Whirlwind MD-1 User Manual

www.whirlwindusa.com

MD-1 MIC TO LINE DRIVER / HEADPHONE MONITOR

The Whirlwind MD-1 is the culmination of many years of joint product development
between our engineers and suggestions from professionals such as you. This compact
portable unit combines a high quality microphone preamp -line driver with a headphone
amplifier designed to monitor the mic preamp or be used independently. The MD-1
employs two 9-Volt batteries and provides enough gain to drive both headphones and
long signal lines and allow for maximum headroom in the mic preamp.

FEATURES

• Up to 66 dBm of gain adjustable with analog style potentiometer.

• Finest quality mu metal shielded input transformer.

• Wide range frequency response.

• Low input noise and harmonic distortion.

• Low susceptibility to radio frequency interference.

• Balanced, transformer isolated input and output connections.

• Headphone section monitors main input, separate balanced headphone line input
or mix of both.

• Rugged construction.

• 9 Volt battery power with reasonable battery life.

CIRCUIT DESCRIPTION

The input is a via a female XLR connector with a switch for mic or line level mode. In
MIC mode, a mu metal shielded input transformer provides 20dBm of impedance gain
with low THD. The MD-1 provides 18VDC phantom power when the MIC/LINE switch
is in the MIC mode and the phantom power switch is on. In LINE mode, the audio signal
bypasses the transformer and is presented to a standard active balanced input
configuration. After the input section, the signal is routed to the active gain circuit.

The active gain section uses a low noise IC to produce up to 26dBm of forward gain. The
GAIN control provides both attenuation and gain to the user. With the gain control fully
CCW, the circuit provides 50dBm of attenuation (line input mode). With the gain co ntrol
fully CW (mic mode) the unit will provide 66dB of forward gain from input to output
(18dB from the input section, 28dB from the active section and 20dB from the output
transformer). After the active gain section, the signal is sent to both the output driver and
the headphone amplifier. In the output driver circuit, the signal passes through an IC that
buffers the signal, drives the output transformer (which adds 20dBm of impedance gain),
provides isolation and helps to maintain maximum common mode rejection. The
transformer output is connected to the DRIVER side of the output selector switch. In the

DRIVER position, the output male XLR connector receives the amplified balanced
output of the MD-1 circuitry. With the switch in LOOP position the output XLR is
connected directly to the female input XLR. This allows the MD-1 to monitor a signal
looping through the unit without altering it. In the LOOP mode the MIC/LINE switch and
GAIN control only affect the headphone amplifier.

After the gain stage, the signal is also presented to the headphone gain and driver circuits
via the MIX control. In full CW position, only the input signal is passed to the
headphones. In the full CCW position, only the signal from the ¼” TRS balanced LINE
IN jack is passed to the headphones. Adjusting the MIX control for the desired balance
varies the level of each signal in the mix. The blended signal passes through the
headphone gain section, which has an audio range of -50dB of attenuation fully CCW to
+56 dBm of gain in the full CW position, and then to the headphone driver circuit. The
headphone driver will produce -6dB of clean audio into 30Ω headphones, which is quite
loud! With higher impedance phones the driver can produce 18 Volts peak to peak. Using
30Ω headphones at very loud levels continuously can consume a set of batteries in about
2 hours. Battery life can be greatly extended simply by turning the headphone volume
control to the off or CCW position when not listening to the headphones.

Power to the M D-1 is supplied by two 9 Volt batteries accessible via slide out
drawers on the side of the unit. Power is applied to the unit via the power switch. When
the battery voltage falls below 13VDC, the BATT OK LED will not illuminate (the unit
will still work but with reduced headroom).

The MD-1 is housed in a robust steel case that provides superior shielding and durability.
A belt clip is also included and riveted to the enclosure.

CONTROLS AND FUNCTIONS

PH 18V button applies 18VDC phantom power to the input XLR for powering standard
condenser microphones. Operates in MIC mode only.

MIC/LINE button matches the input circuit (rear female XLR jack) to either microphone
or line level sources. Position is IN for mic mode and OUT for line mode.