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| The MAX Audio Processor For Amateur Radio, LPAM, And Podcasting | |
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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 – Leveraging the Available Configuration Options
Audio Processor Signal Flow Overview:
The block diagram on the intro page gives a general idea of the processor modules.
Input pre-amplifier, either one or all of two microphone and two line inputs, via a summing network to-
Controlling the Signal Path:
The processor has many breakout headers that enable the user to customize the signal flow, and thus the
behaviour and performance of the processor. Each functional module is interconnected via a header.
The first is a ten-pin summing header labeled INP. Four of the pins receive signal from one of the input
amplifiers, and the other four pass the signal through summing resistors to the next stage of the processor.
Here is how these pins function:
Pin 1 - Input from Mic Preamp 1
The basic configuration would include a jumper from pin 1 to pin 2 to enable Mic 1 into the mix, then jumper
3 to 4 for Mic 2, etc. Thus it is possible to enable any or all inputs with just jumpers, no wires are needed.
As a mixer, each input gain control would determine whether that input is enabled or disabled. Conversely,
individual switches or a rotary switch could be used to alter the path, singularly or in multiples. This allows
the gain controls to remain at a preset level, and quickly select or deselect any input. (This might be a case
where all input levels are defined by on-board trim-pots, and the input is selected by a single rotary switch.
This would make a simpler, less-cluttered user interface.)
A possibly more important reason to decouple the inputs from the mix is to allow any input (and output) to be
used to provide an interface to external devices, AKA, an Insert with Send and Receive lines.
Since it is similar in function and form, we shall skip ahead to the analagous output interface, the OUT header:
Pin 1 - Input to Utility output 2
Again, jumpers across adjacent pins direct the signal to any or all outputs. A switch, or simple jumper wire, could
"pick off" a signal from anywhere in the chain to be used for monitoring, troubleshoting, etc. If the chain is
"broken" by removing a jumper anywhere in the signal path, the Insert send and return function could be realized
with any pair of input and output circuits. The suggested location to insert a graphic equalizer is at the output
of the all-pass filter (pin 4 of header APF), and return the signal to the input of the pre-emphasis network
(pin 3 of header APF).
A "standardized pinout" has been defined for each of the main modules in the compressor, providing a simple and
error-free method of including or excluding any or all of the modules from the chain. The following convention
is used for this list of modules:
Pin 1 - Ground (available in the event it is desirable to use shielded interconnect cables)
Modules that follow this standard are:
PHZ - Phase Inverter
The idea is that any module can be included in the chain (enabled) by jumpering pin 3 to 4 on that module's header.
Conversely, it may be excluded (disabled) by jumpering pin 2 to pin 3, thus passing the signal from the prior module,
around the current module, directly to the next module. Sufficient jumpers for this functionality are provided
in the parts kit.
Another convenient option is to implement a single-pole double-throw switch instead of using on-board jumpers.
Run three Dupont wires from the header to the switch, with the common switch terminal connected to pin 3. Now the
configuration may be altered on-the-fly from the control panel. Dupont wires are also included in the kit. With these
low-impedance signals, shielding has not been found to be necessary for Dupont wires of 200 to 300 mm in length.
While these switches may be applied to all seven of the above modules, there is virtually no need to apply a switch to
either the COMP (compressor) or the CLP (clipper) module. Instead, there are headers related to these functions that
control the internal configuration of these modules, making it unnecessary to remove the entire module from the chain.
Although it is obvious that the header interconnect methodology used here allows almost unlimited redirection of the signal
path through the various modules, the intent is simply to allow the inclusion or exclusion of any module in a flexible manner.
It is NOT intended to allow indiscriminate re-ordering of the modules within the internal audio chain. A great deal of time,
effort, and on-the-air testing has been expended to ensure the best possible configuration, consistent with clear, clean,
and loud audio which will provide the best possiblity of reliable communications during marginal band conditions. On the
other hand, when the band is wide open and marginally occupied, disabling such functions as the low-cut filter, all-pass
filter, and pre-emphasis will allow the user to "open up" his sound signature. At these times, a wider audio passband
and wider occupied bandwidth are appropriate and invaluable to achieve the maximum operating pleasure.
.
The microphone inputs in the Max Audio Processor are designed to function as low-level
balanced differential preamplifiers. However, the circuitry and PCB artwork are flexible
and can be altered to function as single-ended high-impedance stages. This was also done
with the version 3.0 processor, which had but a single stage op-amp to provide all the
required gain. The version 4.0 uses two stages in cascade, offering more stability, lower
distortion and noise. However, at this time, it has not yet been tested for high-impedance
operation, so the parts list defines only the components necessary for implementing the
balanced low-level function. In the near future additional information will be provided for
the alternative mode, but thorough testing is the first priority before providing the
component value specifications for implementation.
Input and Output Interconnection Data:
All main inputs and outputs use three-pin headers.
