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Cover for Automix™
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E N G L I S H
AUTOMIX™
Thank you for purchasing the Automix™! The Architectural Acoustics Automix is a high quality automatic
mixer with eight transformer balanced, Mic/Line inputs. Each Mic/Line preamp provides a gain control,
phantom power (mic inputs), low cut filter, activity/clipping LED, and a choice between manual or automatic
operation. Each channel can be muted externally or multiple channels can be muted externally via an
assignable mutebus. In addition, channels one and two provide adjustable priority and a signal processor
loop. The master section provides a gain trim control, three sweepable notch filters, a downward expander, a
transformer balanced output, and a remote volume port.
The Automix has been designed so that multiple units can easily be linked together to form a single mixer
with many more inputs (16, 24, 32...). The Automix is supplied with a plexiglass security panel to prevent
changes to the installers settings. The Automix is packaged as a stand-alone unit, but will also rack-mount
into a standard 19" rack with optional rack-mount kit.
THEORY OF OPERATION
The Peavey Automix is a classic "automatic mixer" combining several time proven techniques to deliver
maximum gain before feedback in an easy to setup and easy to use configuration.
The Automix uses VCAs (voltage controlled amplifiers) and gain computing circuits in each channel to ride
the gain downward as more microphones become active. By dropping the gain 3 dB every time the number
of active microphones double, the overall system gain remains at unity for ideal gain before feedback
performance.
A precision rectifier and logarithmic converter in each channel computes the amplitude of the audio signal
present at each microphone in real time. This channel amplitude, in decibels, is then compared to the
amplitude of the sum of all the channels, also in decibels. A simple computing circuit calculates the math-
ematical difference between these two amplitudes and feeds the product to the VCA as a gain control signal.
To better grasp how this works, let me offer a few examples. If we take the case of one person speaking into
a microphone, the amplitude in that channel will be virtually identical to the "sum of all channels" amplitude.
The difference between these two equal amplitudes is "0." When 0 dB is fed to the VCA control port, the
result is unity gain for that channel. All of the other channels, with no one speaking, will exhibit significantly
lower amplitudes. When compared to the "sum," which contains a nominal signal, negative numbers result.
These negative numbers, presented to the VCAs of non active channels, command further attenuation of the
noise and leakage present at these non active channels.
A second more interesting example is two people speaking simultaneously. To simplify this example, let's
assume they're both speaking with the same loudness. As these two sources are incoherent, i.e. not identi-
cal, they will sum as the square root of the sum of the squares or 3 dB more than either individually. When
each channel's amplitude is compared to this +3 dB sum, it's VCA will be commanded to attenuate 3 dB. As
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before, the non active channels will be further attenuated. The elegance of this approach is really apparent
in this next case. Assume a person is stationed exactly equidistant from two microphones. The identical
signal arriving at both channels, being coherent, will sum linearly to +6 dB in the sum of all channels. This
results in the two channels being turned down 6 dB. Being coherent, these two identical signals summed
at -6 dB will add up to unity.
These gain relationships hold true no matter how many microphones are active and whether or not signals
are coherent. Since all computations are done in the Log domain, it doesn't matter what the actual ampli-
tudes are, only the relative amplitudes between signals.
Front Panel:
LED
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FRONT PANEL FEATURES
CHANNEL LEVEL (1)
Manual Mode: In the manual mode the Channel Level controls provide 6 dB of gain above nominal unity
and 50 dB of attenuation.
Auto Mode: In auto mode the controls still provide 50 dB of attenuation, but the control does not add gain
to the overall system level. Instead, when the level is adjusted above 0, the other channels are attenuated
to make the channel sound louder in the mix without adding to the system gain.
MASTER VOLUME (2)
The “Master Level” controls the overall output level. The range is set for 10 dB of gain above and 40 dB of
attenuation below, nominal unity.
Note: The operating range of this control is reduced by the setting of the internal gain trim.
POWER SWITCH (3)
Depress the switch to the "on" position. The red LED will illuminate indicating power is being supplied to the
unit.
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Internal Front Panel:
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11 12
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9
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INTERNAL PANEL FEATURES
PRIORITY (Channels 1 and 2 only) (4)
Turning the “priority” control clockwise allows one channel to override the others. It does this by tricking the
gain computing circuits into thinking this channel is louder than the others. Up to 9 dB of priority is available.
GAIN TRIM (5)
This control sets the input gain in each preamp. The amount of gain is adjustable from +25 dB to +55 dB
(Mic Input).
