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  1788A Remote Controlled Mic Preamp System, Aphex

Aphex 1788A


 
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Product Code : 1788A

Description
 
1788A, Remote Controlled Mic Preamp System, Aphex

The Model 1788A 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 affects. 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.

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.



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