+4dBu line level inputs with Zoom R16

    So, you bought a Zoom R16 (or R24 for that matter) and you're happy as a pig in a ditch 'cause now you can record your band playing live.  Well, yes, and no.  If you want to run a whole bunch of mic cables and hang mics in front of everything on stage you can do that just fine.  But wait, you say in your happy delirium, we already mic everything on stage so I can just run the sound board direct outs to the R16 line ins since the Zoom website says the R16 can handle line or mic level input.  Wrong bassoon breath!  The Zoom R16 (and R24) can handle consumer level line input.  What's that?  That would be the ever popular -10dBV.  For an explanation, see here.  What happens when you run +4dBu professional level line output into the Zoom R16 which is expecting -10dBV input?  It overdrives the input.  In many cases, even when you have the gain knob set to the absolute minimum.  So, what's the solution?  You could kindly ask your sound engineer (if you have one) to please set the input levels for each stage mic really low.  I'm sure he (or she) won't kill you more than once.  Another solution is to go out and buy eight, 40dB or 50dB L-pad attenuators.  The cheapest I could find came in at seventeen dollars US (of course your mileage may vary).  The cheapest solution is to make your own 40dB or 50dB L-pads.  There are plenty of places on the web that have the instructions.

    Since I have two R16s that I link together, and my band has between 10 and 16 separate inputs (depending on what we run), either mic or line, I decided that I would build my own L-pads.  So, having made this momentous decision I had to think up a simple way to do this without going broke.  Most examples show single L-pads built into XLR tubes.  Since the tubes cost almost as much as the pre-built attenuators this didn't seem like a great idea.  I did find one example where they said you could just put the L-pad in a small, metal project box.  Still too expensive.  How about eight L-pads in a single, large metal project box?  That should work.  A plastic project box would be even better (read cheaper) but might be vulnerable to noise from external sources.  Since I used to build my own guitars from scratch (pre-Katrina, which took all my tools) I had some copper foil shielding tape laying around that I didn't need.  Aha, I can use a plastic box and shield it with copper foil tape (actually, the metal tape that is used on air conditioning ducts works just as well).

    So, what do I need?  Sixteen 1/4" jacks (GLS Audio 1/4" Jacks Female TS Mono Panel Mount Jack - 20 PACK, twenty dollars, or a buck apiece, from Amazon).  Some wire (I had some scraps that I could use).  A plastic project box large enough to accommodate sixteen 1/4" jacks and some wire and resistors (Hammond 1591ESBK ABS Project Box, 7.5" x 4.3" x 2.2", Black, about ten bucks at Amazon).  A good soldering iron and some solder (most people have these).  Shielding tape (free since I already had it).  Oh, I almost forgot, sixteen resistors (about 14 cents apiece from DigiKey).  After reading a bunch of things on the good old interwebs about overdriving R16s I decided that I would make 50dB L-pads using the following circuit:

+Line level input ---------- R1 ----------+--+ Mic level output
                         +------- R2 -----+
Ground (input) ----------+-------------------- Ground (output)

R1 = 33K ohms
R2 = 100 ohms

That probably looks familiar if you looked at the links in the first paragraph ;-)  I used 1%, 100 ohm metal film axial resistors for R2 and 5%, 33K ohm metal film axial resistors for R1.  The power level doesn't matter much but mine were 1/4 watt for R2 (DigiKey part number S100CACT-ND) and 1/2 watt for R1 (DigiKey part number CF12JT33K0CT-ND).  So, money out of pocket so far is about thirty-five bucks for a single box (seventy bucks for me since I was making two boxes).

    Important note - I am not an electronics wizard (I'm a programmer).  Warning - the following is not pretty ;-)

Inside of project box

    As you can see from the above picture, the 100 ohm resistor is soldered across the terminals of the jack (tip terminal to sleeve terminal) on one side and the 33K ohm resistor is in series from the tip connector on one side to the tip connector on the other side.  As long as nothing is grounding out or touching another line there shouldn't be any problems.  If you're anal retentive you can use heat shrink tubing and/or electricians tape and make it much prettier.  I don't think any of the wires in the box will move unless I decide to throw it at a wall at very high velocity.  One nice thing about these is that you can completely solder up all of the wires outside of the box and then insert the jacks into the box (so you don't have to be an ambidextrous octopus [or would that be octodextrous] to make this).

    Since it's kinda hard to write on black plastic I needed some way to differentiate between the R16 (downstream) and the main board (upstream) sides of the box.  As can be seen in the following picture, I used a green Sharpie to color the nuts on the jacks on the upstream side and, looking at the above picture, a red Sharpie to color the nuts on the downstream side.

Exterior of box, green side

    In hind sight (which, as we all know, is twenty-twenty) I probably should have gone for 40dB attenuation.  The 50dB really cuts the signal quite a bit.  I can still overdrive the input though if I crank up the gain on the R16 so I guess it's OK.  If I find that it's too much attenuation I can always go buy some 10K ohm resistors and swap them out for the 33K ohm resistors.  That will cost me about 4 bucks per box (shipping is a bitch).

    If you have suggestions or improvements, email me - eviltwin69 AT cableone DOT net.  Kinda silly to obfuscate the email address since I'm already getting spammed half to death ;-)

Jan Depner
September 15th, 2013