1 (edited by humina 2010-06-12 08:59:30)

Topic: hardware (speaker)

I had a few hardware questions:

Does the chumby one come with two speaker driver chips (TPA6211A1DRB)?  Is it possible to easily add a second speaker to get stereo audio output?  Looking at the spec sheet for the chumby one and the TI chip it looks like a second resistor and capacitor could be added to create a band pass instead of a high pass filter to improve audio quality (not sure by how much but it's worth a shot).  It would be a single ended version of figure 31 from the spec sheet (http://focus.ti.com/lit/ds/symlink/tpa6211a1.pdf) instead of figure 30.

Does the chumby one use the headphone detect to switch between mono and stereo audio output?  If so then simply adding a second speaker wouldn't work and would require some software hacking as well.

Thank you chumby for making this project open source.  I totally wasted my morning looking at schematics and spec sheets.

Re: hardware (speaker)

I actually spent a little bit of time looking at the speaker setup with he texas instruments chip.  The high pass filter with the speaker gain as set up with the chumby has a 3db cutoff frequency at 44Hz (R = 15kohms C=.22microfarads).  The capacitor could be decreased to .106microfarads in order to move the 3db frequency closer to 100Hz (the lower end of the audible range).  A capacitor from the input to the output would create the low pass filter.  This could be set at 1 nF.  If you want to tweak the audio more, the higher frequencies could be decreased at 40 db/dec instead of 20 by adding another RC pair using equations 10 and 11 from the spec sheet.

3 (edited by humina 2010-06-11 13:45:04)

Re: hardware (speaker)

I'm just going to keep replying to my own posts...To hear what frequencies are being filtered out it is nice when designing the filters.  The wikipedia page on it is nice for that:

Hearing the 65Hz frequency on that page leads me think that filtering at 100Hz might be too high and thus the current input filter can remain untouched.  Adding a capacitor for the high frequency would still be super easy to try out.  Of course the speaker itself will have a specific frequency response regardless of how much tweaking the circuit gets.  Anyone know the part number/manufacturer of the speaker?  All I know is that it is a 2W speaker.

Here is a picture of a sample speaker response versus frequency.  A great speaker would just be a flat line:


If I knew the part number of the speaker in the chumby I could possibly find the frequency response of the speaker.

Here is a picture of a speaker that would have a better frequency response than the the image above: