Bluetooth Bench Mixer

Readers: If you want to build the project featured in today's post, please go to PCBWAY's Community pages--gerber files (front panel, main board) ready to download and/or fabricate as well as STL and FreeCAD for the breakout board tray, KiCAD pro/pcb/schem files and a BOM are here.  

You can also help out this site immensely by checking out PCBWAY using the link here. Thanks!

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This time I decided to do something old school: a simple mixer circuit that isn't crammed onto the head of a pin. Yes, this 4x2 mixer doesn't have a lot of features, but does sport Bluetooth audio inputs derived from a popular breakout board: the XY-BT-MINI.

Did it work first time? Yep--one stupid mistake when sizing the pcb--but it all worked.

This mixer was designed around the Eurorack standard. It's 14HP , meaning it's pretty wide for a simple 4 x 2 Eurorack mixer. 

This meant less space for more and more Eurorack gizmos on my bench. But enough is enough--you don't need tiny hands to use this module--and sometimes it's OK when the thing gets big.

BLUETOOTH BREAKOUT BOARD

The star of the show was an inexpensive breakout board available from Amazon as well as from some of the larger Chinese electronic marketplaces, the latter of whom I no longer do business with, after having gotten ripped off by those guys too many times.

I was introduced to this board from the video found here although the content creator never says which board he used. 

Matching the look of the board in the video against the many BT boards out there I determined it had to be the "XY-BT-MINI" so I got a few from Amazon for next to nothing. (Amazon link that works as of 7-7-24 is here) 

I couldn't find a datasheet nor a schematic for the XY-BT-MINI, nor even figure out what main IC the breakout board uses, but, at about $1.80USD per breakout board how risky could this be? 

I wired it up on my bench (5v, left out, right out, ground) and the bluetooth board worked great.

From here it was a matter of designing the XY-BT-MINI into a simple 4x2 mixer.  

Using Kicad and FreeCad I did just that and in a few days the boards were back from my patient sponsor, PCBWAY:

 

Simple enough to build:

For hookup wire I used JST connectors purchased from Tayda. These came recommended from other techs, who said JST's can't be beat in terms of price, reliability, and ease of use. Indeed! The JST's were extremely easy to incorporate into the build.

I used FreeCad to design a tray for the XY-BT-MINI board. 2mm nylon screws and nuts were used to fix the breakout board to the tray; 2.5mm screws and nuts to affix the tray to front panel. Once I knew I had a good stl file I uploaded the 3D stl file to PCBWAY for 3D printing; the prints came out great.

For pots I used 12mm A250K's because it's all I had in my junk box, but if you have A100K's use them instead (12mm or 15mm panel mount will do the trick)

Not a bug--a feature: I normalled the BT outputs to pairs of mixer inputs: left in 1 to 3, right in 2 to 4. To get a bit of extra boost from the BT module I could increase volume on 2 pots in series but of course this benefit was lost if I use inputs 3 and 4 for other purposes. 

Not completely ideal, but this avoided having to use more amplification for the XY-BT-Mini's slightly weak output (see "improvements" below). With all pots all the way up, the XY-mini's output better matches the output of the 10V P/P modules found on my bench.

MISTAKES? Yep. One BIG one.

I spent a lot of time and energy making sure the 3d printed tray fit onto the front panel, the XY-BT-MINI could be screwed to the tray, and the hardware (JST's, regulator, 3.5mm jacks) were correctly sized for the pcb. 

However I forgot to check if the width of the PCB was too wide for the front panel. 

Turns out the PCB stuck out about 2mm from the left side:

DOH!!!

Not a big deal; this issue was corrected via the next PCBWAY PCB run; the better fitting PCB was then uploaded to the PCBWAY community site. Thanks to PCBWAY for their support and for turning around the improved boards in only a few days for testing.

 Fixed!

CONSTRUCTION:

Eurorack designs often try to accommodate "Skiff"modules, allowing shallow enclosures for all your modular synth crap. 

Great for big synthesizers in small studios, but for prototyping: not always the best approach. 

For instance:what happens if I wanted to make a simple modification? What if I had the wrong jack label for a given silkscreen?  

"Skiff" is no good for "What if," I have found.

A certain amount of hook up wiring, that I could cut and resolder might be helpful, and least until my design was finalized.

Thus for this project I used through hole parts, a relatively large front panel and main PCB, and a lot of 22 gauge hookup wire. 

Think of construction techniques one might see on a Large Format module? 

I assumed that that would make any improvements or modifications, followed by a "rev 2" of the project, easier to resubmit and retest. 

Eventually I'd get it right!

Fortunately other than the BIG mistake mentioned above it all worked great--no trace cutting and bodge wires were needed. 

Overall this was a refreshingly simple project, super easy to build. 

IMPROVEMENTS:

As previously mentioned: the line out of the xy-bt-mini is a bit low--maybe 2-3v peak to peak. To match the levels of other audio devices found in my rack--mostly 7 to 10v p/p--it might make sense to gain the xy-bt-mini up by say 12db and bias it for bipolar operation (which would eliminate the need for the 4.7uF decoupling caps) using a TL072 or equivalent dual op amp configured for non-inverting gain.  However, I am not sure this would be what other users want or need. 

Also: I could bring the stereo output jacks to the front panel; maybe in parallel with the rear mounted screw terminals. This would allow it to be used on the bench or in a standard Eurorack setup. No need for that right now, for me anyway.

A male plug to female 3.5" jack could be employed to bring the BT module's headphone output to the front.   

Different jacks? That would probably be an improvement. There are ongoing issues with the footprint for the Switchcraft RAP35ACV jack. I am yet to find a 3.5" horizontal jack and footprint that works in 100% of my designs, and I never had one I like for Eagle.

Update 7-25-24: I am still working on revising my RAP35 Kicad custom footprint. I might update the gerbers posted to the PCBWAY community site down the road if I can come up with something I really like for this useful component but for now what is posted works. 

Update 7-29-24 Thanks to PCBWAY for getting me more boards to sort the RAP35ACV issues with fit. This time around I had luck using 6mm spacing between round (not oval) pads for this jack. This isn't the pad spacing found in Switchcraft's datasheet but my footprint for this part seems to work well for holding the jack in place during assembly.  I also modified pad 1 (ground), making it asymmetrical, to allow for better edge cuts and a better, more flush front panel fit. I uploaded the newest gerber and Kicad files to PCBWAY's community page; these updated files incorporate these and a few other minor changes and fixes (here). Should be good to go.

Update 8-1-24: more minor cosmetic fixes/silk screen improvements uploaded to the PCBWAY Community. Have fun!

OUTTRO:

Now I have integrated BlueTooth on my bench that I can mix with other audio sources. 

Funny, I didn't think a lot about the practicality of this module while designing and building it, but I really like how this module turned out, and find myself using it a lot more than expected.  Like: for listening to music while building, without cluttering up my bench with more cabling.

Overall what I created for now--good enough, for me anyway. Keeping it simple is always stupid?  It's not stupid if it works? Something like that.