Presentation

This is a very short, but dense review of the Stax Lambda, one of the “right sounding” headphones that I currently own, and which I’d blindly suggest to any audiophile on a budget. The SR-Lambda came with bias voltage of 230V (or “normal”), and 6 pins termination and were the first in the Lambda line. They were developed in the 1977, after Mercedes-Benz request to check the low frequency noise in their cars.
They share the same 2 um thick driver with the Stax Sigma Panoramic headphone. The Lambda design is called “Semi Panoramic”, and albeit not having as wide soundstage as his ancestor, the Lambda has a more balanced frequency response and more linear bass than the Sigma.
In 1987, later versions of the normal bias Sigma (from which the Lambda was derived) took advantage the same flat black cable used with the SR-Lambda that were in production at the same time, which offered lower capacitance than the fabric coated, round bodied cable used by the first Sigma models.

Sound-wise

The SR-Lambda (also known as Lambda Normal Bias) is a “best value” headphone that has a balanced, involving sound and high sense of pace. They can work great with a SRM-1 Mk2 amplifier (and up, with SRM-T1S being an ideal sweet spot – for cost/sound quality).
There is a slight forwardness in the midrange, which is explained by two peaks at 2.5 kHz and 5 kHz. Such midrange translates in involving, “in your face” type of sound, with zero aggressiveness or abrasiveness, and adds up to involvement and sense of liveness (with vocals). But, on the other end, the same peaks produce a sense of “shiney-ness”, a slight washed out tone, when compared with Stax Omega 2, which indeed have the perfect tone saturation.
The Lambda love warm, vynil-like sources, like the Museatex Idat-44.
The detail rendition is excellent, low level details are pleasantly presented and almost never intrusive. The soundstage is somehow less natural than the Stax Omega 2, since, the sound space can be wider than the Omega’s, but there is a bigger concentration of instruments in the very center of the listening area (“in your face”, as previously mentioned).
They have big ability to scale with different source. The resolution power, attack, decay and sense of rhytm are all excellent.

The bass is very textured, clean, has presence, provided by a hump at 100 Hz, while it gradually drops off till 40 Hz. At 30 Hz the level is down -10 dB or so. Hence, they lack the same bass presence that the Omega 2 (or dynamic cans like LCD2/3 and Sennheiser HD600/650) have, while the slight bump in the midbass tries to make up for it.
They are never mushy, and they are almost never edgy. They manage to be musical, cohesive, and very very involving without spoiling listening, unless being spoiled with real bass (the difference between them and the Omega 2 is huge when the music requires so).

They are considered, by some, the best and most musical of the lambda series and are one of the best bargain in electrostatic (and all headphones) world. Other notable mentions would be the Lambda Signature, the Lambda Nova Signature, the SR-507, the Airbow SC-1 (a cryo treated and better sounding SR-404), the SR-404LE (limited edition, reported to be similar sounding to the SC-1) and the late, misterious and very expensive Airbow SC-2.

Further info: STAX SR-Lambda Frequency Response and CSD Waterfall Plot Measurements – Changstar.com