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

• Input Level - Input levels should be set so that the yellow NORM indicator is mostly on and the red PEAK indicator only briefly illuminates. The "Mic Input Gain Set" trimpot on the circuit board can be adjusted to vary the gain broadly if you are using the microphone input. The microphone input can also be used as a second line input by reducing the gain to minimum.

• Phase Rotator (Recommended Setting: ON) - The "Dominant Polarity" indicators will show whether the input signal has greater negative (blue) or positive (green) peak energy. Many users believe that the source audio should always be positive and will rewire the microphone connector to make it this way. Since the polarity of most voices vary with speech, the alternative is to enable the Phase Rotator to reduce the opposing variations. The blue and green indicators will still flash alternately due to the sensitivity of the circuit, but the elimination of the extreme variations will increase perceived loudness by decreasing gain reduction.

  NOTE: Avoid using headphones to judge the effect of the Phase Rotator as mentioned above in the circuit description.

• Low Gain (Recommended Setting: 12 o'clock) - This function provides up to 12dB of continuously variable low frequency cut (counterclockwise from 12 o'clock) and boost (clockwise from 12 o'clock). A good starting point is at the 12 o'clock position for a flat low end response. Cutting lows can be extremely helpful for increasing loudness when using low bandwidth settings, with microphones having excessive low frequency (bass) response, or with transmitters that have poor low frequency response. In the latter case, the use of this filter will greatly increase average modulation while reducing distortion.

  Adding a reasonable amount of low frequency boost can help compensate for microphones with excessive low-end rolloff such as a D-104 or most hand held microphones although it is a better option to use a microphone with sufficiently flat frequency response. In either case, it's important to recognize that some transmitters will not pass low audio frequencies and the use of excessive low boost will sacrifice loudness.

• Notch (Recommended Setting: Full clockwise) - This control provides attenuation 250-500 Hz range, the "mud" frequencies, with the greatest null of -12dB around 300 Hz when turned full counter-clockwise. The primary benefit is to reduce the frequency range that contributes little to intelligibility, particularly for baritones or users with microphones that are overly sensitive at these frequencies. Users will also find that this medium Q filter can accentuate the effect of the Low Gain and Pre-Emphasis controls so less boost of either will be needed.

• Pre-Emph (Recommended Setting: 3 o'clock) - Pre-emphasis is beneficial in almost every application of the MAX Audio Processor in order to overcome the effects of narrow bandwidth receivers. It also boosts the important "presence frequencies" to improve clarity. A fully counter-clockwise setting produces a flat response (no presence or high frequency boost) with an increase occurring as the control is turned clockwise. The 3 o'clock position produces a response similar to the fixed pre-emphasis of earlier models and is recommended as a starting point. The boost of higher frequencies can be limited using the Bandwidth controls discussed below.

• Gain Reduction Detection (Internal Setting) - The MAX Audio Processor offers either full-wave or half-wave detection of the audio waveform to obtain a gain reduction control voltage.

  Most off-the-shelf processors use full-wave detection which allows the amplitude of both positive and negative peaks to control gain reduction. This is fine for studio work or FM transmission but can result in the loss of perceived loudness for AM transmission resulting from asymmetric positive peaks causing excessive gain reduction. Diode D102 should be removed (or disabled by unsoldering and lifting one lead) for AM or SSB use, or installed for FM or studio production use which require full-wave control voltage detection. With this done, a positively polarized asymmetric waveform could gain several dB in perceived loudness. By contrast, a negatively polarized asymmetric waveform would have the same perceived loudness as when both halves of the audio cycle. It's now fairly important to ensure that your audio source (microphone) is connected so that the green Dominant Polarity LED lights up more than the blue LED in order to take advantage of this modification (click image for a larger view).

• Dynamic Processing (Recommended Setting: ON) - A lot of thought went into designing this function with the goal of maximizing the signal-to-noise (S/N) ratio. Without it, the processor operates "off the leash" where everything is maximized. It may be worth trying, but we are pretty sure you'll turn the Dynamic Processing back on (or others will ask you to do so).

  Interestingly, the amount of audio energy processing is the same whether Dynamic Processing is on or off. The difference is that background noise is *not* processed when Dynamic Processing is on, thus it is the highly recommended setting.

