The MAX Audio Processor For Amateur Radio, LPAM, And Podcasting
A Collaborative Effort of W8KHK and N1BCG

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Revision Date: 200417 v3
Information contained on these web pages copyright W8KHK / N1BCG. Amateur or individual use is encouraged.
Commercial use of any kind is prohibited without the express written permission of the author, Richard A. Maxwell

Max Audio Processor - Calibration

Users can make changes to suit personal taste, but the following are how the MAX Audio Processor is supposed to be set for optimal results.

  • Polarity Indicator Balance

    Input: Pin 3 of "APF" header - Connect a signal generator (1 kHz @ -12dBm = 193 millivolts RMS)
    Output: n/a

    Adjust R168 (Balance) so that both the + and - Polarity LEDs are either both on or both off. This is a very sensitive adjustment.

    The adjustment may also be made measuring the comparator input (LM339 U152, pins 4 and 7) with a dual-input oscilloscope set for differential mode. Adjust for null, such that both signals are equal in level.

  • Compressor Drive

    Input: Pin 3 of "APF" header - Connect a signal generator (1 kHz @ -12dBm = 193 milli-volts RMS)
    Output: Pin 1 of J102 (SSM2166 input) - Connect 1M oscilloscope input (500uS/div, A.C. coupling)

    The red "Peak" LED threshold should be close to an input signal level of -12dBm. Adjust R101 (Compressor Drive) for a 200mV PtoP waveform on the oscilloscope, or a measured level of 70.7 milli-volts RMS on an AC voltmeter. This provides 18dB of input clipping headroom needed for the pre-emphasis circuit.

  • Clipper Drive

    Input: Pin 3 of "APF" header - Connect a signal generator (1 kHz @ -12dBm)
    Output: Pin 2 of SCAF header - Connect 1M oscilloscope input (500uS/div)

    The amount of clipping to use is subjective and these settings are considered a good starting point. Adjusting R202 (Clipper Drive) will provide up to 1.9V PtoP as shown on the oscilloscope. This represents fairly aggressive clipping. Setting R202 for a 1.3V PtoP is fairly light clipping. While the amount of clipping change seems large for a very small change in voltage, the limiter function of the SSM2166 maintains a constant clipping level over a wide range of input levels.

    A good starting point is around 1.6V PtoP