Monday, February 9, 2009


In our last episode of Soldering Iron Justice, our intrepid technician was befuddled by a capacitor that was really a thermistor. Could the part be located? Would it in fact enable the repair? Could sonic mojo be restored to the Stax Earspeaker Energizer, SRD-7?

It turns out, the answers were and are: yes.

Mouser Electronics to the rescue! They sell Murata thermistors, in this case 4.7 ohms @ 140 volts. They are a fraction of the size of the torched thermistor, but I can live with that especially since the prior examples were bent to fit and one lay across a wire wound resistor which contributed to its demise. I purchased 2 for the sake of symmetry, a buck each.

My original effort on the SRD-7 was to replace the pair of 1 mfd 160 volt electrolytic caps with fairly compact polpropylene 250 volt caps. The new thermistors were a bit tricky to install because they were nearly too small for the leads to reach, but with a bit of gentle coaxing they reached and the solder grabbed.

In the photo above, the thermistors are the brown disks above the left hand transformer. The right one is torched. A black wire wound resistor is next to each thermistor. The 1 mfd film caps are the white lozenges in the middle of the board. The small, loose caps at the top were the originals, now removed. Time passed... and the thermistors were subsequenty replaced.

I brought the SRD-7 up on the variac without any smoke so I hooked up the pair of Stax SR-X Mark 3's. The energizer was driven by the trusty Advent 300 and when it was lit I heard sweet, sweet music.
Electrostatic headphones are ear candy deluxe. If everyone on earth had a pair connected to an Advent 300 there would be no war. Tripping Daisy would be everyone's favorite band. The sound is effortless and extended. Listening to Austin's classical station, KMFA, I could clearly hear the crappy digital sideband hiss, but the music sounded so good I really didn't care. I am not sure I can give higher praise without an eargasm.

Given the age of the SRD-7 (introduced in 1971) it would seem to me that the first order of business for anyone who owns one should be to replace the two electrolytic caps. I also wonder about the location of the thermistors since the originals are large and easily bent over to touch the large resistors next to them. Not good. Only 4 screws separate one from peace of mind, so open it up and make sure the thermistors are not bent over, touching the resistors. Only 2 more screws need to be removed to actually work on the unit.

If you have a chance to acquire a pair of Stax headphones, I highly recommend that you do. You will thank me. And you're welcome.


Anonymous said...

Hello Nat,

I've used Stax headphone for years -- SR-5, SR-X Mk III, SR-404 Signature, SR-40 (electret). The SR-404 signatures require Pro (580v) bias so one is either forced to buy a Stax pro-bias amp OR the lone SRD-7 Professional energizer which allows you to select the amplifier of choice. Stax also made some energizers for standard bias that were self energizing and did not require a plug in to any AC socket. Glad to see you have discovered the magic of Stax and may your collection grow. Electrostatic headphones are really something special.

Paul B.

Eoin said...

Thanks for you write up! I have just got hold of an SRD-7 and a pair of SR-X earspeakers and 'ear candy' is about right. I've had to make a set of replacement earpads as the original foam and vinyl pads are no longer available.

I want to do the capacitor replacement you describe. All I need is two polypropylene 1 microfarad capacitors rated at 250V then?

Nat said...

Yeah, this is a very simple modification. There is some variation in cap size, but most 1 mfd film caps rated at or above the original voltage will fit nicely. Unless you get seriously boutique-ey and buy honking huge caps this is an easy replacement that will benefit both your ears and peace of mind.

Anonymous said...

Hello Nat,

I've just got the same SRD-7. I've noticed that the two caps in it are dead. The sound of my SR-X Mk3 isn't good and lacking bass. Could it be the cause? Otherwise, what's the effect of two bad caps in the SRD-7?

Thank you, Louis