  A companion function controls the density of the audio by setting the aggressiveness of the processor. When set to "Dense", the processor speed is highest while "Open" gives a much more natural sound. The effects are most pronounced when the Dynamic Processing is off while offering more subtle control when Dynamic Processing is on. This allows users to find the optimal combination for their needs.

• Bandwidth - The options give the user the ability to not only adjust the amount of high frequency audio boost but also tightly control the bandwidth of the transmitted signal. Standard settings are 3, 5, and 7 kHz of frequency response (6, 10, and 14 kHz bandwidth) although these can be changed by swapping out different value resistors on the board.

  The transmitted bandwidth might be limited by the transmitter. In this case, the bandwidth setting will only have an effect at narrower settings. It's also important to consider the frequency response of receivers where most are far more limited than even the wide (7 kHz) setting of the MAX Audio Processor. Focusing the transmitted audio into a narrower bandwidth can actually increase perceived loudness.

• Asymmetry (Recommended Setting: 100% + Peak) - This controls the maximum positive peak level where the transmitted waveform is symmetric (100% + Peak) or asymmetric (around +130%). The description of the peak control circuit above covers the advantages of a symetric signal so the recommended setting is "100% + Peak".

• Peak Control Indicators - The red "NEG" and "POS" indicators show the amount of peak limiting for the negative and positive halves of the audio cycle. The drive to this circuit is regulated for the optimal combination of loud and clean. There will typically be a correlation between the activity of these indicators and the blue and green indicators showing Dominant Polarity. Enabling the Phase Rotator will reduce the difference in activity between the two red indicators since the signal will be more symmetric.

• Positive Peak Limiting (Internal Setting - MAX models 510 and up)

  The R130 trimpot has been added to set the positive peak limit from +100% (fully counterclockwise) to around +130% (clockwise) which is the default. The PEAK switch on the front panel selects between +100% (symmetric) and the R130 setting (asymmetric). An accurate peak indicating modulation monitor is needed for accurate adjustment.

  NOTE: While it is possible to completely defeat positive peak limiting, we do not recommend this as it removes protection against transmitter and/or modulator damage due to voltage spikes. Also, there is no loudness advantage of having unrestricted positive peaks since there is no increase in waveform density.

• Second Stage Negative Peak Limiter (Internal Setting - MAX models 510 and up)

  While the positive and negative limiters should provide sufficient peak control, there may be an extreme case where even tighter limiting of the negative peaks is required. Trimpot R140 provides a means to add a second stage of negative peak limiting if desired. An accurate peak indicating modulation monitor is needed for this adjustment. Trimpot R140 should be set fully counter-clockwise by default to defeat this function.

• Balanced Output(s) - This combined high and low level output (Main Output terminals on the board) can drive a variety of loads depending on how the connections are made to this output. High level connections will give up to +4 dBm of drive for transmitters requiring professional levels while low level connections will provide microphone level (-60 to -30 dBm) and consumer line level (-30 to -10dBm) audio.

  * Balanced, High Level - Connect to the + (pin 2) and - (pin 3) output terminals.

  * Unbalanced, High Level - Connect the shield to G (pin 1) and the hot lead to either the + (pin 2) or - (pin 3) output terminal depending on the polarity needed for the AM transmitter. The + terminal should be used by default for SSB or FM transmission.

  * Unbalanced, Low Level - Connect the shield to G (pin 4) and the hot lead to the V (pin 5) output terminal. The low level trimpot R193 on the board can be used to adjust this output from zero (center setting) to any microphone or consumer unbalanced level by adjusting from the center in the direction that provides the desired polarity and level.

  NOTE: Some applications will require a D.C. block such as when connecting to a transmitter designed to be used with an electret or condenser microphone. In this case, connect the hot lead to C (pin 6). The polarity and level adjustments are made the same way as when using pin 5. Use the C (pin 6) connection unless you are very sure that there is no need for D.C. isolation.

• Utility Output (MAX models through 530) - This is an unbalanced output with both a fixed level around 0dBM at F (pin 3) and an adjustable level at V (pin 2). The shield should be connected to G (pin 1)