Instruction Manual for HPA4 with 1.0.X Firmware – REV B Page 29
THX-888 Amplifier
The headphone outputs are driven by the
flagship THX-888 amplifier. This headphone
amplifier incorporates the same patented THX
AAA™ technology that Benchmark uses in the
AHB2 power amplifier. The HPA4 is designed
to deliver power, audio fidelity, speed and
accuracy over an utterly silent background.
THX AAA™ reduces harmonic,
intermodulation, and crossover distortion by
20 to 40 dB to guarantee a realistic and
fatigue-free listening experience. It
accomplishes this using a patented feed-
forward topology to null conventional
distortion and noise mechanisms. THX
AAA™ allows the amplifier to reach its
maximum output power and sound pressure
level (SPL) without producing the distortion
that normally accompanies increased output
levels in traditional amplifiers.
Headphone and Hearing Warnings
CAUTION: The THX-888 headphone
amplifier in the HPA4 is capable of delivering
peak-to-peak voltages of about 25 Volts. It is
also capable of delivering 1.5 Amps. These
levels are high enough to damage most
headphones (and your ears). Use caution
when adjusting playback levels.
CAUTION: Do not drive a set of headphones
while they are sitting on a desk or stand. It is
easy to accidentally overdrive them. This is
especially true when you have set the
headphones aside while listening to speakers.
Mute or unplug your headphones when they
are not in use!
Soft-Start Circuit
The THX-888 uses relays to mute the
headphone output until the power supplies
have reached normal voltages.
Protection Circuits
The THX-888 headphone amplifier has
several circuits that are intended to protect
the amplifier from overloads and short
circuits. Fault conditions will disable a pair of
output relays which will disconnect the
amplifier from the output jacks. The output
relays are only energized when all conditions
are normal.
Over Current Protection
If the output current exceeds 1.5 Amps on
either output channel, the protection relays
will disconnect the amplifier from the output
jacks. The connection will be restored after a
few seconds. The protection cycle will repeat
if necessary. This protection mode may
engage if headphones are plugged into the
1/4" TRS jack while audio is playing. TRS
plugs can cause a temporary short-circuit
condition while the plug is being inserted.
Clip Detection
If the headphone amplifier is driven into
clipping, the protection relays will disconnect
the amplifier from the output jacks. If clipping
ceases, the connection will be restored after a
few seconds.
DC Detection
If DC is detected on the output of the
headphone amplifier, the protection relays
will disconnect the amplifier from the output
jacks. This protection circuit is intended to
protect the headphones in the unlikely event
of a catastrophic component failure.
Temperature Limiting
If the THX-888 is severely overloaded for a
period of time, component temperatures may
start to climb. If safe operating temperatures
are exceeded, the protection relays will
disconnect the amplifier from the output
jacks. After the drivers have cooled, the
connection will be restored. This cooling cycle
may take several minutes.
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