This Myth Goes Something Like This:

"Balanced headphone amplifiers are better."

"If balanced line-level connections work well, balanced headphone outputs should also work well."

We disagree!

Benchmark does not offer voltage-balanced headphone outputs on any of its products. The reason for this is that a voltage-balanced interface serves no useful purpose when driving headphones. The truth is that a conventional single-ended headphone drive is technically superior to a balanced drive. This paper explains why single-ended headphone amplifiers are inherently more transparent than balanced headphone amplifiers.

We do offer a 4-pin XLR connection on the HPA4 headphone amplifier, because this 4-pin connection offers lower contact resistance than a traditional 1/4" TRS connection. For performance reasons, this connector is not driven with a voltage-balanced signal. Instead, it provides isolated left and right ground returns that are individually connected to the ground reference points in the left and right amplifiers. This high-quality XLR connector provides very low contact resistance and this eliminates the distortion that can be produced by the much higher contact resistance of a TRS connector. The THX-888 headphone amplifier in the HPA4 is virtually distortion free and this makes it easy to measure the distortion caused by poor TRS connections. For convenience, we also provide a TRS jack. If the TRS jack must be used, we recommend using a gold-plated plug that shows no signs of plating wear. We find that the gold plating on TRS plugs wears quickly when headphones are plugged and unplugged.

Please understand that Benchmark strongly encourages the use of balanced line-level interfaces between audio products. All of our products are equipped with fully-balanced XLR analog inputs and outputs. In these box-to-box line-level connections, a balanced interface can provide substantial performance improvements. Balanced line-level interfaces reject hum and noise while providing a higher voltage level. The higher signal levels can improve the SNR (signal to noise ratio) of the audio system.

If balanced connections work so well between audio boxes, it seems reasonable to expect improvements when driving headphones with a voltage-balanced feed. This expectation provides a marketing opportunity for any audio manufacturer that is willing to exploit customer expectations. Many are willing, and this is how audio myths are born and nurtured. Benchmark does not promote these myths nor does it embrace trends that do not contribute to transparent sound.

This expectation that balanced headphone amplifiers should be superior to single-ended amplifiers, ignores some of the most basic laws of electricity. The current into and out of a two-wire network is always equal and opposite. This means that the two-wire transducers used in headphones always see fully-balanced current. It also means that the headphone drivers are completely ignorant of the difference between single-ended and balanced-drive voltages.

Let me spell this out again: Headphone transducers are balanced devices. They have two wires. The electrons that flow into one wire must flow out of the other. The current is always balanced. The headphone transducers cannot detect the difference between a single-ended drive and a voltage-balanced drive. The system is balanced with either type of voltage drive.

But, this does not mean that balanced and unbalanced headphone amplifiers will sound the same! In many cases, voltage-balanced headphone amplifiers will produce more noise and more distortion than single-ended amplifiers of an identical design. The reason for this is that two separate output amplifiers are required in a voltage-balanced amplifier, and each must drive one half of the transducer's load impedance. The output noise will double because there are two amplifiers instead of one. The damping factor will degrade by a factor of 2 because both amplifiers contribute to the source impedance of the balanced amplifier (output impedance is doubled). Distortion will usually increase because each amplifier is required to drive half of the impedance that would be seen by an unbalanced amplifier. Power consumption will increase by a factor of 4 for a given output level (assuming the power supply voltage rails remain unchanged).

In fairness to balanced designs, I should point out that a balanced output can deliver 4 times the power for a given power supply voltage. However, this would only be necessary if the product was battery operated. The balanced output would offer no real advantage in an AC powered amplifier.

Please note that distortion is not always a bad thing (see Two Distinct Types of Audio Products). Many audio products are designed to add some harmonic distortion in an effort to add warmth to the audio reproduction. This is an artistic choice that will change the way a recording sounds. If the goal is to add distortion, a balanced headphone amplifier may produce more harmonics than a single-ended design.

In contrast, Benchmark products are designed for maximum transparency. Our goal is to build products that faithfully capture and reproduce music. This means that our products are not designed to alter the sound of a recording. This transparency is critical in a studio monitoring chain. We feel that this also makes our products well-suited for home playback when a listener wants to hear the music the way it was heard in the studio. A voltage-balanced headphone amplifier would run contrary to our goals.

The Benchmark DAC1 and DAC2 converters include Benchmark's highly-transparent HPA2™ headphone power amplifier. The near-zero Ohm output impedance of the HPA2™ gives it outstanding control over the headphone drivers, while its low distortion keeps the presentation true to the original recording.

The all-analog HPA4 headphone power amplifier is Benchmark's flagship headphone amplifier. It incorporates the same THX-AAA technology that we use in the AHB2 power amplifier. The HPA4 is built around the flagship THX-888 headphone amplifier and is driven from a Benchmark relay-controlled attenuator and input stage. The HPA4 provides higher output levels than the HPA2 and it can drive lower impedances. The HPA4 is virtually distortion free, even when driving difficult impedances and phase angles.

Note: Updated 10/10/18 to clarify the function of the 4-pin XLR on the HPA4 - JS