The Sigma was the first true “earspeaker” made by Stax, in 1977. It was before the Lambda era (first Lambda released two years after). The Sigma “offers a panoramic view to the sound”, with spaciousness and extremely angled drivers.
Housing is one of the deepest I’ve seen – around 5cm, no chance of having your ears touching the drivers. The Σ is also very insensitive, requiring much more gain than any Lambda or Omega. The comfort is pretty good – light clamp, plenty of space for the years, and it’s not that heavy.
○ Release : 1977
○ Current status : Discontinued
○ Type : Over-ear / Open-back
○ Measured weight : 395g
○ Impedance : 140k Ohms
○ Sensitivity : 94dB/100V
○ Average used price : $450-800
The Sigma I owned was a normal bias Sigma, converted into a Pro Bias out (580V) and original drivers replaced by SR-404 drivers. So it might probably sound a bit different than the original normal bias Sigma. Tonal balance is dark
Bass is thin and anemic below 100hz, but upper-bass is quite boosted, giving a good amount of body to the sound. Midrange is very warm, mostly 200-1000hz emphasized. Very lush sounding, with high resolution.
But the balance worsens a lot above that. Upper-mids to treble area is extremely recessed, more than an Audeze. Clarity / presence region is muffled. It takes away a lot of energy for any music but creates a very atmospheric and warm sound, combined with the huge soundstage, the presentation is quite unique.
The Sigma is the only headphone I’ve heard that competes with the Sennheiser HD800 for soundstage width and depth. I couldn’t tell, in A/B listening, which one was the winner. HD800 however sounds much more open. The Sigma still have extremely extended treble and absurd amount of sparkle. I was running my pair with a 323S, and I had to give it power to be loud. I would pair any Sigma with a powerful solid-state amplifier, giving enough voltage and current swing.
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