Balanced headphone amps are not new, but iFi are atypical in their use of TRRRS connectors.
Recent hi-fi purchases from iFi seem to bristle with TRRRS Pentaconn sockets for balanced inputs and outputs, including headphone outs. I’ve never seen such a beast and I can’t find any sensible comments on the Internet. Something within me rails at the thought of yet another headphone connector that’s not a quarter‑inch TRS plug! Am I missing something here?
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: iFi are the only company I’ve met to date that are using the Pentaconn TRRRS [Tip /Ring /Ring /Ring /Sleeve] connector for stereo balanced audio. In the pro world, people tend to use either separate TRS/XLR plugs or five‑pin XLRs for that purpose, but I can see the attraction, even if I’ve no idea how robust the 4.4mm Pentaconn plug/socket will turn out to be in practice.
As for balanced headphones, yes that’s definitely ‘a thing’, but it’s often very poorly explained and understood, and in most cases it’s not actually balanced!
Conventionally, headphones have three connections: positive from left amp to left earpiece, positive from right amp to right earpiece, and a shared return to ground from both earpieces’ negative connections. Hence requiring a TRS plug (either quarter‑inch or 3.5mm). But the shared ground wire has a finite resistance, so the signal from the left amp will inherently create a small voltage across that common return wire. That voltage then adds to the signal from the right amplifier, causing a low level of crosstalk between channels. In practice it’s irrelevant and inaudible in all but the most extreme situations, but some people don’t like it and to be fair it is poor engineering.
A better solution is to keep the wires to each earpiece separate, and that requires four wires: positive and negative for the left and the same for the right. The negative sides are usually both grounded at the amplifier, so it’s functionally the same as the three‑wire connection, except that the grounding is at the right place to prevent unwanted crosstalk.
This four‑wire connection is often achieved with a four‑pin XLR or sometimes two three‑pin XLRs — or, now, the Pentaconn TRRRS plugs.
This four‑wire connection is often achieved with a four‑pin XLR or sometimes two three‑pin XLRs — or, now, the Pentaconn TRRRS plugs (adding an overall screen to the cable). Few headphone manufacturers wire their headphones with four‑core cables to allow this, but several of Sennheiser’s HD models do, for example, joining the negative sides only in the quarter‑inch/3.5mm plug itself, thus avoiding any risk of crosstalk.
It’s also possible to drive headphones with a properly balanced (ie. symmetrical/inverted) signal. This requires a ‘bridged amp’ configuration where the positive connection is driven by one amp and the negative connection by another, with the latter using an inverted signal. So you then have two amps per earpiece working in a kind of push‑pull arrangement. There’s no ground connection at all at the headphones, no crosstalk between channels, and potentially twice the voltage swing for a given power‑rail voltage. It is a technique used occasionally for battery‑powered devices to get more level into the headphones than would otherwise be practical — and sometimes by manufacturers looking for a USP (unique selling point) to distinguish their offerings from the crowd!