Matrix Audio HPA-3U - Manual (Page 7)

Settings & Usage
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Power On/Off
Connected the power plug to the HPA-3U, turn on the power switch, the
working LED indicator will light up about one second, then the HPA-3U will
enter the working status.
Switching Input Channels
Using front panel input source switch for switching between the RCA input
and USB input.
Gain Control
The gain switch on the front panel, can set the gain at 5dB, 10dB or 20dB.
Volume Control
Using the volume knob to adjust the volume, to avoid excessive volume
causes hearing damage, please make sure the volume knob on the suitable
position before power on.
Headphone Output
The 1/4" headphone jack is a stereo interface, please do not connect the
mono plug into it, otherwise it will have the risk to damage your device.
The rear panel line output will automatically muted when headphone jack
is plugged in.