I like high quality (and loud) audio and wanted to enjoy it outdoors or wherever there is a party. A powerful portable boombox was needed. Well, ghettoblasters like that are not readily available on store shelves. There are nowadays several options available, but they tend to fall short in bass response, sound pressure or sturdiness. The only choice was to design and build one myself!
I have always been interested in electronics and building things in general. The project started sometime around the year 2003. I began by researching what kind of components and circuits were available and how to fulfill the requirements I had in my mind:
Philips Semiconductors (now known as NXP) has some quite interesting integrated power amplifier chips such as TDA1562Q that contains a built-in charge pump for doubling the operating voltage. It is also a so-called Class-H -amplifier that internally switches the voltage between 12 and 24 (doubled) volts on the fly. That should increase efficiency significantly. Immediate benefits are longer lasting battery and a smaller heat sink.
Impedance of speakers is usually 8 ohms. It's quite a problem in a battery powered application because Power = Voltage^2 / Resistance. Increasing operating voltage in battery powered applications is certainly possible but not very trivial. However, car speakers appear to be 4 ohms mostly! Peerless CDC-165C speakers, which, unfortunately, are nowadays a discontinued product, have nice frequency response and great sensitivity of 91dB/W.
To maximize sound quality and output power, I chose to proceed with an active crossover and bi-amped speakers. PA-tweeters are a somewhat radical choice, but they have an incredible sensitivity (105dB/W) and good power handling capacity. Yes, volume had to be as loud as possible - and it's practically impossible to burn them. Their frequency response in the high end is a bit limited though and frequencies over 15kHz are practically nonexistent. More expensive tweeters have a better frequency response, but the budget would have exploded then.
Now I had figured out essential components of a powerful ghettoblaster. The next step was to design and build an enclosure for the system. After many many iterations and modifications I finally had a design:
The volume of the enclosure is about 22 litres. With 11 litres per woofer, it was possible to tune the bass reflex system down to 40 Hz. A bigger enclosure would have been better for deeper bass output but hey, this had to be portable! The heat sink is overly big and prominent, but it looks pretty cool (eh heh) and fits into the overall design quite nicely.
6,5mm plywood is a natural choice for an enclosure which should be quite light but durable enough the same time. Construction didn't take much time if compared to the design phase. I spent about two days sawing, drilling and screwing. Waiting for paint to dry was frustrating.
Ok, something essential was still missing.. the electronics! This phase took a long time - building and soldering are fast and easy, but designing printed circuit boards is very time-consuming - almost frustrating. All PCB:s are my design. Some of the circuits are fully designed by me, but the rest are mostly inspired by datasheets and especially Elliot Sound Products' schematics. Unfortunately, I have lost all the original PCB design files. I have them only in printed form... plus a few low resolution png images.
TDA1562Q has a diagnostic pin that gets connected to ground when the chip is overheating or THD goes beyond 10%. Cool, I can have a red led blinking when the signal is too loud! However, there are also more practical usages for that. I designed a limiter circuit that compensates distorting peaks. Surely, the output gets very compressed when someone turns the volume knob to maximum, but that effectively eliminates nasty overdriving and distortion. The design phase took quite a while as I tested different ideas for the controller and VCA. Final schematics for the controller part are avaiable here. The controller modulates the signal using an SSM2164 VCA.
The bass power amplifiers contain multiple electrolytic capacitors to compensate voltage fluctuation caused by battery's internal resistance. Voltage doubling charge pump requires quite a bunch of capacitors as well. Lead batteries are capable of outputting an incredible current, and the power switch was in danger to melt when switched on! I had to design a soft start circuit to prevent that. The current implementation is quite lousy however, and I have yet to build a better one.
In early 2005 all the necessary boards were ready:
The final step was to assemble the console and the power amplifier block. All the electronics are attached to those two modules that are easy to unmount for service, tuning or whatever.
The project lasted over two years, although I had several long pauses with it. Last finishing touches were made in the spring of 2005. And here is a summary of my finished creation: