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
