was working normally. I am less likely to keep the Stax if it cannot.

Given that background, should I bother messing with the Stax?
a) Is the HPA4 likely to be able to drive it, effectively?
b) Is it ever going to compete with the new Focals?
c) Is there any market for the Stax as-is? I also have a nice, long extension cable for it.
d) Is the resale value after refurbishing it going to be significantly more than as-is?
e) If I refurbish it, would it be appropriate to increase the drive voltage a bit, say 10 or 20%? That would be by using the next higher value diodes in the bias string (up from 100V to 110 or 120). Is that likely to be beneficial? Is it likely to damage the (old) SR-X phones?

Depending on what you all can advise me, I might have a few follow up questions to wrap it all up.

Thanks!

 

[isnotdynamic]

I think refurbing the SRD7 is an option, the phones probably just need a bit of charge to get them back to normal sounding. If they were stored for a long time and the bias is low, the phones won't sound good. Their age isn't generally a problem so I wouldn't mess with them.

 

[Jimbtoo]

Hi!

Thanks for the reply. I have since read a bit about bias recharge time and have given the phones more time and listened more. They don't have much bass, but pretty amazing highs. They are not how I remembered them, but still clearly have really special characteristics.

Having now looked "into" it, the SRD-7 bias circuit is quite different than the MkII. The original uses a full-wave V doubler on a 100V diode, which will yield about 170V. Weak, old caps might be contributing to my, even lower, low bias. I'm inclined to up-scale it to get closer to the 230V that a MkII would provide and see where that takes it. The condition of my phones seems 'ok'. They are starting to loose the surface layer on the pads, but even that is mostly still intact. If I do get into refreshing the bias circuit, I'll update people, here.

Now, can anyone tell me anything about the risks and benefits of just using a 4.7 ohm resistor in place of the 4.7 ohm thermistor/posistor type limiters in the primary audio path of Stax converters?

 

[Jimbtoo]

To wrap this one up, I did refurb the bias circuit to MkII values, but boosted the output to 260V (original ~170V and MkII =230V). The headphones sound fine (more efficient), but still not what I was hoping for - not competitive with Focal Utopia, overall. I'll be writing up the details of the changes in a separate post, in case anyone else wants a relatively easy way to refresh their stock SRD-7 to MkII values (+). It's actually just as easy to get about 235V with a single component value change, but I wanted to try a bit more. I'll likely sell the set (once I figure out how to accept payment).