Benchmark HPA4 - Headphone Outputs; Driving Two Sets of Headphones; Standard 1;4 TRS Connector; High Performance XLR4 Connector

Instruction Manual for HPA4 with 1.0.X Firmware – REV B Page 28
Headphone Outputs
The left-hand jack is
a traditional 1/4" TRS
headphone jack. The
L & R channels share
a common return.
The right-hand jack is
a 4-pin XLR
headphone jack that
features dedicated
returns for each
channel.
Driving Two Sets of Headphones
The HPA4 has more than enough power and
output current to drive two sets of
headphones simultaneously. The two
headphone jacks can be used simultaneously,
but the two sets of headphones need to have
similar sensitivities. If the sensitivities are
different, one set of headphones will play
louder than the other.
The main purpose of the second connector is
to provide a high-performance alternative to
the traditional TRS phone jack.
Standard 1/4" TRS Connector
The HPA4 included a traditional 1/4" tip-ring-
sleeve (TRS) headphone jack. This provides a
convenient output that is compatible with
most headphone cords. Slightly higher
performance is available from the XLR4
connector.
In our tests we have found that the TRS
contacts can occasionally produce more
distortion than the entire HPA4 headphone
amplifier. We have selected one of the
highest quality TRS jacks available, but the
performance of the TRS jack is highly
dependent upon the quality and condition of
the headphone plug. Gold plated plugs are
highly recommended.
The traditional 1/4" TRS connector uses the
sleeve as a common ground for the left and
right channels. This connection carries twice
the current carried by the other contacts.
High contact resistance at the sleeve
connection can cause distortion on both
channels.
With many TRS plugs we have found that the
tip connection is somewhat less reliable than
the ring connection. This means that there is
a higher chance of distortion being added to
the left channel.
Tip: If the TRS plug is gold plated, clean and
in good condition, the TRS jack can match the
performance of the XLR4. When conditions
are less than ideal, the TRS contact resistance
can be the largest source of distortion in the
entire HPA4 headphone system.
High Performance XLR4 Connector
The 4-pin XLR connector provides dedicated
pins for the left and right returns. There are
no shared connections. In the HPA4, the left
and right outputs are differentially driven
relative to their respective returns.
The separate returns eliminate any interaction
between the channels. More importantly, the
XLR4 connector provides much lower contact
resistance than the traditional 1/4" TRS
connector.
Our tests have shown that XLR jack and plug
combinations provide very reliable electrical
connections. No measurable distortion was
produced by the XLR4 connector in any of our
tests.
Tip: The Volume Control simultaneously
adjusts the level for both headphone jacks. If
two listeners will be simultaneously using the
headphone outputs, we recommend using
headphones with identical or very similar
voltage sensitivities.