AMZ Q&D Compressor 2 PC BOARD

This is the AMZ Q&D 2 Compressor PC Board. It is a revised and improved version of the Q&D Compressor 2 that was previously posted on my web site (in 1998). The circuit has been revised and updated, and a new pc board layout was created for the project, to make construction easier.

The DIP through-hole version of the SSM2166 chip has been discontinued, but Small Bear Electronics has bought some of the surface mount versions of the 2166 and had them soldered to DIP adapters. This makes handling easy, without having to solder fine pitch pins, which many builders may not have practiced.

NOTES

The potentiometers are shown connected to the pcb by short wires and this is an acceptable way to wire them, but the pcb was designed so that special pots designed for direct mounting to circuit boards can be used, as illustrated below. When the pots are mounted in the holes in the enclosure, the pcb is held suspended above the board by the legs of the four pots.

These pictures are not the SSM2166 pcb but the mounting method for the pots is exactly the same.
   
Here is a drilling template for the board-mounted 16mm pots. It is 300 dpi and when printed at 100%, will provide proper spacing of the components. The cross above each pot name is the mark for the center of the shaft. I print on clear laser stock and then cut out the board outline so it may be positioned on the box, then use a sharp nail or punch to mark the holes on the metal so that they can be drilled.

BRIGHTER SOUND

The C13 capacitor is optional and is used to add a slight high frequency boost. The squashing of the signal in a compressor often gives the listener the impression that some highs are being lost, and this capacitor can restore the sparkle. A starting value of 0.01 to 0.022 uF is recommended if you chose to use this feature.

WIRING DIAGRAM

Here is a diagram for basic stompbox wiring that shows how to add a footswitch and power jack.

POWER REQUIREMENTS

Since the SSM chip requires 5v DC for proper operation, a 78L05 has been used to provide a clean stable voltage source for the circuit. The power input to the 78L05 can be any 9v or higher dc source, such as a wall wart. Current requirements are quite low (less than 15ma) and a 9v battery can easily power the project.

TROUBLESHOOTING

The first step to take when troubleshooting is to measure the DC voltages of the circuit to get a starting point. Set the DC voltmeter on a convenient range (10v to 25v) and put the black probe (negative) on a ground point in the pedal or on the pcb. Use the red probe (positive) to measure the voltages at the places shown on the image to the left. The voltages do not have to be exactly the same, but should be close to those listed.

A good ground connection is essential, and the GND pad on the pcb should be 0v since it is connected to a common ground in the pedal, such as the ground lug on the input or output jack.

POWER SUPPLY HUM

When using a cheap AC wall adapter, hum from the power supply will sometimes creep into the circuit, especially since it is high gain and will amplify any noise on the voltage rail. The value of C8 may be increased to 100uF, or even larger, to help with this problem. This mod should never be required when using battery power only.

REDUCING INPUT GAIN

Some guitars may not need the +2 gain that is provided by the internal opamp of the SSM2166. If your guitar has high output pickups, or you plan to run a booster or other pedal in front of the compressor, the gain can be reduced to unity (gain=1) by removing C4. You can also remove R6 as it will not be part of the circuit when C4 is deleted but that is optional.

DECAY TIME MODS

While the attack response of the compressor is fixed by the internal circuitry, the value of C6 is controlling the decay time of the VCA. You can increase the value to lengthen the decay, or make it smaller for faster response. I would definitely change C6 to 10uF for use with bass guitars.

MUTING THE AUDIO

The mute function could be controlled with a footswitch also. You can even use the blue 3PDT and ignore the other switch poles that are not being used. The AMZ DPDT soft touch switch is also a good way to activate the mute control. Connect switch contact 1 to the mute pad and pad 2 to ground. The microprocessor will activate the relay that toggles the muting.

If the Mute function is not used, then R11 is not necessary to the circuit and can be omitted.