The “Black Box” is a sound-art piece by Flingco Sound System. It’s a lo-fi playback device with 9 sonic adventures onboard. We’ve been researching low cost audio devices lately, and thought a look inside would be educational.
It uses 2 AAA batteries, has a rotary potentiometer with switch for on/off, and an 8ohm speaker for the output. There is also an 1/8″ jack for headphones. The main processing is done under the epoxy blob, so there isn’t too much to mess with, and there are no signals on the unused pins.
Figure 1 – Internals of the Black Box, showing PCB, speaker, and case.
The data is stored in 8bit/40ksps format, with an interesting PWM output format. There are 2 PWM output pins, with one going to each side of the speaker. These output 7bit PWM signals at 40ksps, with only one output active at a time. The 8th bit is used as a sign bit, so if its high, the positive PWM output is used, and the negative is held low. If the sign bit is zero, then the negative PWM output is used, and the positive is held low. This effectively gives you one more bit of resolution for a given sampling frequency – a neat trick!
The first thing i wanted to do with this PCB was to put a better volume control on. The potentiometer doesn’t go quiet enough, and the speaker distorts heavily. Even worse, listening on headphones was downright painful. This was easily fixed by cutting a trace and soldering a 10k resistor in series with the speaker. This made the headphone output perfect, but reduced the onboard speaker volume too much. unfortunately, without replacing the onboard pot, there isn’t a happy medium.
Figure 2 – Visual schematic of mods.
Since there aren’t any data pins external to the chip, the only control parameter is the supply voltage. Luckily, dropping this causes the playback speed to slow down. The easiest way of accomplishing that was to put a potentiometer between the battery and the IC. A 500k audio taper pot was used here, but a 100k might have been a better choice. A 100uF capacitor is put onto the powersupply line to stabilize the voltage as current draw varies. This creates a nice effect of the playback slowing down gradually as the sound becomes more dense.
Finally, a feedback potentiometer is used to modulate the powersupply with the audio. A 500k audio taper pot worked well, and an AC coupling cap of 1uF was used. The coupling cap can be varied to allow different frequencies through. The larger the cap, the more low frequencies will effect the modulation. The final assembly is shown below.
Figure 3 – Mods to the Black Box, with potentiometers glued to the case.
There are a lot of fun sounds to be had inside of the Black Box. You need to set the speed knob to a little down from normal speed to get any effect from the feedback knob. And, the speed knob takes a little time to change state (due to the 100uF capacitor), so turn it slowly. If the device seems to lock up on you, and not make any more sound, just toggle the track select button, and it will spring back to life – or at least as lively as it can muster.
Figure 4 – Final assembly showing 2 pots on front for control.