The amplifier is a speaker dock for MP3 players. It's just a cheap single-chip amplifier with two cheesey speakers from Ebay, and is of no real consequence, but I would like to understand why the designer may have made certain changes from the example schematic shown in the datasheet, and what the consequences would be of changing back to the datasheet version. The as-built amp produces very little output volume, and appears to be capable of much more, and I'm curious why they elected not to let it do that.
The datasheet and a circuit drawing of the amp as received are here:
It appears the designer went way out of his way to reduce the input signal and the amp gain. I don't really know why. The actual circuit differs from the drawing in the datasheet in three ways:
- The input signal first goes through a 50K pot to ground, and the wiper off that then goes through another divider circuit of 39K/4.7K to ground, with the chip's input pin connected at the junction. I really don't understand this at all. I even wonder if the 39K resistor was intended to be 3.9K. (It is indeed 39K, on both channels.) In any event, I don't see why either of these resistors need to be there. It appears to be the Homoepathic version of the input signal - diluted down so much that there's hardly anything left. I tried jumpering around the 39K, and got nice loud volume out of the speakers.
- The resistor labeled Rf in my drawing is the feedback resistor. It's referred to in the datasheet as reducing amp gain. Again, it's not clear this is really needed, but I assume the intent is to reduce distortion.
- Capacitor C8 in the datasheet is not present in the actual circuit. I don't know what this does. I don't know what "bootstrap" does here at all.
Obviously I'm not an EE, and analog isn't exactly my strong point. Well, at least it's not RF. But I would like to understand this if I can. Any help would be appreciated.