Table of Contents
Advertisement
HPA2™ Headphone
Output
The output of the solo bus is directed to the
"monitor" fader and then to the headphone
and "control room" outputs.
The headphone output is driven by
Benchmark's signature HPA2™ headphone
amplifier. This high-current, high-output
amplifier has an output impedance of 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.
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
PRE420 Instruction Manual – rev. E
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
load, and 30-Ohm headphone load, and 600-
Ohm headphone load 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.
A
Page 14
Advertisement
Table of Contents
