MPA685 Owner’s Manual
3/2012
4
INTRODUCTION
The Ingram Engineering MPA685 is a dual channel audio preamplifier designed for recording
amplified or instrument level signals with the highest quality and flexibility.
Microphones of any type and with any output impedance may be used. The preamplifier has
variable input impedance that can be presented to microphones for maximum flexibility in
capturing the audio accurately or in shaping the tones to taste. Two ranges of impedances are
possible, and the range is installed at the factory. Your pre-amplifier has been configured at
the factory for the high impedance range, and its model number is 110685-20, referred to as
“Option 2” in this Owner’s Manual. This range consists of 600, 1.5k and 2.5k Ohm
impedances that may be presented to microphones. This impedance range is particularly well
suited to ribbon microphones, as they typically require higher impedance load for optimum
performance. However, the extremely accurate handling of transient signals, low noise,
extended low and high frequency response and other features enable this preamplifier to be
used with any microphone type and any audio signal with outstanding results.
DESCRIPTION
The MPA685 provides two channels of 100% discrete semiconductor-based amplification of
microphone and instrument signals.
Audio content can be input to the MPA685 via the balanced XLR input, pin 2 Hot, on the rear
panel, or to the ¼” jack on the rear panel, labeled “Opt”, on your unit. The 1/4” jack is fully
balanced.
The balanced microphone input uses an input transformer for high CMRR, optimum noise and
impedance matching and smooth tonal character over a wide range of microphone types.
Your pre-amplifier is configured with a custom Sowter transformer, designed for and exclusive
to Ingram Engineering, The pre-amp is designed to provide the input transformer with constant
impedance load, independent of pre-amp settings, therefore guaranteeing consistently
excellent transformer transient, frequency and phase response, as well as impedance and
noise matching.
The impedance load presented to the microphone can be varied so that tonal and dynamic
characteristics of the audio can be adjusted to taste. Different impedances are selected using
a front panel switch. The impedance seen looking into the balanced microphone input is
switched between 600, 1.5k and 2.5k Ohms.
