MPA685 Owner’s Manual
3/2012
18
If a multimeter is accessible, it can be used to check for AC or DC voltage on the DI cable
shield coming from the instrument.
Note that the impedance of a circuit connected to the direct instrument loop-through will be
in parallel with the pre-amplifier Direct input 1 Meg
Ω
impedance. The load impedance of
the device connected to the loop-through may therefore dominate the instrument load if the
loop-through load is less than 1 Meg
Ω
. This is why the suggestion is made to remove the
loop-through connection and check the specifications of the device that was connected to
the loop-through if signal degradation is experienced when using the loop-through
connection.
5. Hum or Buzz is Audible
A ground lift toggle switch is included on the rear panel to isolate input and/or output XLR
Pin 1 from chassis. If hum or buzz is audible, try changing the switch position. Note that
Pin 1 is permanently connected to audio ground for maximum stability with highest gain
settings, so the toggle switch only floats audio ground from chassis ground. In the event
that the ground lift toggle switch does not eliminate hum, Pin 1 of the cable connected to
the pre-amplifier
output
may be disconnected at the connection to the pre-amplifier.
If hum or buzz encountered when using the DI and/or loop-through is not solved by toggling
rear panel ground lift switches, ground system currents may be flowing through the
instrument to pre-amplifier chassis or instrument power amplifier, if used. If possible, pre-
amplifier, instrument and power amplifier AC power should be provided from the same
electrical service. In the cases where a power amplifier is used, this can be done by
placing the power amplifier head in the vicinity of the pre-amplifier and then running the
amplified speaker signal to the speaker cabinet in an isolation booth. If the power amplifier
must be placed a distance from the control room and pre-amplifier, an alternative is to use
an external isolation transformer on the DI signal, installed between the interface of the pre-
amplifier to the cable that connects the pre-amplifier to the power amplifier.
CONTACT INFORMATION
Ingram Engineering
PO Box 5870
Atlanta, Ga. 31107, USA
Phone +1 (678) 685-9838
Fax +1 (404) 622-0036
URL: http://ingramengineering.net
Email: [email protected]
