USER’S MANUAL
Electronaut M63 Vacuum Tube Microphone/Instrument Preamplifier
11
E L E C T R O N A U T C O M P A N Y
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VU Meter calibration
The VU meters in the M63 can be set to the old standard of 0VU = +4 dBu. Alternatively, the meters can be set
to a higher reference level so that the meters reach full deflection at or near the maximum level of modern digital
recording systems.
To set the desired reference level, follow the procedure shown below. WARNING: This procedure requires setting
an internal control while the M63 is powered on, and should only be done by a qualified technician who is familiar
with the hazards of working with high voltages!
1) Remove the top cover of the M63 and set it aside.
2) Attach the power cord, plug it into an AC MAINS outlet, turn on the M63, and let it warm up for about five
minutes.
3) Turn the TRIM and ATTENUATE controls on both channels fully counterclockwise.
4) Connect a signal generator to the left channel INSTRUMENT input on the M63. Select the INSTRUMENT
mode on the Input Mode Switch.
5) Connect the output of the left channel to the input of the recording device. Set up the recorder so that the
metering system on the recorder is activated.
6) Configure the signal generator so that it produces a sine wave of approximately 1kHz at -30 dBu.
7) Adjust the TRIM and ATTENUATE controls until the VU meters reach 0VU. Compare this to the level shown
on the recorder.
8) Using a small flathead screwdriver, adjust the Meter Calibrate (see Figure 1) potentiometer for the appropriate
channel until the level shown on the VU meter better matches the level shown on the recorder.
9) Repeat this procedure for the right channel.
10) Put the cover back on. Do not tighten the top cover screws with a screwdriver until all the screws have been
inserted and lightly tightened by hand. Do not over-tighten the screws.
The VU meters are driven by separate amplifiers built around TL072 op-amps, which present a very high input
impedance to the output of the the M63, providing isolation and avoiding the additional load the meters would
otherwise present to the outputs. (No, those little ICs are not in the signal path!)
Phantom Power adjustment
The Phantom power voltage adjuster has been preset at the factory to 48.0 volts, and is not intended for end-
user adjustment. It has been included merely for the sake of maintaining accuracy between the actual phantom
power voltage and the faceplate designation of 48 volts. In general, phantom power is a very loose specification,
and the absolute value of the DC voltage has no effect on the performance of condenser microphones.
Configuring the Ground Lift Switches
Each channel has its own ground lift switch which is set at the factory and should not need adjustment. How-
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