Benchmark DAC2 DX Manuel d'instructions télécharger pdf (Page 36)

DAC2 DX Instruction Manual Rev B Page 36

HPA2™ Headphone Amplifier

The DAC2 headphone output is driven by

Benchmark’s signature HPA2™ headphone

amplifier. This high-current, high-output

amplifier has an output impedance of near 0-

Ohms. It is designed to drive loads as low as

30 Ohms without any increase in distortion.

It also has sufficient amplitude to drive low-

sensitivity 600-Ohm headphones.

The HPA2™ includes current-limiting circuits

that fully protect against damage from short

circuits. This is important because the right

channel of a headphone amplifier will

experience a short whenever a mono phone

plug is inserted into the stereo headphone

jack. Shorts may also occur when a plug is

partially inserted.

'0-Ohm' Output Impedance

Most headphone amplifiers use series

resistors to maintain stability and protect

against short-circuit conditions. These

resistors are usually at least 30 Ohms, and

have a negative impact on performance. A

headphone amplifier with series resistors may

measure very well when driving resistive

loads. However, the same amplifier will

measure very poorly when driving a

headphone load. Unfortunately, most

manufacturers do not specify headphone

amplifier performance with anything other

than ideal resistive loads. Our measurements

show that headphones do not behave like

resistive loads.

Headphone Performance

In our tests we have measured substantial

distortion across resistors that are wired in

series with headphones. We conducted

measurements with a variety of headphones.

In general, distortion increases as headphone

impedance decreases. This distortion can be

eliminated with a properly designed 0-Ohm

headphone amplifier.

The performance of the HPA2™ does not

change when headphones are driven. THD+N

measurements for no-load, 30-Ohm resistive

loads, 30-Ohm headphone loads, and 600-

Ohm headphone loads are virtually identical.

The HPA2™ will substantially improve the

sound of 30 and 60-Ohm headphones. It will

make very noticeable improvements with

600-Ohm headphones.

Differential Amplifiers

Differential amplifiers remove common-mode

distortion components from the D/A converter

outputs. This feature is critical for achieving

low-distortion in down-stream devices.

Benchmark addresses common-mode

distortion so that it will not cause distortion in

power amplifiers and other connected

devices. Common-mode distortion can cause

audible distortion while escaping the scrutiny

of an audio analyzer. The balanced and

unbalanced outputs on the DAC2 deliver very

similar performance.

Many D/A converters omit the differential

amplifiers after the converters.

Specifications usually ignore common-mode

distortion. A balanced signal with high

common-mode distortion can measure just

fine when feeding a precisely balanced input

on a high-quality audio analyzer. However,

any imbalance in a downstream device will

expose the common-mode distortion.

UltraLock2™ Clock System

Accurate 24-bit audio conversion requires a

very low-jitter conversion clock. Jitter can

very easily turn a 24-bit converter into a 16-

bit converter (or worse). There is no point in

buying a 24-bit converter if clock jitter has

not been adequately addressed.

Jitter is present on every digital audio

interface. This type of jitter is known as

‘interface jitter’ and it is present even in the

most carefully designed audio systems.

Interface jitter accumulates as digital signals

travel down a cable and from one digital

device to the next. If we measure interface

jitter in a typical system we will find that it is

10 to 10,000 times higher than the maximum

allowable level for accurate 24-bit conversion.

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Benchmark DAC2 DX Manuel d'instructions Page 36

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