Ingram Engineering MPA685 Microphone Preamp Review

Sep 1, 2011 9:00 AM, By Barry Rudolph



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Ingram Engineering’s MPA685 dual-channel microphone preamp is a large, two-rackspace (14 inches deep) unit that’s handmade in the U.S. and features three switchable input-impedance choices and up to 76 dB of gain. Changing the input impedance presented to a microphone by the preamp alters the mic’s sound and operation. For the recording engineer, having this ability expands the range of sounds possible from any microphone.

You can order the retro-looking MPA685 in either of two impedance range options. Option 1 has 60, 200 and 600-ohm impedance choices for passive or active mics that sound best working into lower impedances. Option 2 (the unit reviewed here) has 600, 1.5k and 2.5k-ohm choices, and is better suited to condenser or ribbon mics that require higher impedances.

Input impedances are selectable using a front panel rotary switch that alters a switched resistor network on the secondary winding of a custom Sowter mic input transformer. This method “reflects” the impedance change back to the primary and provides the subsequent preamplifier section with a constant impedance load for good transient response, low noise, phase coherency and flat frequency response over a wide gain range.

Each channel of the MPA uses two Ingram-designed, J-FET-based, Class-A gain block stages, while a third module and an electronically balanced output buffer amp drives a Jensen output transformer. These three encapsulated modules use all-discrete components and the same pin-out and voltages as API’s 2520 and Jensen’s 990 modules.

A front panel 24-position Goldpoint rotary switch sets the gain of the second gain block to optimize its signal-to-noise, dynamic range and transient response predicated upon the mic’s output signal level. The knob’s “0” center position is unity, and rotating clockwise adds gain while turning the control counterclockwise attenuates gain. The front panel markings are for reference and do not indicate gain in dB.

There is 65 dB of gain adjustment in steps that are coarser (6.5 dB) at the low- and high-gain extremes and finer (2dB steps) in the more used, middle-gain range. The MPA also has a regulated +48-volt phantom-power on/off switch and a 1-megohm FET-buffered DI ¼-inch input with loop-through.

For each channel, there are front panel overload LEDs that light when 1-percent THD is reached. Another rotary switch selects the corner frequency for a 6dB/octave highpass filter. Choices include bypass, 70 Hz or 140 Hz. Finally, a continuously variable output gain control finely adjusts total gain.

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