Manley VOXBOX - MVB 1996 - 2/2000 and MVBX 6/2000 - 4/2003 - Manual (Page 10)

OVERVIEW and BLOCK DIAGRAM
TRANSFORMER
SYMBOL
SWITCHING
PHONE JACK
This block diagram is intended to give you a reasonable view of the signal flow in the VOXBOX.
A few interesting aspects can be seen at a glance. First notice that when an instrument is plugged it that
it disconnects the LINE input. After the INPUT (level) control (which is a conductive plastic pot) the
signal goes into the COMPRESSOR which is the first major block in the chain. Normally this is impossible
because mic signals are so low that the noise introduced by a compressor would be a problem even if the
the threshold control could see such a low signal. How do we do it ? We use a light dependent resistor called
an opto-isolator. This part is designed for audio applications and is the same part we have been using in
the Manley Electro-Optical Limiters. When light shines on this special resistor some of the signal is
shunted to ground which reduces the level. Adding this part to the basic mic pre reduced the preamp gain
by 0.1 dB and did not affect any other specification even when compressing 15 dB. From the compressor
the signal goes to the MIC PREAMP. Uniquely this compressor up-stream can prevent mic signal clipping
before the first tube. The PREAMP is where the mic signal gets boosted to line levels. This all-tube gain
block is the same as the Manley 40 dB Microphone Preamplifier and quite similar in topology to the circuit
in the Manley Pultec EQs and Opto-Limiters. Notice how the GAIN is a feedback circuit, getting the signal
from the output and sending it back to the input. The signal is injected out-of-phase so that it acts to reduce
gain, distortion and noise. We chose a respectable range of operation for the feedback select. Not too little
and not too much which can cause problems with transient accuracy and imaging. You get to choose the
optimum amount for "your" sound. It is also not in the main signal path which we try to keep simple and
pure. At the end of this section is a transformer, that you can see, gets switched out to prevent any loading,
if you use the phone jack output. These outputs are intended to send to tape.
The first thing in the EQ section is a switch to select the input. You can choose the LINE INPUT
(normally going to the first stage Mic Preamp) or the output of the Preamp or a third input called INSERT.
INSERT is intended to be used so that this section is in the monitor path between the tape output and the
console. This way you can EQ and de-ess safely and not record them. Maybe save it for the mix.. INSERT
is also used to "insert" an outboard processor in the middle of things. Another use for this input select
switch is to compare the LINE in with the PRE OUT because there is no "make-up gain" control (rather
than just using the "Bypass"). This is a good, level consistant, alternative. Yet another gain control here
would have compromised the signal integrity and maximum available gain on the mic preamp. Too bad.
The signal goes into the EQ and actually loses gain because the EQ is a simple passive circuit.
The signal later gets amplified by the next tube stage so that the full signal comes back to normal levels.
But first..... it hits the DE-ESSER / LIMITER. Once again, this is a passive stage with far less signal quality
loss than outboard rack mounted devices. And once again we use our proven tube amplifier circuit to
restore levels and drive long cables.
TUBE GAIN
STAGES
DE-ESSER
LIMITER
PASSIVE EQ
PULTEC STYLE
BLOCK DIAGRAM GAIN
SWITCH
BALANCED
OUTPUT BALANCED
OUTPUT
UNBALANCED
OUTPUT
TUBE CIRCUIT
LINE AMPLIFIER
PASSIVE OPTO
EQUALIZER
INSERT
INPUT
UNBALANCED
OUTPUT
OUR 40 dB
TUBE PREAMP
MIC PRE
INPUT
LEVEL
NEW PASSIVE
OPTO CIRCUIT
COMPRESSOR
PHASE
INST IN
MIC IN
LINE IN
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