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<title>Kevin Boone: Snake oil: balanced headphones</title>
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<h1>Snake oil: balanced headphones</h1>
<p>
<img class="article-top-image" src="img/snakeoil.png"
alt="snake oil"/>
In recent years there has been a trend for high-end audio equipment
-- even portable music players -- to provide "balanced" outputs
for headphones. There's very little evidence that the use of
such outputs improves sound quality in blind testing, although it
adds significantly to the cost and complexity of the equipment.
</p>
<p>
There doesn't even seem to be a clear understanding among the buying
public about how balanced headphones are supposed to work. The
term "balanced" is often used to describe a scheme where the left
and right speakers of the headphones have separate amplifier connections.
As I will explained, a genuine "balanced" headphone connection
requires more than this -- and still probably won't provide
any benefit.
</p>
<p>
My suspicion is that the manufacturers of equipment of this type
are exploiting the fact that fussy, technically-savvy buyers will
already be aware of the (perfectly legitimate) use of balanced signal
connections between other audio equipment. However, the fact that there
are, in fact, perfectly appropriate applications for balanced audio connections
does not mean that that headphone amplifiers represent such an application.
</p>
<h2>Conventional three-wire headphone connection</h2>
<p>
The figure below shows how typical inexpensive headphones (and
some expensive ones) are connected
to an amplifier. There are three conductors between the amplifier and
the two headphone speakers -- one conductor supplying terminal of
each speaker, and one conductor acting as the common return path
for current.
</p>
<p align="center">
<img src="img/headphone1.png" alt="circuit diagram" width="650px"/>
</p>
<p>
I've included some figures for the equivalent resistances of the
various parts of the circuit; they aren't intended to be accurate:
they're just a starting point for understanding the circuit
behaviour. I'm also aware that neither the cables nor the speakers
are true resistances, but that doesn't really change the analysis.
</p>
<p>
V<sub>L</sub> and V<sub>R</sub> are the voltages at the left and
right amplifier outputs. These are measured with respect to the
common conductor which, for simplicity, I will take to be zero
volts.
</p>
<p>
If the right output were not connected to anything, then the left
speaker would be (in this simple representation) a resistance of
ten ohms in series with the cable
resistance of two ohms. This circuit would effectively be a voltage
divider in the ratio of 10/12. Apart from the slight loss of
volume, the cable resistance has no effect on audio reproduction.
</p>
<p>
When both the right and left outputs are connected, however,
the voltage V<sub>common</sub> at the common conductor will depend
on <i>both</i> the left and right voltages, as both contribute
a flow of current in the cable resistance in the common path.
This means that both speaker voltages will depend, to some extent,
on both output voltages. This is <i>cross-talk</i>, and is rightly
considered to be a Bad Thing.
</p>
<p>
With the resistances shown, the cross-talk is likely to be
substantial. Very approximately, about 10% of the voltage
seen by the left speaker will come from the right output.
In reality, though, two factors make this effect less
significant. First, in most music the difference the left
and right output level is not that great most of the time.
Of course, it's great enough to make it worth recording in
stereo, but it's still comparatively small.
</p>
<p>
Second, and more important, the cable resistance won't be anything
close to one ohm. With decent-quality cable and connectors,
the resistance will be of the order of milliohms. The amount
of cross-talk will be small, in a context where the difference
between left and right signals is already quite small.
In practice, cross-talk can't be eliminated: in any headphone
there will be some unavoidable cross-talk
through the listener's skull. With open-back
headphones, there will also be cross-talk through the air around
the skull. Without elaborate suspension arrangements, there will
be cross-talk across the headphone's head-band. In short,
we're looking at electrical cross-talk that is likely to be
smaller than unavoidable mechanical cross-talk. It's worth thinking
about how much trouble we should go to to eliminate electrical cross-talk,
when mechanical and acoustic cross-talk is inevitable.
</p>
<h2>Conventional four-wire headphone connection</h2>
<p>
While not all sources of cross-talk can be eliminated, there's
no harm in trying to mitigate the problems that we actually
can mitigate, if it's practicable to do so.
A significant improvement
can be made simply by using a four-wire connection between the
amplifier and the headphone, as shown below.
</p>
<p>
The cable resistance still forms a voltage divider with the speaker
coil, but now there's no resistance in a common current path.
</p>
<p align="center">
<img src="img/headphone2.png" alt="circuit diagram" width="650px"/>
</p>
<p>
Note that this is <i>not</i> a "balanced" design. Although balanced
designs use four (or more) headphone conductors, not all four-wire
arrangements are "balanced" in the sense under discussion here.
