I am a lifelong on-and-off amateur electronics hacker who has recently gotten back on with microcontrollers. As such I have started collecting junked electronics for parts and I have a pile of speakers -- raw 3" drivers, a bunch of low-end cabinets, and a few older mid-range cabinets -- I have been wanting to do something with. I also have a few generic TDAxxxx amps from scrapped TVs I have working on breadboards. Yesterday I got the idea to use these as halloween decorations -- whip up a simple WAV file player and place them around the yard. The speakers will be placed in construction-grade trash bags and then concealed. I have a number of rejuvenated rechargeable battery packs (NiMH, Li-ion, SLA) for power to avoid mains current and wires. The WAV file will live on an old 64 MB uSD card in a hacked uSD -> SD adapter. The initial plan was to use a simple R-2R resistor ladder DAC to an op-amp buffer/pre-amp and then into one of the TDA amps. As I was breadboarding this, I realized it requires quite a number of components, and thought things might be a whole lot simpler just driving the speaker with PWM and a MOSFET. It didn't take much research before the light bulb came on and I realized, "Oh, so that's what a Class D amplifier is!" So I did a little bit of research on simple uC/PWM-driven, Class D amplifiers and didn't come up with much. This maks sense given that an analog/comparator driver makes a whole lot more sense when you are dealing with an analog input signal. But since I will be storing a digital signal to begin with, and I am in no way concerned with fidelity, the uC approach seems to make a lot more sense. I'm looking at a simple, single rail, single mosfet design. Nothing more than uC PWM pin -> gate driver -> MOSFET -> L/C filter -> speaker. Hardly optimal, but I think this should get the job done. What I am unsure about are PWM frequency and inductor selection. 200 kHz seems an oft-referenced frequency in the analog designs, but it is easy enough to use much higher frequencies with hardware PWM generation in the uC. I would also like to try to use some of the inductors I have been salvaging as well -- most of which are unmarked -- and I am guessing that I might just want to select a PWM frequency based upon the inductors I have available. I do not have an inductance meter, but I have been thinking about building one, and this might be the time. Most of the L-meter designs I have seen are fixed-frequency, fixed-current but what I think what I really want is an L-meter that can vary frequency and current, sending the results back to a PC so that I can graph inductance vs. frequency and determine saturation at various frequencies. This should be not terribly difficult with Perl/Python and some graphing libraries -- but has anyone done this yet? But in any case, is this actually necessary? Would just hand-tweaking PWM frequency and capacitor values for optimal sound quality be at all possible? I figure if I start with a low voltage, current-limited power supply and a large speaker I'm not likely to fry anything before I get values into the ballpark. Class D really does seem to be the way to go in this application since quiescent current will be practically zero (in the dead-time between sounds) in addition to providing good efficiency when actually driving the speaker. Many thanks in advance for any comments/suggestions/URLs I missed! -p. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .