I had a vision about a portable boomblaster which would leave JVC Boombox and similar stuff far behind. Of course ghettoblasters like that are not readily available at shops. Ok, nowadays there is iPod HI-FI, but it does not have exactly the same design principles ;)
I have always been interested in electronics. Building mechanical things has not been hard either. Project started somewhere around the year 2003 and I began the work by researching what kind of components and circuits were available and how to fullfill the requirements I had in my vision:
Philips Semiconductors (now known as NXP) has some quite interesting integrated power amplifier chips such as TDA1562Q which includes a built-in charge pump for doubling the operating voltage. It is also a so called Class-H -ampifier that internally switches the voltage between 12 and 24 (doubled) volts on the fly. That increases efficiency significantly. Immediate benefits are longer lasting battery and smaller heat sink.
Impendance of good quality speakers is usually 8 ohms. It's quite a problem in a battery powered application because Power = Voltage^2 / Resistance. Increasing operation voltage in battery powered applications is not trivial. However, car speakers appear to be 4 ohms mostly! Peerless CDC-165C speakers have a nice frequency response and great 91dB/W sensitivity.
To maximize sound quality and output power, I chose to proceed with an active crossover and bi-amped speakers. PA-tweeters are 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 ;)
Now I had figured out the most essential components of a powerful ghettoblaster. Next step was to design an enclosure for the system. After many many iterations and modifications I finally had a design:
Volume of the enclosure is about 22 litres. With 11 litres per woofer it was possible to tune the bass reflex system to 40 Hz. Bigger enclosure would have been better for bass output but hey, this has to be portable! :)
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 is 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 which gets connected to ground if the chip is overheating or THD goes beyond 10%. Cool, I can have a red led blinking when signal is too loud! However, there are also more practical usages for that. I designed a limiter circuit which compensates distorting peaks. Design phase took quite a while as I tested different ideas for controller and VCA. Final schematics for controller. Controller is connected to SSM2164 VCA.
Bass power amplifiers have a lot of electrolytic capacitors to compensate voltage fluctiation caused by battery's internal resistance. Voltage doubling charge pump requires quite a bunch of capacitors as well. Lead batteries are capable to output an incredible current and the power switch was in danger to melt with great looking sparks! I had to desing some kind of soft start circuit to prevent that.
In the early 2005 all the necessary boards were ready:
The final step was to build the console and power amplifier block. All the electronics are attached to those two modules which 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 my creation:
