 |
The Model 1788 Microphone Preamp and Model 1788RC Remote Controller form a complete Microphone preamp system with
both analog and (optional) digital outputs available simultaneously.
|
|
|
| The Problem |
|
Why place the preamp near the Mic? Long mic level lines, mic splitters, and multiple inputs reduce the
sensitivity of the microphone and change its frequency, phase, and transient response. This performance degradation is the result of shifts in impedance,
capacitance, and inductance values and is quite audible.
Phantom power on a mic line causes the line itself to become a microphone. Jiggling the line will create
audible effects. Having the phantom power source as close as possible to the microphone also reduces the chance of phantom voltage loss within a very
long cable run.
Most modern preamplifiers have excellent common mode rejection ratio, often approaching 110dB. If there
is common mode signal (noise) built up in cable run and the gain of the preamp is 60dB, the residual common mode noise is only 50dB down. While common
mode signal can be present in a short line as well as a long line, there is a higher probability that there will be more common mode signal in the
longer line, especially if that line is crossing power and lighting cables and a high RF field.
|
|
| The Solution |
|
 |
By placing the 1788 close to the mics and controlling it remotely the need for long mic level runs is eliminated.
|
|
The noise picked up by long cable runs is no longer boosted at the end of the lines since the preamp gain has
already been taken at the input of the line. |
|
By providing 5 outputs (2 Analog and 3 Digital) that are usable simultaneously, the need for splitter boxes
is eliminated. |
|
| Why Remote Control? |
|
|
|
|
 |
|
Almost all live applications require the monitoring of mic preamp gain. If there isn't an engineer
sitting next to the preamps on stage, there must be a way to control them from a remote position.
All parameters of the 1788 are remotely or locally controllable, and setups, snapshots, and recall
of presets are quickly and easily available.
The remote control protocol for the 1788 is MIDI and up to 16 units (128 channels) can be controlled
with only one control line.
The picture to the right shows the 1788, 1788RC, and the Power Supply / I/O box.
There are three ways to control the 1788 remotely:
|
|
| 1) Via the 1788RC - Hardware Remote Controller |
|
| 2) Via a PC running the 1788SW controller software (W95/98/ME/2000 compatible) The control software
has a screen which displays all parameters and metering of one unit at a time and up to 16 units may be called up. All channel status information and
metering are displayed in real time. A channel can be selected and settings modified. Scenes can be saved, modified and recalled in the control software.
The software also contains a 'learn' function in which the channel(s) adjust the preamp gain to a definable peak value based on the input level during
the 'learn' time. |
|
| 3) Via any device that generates MIDI, such as a MIDI controller, mixer, sequencer, or show control
system |
|
| FEATURES |
|
Size - The Model 1788 contains eight preamps in 2RU space. Since there is a fair amount of heat
dissipation, there should be at least 1RU spacing between every preamp. In that configuration, they may be stacked to provide 40 channels of high quality
preamps in a standard 16RU road case.
|
|
Microphone Output Limiter - All preamps have a maximum input level. Once that level is exceeded,
there is no way to remove that very unpleasant distortion. In order to avoid that occurrence, many engineers set the preamp so that the expected peak
level at maximum sound level is still at least 12dB below the clip point. While this provides some insurance against the preamp clipping, it causes
a loss of noise performance and, in the case of digital, a loss of resolution. The Model 1788 has a unique limiter (patent pending) in the front end
of the preamp which limits the output level of the microphone by as much as 20dB, hence the name 'Microphone Output Limiter'. This allows the engineer
to get maximum noise performance and also allows maximum resolution for an analog to digital converter, all without worrying about crashing the preamp.
|
|
26dB Pad - The Model 1788 has 39dB of adjustable input gain and fixed gain of 26dB for a total
maximum gain of 65dB. There is a 26dB pad which will allow a preamp to be run as a unity gain buffer stage. For example, this allows the line level
output of a wireless mic receiver to be fed into the preamp.
|
|
Adjustable Maximum Output Level - The maximum output level of the Model 1788 is +24dBm (loaded
into 600 Ohms) balanced. There are, however, many input stages which have odd maximum input levels (e.g.-+18dBu). In order to retain the function of
the microphone limiter and maximize noise performance of the entire audio system, the Model 1788 has output level trim of up to 24dB. Once the maximum
input level of the device following the Model 1788 is defined, the output level of the 1788 can be trimmed to that exact level. The input gain of the
1788 is then adjusted to achieve optimum performance. In addition to the 24db of output adjustment, each output may be muted.
|
|
Separate Analog Outputs - Microphones are often passively split and then fed to two or more locations.
The Model 1788 has two separately trimmable, buffered analog outputs. The main output on XLR is transformerless servo-balanced and the second output
is tranformerless balanced on a DB25 multipin connector. This allows two separate inputs to be fed at optimal levels from an optimized mic preamp.
|
|
Digital Option - In addition to the analog outputs, the Model 1788 has three optional digital
outputs. The analog to digital converters are 24 bit with selectable sampling rates. The sampling rates can be generated from an internal clock or an
external source. The Model 1788 employs a drift stabilized A/D converter circuit ( patent pending) which eliminates DC from the output without incorporating
a digital high pass filter. The digital outputs appear on two DB25 connectors, one for AES/EBU and one for the T/DIF format. There is also a TOSLINK
optical output for the ADAT format. There is an AES/EBU input on an XLR and a Word Clock on a BNC connector for external clock input. If the unit is
being used as a master there is an AES/EBU output on an XLR.
|
|
Test Tone Generator - There is a 700Hz test tone at two different levels which can be bussed to
any selected preamp channel. The two levels are at 0dBfs (maximum output level) and -20dBfs. The test tone at 0dBfs allows easy set up for finding maximum
peak input into following stages. The -20dBfs tone is useful for digital systems which use a -20dB reference tone for '0Vu'.
|
|
Display - The Model 1788 has LED indicators for the status of each function for each channel.
