Audio DIY

I just learned something interesting. It's "hard" to get lots of power output from a 12V power supply. All those Alpine V12 subwoofer amplifiers actually have a beefy DC-DC inverter inside to convert your run-of-the-mill 12V to something more interesting, like +/- 40V for the amplifier.

With 12V, you only have +/- 6V swing, or about 4.24V RMS. If you're driving a 4-ohm load (typical car speakers), V = IR and so I = V / R = (4.24V / 4 ohms) = roughly 1 ampere. Since P = I^2 R, maximum power is a measly 4 watts or so.

Now if you don't want the expense and complexity of a DC-DC inverter, you have to bridge two amplifiers (driven 180 degrees out of phase). So when one of them is swinging to +6V, the other one is swinging to -6V and you get double the voltage swing, which translates into four times the RMS power.

So that's around 15W RMS power. Pretty respectable, but nothing like the 75W RMS into 4 ohms that something like the Alpine MRV-F345 gets. But then again, you only need lots of power because subwoofers are notoriously inefficient. 15W is plenty if you're more into Sarah Mclachlan than Metallica.

Which brings me to the STMicro TDA2005M audio IC. It's two power op-amps in a single package, runs off 12V, and bridge-able. RS Philippines quotes 250 pesos for one of these. So it should be cheaper at Alexan. And it is in the Alexan product list, too.
And here's the suggested circuit from the data sheet. "Low cost" indeed -- look at that component count. A whole bunch of cheapo passives.

I'm quite pissed off at the ancient Alpine head unit in the car. It's finally giving up the ghost. To replace it ghetto-style, I'd have to build four of these TDA2005M-based amplifiers, total price probably around 1500 pesos, remove the misbegotten Alpine (and its equally ancient 6-disc CD changer in the trunk), and just drive the "amplifier array" using a cheap 128MB MP3 player that was a freebie from Canon.

On the other hand a decent Pioneer MP3 head unit is like 5500 pesos. And it doesn't look like something from Weird Science.

What You Can Buy For P 150

I paid a visit to "old reliable" Alexan yesterday. Apparently the prices at RS Philippines are much higher than Alexan prices. All these goodies in the picture cost a grand total of P 152, including a TL082 operational amplifier, whatever replaced the Allegro UGN3120 Hall Effect sensor, an OnSemi MC7805CT voltage regulator, and some assorted passive components.

The TL082 and the assorted passive components are for my latest project-in-waiting, an oscilloscope buffer circuit for a sound card. With one of these, you can provide a 1M input impedance for a sound card, just like "the real thing." Unfortunately Alexan didn't have any of the 1N914 diodes. With the circuit below, and the XOscope application, you can use your ordinary PC and sound card as a low-speed oscilloscope.


Nothing ground-breaking here, but I can use it to scope the NE and G signals on the car's distributor. Actually I found a pinout for the ECU (engine control unit) itself, so I can scope the signals with a notebook PC from inside the car.

And then, if the G signal is well and truly gone, I'm planning to remove the distributor, take it apart, and somehow jury-rig that Allegro Hall sensor inside it. If I can repair the distributor (an all-too-common failure mode on the 626's FS-E 2.0L engine), I know a lot of people with 626's with flaky distributors. I'll be rich! rich, I say!