In this four-wire arrangement, there is still a common point
in the amplifier, but it splits in the headphone jack. So long
as the amplifier's output impedance is low, no voltage is developed
at the common point that can contribute to cross-talk.
</p>
<p>
There's nothing clever here: most full-size loudspeakers are connected
using this four-wire set-up, with a common point inside the amplifier.
Nor is it original: many decent headphones already use separate left
and right connections all the way from the headphone jack to
the headphone's speakers.
</p>
<h2>True balanced headphone connection</h2>
<p>
A true balanced design eliminates the common point completely.
Each speaker is (usually) supplied with an equal and opposite
pair of voltages. There is still a "zero volt" reference point
somewhere in the amplifier, but it isn't directly applied
to any headphone connection. In the diagram below, the "+" and
"-" terminals are supposed to represent the complementary
outputs of the amplifier.
</p>
<p align="center">
<img src="img/headphone3.png" alt="circuit diagram" width="650px"/>
</p>
<p>
I'm not going to go into any details about how a complementary-output
amplifier is designed; it's pretty conventional stuff, although somewhat
more complex than a "single-ended" amplifier that does not have
these balanced outputs.
</p>
<h2>But does it work?</h2>
<p>
What advantage, if any, is offered by the use of balanced
outputs (compared to a four-wire connection with a common point)?
Proponents claim that these designs reduce cross-talk.
However, we've already established that the use of a conventional
four-wire connection
will more-or-less eliminate electrical cross-talk,
provided that the output impedance of
the amplifier is low enough. And nothing can be done electrically to
reduce acoustic and mechanical cross-talk, which could be much larger
in practice. Despite vague claims, there's no reason to think that
anything additional is gained by using
balanced outputs. Why? In the end, the headphone speakers are
completely independent of one another; each will "see" the
same potential difference whether we express that as
"one volt relative to a common point" or "0.5 volts relative to -0.5 volts".
That's the crucial point here: headphone speakers are
not electronic components
operating with some kind of common electrical supply -- they are
completely separate, electro-mechanical components.
</p>
<p>
This, I think, is where the confusion comes in. It's very common
-- often necessary -- to use balanced connections when supplying
an electronic amplifying device from some other device, or
from a transducer. For example, studio microphones
almost always use balanced connections. Why? Because electrical
interference that is induced on the microphone cable will tend to affect
both "arms" of the balanced connection equally. So when the balanced
microphone feeds a balanced microphone amplifier, the subtractive
operation of the amplifier removes, or at least reduces, the
interference.
</p>
<p>
Unlike a microphone amplifier, a headphone's loudspeakers are not
subtractive devices -- they simply respond to the potential
difference between their terminals. So even in the unlikely event
that interference could be induced onto a headphone cable that is
loud enough to hear, the use of balanced connections will
<i>do nothing at all</i> to ameliorate the situation. The benefits
of balanced connections appear only when the receiver of the
signal is subtracting the balanced voltages, relative to
some common voltage point, to remove the interference.
</p>
<p>
So why do some people claim -- and honestly believe -- that the use
of balanced connections improves headphone performance? Well, of course,
there's always the
"hi-fi placebo" effect whenever someone spends a lot of
money on complicated equipment. It's possible that the balanced
headphone design will use generally superior electrical
connections and cables, with a lower overall impedance. It's also plausible
that the manufacturers of balanced headphone equipment will have made
a better job of the design overall, compared to traditional
headphone amplifiers. My gut feeling, however, is that this
is somewhat unlikely -- balanced amplifier designs are more complex
than single-ended designs, and the higher component count leads to
higher noise and increased output resistance. Even in situations where
balanced connections <i>might</i> be expected to have benefits, their
benefits have to be weight against their costs. Electric guitars,
for example, almost never provide them, although the technology to
do so certainly exists.
</p>
<h2>Summary</h2>
<p>
Balanced headphones are snake oil. Although balanced operation has no
benefit in itself, it is arguably the case that manufacturers will
use superior design and components generally, to accommodate discerning
buyers. However, if this practice leads to improved audio
quality, the connection between the improvement and
the balanced operation is merely incidental. Balanced designs require
increased complexity, and considerable attention to detail is needed
even to <i>equal</i> the performance of a single-ended design,
let alone exceed it.
</p>
<p>
In a way, balanced headphones are marketed in a way that is analogous
to the marketing of the
(also ineffective) "bi-wiring" of loudspeakers. It's possible to make a
technical case for bi-wiring that has some vague plausibility,
but it doesn't stand up to analysis.
The difference, though, is that providing bi-wiring terminals on a
loudspeaker adds only pennies to the cost; balanced headphone
amplifiers are necessarily more complex, and thus more expensive,
than traditional designs.
</p>
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