A twelve segment LED meter displays headroom. Two eight segment LED's show input gain or maximum output level on either of the two analog outputs. All
indicators and metering can be monitored at the remote location.
|
|
Headphone Output - As an additional feature, the Model 1788 has a front panel headphone output.
Any channel can be selected and the output level adjusted.
|
|
 |
| Shown
above is a typical setup for the 1788 in a Live Sound Application |
|
| Aphex Proprietary Technologies |
MicLim™ is a patented limiter circuit at the mic input - a mic level limiter that speeds setup and makes the 1788
virtually crash-proof. MicLim allows you to set for maximum noise performance without having to back off for headroom, and eliminates the need for
constant gain riding...plus fiber or digital systems can be driven at levels that yield maximum resolution without fear of overload.
LeNA™, the digitally controlled Low Noise Amplifier, allows the 1788's input gain to be adjusted without noise or
glitches. This patented Aphex circuit changes level in 1/4 dB steps with 1dB stops. Other remote controlled preamps use relay switching with step
sizes as high as 10dB. Even a step size of 1/4dB without LeNA will cause an audible click or pop that at high gains could be explosive!
|
|
| Audio Quality |
The 1788 provides 8 balanced Jensen® transformer-coupled microphone preamplifier inputs which offer audiophile sound
quality far in excess of the mic preamps found in most mixing consoles. It also provides two separately trimmable analog outputs for each channel.
The optional digital output module allows simultaneous output of 24 bit AES/EBU, T/DIF® and ADAT® Optical.
These A/D converters are selectable up to 96 kHz and are Aphex proprietary "Drift Stabilized" (patented) which
eliminate the need for high pass filtering in the digital domain. Because the inputs to the A/D converters are trimmed exactly to the outputs of the
preamp, digital resolution can be maximized without fear of crashing. This makes the 1788 the perfect front end to any DAW, fiber system, or digital
console.
|
|
 |
| 1788 Rear Panel Shown with Digital Output section, Option No. 1788-1 |
 |
| 1788 Remote Controller panel shown |
|
| |
Specifications - Aphex 1788 |
Remote Controlled
Microphone Preamplifier System |
|
|
| AUDIO |
|
| |
Frequency Response |
10Hz - 38 kHz (± 0.5dB) |
| |
Hum and Noise |
-125 dBu EIN |
| |
THD + N |
.003% @ +4dBu out |
| |
SMPTE IMD |
<.02% |
| |
DIM |
<.005% |
| |
Crosstalk |
<-96dB |
| MICROPHONE INPUTS |
|
| |
Connector |
XLR-3F |
| |
Type |
Jensen® Transformer coupled |
| |
Impedance |
Balanced, 1.6k ohms |
| |
Maximum Input Level |
+27 dBu (with 26dB pad engaged) |
| |
CMRR |
better than -99dB @ 60 Hz, -72 dB @10kHz |
| |
Phantom Power |
48 VDC on pins 2 and 3 |
| ANALOG OUTPUTS |
|
| |
Connector |
Main: (8) x XLR-3M; Aux: DB-25 (Tascam) |
| |
Type |
Main: Servo Balanced; Aux: Balanced |
| |
Impedance |
Balanced , 65 ohms |
| |
Maximum Output Level |
+27dBu |
| |
Test Tone |
700 Hz 0dBfs, -20dBfs |
| |
Headphones |
8-600 ohms recommended load impedance, mono or stereo |
| DIGITAL OUTPUTS |
| |
Connector |
AES/EBU: DB-15 Male
ADAT Optical:TOSLINK
T/DIF: DB-25 |
| |
Type |
Drift Stabilized 24 bit |
| |
Impedance |
75 ohms (AES/EBU) |
| |
Clock |
44.1, 48, or 96kHz Internal or External |
| |
Clock Connector |
BNC |
| REMOTE CONTROL |
|
| |
1788RC Hardware Remote |
Full remote control functions, full metering and preamp status, Select one of 16 units, Create, Save, Edit,
Recall up to 60 presets |
| |
1788SW Software (W95/98/2000) |
Full remote control functions, Full metering and preamp status, Select one of 16 units, Create, Save, Edit,
Recall up to 60 presets, Assignable up to 20 scenes |
| |
MIDI Controller |
MIDI command protocol for full remote control, metering, and status |
| |
Connection: |
|
| |
Remote to 1788 |
RS232, RS422, MIDI |
| |
Unit to unit linking |
RS422 - up to 16 units can be controlled from one interface (128 channels) |
| |
Remote Functions |
Input Gain, Main Out Gain, Aux Out Gain, Polarity, Mute, Low Cut Filter, Mute, Pad, Phantom Power, Sample Clock,
Test Tone, Group Assign, Snapshot, Recall Presets |
| OTHER SPECIFICATIONS |
| Power requirements |
90-260 VAC 50-60 Hz 60 W |
| Dimensions |
19"W x 3.5"H x 13.58" D 2 Rack U (482.6mm
x 88.9mm x 345mm) |
| Net Weight |
17 lbs (7.7 kg) |
|
|
|
|
|
|
|
|
|
|
