Introduction: Audio Mixer Upgrade

About: I'm a jack of all trades and a master of none. I like to tweak, mod and improvise whenever possible!
In this instructable i'll show you how i will upgrade my Behringer Eurorack MX802A. The Behringer Eurorack MX802 is a nice 8 channel mixer i picked up a few years ago on Ebay for about $50. It was purchased for use as a portable Podcast Audio mixer and is now serving as my Microphone input for Skype calls. As my Podcasting audio mixer it was carried around in a case allowing me to record a podcast from anywhere with up to 4 people. The problem was i had to use my old Creative Soundblaster MP3+ USB Soundcard to connect the mixer to the laptop and even my home PC. That sound card is still being used as the primary sound card for my PC since the on board sound died. Now days portable audio mixers come with USB audio built in. So to help reduce the clutter around my PC and update the mixer to fit the specs of it newer cousins i figured it was time to Integrate the Soundblaster into my Mixer.

Materials Needed:

- Behringer Eurorack MX802A (any Audio Mixer will do)
- Creative Soundblaster MP3+ USB Soundcard (any USB Audio device with stereo inputs and outputs will do)
- Speaker Wire
- 1/8" Phone Jack
- Set of standoffs with screws

Tools Needed:

- Soldering Iron
- Phillips Head Screwdriver
- 3/32", 1/4 ", 9/64", 7/32" and 3/8" Drill bits
- Jewelers files
- Scribe
- Tin Snips
- Needle Nose Pliers
- Razor Blade
- Wire Cutters/Strippers
- Painters Tape

Updated 1/27/2012 with wiring fixes and a new feature. See Step 8 for the update. 

Step 1: Current Mixer Configuration

As it stands right now the Main Output of the mixer goes into the Input on the Soundblaster. I then have the output of the Soundblaster running to a 1/4" Headphone splitter. One side take the audio out ot my PC speakers and the other dumps the audio output back into the Mixer through the Tape In. This allows me to record any channel from the mixer i have connected as well as any audio i have coming from the PC. This was handy when my Pod casting partner was on Skype. Sure there are software packages that will record both the input on the PC and Skype but there where no free options at the time. At the time i felt i had greater control over the overall sound quality with this setup. Weather there was a real difference or not, doesn't really matter to me. Plus it allowed me to monitor everything on my headphones plugged into the Phones jack on the mixer. I like to keep the Soundblaster tucked away and out of sight. If i used the Headphone jack on it the sound to the speakers would cut out and i didn't want to keep reaching under the desk to switch out the cables.

The idea now is to integrate the Soundblaster into the Mixer. Giving the Mixer the added ability to acts as a PC sound card. Allowing the mixer to receive audio from the PC, mix with outside sources and send it back to the PC. Not to mention eliminating all the clutter of cables and adapters used to connect everything together.

Step 2: Open the Mixer

First thing I did was open up both the Mixer. I had to see how accessible the pads are on the mixer under the Tape In and Main Out jacks. I also needed to identify and mark the ground and signal pads of the jacks. Not to mention marking the Left and Right Channels.

To open the case i removed the 6x screws from the side of the Mixer (3 per side). Then there are 6 more screws on the back side of the unit. Three along the top and Three along the bottom. After pulling the sides apart i noticed there is a secondary power board attached to the bottom plate. It is attached by a couple of screws with nuts clamping the top and bottom of the board. There is also 3 MOSFETs screwed down to the bottom plate. I'll be able to work with it without removing the board and MOSFETs

Now I can see the underside of the Mixer's main board. Time to mark the Channels, Ground, and Signal pads i want to use. You can see i marked the board with a sharpie so i could remember what goes where. 

Step 3: Stripping the Soundblaster

Now its time to open up the soundblaster. I removed the 4 rubber pads to reveil the screws. 

I marked the Input and output sides as well as the Ground, and Signal for each jack. I'll leave the USB connector as is and mount the board in the Mixer so the USB connector can stick out of a hole in the back of the case. 

I had considered removing all the connections that where not need, such as the RCA, Optical and Headphone jacks. But decided against it. Ultimatly i have no need for the mic input and headphone jack on the board but why risk damage to the board? I don't plan to use the RCA connectors to wire it to the Mixer. I plan to solder wires in to make a solid connection. 

Step 4: Modding the Case

There are a few holes that need to be cut into the Mixer's case. I need to determin where the Soundblaster will sit and where the USB connector will patrude from the Mixer's back plate. I need at least 2 holes in the base plate to mount the Soundblaster too. I also need a hole to mount the 1/8" female audio jack for the PC speakers to plug into. There is also a switch on the same side as the USB plug that toggles the board between analog and digital. Its a bit long and i have no need to toggle it to i'll cut the end of the switch off. 

To start off i need to figure out where the board needs to sit so the USB jack is just barely protruding from the back of the mixer. I found the edge of the board needs to sit 1/8" away from the edge of the base plate. Which means one of the mounting standoffs will need to sit about 3/8" away from the edge of the board. So i'll need to notch the metal to allow clearance for the standoff. I'll scribe a mark indicating the corner of the board that will be closest to the side of the Mixer then scribe a mark for where each standoff will go. Its hard to see the scribe marks in the pictures. I'll drill the standoff holes with a 9/64" bit. 

