QRP-LABS QMX+ SPEAKER MOD

WITH BOB NAGY'S HELP!

I recently saw a post on Facebook where Bob Nagy (AB5N) had fitted an internal speaker to his QMX+ and I was keen to do the same, so I ordered the same Audio Amplifier and what I believed would be a suitable speaker.

When the components arrived, I realised that it wasn't immediately obvious where to make the connections between the audio amplifier and the QMX+ PCB, so I contacted Bob and asked him for help with the connections.

Very kindly, Bob obliged and told me where the connections go. Honestly, this is the sort of help that few people can be bothered providing and I gotta say that I am extremely grateful for his assistance and guidance 👍👍👍.

So the first job of course was to dismantle the QMX+ and decide where to place the speaker. There's plenty of room in the cabinet, but I was still keen to position it in a location which gave it the most clearance. One thing that crossed my mind was "Might there be an additional upgrade pcb from QRP-Labs in the future that might force the removal or relocation of the speaker"? Who knows? 

Anyway, I picked a spot and marked four 5mm mounting holes for the speaker. Then I drilled another series of holes for the sound to pass through. To prevent bits of dust and crap falling through the holes and sitting on the speaker cone, I fitted a fine black gauze over the speaker (my apologies to the wife - she's convinced she had a new pair of black tights somewhere) 😂.

It's worth noting that I used a pillar-drill when drilling the holes - using a hand drill will almost certainly lead a messy set of misaligned holes. And make sure you use a new, good quality metal drill bit. 

You need a short length of thin shielded cable to connect to the headphone sub-pcb.  I say thin because there’s already almost no clearance between the pcb and the bottom of the enclosure, so adding a cable makes it even tighter!

You also need some regular cable to connect to the speaker and to a suitable power feed. 

Bob had chosen to pick up the power for the audio-amp from the QMX’s power-connector and I did the same, but I’m sure there’s other options available if you know how to read the circuit diagram. To make it simple for others wanting to try this simple mod, I've done a sketch showing the layout of the wiring...

Figure 1

With the QMX+ dismantled, attach the shielded cable to the underside of the mini-pcb and feed the cable through one of the holes of the main pcb as shown below.

Connect the other end of the shielded cable to the IN and GND of the amplifier board as shown in Figure 1, putting the shielding to GND and the center-core to IN.

When you've completed the soldering, use a double-sided sticky pad to attach the audio amplifier to the back of the radio's rear panel (middle top) and start to reassemble the radio.

All that's left to do is pick a spot for the loudspeaker and drill the holes. Unfortunately, I had no black bolts for my speaker mounts, so I had to fit silver for now. I've ordered some replacements to make it look nicer.

That's it! All working well. I should point out that Bob said to change the Volume Increments to 2db because the QMX+ will over-drive the little amp. During final setup and testing, you might want to look at adjusting the trimpot on the amp for best volume levels. Just experiment 👍

Just a note on the feed from the underside of the PCB - I think (now) that it might be possible to pick up a feed from ABOVE the board to save squeezing that shielded cable underneath. See the diagram below and make your own mind up...

Anyway, it's all done for now and I'm happy with the mod and the way it sounds. Thanks again to Bob Nagy for his help 👍.

73, Tom, M7MCQ.