Mini7bt - A Minimus 7 Portable Bluetooth Speaker

Bluetooth (BT) speakers have become ubiquitous and with good reason. Just about every phone is a BT device and can play tunes. Paired with a BT speaker, the cell phone benefits from an expanded listening range, higher volume and better sound quality.

What is inside a BT speaker? It's a driver with a BT receiver, an amplifier, and usually a battery pack for cordless operation. They are available just about anywhere. In the name of portability, they tend to be small. Small usually means that the speaker is just a single full-range driver. While better than a cell phone's speaker, these small driver do tend to compromise quality, especially for the lower cost models.

If cost, size and weight were secondary to sound quality, can a decent bluetooth speaker be made with minimal effort? We would need:

• Bluetooth receiver
• 2-channel summing network to combine the left and right channels from the BT receiver
• An amplifier
• Batteries and charging circuit
• Controls
• Decent speaker with crossover

The Mini7bt is a Radio Shack Minimus 7 combined with a Dayton Audio bluetooth receiver to form a high quality portable bluetooth speaker. Obviously this can be done with any speaker. I used a Minimus 7 because I happened to have one on hand - I found it at a yard sale for $1. It was an original cast aluminum grey colored model that had been exposed to fertilizer (probably sat in a shed or garage) so the aluminum was pitted. It was missing the original metal grill. I sanded the exterior to reveal a "brushed" aluminum look. The Minimus 7 is a classic fan favorite sold by Radio Shack in the late 1970's. For its size, this is a surprisingly good sounding speaker. Used copies can still be found on eBay and because of the rubber surround, there's no surround rot to worry about. Over the years, hobbyists have hacked the simple crossover (just a single cap for the tweeter) to improve the sound quality.

For the crossover, I used the popular "Zilch Mod". Unfortunately the original site seems to be gone so I've reproduced the circuit here. I found all the parts I needed in my junk parts box.

For the bluetooth electronics, I used Dayton Sound products from Parts Express. This is pretty much a pre-packaged set of parts with some options here and there. Their product line is constantly evolving so it wouldn't surprise me if there are better solutions in the future.

• KAB-100Mv2 bluetooth receiver and amplifier. This also performs left/right summing so I can drive a single speaker.
• KAB-BE (battery extension). This houses three standard 18650 LiOn batteries for portable use. The amplifier's power output depends on the supply voltage. Three cells does not max out this amplifier but it's ok for my use.
• KAB-PMV3 panel. This provides the wiring harness and panel for indicators, controls and I/O.

Assembly is really a matter of aesthetics. What do you want it to look like? How big or how small? I ended up with a wooden (walnut) case holding the Minimus7 speaker and electronics. I hand-cut dovetails and finished the surface with Danish oil. The updated crossover is inside the speaker itself. A new grill is held in place by friction. The electronics are bolted to the back of the speaker and protected by a sheet of acrylic. Everything is removable - just in case I need to replace the rechargeable 18650 batteries. The charger is a re-purposed wall wart from some long-dead product. I keep a supply of old chargers around for this very reason.

I really like this BT speaker. It sounds great, is very easy to use; generates decent volume. It's a bit heavy - the Minimus 7 was not a cheap piece of junk! I wish I had built some sort of handle as it's not shaped for easy carrying. I also wished the battery pack was larger for more power and volume.

As noted earlier, this is something that can be done with any speaker. The main limitation is the cost. The electronics I bought cost more than most cheap bluetooth speakers. But what I'm left with is far better than anything off-the-shelf of comparable size.

Spring-2022

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