Output and Distortion of the AD-2

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Back to Home Eliminating Clipping Distortion, and Increasing Audio level from the AD-2 Audio Delay Board
By Jeff DePolo WN3A

The ACC AD-2 audio delay board has an inherent design flaw that may manifest itself as receiver audio clipping in some installations. If you find that you cannot get enough receive audio level through the controller without having problems with clipping or distortion, or you find that you need to turn the repeater transmit audio pot on the RC96 up very high and turn down the other audio sources (synthesized speech, tones, autopatch, etc.) very low to correct for the low receiver audio level output, this information may help you.

The AD-2 audio delay board patches into the receive line audio path. The installation of the AD-2 involves removing a 4053 CMOS analog audio switch IC that is in the receive audio path on the main board, and installing the AD-2 board piggyback-style in its place. The AD-2 provides audio delay and also provides the audio gating that the original 4053 switch on the 85/96 motherboard had been doing before the retrofit. The AD-2 plugs into the main board in place of U4 on RC85 controllers, U9 on RC96 controllers.

The problem with the AD-2 is that there is a 10:1 voltage divider at the output of the AD-2. This results in approximately 20 dB of reduction of audio level leaving the AD-2 controller. Attempting to correct for this large attenuation by turning up the repeater receiver level control on the main controller board can often result in clipping being generated on the AD-2 board.

The voltage divider is compromised of R6 (10K) and R11 (1K) on the AD-2 board. Both are clearly labeled on the silk-screening. Increasing the value of R11, or removing it completely, will increase the audio level leaving the AD-2.

After this modification, there will be plenty of receive audio available leaving the AD-2 that you should easily be able to balance all of the audio sources and not have any problems with receiver audio clipping or low levels.

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Jeff DePolo WN3A Engineering Consultant

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