DIP SWITCHES
Each channel has a four station DIP switch that controls the following functions in each channel.
MAN/AUTO (6)
This switch determines whether the channel is operating in the automatic or manual mode.
OFF/MBUS (7)
This switch is used to connect the channel to the system mutebus. This allows multiple inputs to be muted
under external control. NOTE: See MUTEBUS (21).
LO CUT/FLAT (8)
This switch selects the low cut filter. The low cut filter provides a low frequency roll-off that will help minimize
unwanted noise (handling of mics, bumping of tables, etc.). The roll off starts at 75 Hz (-3 dB) and is a 6 dB
per octave filter.
OFF/PHAN (9)
When this switch is in the PHAN position, +12 volts of phantom power are supplied to the Mic ± terminals.
This provides power for condenser mics and should be defeated when using dynamic microphones or
unbalanced inputs.
ACTIVE/CLIPPING LED (10)
The LED will glow green when the channel is active and the status output is low (0 V). The LED will start to
glow red 6 dB before the preamp begins to clip.
NOTCH FILTER LEVEL AND FREQUENCY CONTROLS (11 and 12)
The Automix notch filter consists of three sweepable 1/3 octave filters with 0 to 15 dB of cut. These filters
can be used to attenuate frequencies that are prone to feedback without adversely affecting the tonal quality
of the system. Equalizing these problem frequencies out of the room's frequency response increases the
sound system gain before feedback.
Notch Filter Level Control (11): The notch filter level control adjusts the amount of cut at the frequency
selected by the corresponding frequency control (12). It is adjustable from 0 dB to 15 dB of cut.
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Notch Filter Frequency Control (12): The notch filter frequency control is used to select the center
frequency of the notch filter. The bottom filter has a range of 40 Hz to 925 Hz. The middle filter has a
range of 260 Hz to 6 kHz. The top filter has a range of 500 Hz to 12 kHz.
DOWNWARD EXPANDER (13)
The downward expander can be used to attenuate the system gain when all of the input signals are low.
This can be used to prevent background room noise from being amplified.
Setting the Downward Expander:
Have someone speak into a microphone at the softest level you expect to encounter. Slowly turn the down-
ward expander clockwise until the background noise between words is attenuated. Be careful not to go too
far. The more downward expander that is used, the less “natural” sounding the system will become.
This method will get you to a good starting point, but the best way to set the downward expander is during
an actual meeting or event. While doing this, the downward expander can be adjusted for the best sound. If
you have to deal with both loud and quiet events, you will need to trade off between natural sounding ambi-
ent and background noise.
MASTER GAIN TRIM (14)
The Master Gain Trim sets the maximum overall system gain. This control should be set with the front panel
master gain at maximum (all channels at unity) and external power amps turned up and locked at opera-
tional settings. When trimmed to a safe margin below feedback, no combination of user controls can cause
feedback. This trim provides up to 25 dB of attenuation.
Back Panel:
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BACK PANEL FEATURES
MIC INPUTS (15)
For use with low impedance microphones or low level sources. This is a transformer balanced input with an
impedance of 2,000 ohms.
LINE INPUTS (Channels 3-8) (16)
These allow line level inputs to be used in channels 3-8. This is a transformer balanced input through a
30 dB resistive pad. Input impedance is 2,000 ohms.
DIRECT OUTPUTS (17)
Each channel has a set of direct output terminals that can be used for recording or anytime the output of an
individual channel is required. The level of this signal is independent of “Automix” gain manipulations.
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MUTE (18)
Channels can be muted individually by shorting this terminal to ground. It provides approximately 45 dB of
attenuation.
figure 1 Mute
STATUS OUTPUT (19)
The status is a DC logic output that is low (0 volts) when the channel is active and high (3.8 volts) when the
channel is not active. This DC voltage can be used to key video cameras or trigger key lights on active
microphones.
LOOP (Channels 1 and 2) (20)
This is a signal loop that allows an external device such as an EQ to be inserted into the signal path of the
first two channels. When used without external processing these loops are hardwired.
CEQ 280
IN
OUT
figure 2 Loop
MUTEBUS (21)
The Mutebus is a control port that when shorted to ground will mute all channels that are assigned to the
mutebus (approximately 45 dB). The channels to be muted must be attached to the mutebus using the
internal switch marked OFF/MBUS. (See number 7).
figure 3 Mutebus
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