Microphone Preamplifier System






Unpacking SAVE THE PACKING MATERIAL for return shipping or carrier inspection and notify Gordon Instruments of any damage. Enclosed with each preamp is an accessories pouch.

Rack kits (a pair of rack ears and four mounting screws) are attached to the sides of the preamp (kit with handles) and gain control (kit without handles) for rack mounting.

Outriggers (a pair and four mounting screws) are attached to both ends of either preamp side panel for freestanding applications. The rack ears can self-store by mounting on both ends of the side panel opposite the outriggers.

Placement For best results, the preamp should be located as close to the microphone or instrument as possible, avoiding high vibration or sound pressure levels, with the gain control at the monitor location. Audio cable length and connections should be kept to a minimum, particularly between the microphone and preamp input. In addition to cables for the two audio outputs, a third cable is required for control signals to each preamp from the gain control.

Preamp installation The Gordon preamp runs warm and, like any electronic gear, has better reliability when kept cool; however, there are no special cooling requirements. In a rack or case, when possible, maintain a 1U rack space above or below for cooling and avoid installing the preamp next to warmer equipment.

The front (audio) half of the preamp should be located away from any magnetic interference sources. The power supply magnetic components have magnetic shields (to contain magnetic interference); however, the audio circuitry does not. Magnetic materials (including shielding, typically steel) in or near the audio signal path are also a source of magnetic interference and have been omitted in the Gordon preamp. Indications of magnetic interference are distortion (less clarity) and noise (hum); actual use has shown these are rare.

Cooling and magnetic interference are not a problem with freestanding applications.

Gain control installation The gain control does not handle audio signal and has no installation restrictions.

Preamplifier front panel

GAIN CONTROL connector is a male three-pin XLR for connecting a standard microphone cable as a control line to the gain control. The control signal is a dc voltage, capable of tolerating noise and several hundred meters of cable.

Pin 1 Screen/return/case
Pin 2 Channel A control signal
Pin 3 Channel B control signal

INPUT connectors Parallel, balanced " phone and female three-pin XLR accept balanced and unbalanced sources. Pinout:

Sleeve/pin 1 Screen/common/case and negative phantom supply
Tip/pin 2 Positive signal and positive phantom supply
Ring/pin 3 Negative signal and positive phantom supply

CAUTION - Mute before changing input connections.

LOW INPUT Z switch changes the input bridge impedance from 2M ohms to the low input impedance noted on the rear panel (1k ohms standard) when depressed.

Input impedance The Gordon preamp input is voltage controlled and requires neither current from the input source nor an impedance match (no input transformer). This permits a high input impedance (LOW INPUT Z switch out) that eliminates current-related distortion from mics and interconnections and is suitable for DI sources. The low input impedance (LOW INPUT Z in) diminishes this characteristic and can provide damping, if desired, for some microphones.

+48V switch turns on the phantom supply, applying +48 volt dc to the input via 6.81k ohm resistors.  Audio out is muted briefly when the phantom supply is turned on or off.

CAUTION - Do not change input connections while the phantom supply is on.

OUTPUT connector is a balanced male three-pin XLR with direct-coupled line drivers on pins 2 and 3. Unbalanced loads should be connected across pins 2 and 1 (pin 3 open) and will lose 6dB of gain. Sensing for load compensation and the peak indicator is on pin 2. Neither pin 2 nor 3 should be grounded, as electronic protection will clamp both in case of an overload. The outputs feature soft power up and power down with an impedance (to ground) of about 20 ohms when the preamp is off.

Pin 1 Screen/common/case
Pin 2 Positive signal
Pin 3 Negative signal

With INVERT PHASE switch in:

Pin 1 Screen/common/case
Pin 2 Negative signal
Pin 3 Positive signal

Output load compensation The operating parameters of the output stage can be varied to optimally drive any output load. The Gordon preamp senses the combined impedance connected to the output (determined by cables, input impedance, etc.) and sets the operating parameters for lowest distortion automatically.

STATUS lamp indicates normal operation of the preamplifier and gain control. The STATUS lamp is off when the ac supply is interrupted and flashes under the following conditions:

  • Ac supply turn on/low line enables the mute function for thirty seconds at turn on and for the duration of ac low line.
  • Phantom supply turn on/off enables the mute function and lasts for less than thirty seconds.
  • Gain control fault enables the mute function and can be caused by an ac-coupled, shorted, broken, or disconnected control line.
  • Output overload enables electronic protection to limit the output signal.

Standby is enabled by a lengthy mute and is indicated by a shorter flash than other conditions.

A flashing STATUS lamp in the absence of the above causes may indicate an internal fault.

Rear panel

IEC-320 power connector/fuse holder Connect the supplied power cord here and to a grounded ac line with voltage as marked on the rear panel (100V, 120V, or 240V). After the turn-on sequence (about thirty seconds), twenty minutes of warm up is recommended. Production burn in (150 hours with signal applied) was performed before final testing and the preamp will continue to "season" with use.

WARNING Do not break the safety ground connection through the power cord.


  • For 100V 0.63A, time-lag, 5x20mm (two required, included)
  • For 120V 0.5A, time-delay, 1/4x1-1/4in (included)
  • For 240V 0.25A, time-lag, 5x20mm (two required, included)

WARNING Replacement fuses (not included) must be the same type and rating.

To replace a fuse, disconnect the power cord at the preamp and locate either of two small slots in the edge of the fuse holder, adjacent to the socket for the power cord. Insert the blade of a small screwdriver and gently pry open the fuse holder cover to reveal the fuse block. Gently pry out the fuse block, change the fuse(s), reinsert the fuse block, close the cover, press until it snaps in flush, and connect the power cord.

Gain control front panel

CHANNEL A, CHANNEL B gain control sets the gain for each channel in 5dB steps from 10 to 70dB.

True gain control Gain in the signal path is the greatest source of distortion in a preamp. The Gordon preamp inserts only the amount selected on the gain control by using switched, variable-gain amplifiers (no attenuators). For each gain setting, the signal path is automatically configured for the least signal processing at minimum distortion and maximum dynamic range. Without attenuators or loop feedback, distortion drops with gain, noise is much less at lower gain settings, and roll-off and balance shift are minimized over the gain range.

+24dBu peak indicator monitors pin 2 out on each channel and flashes a minimum of 100 milliseconds (for transients) up to a continuous one-second blink to indicate level is within 6dB of maximum output.

MUTE switch, when depressed, overrides the adjacent gain control to provide a full audio mute on each

Standby is enabled after a channel has been muted for 10 hours, reducing bias, phantom, and power consumption. Unmute to disable standby.

INVERT PHASE switch inverts the output by reversing the signals to pins 2 and 3.

Rear panel

PREAMP 1, PREAMP 2 connector is a female three-pin XLR for connecting control lines from preamplifiers.

Pin 1 Screen/return
Pin 2 Channel A control signal
Pin 3 Channel B control signal