miniDSP Ltd, Hong Kong /
/ Features and specifications subject to change without prior notice
20
3.5
E
FFECT OF THE AMPLIFIER
(
AND
DAC)
Everywhere else in this manual, it is assumed that the amplifier (and the DAC if you have one in the
measurement chain) are perfect. In practice, this may not be the case.
3.5.1
Amplifier frequency response
Figure 10 shows a typical amplifier (and/or DAC)
frequency response. If your amplifier (and DAC) “droops” in
frequency above 20 Hz or below 20 kHz, this will affect your headphone measurement.
3.5.2
Output impedance
If your headphones have a non-constant impedance
and
your amplifier has a high output impedance (say 10% or
more of the headphone’s nominal impedance), this
may
affect your measurements. The
worst case
variation is
given in this table, where Zamp
is the amplifier’s output impedance and Z
phones
is the headphone’s nominal
impedance:
Zamp/Zphones (%)
10 20 50 100
Worst case variation (dB)
1
2
3.6
6
In practice, real headphones will have less variation than this. For example, planars have an almost constant
impedance and so will not show variation due to the amplifier output impedance.
3.5.3
What to do
To make headphone measurements that are not affected by the amplifier, a solid-state headphone amplifier
with a bipolar power supply (and no coupling capacitor on the output) will usually be a good choice. The criteria
are flat frequency response to the measurement limits and an output impedance less than 10% of the
headphone’s impedance.
In some cases, you may wish to do a measurement and then EQ that
includes
the effect of the amplifier. That
way, the EQ compensates for the amplifier as well as the headphones. In that case, measure using the same
amplifier that you plan to listen with.
Figure 10. Typical DAC+amplifier frequency response (exaggerated)
