Every Stax headphone, or earspeaker as Stax calls them, is an electrostatic. None of them plugs into a headphone jack. Each needs an energizer, an amp that supplies a high bias voltage and drives the diaphragm, or an adapter box that runs from a speaker amp. Over the years Stax used two bias standards, and buying a vintage Stax means matching the headphone to the right one. Here's how to tell them apart and what each energizer takes.
The two standards
- Normal bias. About 230 volts, with a 6-pin plug. Stax's electrostatics of the 1960s and 1970s use it, including the SR-3, the SR-X line, the SR-Sigma and the original SR-Lambda.
- Pro bias. About 580 volts, with a 5-pin plug. It arrived in 1982 with the SR-Lambda Pro and the SRM-1/MK-2 Pro energizer, Stax's first Pro-bias system. Owners say every Lambda made since has been Pro bias, and current Stax energizers are Pro only.
The plugs are different on purpose. The 5-pin Pro plug is the 6-pin layout with one pin left out. That means a Normal plug, with six pins, won't physically go into a Pro-only socket, while a Pro plug will go into a Normal socket.
What happens when you mix them
A Pro headphone in a Normal socket works. It plays at the lower bias, so it's quieter and softer. Stax's own catalog for the SR-Sigma Professional says so. The caution is to keep the volume down, because if the headphone does arc at that point, the discharge current is higher and more likely to damage the diaphragm's coating.
A Normal headphone in a Pro socket isn't possible without an adapter, and an adapter is the risky move. The higher voltage makes arcing much more likely. If you own a Normal-bias Stax, find an energizer with a Normal socket instead.
How to tell which one you're looking at
Sellers often just write "Stax Lambda", so do the checking yourself.
- Count the pins. Ask for a clear photo of the plug face. Six pins means Normal and five means Pro. It's the first and easiest check.
- Read the nameplate. On a Lambda, the nameplate sits on top of the cup, under the headband arc, so ask for a photo with the arc lifted. A Normal-bias SR-Lambda has a blank plate, while Pro and Signature models are named.
- Don't trust the plug alone. Cables can be swapped. One owner found his Normal-bias Lambda had been rebuilt with a 5-pin Pro cable. If the plug and the nameplate disagree, ask the seller what's been done.
- Know the later versions. Some models came in both standards. The SR-X Mk3 of 1975 is Normal, and the SR-X Mk3 Pro of 1985 is Pro. The SR-Sigma of 1977 is Normal, and the SR-Sigma Professional of 1986 is Pro.
Which energizers take which plug
Stax's own product history and its catalogs give the sockets on the vintage units. The useful surprise is how many later energizers kept a Normal socket.
- SRD-5 (1968), SRD-7 (1971, two sockets) and SRD-6 (1973, one socket). Adapter boxes that run from a speaker amp. Normal only.
- SRD-6SB and SRD-7SB (1978 and 1979). Self-bias versions that take the bias from the amp's signal. Normal.
- SRD-X (1979). A portable box fed from a headphone jack. Normal.
- SRM-1 (1979). Stax's first solid-state energizer, two Normal sockets at 230 volts.
- SRM-1/MK-2 Pro (1982). One Normal and one Pro socket, and it can run one of each at once.
- SRD-7 MK2 (1985) and SRD-X Pro (1986). Both have a Normal and a Pro socket.
- SRM-T1 (1986 in Japan) and SRM-T1S (1993). Two Pro sockets and one Normal.
- SRM-1/MK-2 PP (1989 in Japan). Two Pro sockets and no Normal one.
One box to avoid for these models is the SRD-4. It was an adapter for Stax's electret earspeakers, which carry their own charge, so it supplies no bias and won't drive an SR-3, SR-X, Sigma or Lambda.
A quirk of Normal bias
On Normal-bias models each side gets its own bias feed. After you switch on, one side can play louder for a while until both diaphragms are fully charged. That's normal and settles down. An imbalance that stays is a real fault.
Repairs and parts
This is the part that changes how you buy. Stax Japan lists the SR-Lambda, SR-Lambda Pro, SR-Sigma, SR-Sigma Pro, every SR-X version and the SR-3 as repair service ended, with no drivers, pads, cables or headbands available. It doesn't accept repairs or questions from outside Japan and sends buyers to local distributors. It also refuses to convert a Normal-bias headphone to Pro bias. So a vintage Stax with a dead or noisy side stays that way unless you find an independent specialist, and worn pads mean third-party replacements.
Stax also says electrostatics are commonly thought of as sensitive to moisture. Its own earspeakers are protected well enough for normal living conditions, but it tells owners not to use them with wet ears or hair, after a shower for example. Dust in the drivers is the other enemy, so a pair that's been stored covered and dry is the better bet.
Not every electrostatic is a Stax
The Koss ESP/950 is an electrostatic too, but it's a separate system. It runs at a 600-volt bias from its own E/90 energizer and ends in a custom 5-pin rectangular plug that isn't a Stax plug. Buy it as a complete set.
A buying checklist
- Know which bias your energizer supports before you shop, or buy the headphone and the energizer together.
- Ask for photos of the plug face and the nameplate, and match them to each other.
- Ask the seller to confirm both sides play evenly with no crackle or hiss once warmed up.
- If an SRD box comes with it, check its model number against the list above and remember it needs a speaker amp.
- Skip adapters that put a Normal headphone into a Pro socket.
For how electrostatics compare with the other headphones here, see the electrostatic page and our guide to impedance and your amp. Every Stax we cover is on the Stax page.
