This instructable is an add-on/update from Dan's Off-Grid Party Sound System focused on how to modify the currently available amplifier circuit board to include pre-amplification on-board. Off-the-shelf, the Sure Electronics 2x-100W Amp Model: AA-AB32971 requires a pre-amplifier in order to reach full volume. That is, if you plug your cell-phone or mp3 player directly into this amp there is barely any audible volume. This is because your mp3 produces signals at max volume that only reach maybe 150mV, where the amp is expecting to receive 4V peek-to-peek signals. Without amplification you're hearing about 1/30th the volume you could actually produce. A pre-amp mixer would step up this voltage for you, but that's one more thing to buy. Instead, you can adjust the feedback resistors of the on-board input buffer/amp circuit to do the amplification for you. Dan explains this on Step 4 of his page, but he doesn't provide details about how/why and that board he shows is no longer available. The newer "updated" board has a slightly different layout. This instructable explains how to modify your amp to include pre-amplification.
Step 1: Check Your Voltage to Determine Amplification Requirement.
First step is to measure the voltage coming from your device. Put a multimeter on to one of the channels and measure across ground and the output signal. When I measured from my android device I measured a 150mV peak to peak signal. To get to 4V I found I needed to amplify 4V/0.150V = 26.6, so I need about a 26.6x amplification.
Step 2: Find and Replace the Resistors
Your amp ships with a fan and heatsink and the actual audio driver chip is located underneath this heatsink. Remove the fans, then the heatsink.
The audio driver chip is a Tripath TC2001 (datasheet should be attached to this page). Pages 18-19 explain how the input amplifier stage is driven by a feedback resistor Rf that is attached across the OAOUT1 and INV1 pins (22, 23) and also OAOUT2 and INV2 pins(27, 28) for each of the channels.
Looking on the board the Ri resistors are 10K resistors (marked as 103). In order to get the 26x amplification we need a 260K feedback resistor.
It turns out I didn't have this, all I could find in my stash was a 180K, but that's still much better than where I started. Also, my resistors were a little big and difficult to solder, so I just set them up on their sides so I could access their conductive pads and then soldered them down to the board.
There, you're all amplified now.
Now be sure to put the heatsink back on carefully: maybe makesure there is still some good heat-transfer goop (thermal paste) on the chips and evenly bolt the heatsink down. then reattach the fan if you intend to use it.
Now you're all done. Happy party-time.