Stax · Electrostatic · 1977 (Sigma Pro from 1986)
Stax SR-Sigma (Normal bias)
Stax released the SR-Sigma, officially the SR-Σ, in 1977 as its panoramic-sound earspeaker. Instead of sitting over the ears, its big drivers are set forward of them, so the sound arrives from in front, more like a pair of speakers than headphones. That spread of sound was the effect Stax was after. It's a Normal-bias design with a 6-pin plug. In 1986 Stax followed it with the SR-Sigma Professional, a Pro-bias version with a diaphragm half as thick and PC-OCC wiring. Stax says the Sigma Pro can also run on Normal bias, a little quieter and softer, but the original can't go into a Pro socket. Stax no longer repairs either.
SR-Sigma (1977, Normal bias)SR-Sigma Professional (1986, Pro bias)
Spec at a glance
- Maker
- Stax
- Type
- Electrostatic
- Made
- 1977 (Sigma Pro from 1986)
- Impedance
- Not documented (Sigma Pro: 129,000 ohms)
- Bias
- Normal, about 230 V, 6-pin plug
- Design
- Large drivers set in front of the ears
- Type
- Electrostatic earspeaker
- Made in
- Japan
- Also listed as
- SR-Σ, SR Sigma, Stax Sigma
Before you plug in
What it needs, and what wears out
- Drive it fromNeeds an energizer
- Needs a Normal (6-pin) socket, such as an SRD-6 or SRD-7 box, an SRM-1, an SRD-7 MK2 or the Normal socket on an SRM-1/MK-2 Pro, SRM-T1 or SRM-T1S. Owners often use an SRD-6.
- Pads
- Not documented, and Stax no longer supplies pads for the Sigma. Owners often find the black cloth mesh on the sides missing or worn.
- Cable
- Wired in, ending in a 6-pin Normal-bias plug. The Sigma Pro has a wider 5-pin cable.
Should come with it
- A Normal-bias SRD box or energizer, unless you already own one
Check the listing photos against this list, and ask the seller about anything you can’t see. A missing piece should come off the price.
What to check
The black cloth mesh on the sides is often missing or worn. Stax has ended repairs, with no drivers, pads, cables or headbands, and electrostatics dislike damp and dust.
- 01Plug: Count the pins on the plug in the photos. Six pins means the Normal-bias Sigma, and five means a Sigma Pro.
- 02Your energizer: Check that your energizer or SRD box has a Normal 6-pin socket. Without one you'll need to buy a second unit.
- 03Both sides: Ask the seller to confirm both sides play evenly with no crackle or hiss. Stax can't repair it, so a faulty side stays faulty.
- 04Side mesh: Look at the black cloth mesh on the sides of the cups. Missing mesh is common and cosmetic, but it should be reflected in what you pay.
- 05Pads: Look closely at the pads in the photos. They should be whole, since Stax no longer makes them.
New to used headphones? How to check a used pair covers pads, cables, channel dropouts and fakes, and impedance and your amp explains what will drive it.
Find one on eBay
We haven’t been through this model’s listings yet. Search eBay and run every listing through the checklist above. Ask for close photos of the pads, the headband and both ends of the cable, and pass on any seller who won’t send them.
Search “Stax Sigma” on eBay ↗eBay searchCompare with
Stax
ElectrostaticStax SR-Lambda Pro
- Years made
- From 1982
- Impedance
- 129,000 ohms
- Drive it from
- Needs an energizer
$750–$1,125 typical ask
Stax
ElectrostaticStax SR-Lambda (Normal bias)
- Years made
- 1979–1982
- Impedance
- 122,000 ohms
- Drive it from
- Needs an energizer
$550–$1,025 typical ask
AKG
Open-backAKG K 1000
- Years made
- 1989–about 2004
- Impedance
- 120 ohms
- Drive it from
- Runs off a speaker amp
Specs and what to check →