Once the board is mounted i need to go about finding where the center of the USB connector will be at on the back plate. So i line up the bottom plate with the back plate and scribe a couple of line indicating either side of the USB connector. Then i'll take measurements from the bottom plate to the top and bottom of the USB connector and transfer those measurements to the back plate. 

My calipers read .77" between the base plate and bottom of the USB connector. I get about 1.2" from bottom plate to the top of the connector. Looks like the sides of the connector will be 2.5" and 3" from the side of the case. I'll use a small bits to start the hole and work my way up to a 3/8" bit. I'll finish off the hole with my file set to ensue a good clean and tight fit around the connector. Once again i'll need to mask the inside to help prevent damage to the internals of the mixer. I've gotta be very careful since the hole is very close to the mixer's main board. 

Now that the hole is in i can find where the standoff meets the metal lip at the bottom and notch it out with a pair of tin snips. I used a pair of pliers to snap the notched bit off and filed the edge smooth. While doing a test fit i found the stand off near the front of the case is too tall and causes the Soundblaster board to collide with the mixer's main board. So i'll have to remove about 1/4" of the standoff. Since its plastic i'll use a razor to cut it. Then test fit again. 

Now that everything fits we locate a good spot for a set of speakers/monitors to plug in and drill a 1/4" hole for the 1/8" Phone jack.

Step 5: Wire It Up

I'll use 4 wire about 8" long strips of speaker wire to wire the inputs and outputs together. This gives me enough room to work with everything as i try to piece the mixer back together. I'll only need a 2 pieces of speaker wire about 5" long to wire up the speaker/monitor jack. 

I first solder the wires from the Output of the Soundblaster to the Tape in on the Mixer. The Mixer's Tape In uses a single shared ground point so i'll solder the 2 ground wire together first. I'll tin each wire before soldering it to the boards. Then i'll solder the Input of the Soundblaster to the Main out of the Mixer. finally i'll solder  the wire from the Tape In to the Headphone jack on the back. Since the Tape In is receiving the audio output from the PC and can then be put through the mixer then sent back out to the PC. 

With the soldering done i'll do a good inspection of my work. Making sure the solder joints where clean and solid and there are no solder bridges. I'll put a Volt/Ohm meter to the connections to ensure there are no shorts. 

Step 6: Testing

With everything soldered together i wanted to plug everything back in and test things out. I would hate to put everything back together only to find out i burned out something or i missed a solder bridge somewhere.

With everything connected i realized i soldered the Speaker/Monitor jack to the wrong connections on the mixer. So i wasn't getting any sound through the speakers. No big deal. A quick move of the wires fixed that problem and everything works as expected. So its time to put all the screws back where they belong and enjoy my new Audio Mixer. 

Step 7: The Final Product

So here it is. My new and improved Mixer with integrated USB Sound Card!

I went from a cluttered mess of wires everywhere to a more streamline and efficient setup. Now i have a mess of unused cables and adapter to deal with.


There is only one thing i didn't do. I would have liked to move the indicator LED that's on the Soundblaster up to the mixer's top to show the status of the sound card. Unfortunately that would mean removing the main board. Which would entail removing all the nobs, unbolting all the pots and jacks. So it really isn't worth the trouble. Sure i could have it in the back of the unit but i'd rather have it visible. Besides it uses a very tiny SMD Blue LED. It would be a challenge to desolder it and solder in a couple of leads. I'd also hate to use an LED that draws more power and possibly damage the sound card. 

Step 8: After Further Consideration

Having used this upgrade for a while i've ran into a problem i've had in the past. If something is connected or disconnected from the system it causes some lag in the sound card. So while on a Skype call the person on the other end will here and echo of themselves. No surprise since i have the audio coming from the PC feeding into the mixer then it gets fed back to the PC. Since i only need this feature when i want to record audio from the PC as well as multiple outside sources i need to put a cutoff switch in the lines from the sound card output and the mixer's Tape In. This made me realize the way i have the system wired inside is again wrong and needs to be fixed. 

As it sits now the Speaker/Monitor jack is wired directly to the Tape In on the mixer. When it should be wired to the output of the sound card. So i'll have to move that before adding the switch. First i'll clear the old wires and start from scratch on the Output of the sound card and the Tape In on the mixer. Once i have the Speaker/Monitor jack soldered to the sound card output where it belongs i'll go about finding a good spot for the PC In cutt off switch. 

 The switch i'll use needs a 3/8" long and 1/4" wide hole. A 7/32" drill bit gives me a good starter hole and i'll file out the rest. 

The switch i'll use is a SPDT. Since the Mixer and sound card already shear a ground through the sound card's Input and the mixer's Main Out i only need the signal wires running from the sound card's Output to the Tape In on the mixer. Originally the switch didn't come with any screws but has threaded holes. Fortunately my 8 year old son just finished stripping a dead VCR which had screws that will fit. Used a 3/32" bit for the screw holes.

I'll solder the wires to the switch, mount the switch then put everything back together. 
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