While I figure out how to build a pump for the COVID mask, I decided to revisit the boombox project. The though occurred to me to play with the main board of the Mk I boombox to see if I could connect it to the new horn speakers. Otherwise comparing the old boombox to the new requires I compare amplifiers as well.
The one catch is that the old boombox was not working well. I needed to shake some connectors between the tone control and the amplifier to make it work. On Sunday, I took the extraordinary step of taking apart the old boombox to see if I could bypass the old cables (11 conductor bundle of wires). I've been avoiding that chore for years. The difficulty was on obtaining a replacement 11 pole plug and resoldering a rusted out bundle of hair thin wires. But the issue turned out to be not the plug or the wires. The jack on the main board was broken away from the motherboard, and some solder joints broke I didn't find out until I opened the large Subwoofer box.
Waking up an old giant
You see, I had purchased a second Altec Lansing Subwoofer at the second hand shop. That was my "nuclear option" for this project, in case I could not make the horns work. Sadly, the new Altec Lansing box has a blown amplifier. Naturally, since I only paid $6 for it.
But taking it apart revealed how simple the circuit is inside. And it gave me the idea to go ahead and take apart my old boombox.
It's basically a small motherboard built around a TDA 7375 hi-fi amplifier. It's got a two-stage active low pass filter built with Op-amps. Pretty basic stuff. The box is empty otherwise, besides the Subwoofer unit and it's a ported design (Bass Reflex). It's only about 35 watts or so, basically the same as the tiny Sony stereo I show at the start of the thread, except this is an analog, not digital amplifier, which means that the power source of the Altec Lansing is much bigger and much heavier (but probably sounds better). In this case old school is better than new school. On the other hand, the Altec Lansing will not be a match for my Sony tabletop 100 W amp I'm using as a testbed.
In the meantime, I will order the inductors from Parts Express for use as a passive low filter. I'm just tired of trying to make my own inductors. It's so difficult to get the right materials now. What I may do, however is to follow Altec Lansing's example and just build an active Subwoofer amplifier. I'm so impressed to see how small they made that amp.
