ATI MLA400-1 User Manual

Audio Technologies Inc. ■ 856-719-9900 ■ [e-mail] ■ www.

audio.com

© Copyright 2008, Audio Technologies Inc.

MLA400-800 MMA400-

800

LINE AND MICROPHONE AMPLIFIERS

OPERATING AND MAINTENANCE MANUAL

Audio Technologies Inc. ■ 856-719-9900 ■ [e-mail] ■ www.

audio.com

DESCRIPTION

Our engineers insist on including this section in all of our instruction
books. We know it is really just an excuse to blow their own horns by telling you
about all the clever things they have done inside your Multiple Amplifier Array. If
they have really been clever, you will never have to even think about what's
inside the box and this whole section will be superfluous.

Multiple Amplifier Arrays provide you with either four or eight identical Line
or Microphone Amplifier channels depending on the model selected. These
channels may be used as individual independent and isolated amplifiers or they
may be combined at internal summing points brought out to the rear connector to
form mixers, summing amplifiers, distribution amplifiers etc.

Multiple Line Amplifier Arrays (MLA) apply balanced or unbalanced line
level input signals directly to a differential input stage with good common mode
rejection and RC bypassing of any RF signals picked up on the inputs. This stage
will accept line levels up to +22dBm (10Vrms) without clipping and has a typical
equivalent input noise (EIN) of only -96dBm thus yielding a 118dB dynamic
range. More to the point, even with line inputs as low as -20dBm, you will
maintain a 76dB S/N ratio. An input impedance greater than 30,000 ohms is high
enough to avoid loading of even IHF (phono jack output) lines. A bi-fet input IC
(LF347) provides fast and symmetrical slew rate performance along with high
loop gain for minimum transient and harmonic distortion generation.

Additional gain of 41dB and improved noise performance for low-level
signals is provided in the Multiple Microphone Amplifier Arrays (MMA). A dual,
low noise IC (OPA2227) is interposed ahead of the differential stage in an
instrumentation amplifier configuration. EIN of -120dBrn allows good S/N
performance with typical microphone input levels. A very high input impedance of
20,000 ohms for balanced inputs avoids all loading effects on the microphone.
Input DC blocking capacitors allow use of external phantom power supplies with
no fear of input overload. MMA inputs will accept levels up to -20dBu. As a
convenience when mixing high and low level inputs into the MMA, removing
jumper wires J3, J6, J9 or J12 will convert the selected channel into a unity gain,
high-level input, and avoid the necessity for external attenuator pads.

An adjustable gain stage follows the input networks in all models. This
stage utilizes a unique circuit arrangement that allows us to provide a smooth, full
range logarithmic gain control in each channel using an inexpensive (but good)
linear cermet potentiometer. This circuit provides an additional 20dB gain at full
clockwise rotation, unity gain in the 12 o'clock position and tapers smoothly to full
off at the full CCW position. Since we actually vary the gain of the stage rather
than attenuating the input to a fixed gain stage, we can use the channel at low
output levels with very little noise penalty. You can easily match channel outputs

Audio Technologies Inc. ■ 856-719-9900 ■ [e-mail] ■ www.

audio.com

to console medium level inputs (-20dBm) or to drive IHF inputs (single ended at
.10Vrms) without requiring outboard attenuator pads.

A summing junction, isolated from the adjustable gain stage output by a
1K ohm build out resistor, is brought out to the rear connector to allow channel
inputs to be added together and single inputs to drive multiple outputs. Any
channel may be interconnected with any or all other channels. Unused inputs
should have their pots turned to off (full CCW). Each summed channel is
attenuated in proportion to the number of connected channels, e.g. 2 ch = 6dB,
3ch 10dB, 4ch = 12dB, 8ch = 18dB.

The summing junction is also the input terminal of the channel’s output
line driver. The output stage is a balanced driver, which is either transformer or
resistively coupled to the output line. Each channel is programmable for different
output stage gains with a jumper plug connector. The basic stage gain is 6dB
with the plug in the center position; additional gain of 10 or 20dB is selected by
moving the plug. Units are generally shipped set up for +10dB (16dB total stage
gain).

Transformer coupled outputs (-1 models) are recommended when driving
long lines since the low source impedance will minimize high frequency roll off
due to cable capacity and the total primary to secondary isolation prevents AC
power line ground system voltage differentials from adding in with the audio at
the receiving end. Transformer outputs are also best for driving both balanced
and unbalanced lines. The output transformer is a low leakage reactance design
and it is loaded with an RC network to maintain smooth high frequency response
and excellent square wave performance whether it is resistively terminated with
600 ohms or left unloaded.

Active balanced outputs (-2 models) are more economical and are
preferred for short cable runs driving known loads. Avoid driving an active output
into a device with a single ended, unbalanced, one side grounded input since this
condition effectively shorts out the amplifier driving the low output. These inputs,
of which lHF phono jacks are typical, can be driven from an active balanced
output by connecting the load between a HI output terminal and a GND (rather
than HI and LO terminals). Each active output is isolated from the output
terminals by DC blocking capacitors and 150 ohms of build-out resistance. The
build-out resistors are split and bypassed to prevent any RF signals that might be
picked up on an output line from feeding back into the amplifier.

The Multiple Amplifier Arrays operate from an internal bipolar 15VDC
regulated supply. The power supply is designed for minimum susceptibility to
power line transients and conducted RFI using ferrite beads, double capacitive
bypasses and a non-concentric wound semi-toroidal power transformer with a
mu-metal shield.