I already had racked my BOSS DR-110 drum machine and added MIDI, but I was a bit annoyed at how quiet the bass drum was. Actually, since I didn't have any controls on the front panel at all, I couldn't control any of the volumes - usually there is a balance control that lets you control the balance between cymbals/hihat and kick/snare, but I didn't even have that. So I decided to add a mixer.
Power supply
Seeing as how I'm planning to add more voices into this box, i decided to start with 12 channels on the mixer, but leave room on the front panel for 16 if I wanted it. Previously I was running the rack case off a 9V wall wart - if I'm going to be adding more stuff, possibly from the Roland TR-808 drum machine, I may as well have a power supply built in, so I added in a perfectly stock standard +/-15VDC power supply using a 7815 and 7915. The 5V regulator built into the Highly Liquid MD24 MIDI trigger board I'm using remains supplying 5V for the box.
In the picture above you can see a DC distribution PCB by the power transformer, it supplies +/-15VDC, 5VDC, and separate grounds for analog and digital.
Now I had power, and t hings were running as before with no hitches, it's time to add the mixer.
Mixer
For the mixer I went with a design I already had - my active summing amp. This wasn't hard, the tricky part was figuring out how to get separate outs for all the instruments on the DR-110. As it happens, the cymbals and hihats run through the same high-pass filter, so you can't mix these separately.
Here's how to get the individual outs from the DR-110: (it's a bit tricky)
| HH | Lift end of R88 connected to C43, wire goes in the hole. Leave R88 unconnected. |
| CY | Lift end of R89 connected to C42, wire goes in hole. Leave R89 unconnected. Note that the HH and CY have the same signal. |
| CP | Short out R121, lift end of C59 connected to R116 and solder wire to the free end of C59. |
| SD | Disconnect R85, R71, and C36 from C46 and R86 - you need to cut a trace. Wire goes from junction of R85, R71 and C36. |
| BD | Take wire from junction of C46 and R86. |
Here's my measurements of normal signal levels from the outputs of the instruments:
SD = ~1V P-P
BD = ~0.8V P-P
HH = ~0.8V P-P
CP = ~0.8V P-P
CY = ~2V P-P
As you can see, you can give the cymbal quite a bit less gain!
Adding the TR-808 Cowbell
I've always coveted this sound. I love it. To get it, I used microLARGE's PCB which I etched onto copper board using Press n Peel Blue, which I've never used before - it worked OK, but I ruined a $10 sheet attempting to figure out how to get it to work in my office's printer!!
Something to know about the microLARGE cowbell PCB is that it doesn't include the oscillators, you have to build those separately. The circuit uses a hex Schmitt Trigger chip running as astable multivibrators - it fills me with glee to use words I learned in 5th form Physics!! :D Anyway. I added in a dual 250k pot in place of the two trimmers, and left out Q9, C12, R60 and R61 - instead I just ran the IC straight off the 5V supply I already had, and built all six osc's on stripboard. (You actually only need to build the top two of them, but maybe I'll use them later...)
After getting the cowbell circuit running, something was missing. It went 'BONGGGG' and the sound faded away and the dual out-of-tune notes were there, but the envelope wasn't right - there was no 'DONK' at the start of the sound!! *hehe* I added in a trigger circuit I found on Eric Archer's page on TR-808 cloning, and hey presto, the DONK was in full force!
This circuit is pretty important to add before the 'TRIGGER' input to get things sounding right, if you're triggering the cowbell off a logic signal.
It sounds awesome! I'm well impressed with my ever-growing little box of techno...