Active speaker, 70s style

It has already been three years, since I ranted in some pub about not being able to buy a plain and simple active speaker, powered by primary batteries, so you could use it for a few days on a festival campground. (“Power banks” were uncomon then and are still ridiculous now IMHO.) That evening I was immediately told, that I should build one myself.

So I startet to make a plan to relieve my trusted (but worn) Logitech “mm28” with something better. And as my parents just wanted to throw away an old portable radio (the Telefunken “Bajazzo CR1000” was heirloom), I knew where to start.

As no Metalhead needs any radio receiver at all (and you don’t carry a box of tapes around anymore) there was space for a few gimmicks. But first: the basics.

Disassembly of the old radio was easy, because everything was screwed back then.

The PCBs were drawn by hand, the switches made from PCBs in a transparent shell. Unfortunately I had to dremel most of it away:

While the FM/AM/Rec… switches could be screwed back in place the Mic/Osc switches were now hanging in the balance. So I had to make a “thermoplastic mount”.

After testing the original speaker with a small amp-module I decided against keeping it.

The mm28 was already deploying a class D amp (and NXT flat speaker technology), so I was used to getting some volume out of a portable speaker.


As the battery compartment of the “Bajazzo” could absorb six D-cells, obtaining the efficiency of the mm28 (four AA) was optional.

The Visaton FR 4×6 has a “music” power rating of 30W and fits into the Bajazzo’s case quite well. As I have no specs for the mm28 I can’t compare them on paper.

Reichelt, where I ordered the speaker also had the TDA7350 amplifier IC, rated 22W (Bridge-Tied Load). On 2 Ohms (20 W on 4 Ohms) and at 18 Volts. So at least this won’t damage the speaker at 9V. 😉

The schematic for BTL is in the datasheet from ST. I decided against using a PCB, but mounted the bigest heat sink I could find.

After everything worked I connected it to the “On” button and used the hole left by the antenna to conduct the (mono) cable. The mic was not connected, as I had no TRRS jack, but if I ever feel like using the old Telefunken as hands-free equipment it could be done that way.


As I was too lazy to figure out, how the volume-pot worked (and how good that part still was anyway) the gain is fixed for now.

The buttons of the tape deck were later glued on to fill the gaps.

In the next post I describe the add-ons to make this apparatus the perfect festival companion.


(Original Posts:, June 16th 2013,, January 26th 2015 and, March 2015)

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