Line Output Pad and DC-Blocking configuration Headers:
Adding an "Insert" (Send and Return Interface):
As stated previously, a pair of input and output modules may serve as an "insert" for auxiliary devices, for example, a graphic equalizer. The suggested
location for this function is between the all-pass filter and the pre-emphasis network. To add this customization, configure the jumpers as follows:
Compressor Options Jumper Configuration:
The eight-pin header for compressor options (pin 1 through 8, left to right) allows configuration of the
"Gain Gate", Noise Gate, selection of Compress or just Limit mode, and the response delay (dense or open)
Clipper Jumper Configuration:
Note that pins 1 and three are ground, and may be common if using a switch.
SCAF Bandwith (clock frequency) Jumper Configuration:
The four pin bandwidth jumper enables three pre-calibrated bandwith selections, selectable via jumper,
or DPDT toggle switch, or single-pole three position rotary switch. optionally, a continuously-variable
bandwidth may be achieved via a potentiometer (10 to 20 K Ohms).
If a SPDT or DPDT toggle switch is used, connect as follows
Because the narrow bandwidth trim-pot is always connected when using a toggle switch, it is necessary to
calibrate the narrow clock frequency first. Then, when calibrating the medium and wide bandwidth trim-pots,
the parallel resistance is reduced proportionately to achieve the higher clock frequencies required for the
progressively narrower bandwidth.
If continuously variable bandwidth is desired:
Input Amplifier Adjustments
The microphone input amplifiers, using default component configuration, provide in excess of 50 dB gain.
There are two adjustments to manage this gain to achieve the best performance. For Mic 1 and Mic 2,
R10 and R20 respectively, serve as a main level control, which is normally a pot mounted on the front panel.
R14 and R24 are "gain-set" adjustments, which alter the negative feedback, thus controlling the gain of the
first stage. In order to properly balance these two controls, start by setting the front-panel main level
control to between 60% and 75% of full output. Then adjust the "gain set" trimmer (R14 or R24) to provide
the necessary degree of amplification. Using this technique, only the required gain is afforded by the
first stage, thus reducing noise and increasing stability. This is preferable to running the first stage
gain much higher than necessary, then excessively attenuating the signal with the front panel main level
control.
The line input amplifiers include a variable resistor (attenuator) directly at the input. This is
provided to allow reduction of very large signal inputs to avoid overload of the input op-amp stage.
The output of the line input one and two is sent to an attenuator pot (R38 and R48, respectively) and the
output of these pots is summed by 100 ohm resistors, along with the outputs of the Mic one and Mic two
amplifiers. You may find that a variation of the circuit and/or component values may improve the balance of
the mixing function. The schematic specifies 10 K pots, where 1 K or 2 K may provide more balanced mixing.
Another option is to alter the wiring of the pot to the header on the PCB, exchanging two wires such that
the output of the amplifier stage is sent to the arm of the pot (center terminal) and then the input to the
attenuator pad is taken from the top end of the pot. This alternative wiring option is illustrated on page
four of the schematic diagram.
Balanced Output Amplifier Adjustments
The differential balanced output amplifiers also have two separate level adjustments. For Line output 1 and 2,
variable resistors R261 and R281 are the main output level controls, and they function as standard attenuators.
The overall gain of the output amplifiers is set by trim-pots R265 and R285, part of the inverse feedback loop.
Proper adjustment entails setting the output level attenuator to between 60% and 75% of full output, then
adjusting the feedback gain-set trim-pots to achieve the desired output level. Once set, the main output
attenuator may be used for fine adjustment of the signal level to the transmitter. This control may be on
the front panel, or an on-board trim-pot. One advantage of the trim-pot approach is that, once set, it
achieves a balance between the compressor /limiter output level, the peak clipper, and the input requirements
of the associated transmitter. Not only does this technique obtain maximum modulation without overdrive and
splatter, the use of two separate outputs feeding two separate transmitters maintains the adjustment without
the risk of inadvertently changing the setting.
The "CUSTOM" (patch) Amplifier Stage
The Audio Processor contains one "uncommitted" op-amp stage, U221.2. Just like a software jock often
provides a "patch area" for temporary or last-minute hacks, the Max Audio Processor also includes such
a feature in hardware.
Without resorting to cutting traces or adding traces using hair-fine wire-wrap wire, U221-2 is avaliable
for a myriad of applications. Circuit traces are provided to enable this stage to be either an inverting
or non-inverting amplifier, with unity or any other gain level desired. Simply populate the circuit with
resistors and capacitors in the 23X series, as needed, to accomplish your desired circuit. Interface to
the amplifier is provided at header H08 (PATCH). Pin 1 is the output, pin 2 is the inverting input,
and pin 3 is the non-inverting input. Resistors R233 and R235 are defined in the parts list to avoid
the dreaded open input pin syndrome.
