this page is for those who enjoy DIY audio stuff

    It's about time I did a formal writeup on this abomation of plastic, metal, and heat. At the moment, I have no respectable name for this project, so I'll go with a brutally literal description: HEAVY CUBE, A.K.A. my current guitar rig.

 

    Basically, its just a plastic flight case with rackmount rails, allowing for any standard rackmount hardware to be safely (haha) used, stored, and transported. Of course, that doesn't mean I have to use specifically rackmount audio gear. No, the rackmount system can house all sorts of audio gear on top of trays and shelves. Versatile. Inside, I stuck a mini-pc, speaker amp, audio interface, and custom speakers. So that allows two instrument/mic inputs along with MIDI inputs. The system runs in stereo, as well.

 

    There is nothing novel about sticking audio gear inside of a rackmount case or something of the sort and making it your portable guitar rig. No, this has been done many times before me and with better results. Hell, people have fabricated their own rackmount enclosures from scratch. I write this now having been humbed on multiple forums whilst showing off this thing. I attacked this project with enthusaism and showed it to the world, but it was in an admittedly sad state. There was and still is much work to do. I come now with a far better understanding of where my rig succeeds and where it fails. 

 

    I became sort of infatuated with the idea of building a custom guitar rig whose effects ran off of a DAW which itself ran off of a mini-pc. Inegrated within the rig are speakers. This configuration is so far away from any typical guitar rig that you immediately go into DIY mode. Though, it's quite simple: your guitar plugs into an audio interface (focusrite, for example), that signal is passed to a mini-pc over USB, the signal is processed using any sort of DAW, and then sent back out of the interface into a digital amplifier. The amplifier powers the speakers. And that's the whole chain. Now, you just need to develop some sort of configuration within some sort of enclosure that will make the system reliable, resilient, and SOUND GOOD.

 

    So, I settled on using the rackmount system. There's already a ton of rackmount audio gear out there, and a ton of non-audio rackmount gear that is still useful to me. As I said in the opening, non-rackmonut gear can be easily racked with a rackmount shelf and some velcro. Classic! Gear can be held very confidently in this system. Given that this was to be a portable rig, I chose the flight case form factor. A sturdy feeling plastic enclosure lined by metal (aluminum?), each end holding removable covers with locking latches, making a nice seal. This way, all components can stored and covered with some level of confidence.

 

Now I just need speakers. At first, I thought I could get away with just sticking existing speaker enclosures inside, and maybe that’s perfectly viable, but I did not like how it looked, and it was space inefficient. To really optimize it, you’d have to find a speaker as close as possible to 17” wide without being too wide, and then match it’s height to however much rackspace you have vertically. Funny enough, the Kenwood CRS-157 center channel is EXACTLY 17” wide. There I was in our local thrift shop, tape measurer in hand, astounded at the perfect dimensions. I went right home and velcro’d the speaker to a tray and it sounded… alright? I mean, there’s nothing special about a $100 Kenwood. Or a center speaker, for that matter. The most obviously annoying part was the fact it didn’t take up an exact amount of rackspace vertically. I think it was just 3.5 RU tall. It left a nasty gap smack in the middle. After some fruitless searching online I realized that having to find a super specific speaker to fill a super specific amount of space was just not viable. So you just make your own way to mount the speaker.

The first design consisted of an entirely separate enclosure that would “slot” into the existing rack. This enclosure would be totally sealed. But I think the main issue was weight. MDF is heavy, and that would be my material of choice for the first few weeks. Acoustically, I would be great. But my thinking was that I could do away with a separate enclosure and just design a panel or “baffle” that mounted directly onto the rack, I’d cut holes in it for speaker divers, and then allow the flight case as a whole to act as the speaker enclosure. So when the speakers are running, all back waves are fired directly into the case, mixing together (yuck!). Using TinkerCAD I modeled some simple panels and had a friend CNC them out of MDF. I made a few different configurations with MDF, including different drivers and different rack unit measurements. It was either 3RU (5.25" tall) or 4RU (7" tall). I eventually did away with the 3RU variants as they just didnt have enough surface area for mounting larger woofers. With 4RU, I was able to fit a 6.5" woofer with an overall diameter of 6.91".

 

    Here's what I put inside:

Rack unit 1: Sliding shelf with monitor and keyboard+mouse (touchscreen soon)

Rack unit 2: Tray holding mini-pc, audio interface, and amplifier

Rack unit 3: SPEAKERS

Rack unit 4: SPEAKERS  

Rack unit 5: SPEAKERS

Rack unit 6: SPEAKERS

    The backside of the unit currently holds the PDU and also the rest of the sliding tray (don't look at how I ended up mounting that)

    Admittedly, I can't make any strong statements about the quality of the sound given I haven't measured it's frequency response. I can only imagine how bad it is by any audiophile's standards. The high end definitely suffers as anything above 300hz relies on full range cones. With no tweeter, we have a "muffled" high end. The full range drivers are surface mounted, which almost certainly results in nasty refractions around the flanges. Moreover, the FR drivers lack their own back-enclosure, so they share space with a 6.5" sub. I imagine the subwoofer is knocking them around quite a bit. I mean, I can do small amount of EQ, but I fear the most natural qualities of the sound and speaker are lost due to small things like this. As the rackmount enclosure itself is mostly plastic, there is an ungodly amount of vibration (and refraction?) throughout the whole thing. I've laid to rest any fantasy of being able to fully seal or properly port the enclosure, so that it functions perfectly as a speaker. No, I don't think it's possible without loads of material testing. On top of the fact that each driver shares enclosure space, my amplifier lacks EQ that would allow me to do a hard crossover. Unfortunately, the 2.1-channel aplifier I use has EQ parameters for just the sub cutoff+volume, and treble+bass+volume for the L/R channels. The cutoff for the sub seems quite steep and stops at 300hz, but the treble+bass cutoffs for L/R are far more gradual and just don't give you much control, even if you're able to visualize it. A DSP with full functionality would go HARD. I'll need to find something simple and lightweight for this. Maybe one with a built in amp?

    

    Vibration is also an issue at high volumes: mounting points, loose cables, objects resting on the enclosure, the enclosure ITSELF, and attached covers all tend to vibrate at specific frequencies if not properly managed. Everything has to be either very tightly mounted or completely free from touching the surrounding enclosure. Velcro has worked suprisingly well for damping the transfer of vibration. 

 

    Heat will undoubtedly become an issue. After just a couple minutes of running any component near the mini-pc is hot to the touch. Not scalding hot, but like concerningly toasty. I've had a few instances of what sounded like my CPU being throttled (clicking noise, presumably a imposed overrun), I have a suspicion heat builup contributes to that. This problem was only excacerbated as I implemented a closed-baffle system, meaning I put the back cover on while playing, so waaaaaay less heat escapes from the enclosure. I suppose the best solution would be to add fans, which doesn't seem to difficult, but I'm still struggling to envision a thermal system that works functionally and aethetically (cringe, I know) with the current configuration. Trying not to be invasive, so no cutting holes in the top/sides. 

 

    I feel as though a design like this just BARELY toes the line between practicality and versatility. But, I really don't care, I love this thing. I love playing on it and working on it. I enjoy implementing increasingly sophisticated designs each time I work on it. The brainstorming/research alone has been very rewarding, and I've stumbled across great wealths of knowledge in this niche. There are people out there that are as passionate as you are about the same exact things, and those same people are almost always willing to guide through whatever stage of the process you're going through. 

blahblahblah to be continued lots of typos