Manley VOXBOX - MVBXA 4/2003 - Manual (Page 13)

THE MIC PREAMP & COMPRESSOR
The VOXBOX can be divided into two main sections. The first is the Compressor and Mic Preamplifier.
The input signal can be a microphone, instrument or line level signal. The mic and line inputs are on the back
panel. The mic input is transformer balanced with 48 volt phantom power available, switched from the front.
The line input has both XLR and 1/4 inch phone jacks and will accept balanced or unbalanced signal at either
of the jacks. To use the line or instrument input, the phase/polarity switch should be set to the middle position
to disconnect the mic input transformer. The instrument input on the front panel will over-ride the line input
so to use the line input, you must not have a jack plugged into the instrument input. You will probably need to
really turn up the gain to get a reasonable output with a guitar. We also suggest that you rely on the limiter in
the EQ section rather than compress because the impedance may lower from 100K ohms to 50K ohms and this
eats highs. Whichever input you use, the next thing the signal sees is the Filter which allows you to cut lows at
80 or 120 Hz to help reduce room rumble or pops. The next thing is the INPUT control which is the main level
adjustment. This is followed by a passive opto-isolator that does the “work” of the compressor. Up to this point
the signal has only gone through passive components - no gain, no clipping, no noise. The signal then goes to
the all-tube amplifier which is the standard Manley Mono Mic Preamplifier that has been winning shoot-outs
for years. The GAIN switch is not a pad. This control sets the overall gain by setting the amount of negative
feedback. Besides gain, this affects several aspects of the “sound” of a pre-amp including transient speed, &
accuracy, distortion, noise, output impedance and clipping characteristics. At lower gains (40 dB setting) the
Pre Amp has a very clean quality, in some ways like solid state and is the best at minimizing tube hiss.
“50” is
a normal setting because it tends to sound most like the source, and is very musical and real. The
“40” settings
can sound slightly slower, further back and more mellow by comparison. The
60 dB setting uses almost no feed-
back and can give a slightly more punchy and forward or aggressive sound. It may be a little “hot” (more than
simply warm) for some tastes. Experiment to discover your own descriptions. You will likely find a few settings
that you prefer for your style of music. The preamplifier has two outputs. The first is the unbalanced (and trans-
formerless) 1/4 inch phone jack. It interrupts the signal before the transformer balanced XLR output. Use one or
the other, but you cannot use both outputs.
A few words about PHANTOM POWER. It is a rare occurance but a few ribbon mics and PZMs can
be damaged by the 48 volts of phantom power. We suggest to ALWAYS have PHANTOM switched off when
switching mics, cables, patches that involve mics etc. You ONLY need phantom power for SOLID STATE
(FET) CONDENSOR MICS. You should have phantom switched “off” for tube mics, dynamic mics, ribbon
mics and battery powered mics. This is true for all mic pre’s. With this MIC PRE you “PULL THE TOGGLE
to SWITCH PHANTOM”. It is a locking toggle to prevent “accidents”. The second great reason for not using
phantom if you don’t have to is that - if you change a connection with phantom on, then the preamp will be fed
a quick burst of 48 volts (when it normally is amplifying about a hundredth of a volt), which may be monitored
- usually once. After you have replaced your speakers, you have learned a valuable lesson about turning down
the volume of the monitors before changing mics or mic patches. This is a good habit with phantom on or off.
Consider a variation of this - any slightly intermittant mic connection, will be super noisy with phantom turned
on. This goes for cables, patchbays and patch cords.
Suggestion #2 - Avoid running mic signals through patchbays. Some patchbays “ground” all the
“sleeves” which can add a ground loop into your delicate mic signal. Ground loops usually cause hum.
Suggestion #3 - You can set up the VOXBOX in the studio near the mic and use a short mic cable. Why?
Microphones often have “light duty” line drivers and you can lose an audible amount of signal in long cables.
You can get the best fidelity by having the mic pre close to the mic at the “cost” of having to walk into the stu-
dio to adjust a level control. You also avoid almost all of that phantom power / patching problem because now
you have the mic plugged in directly to the mic pre and are patching the hefty line level output at the console.
Suggestion #4 - If it sounds good, no problems - fine - record.
The PHASE switch is very important when more than one mic is picking up a particular instrument. A
good example is a snare drum with a mic on the top and another on the bottom. The two mics are out of phase
acoustically and unless one of the mic signals is flipped 180 degrees one can expect some cancellation of lows.
For vocals, we often assume that the PHASE switch is unimportant (only one mic) and we don’t hear a signifi-
cant difference in the control room - but the singer will. If the headphones are out of phase with thier own voice,
it sounds pretty strange. Flip the switch - ask them what they prefer. This might explain why sometimes you
just couldn’t satisfy the singer’s headphone complaints. A fair amount of older gear (especially mics) may be pin
3 hot (out of phase) because the pin 2 hot standard was not developed yet...
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