A few words about Shielding, Grounding, and Ground Loops:
Using Dupont Wires
Dupont wires and male/female headers are included in the parts kit with the Audio Processor.
Male to Female wires may be use to connect LEDs to the header on the PCB.
They can also be used to connect male header pins to switchs and potentiometers on the front panel.
Either cut to length and solder the wire to the switch terminals, or solder the mail Dupont wire pin
directly to the switch or pot.
Female to female wires may also be used to connect switches and pots using the cut-and-solder method.
In this case, if the distance is short, a single female to female wire might be cut in half and used
to complete two separate circuits.
Some folk have reported that Dupont wire connectors sometimes become "loose" and do not make good contact.
The header pins are square, not round, thus the female Dupont pins must be attached to the male headers
"squarely", or the female connector "fork" becomes stretced, resulting in a loose connection. Moderate
attention to this detail will provide tight and secure connections when using the Dupont wires.
Max Audio Processor - Customization Tips & Tricks
While the MAX Audio Processor default settings provide loud and clean audio which is optimal for most applications,
it also gives users with circuitry knowledge and skills the ability to achieve a highly customized sound. It's
important to note that these modifications are provided as an expanded description of the circuitry and not as an
encouragement to perform any modifications. Doing so is at your own risk! That said...
Unbalanced Input - Using a shielded wire as and example, the shield is connected to the ground (pin 1 "G")
of the input. The center lead is connected to either the + (pin 2) or - (pin 3) terminal. The unused terminal should
be connected to ground (pin 1).
Unbalanced Output - Using a shielded wire as and example, the shield is connected to the ground (pin 1 "G") of
the input. The center lead is connected to either the + (pin 2) or - (pin 3) terminal. The unused terminal should be
left unconnected! It is very important that neither the + (pin 2) or - (pin 3) output terminals are grounded.
Unbalanced connections also make polarity changes easy. If you find that the polarity of audio feeding your transmitter
is reversed then simply use the other output terminal (2 vs 3 or 3 vs 2). The same applies to the input except that the
grounded input pin will end up being swapped with the used input.
Users who want tp be able to adjust the compression ratio through its entire range should connect a resistor or potentiometer
between pins 4 and 5. The following table shows the relationship between compression ratio and the value of the external
resistance:
2:1 @ 8.5k Ohms
The voltage on Pin 2 sets the noise gate threshold and is part of a very high impedance circuit. R108 (1M Ohms) connects between
this pin and Pin 1 which connects to the +5V power buss. Lowering the resistance between these pins will raise the noise gate
threshold. You'll notice a big difference by adding a 100k resistor there. Conversely, adding a 1M Ohm resistor between pins 2
and either 4 or 5 (ground) will reduce or disable the noise gate. Due to the high impedance of this circuit, very high value
resistors will accomplish a lot.
For continuously adjustable noise gate, remove R108 and connect a 1M Ohm potentiometer in its place with one end connected to
Pin 1 through a 100k Ohm resistior, the wiper connected to Pin 2, and the other end connected to ground.
Users who wish to set their own bandwidths can do so by adjusting trimpots R136 (Narrow), R135 (Medium), and R134 (Wide).
An accurate setting requires measuring the clock frequency of the SCAF filters (U141 and U142) which operate at 50 times the
audio response frequency. This can be done using a frequency counter or communications receiver with a short pickup wire for
an antenna placed near the circuit board and adjusting the trimmers for the desired frequencies.
This chart shows the relationship between audio frequency response, total bandwidth, and clock frequency:
Another method for adjusting the bandwidth filter uses a tone oscillator connected to the line input and a VU meter connected
to the Utility output. Set the oscillator for 1kHz at a level between -10dB and 0dB (or any convenient level that lights the
yellow level LED but not the red one). Set the Utility output level trimmer (R251) for 0dB on the attached meter. Set the
oscillator for the desired frequency of filtering and adjust the appropriate trimmer (as descibed above) for 3dB of loss.
Increasing the oscillator frequency will show the filtering as a reduction of audio level.
A Final Word regarding Customization, Troubleshooting, and Support
The Max audio processor was developed by several amateurs in spare time over the past two years, with many revisions and refinements.
They have performed on-air testing, evaluated various components in the mix, and arrived at a point where it is believed that the
results should be shared. If assembled as recommended in these documents, without extraneous customizations, the device should provide
excellent performance. That said, the kits are being made available at cost-of-components plus just enough to recover handling,
shipping, tax, etc. Additional effort has been expended to provide as thorough documentation as possible. The kits are offered
"as-is" with no implied warranty or support. With the modular interfaces, it should be very easy for any amateur to trace the
signal through any of the modules, isolate problems, and identify any parts issues. Through numerous component changes, the
circuit boards have stood up to extensive soldering and de-soldering without any quality issues. It is hoped that the
availability of this device will bring satisfaction to the builder and pleasure in operating on the ham bands. We look forward
to hearing your call and working you in the near future.
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