Redshift187 wrote:The analogue output is capable of higher quality than it comes with.
Note: these improvements involve voiding your warranty.
First, C34 and C48, part of the bandpass filter, also removes any DC component from the audio. However, immediately after that are 4 diodes (in two packages, D12 and D13). These are meant as voltage clamps, but they clamp the now AC signal to between 0.6V and 2.7V (clipping all of the negative half of the waveform, and adding a DC component back that changes with the sound being played). Removing these diodes is the first step.
Second, C34 and C48, being 10uF, work with R20 and R26 to form a high-pass filter, limiting the low end of the bandwidth to approximately 100Hz. If you were to replace these capacitors with 47uF ones, you would have low end down to about 22Hz.
Edit: You're right about the diode network - it will clamp at 1 diode drop below 0V. I'd just written some blah about clamping at the supply rails and then realised that the diodes are after the coupling cap. Hang on though - R20 and R21 form a voltage divider that cut the 3v3/2 negative excursion by about a third, so maybe it's not a big deal. The bav99 is a silicon diode with a Vf of about 0v6, so it shouldn't be too troubled. Bit messy though...
Re the 10u cap: Yes and no.
If the output is plugged into a line input at an impedance of ~10k ohms then the -3dB freq. will be in the order of 1.59Hz (with a caveat regarding my mental arithmetic...) Good enough to listen to that album of whale song.
If the output is supplying a set of low impedance headphones (at say 16-32R) then yeah, it will be rather toppy. I'm not sure there's room on the board for a pair of 1000u caps though! Also, looking at the lpf and whatnot I'd wager the output's not really designed with low impedance headphones in mind anyway. I don't remember the current sourcing ability of the BCM's PWM outputs, but I expect they're fairly non-zero output impedance too.