How to detect if the MP3 player is playing and making an MP3 of my own

It took me longer than I thought it would, but I finally got a circuit to detect audio output. This is the only way I can tell if the MP3 player is actually playing — the computer will need to turn on the MP3 player and then to set it to play. Anyway, the thing that took a while was doing tests with various capacitors and discharge resistors. I wanted at least 15 seconds of silence before the circuit would indicate silence. In retrospect, I probably should have had the computer handle that, but oh well. It's built now. I also added the LED so I can set the trimpot easier — it might have been a better idea to put in a switch or jumper, but I used a large resistor so the LED only consumes a few milliamps.

The pretty simple circuit for detecting output on an audio signal.

The other thing today in audio land was to edit the recording I made the other day and make an MP3. It wasn't too hard although I really have no audio editing software so I had to do it with video stuff. Anyway, it's titled "The Edge Between 15A and 15."

Hacking the MP3 player

I took apart the MP3 player and did some measurements on the switches.

It's pretty convenient that I can detect power-on through the switch voltages. Whew. For playing I'll need to check that there's some signal on the output which could be a bit tricker. I think I'm going to compare to some arbitrarily low voltage and run the comparator output through a diode to a capacitor. The capacitor would discharge slowly with a resistor through the base-emitter junction of a transistor: the collector could be attached to a resistor to 5 volts — while playing, it would stay very close to 0 volts (logic 0) but if there was no ouput, it would slowly rise to 5 volts, eventually tripping whatever logic input.

I wired up little wires to the switches and battery case to hook it to the logic circuits and power. What a pain to solder to this surface-mount stuff:

Here's a super close-up of the play button.

Finally sounds like it's working

I'm using some DC-blocking capacitors to get the MP3 player output (which is grounded at -5V) to match the audio amplifier (grounded at -12V.) Unfortunately, I keep getting 1KHz noise from the switching supply. I tried going to the linear supply but that didn't work either. I even tried a big filter capacitor right at the MP3 player but no joy.

I figured out that the MP3 player was generating clean output but the speaker amplifier had issues with its ground being noisy. I tried a number of filtering techniques and such but came up dry. I decided to try with the LA4445 amplifier again: I figure I can bend the 2mm pins to fit the 0.10" spacing on the circuit boards I have. It dawned on me that I could probably make some kind of common-collector amplifier in a class A-B configuration.

I figure I can connect a resistor from emitter-to-base on each pole and one from base to a common input. The ratio of resistors defines the quiescent current and the values define the amplification. I may avan be able to do a push-pull with the 5-volt source for lower power and higher efficiency. I could set up the output to center around the -5V which is signal ground for the MP3 player. I thought about it for a while and decided to go with a common-emitter design with some biasing resistors and an op-amp — using the op-amp to compensate for the transistor switching.

First I tried the LA4445 because I thought it would be simpler. I spent an hour or so working, got everything wired up, but I took some shortcuts and the thing just won't work. Crap. Back to square one — again.

I decided to tinker with the transistors a little. I got to the point that I could generate output, but they won't stay on all the time in a true A-B configuration so it gets all noisy and such. I considered using some resistors and diodes to keep the transistors biased all the time.

I fianlly got a viable solution. I used the same crappy design I used for the amplifier I built years ago which just drives a couple opposed Darlington transistors with an op-amp as feedback, although I did a better job of fixing the high-frequency oscillation problem. It ends up drawing about 400 mA (5 watts) with the MP3 player in-circuit and the amplifier at volume with both speakers driven in parallel. I suspect that the output is around 2 watts total or so, based on around 2 volts RMS (as measured by the oscilloscope) into 2 ohms. If it hits 2.5 volts, it's at 3.1 watts which I think is about peak.

Based on the circuit I used, the input is already self-centering around the midpoint of the power supply. The transistors need 1.2 volts peak-to-peak to maintain quiescent current, so the theoretical maximum output (going rail-to-rail) is +/- 5.4 volts; RMS voltage is 3.8 volts or 3.6 watts into 4 ohms. I tried to get away with a single op-amp to do voltage gain and to power the main amplifier. It seems to work but is even more twitchy than before … although I guess the noisy volume level is so quiet that you'd have to be right next to the speaker to discern it. Here's the circuit:

I had an op-amp I was using to generate a solid signal ground, but I didn't need it so I took it out. Anyway, at the 12-volt source, the audio section (with the MP3 player draws about 50mA and when it's at full volume, it peaks around 200mA, so I assume the output power is around 1.8 watts total consuming some 2.4 watts at peak volume. It kind of sounds like crap with a bit of fuzzy distortion from the transistor switching noise — if you know circuits, it should be pretty evident why:

A simple but lousy amplifier.

I got the amplifier wired and it works okay from the linear power supply but it's not working off the battery and I'm not sure why. It dawned on me that I can just use one of the op-amp's as a differential amplifier off the MP3 player and feed that into the prefabricated amplifier? I ended up with this simplified circuit:

The op-amp tends to avoid power-supply noise.

The first experiment worked great so I rewired the circuit to take out all that extraneous crap to deal with the transistors and got it working again. It looks like I can get something like 3 watts of audio out of the thing, and the whole powered system typically takes around 2 watts at moderate volume.

It's just too bad I had to ditch the power transistors and heat sinks because they really looked cool.