Changing the gain resistors
The amplifier chip uses two fixed resistors to set the gain, which is how much the
input signal will be amplified. On this board, we're using 150K resistors as
recommended by the datasheet, for a gain of 2. If you would like the output to be
louder, you can install smaller resistors.
The gain equation for this amplifier is 2 * (150K / R). So if you use 100K resistors the
gain would be 3, for 50K resistors the gain would be 6, etc. The datasheet states that
the smallest value you should use is 15K for a gain of 20, but we've gone down to 3K
(gain = 100) with fair results.
The amplifier board has two positions to add your own through-hole resistors. These
are within the white rectangles on the top of the board. You do not need to remove
the existing surface-mount resistors, just put your through-hole resistors over the top
of them. (Leaving the SMD resistors in parallel with the new resistors will slightly
reduce the total resistor value, but this is generally not a problem).
For best results, the two gain resistors should be as closely matched as possible. If
you have a bag of identical resistors, you might measure them all with a multimeter
and pick the two that have the closest resistance.
Tips
The amplifier's output is only designed to be connected to something with a coil (a
speaker or magnetic transducer). Since the output is not a true analog signal, you
shouldn't expect to use this board as a preamplifier, etc.
The differential inputs of this board are safe to connect directly to floating-ground
audio signals such as from the MP3 Shield and MP3 Trigger.
If your audio source and amplifier have different AC power supplies (such as audio
coming from a desktop computer), you may hear a loud hum in the output. To fix this,
connect a jumper wire between the - side of the audio input and the power supply
ground (PWR - header).
Because the amplifier outputs a 250Khz PWM-like signal, it could potentially radiate
interference to nearby sensitive circuitry. For this reason, keep the wires between the
amplifier and speaker as short as possible.
Questions?
If you have any questions or problems, let us know at techsupport@sparkfun.com
and we'll do our best to help you out. We'd also like to hear about the cool projects
you're building with our parts!
Have fun!
- Your friends at SparkFun.
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