Posted by Tom Benedict on 26/02/2016
Some months ago a co-worker and I were characterizing a pressure and vacuum system at our summit facility that changes its output as a function of angle off of zenith. As boring as that may sound, that’s nothing compared to what it actually meant for the people doing the characterization. Until something new comes along it’s my gold standard definition of boredom:
“Moving to plus thirty degrees. … Mkay.”
“Kay… Ok, go to plus fifteen.”
“Moving to plus fifteen degrees. … Mkay.”
“Kay… Ok, go to zenith.”
“Moving to zenith. … Mkay.”
“Kay… Ok, go to minus fifteen degrees…”
It went on like this. And on… And on… We made scans from +75 degrees to -75 degrees in fifteen degree increments. Over and over and over, day after day. By the end of a week of this my brain went off into some mathematical neverland. Positive angles were boring because they were what they were. No depth to them. No nuance. But negative angles… oooh, they’re tricky.
The rotary stage we used doesn’t do negative numbers. It wraps around to 360. So the negatives go from 0 to 345 to 330 to 315 to 300 to 285. The first fifty times or so you can convince your brain there’s nothing wrong. But by the time you’ve done hundreds of scans weird things start to pop out at you. I found myself thinking, “If you take away fifteen from sixty, you get forty five! And if you add forty five to fifteen you get sixty! And seventy five is a multiple of five and five and THREE!!” It was epiphany after epiphany! This stuff was MAGIC!
I think some of this had to do with the inane repetitiveness of what we were doing. I think the rest can be chalked up to oxygen deprivation. Luckily by the time I got down at night my brain had calmed down enough to realize how bizarre my mental state actually was. I opted not to share my new-found knowledge with my kids so they never had the chance to say, “Dad, I think you’re off your rocker. That’s basic math.”
This is why I fear the question every kid asks: “Are we really going to use this math stuff once we’re out of school?” I’m afraid of what I’d tell them.
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Posted by Tom Benedict on 23/02/2016
I made up my mind about powering my EM172 microphones. Ultimately this decision had less to do with how I was powering the microphones than how I was plugging the mics into the recorder. One of the things I discovered when I wrote my last post was that the Tascam DR-70D uses completely different amplifiers for the XLR inputs and the 1/8″ inputs. Different form factor, obviously; different impedance; different gain. It’s that last part that really drove this decision.
The gain ranges on the 1/8″ plug are +3dB, +11dB, +26dB, and +38dB. The XLR gain ranges are +21dB, +36dB, +51dB, and +63dB. While I was performing side-by-side tests I kept having to crank back the gain on the XLR input to match the levels on the 1/8″ input. As I tested with quieter and quieter subjects it finally hit me: +38dB of gain just wasn’t enough to bring up the levels of some of the subjects I want to record. The XLR input gave me more gain to play with. The last test I ran was what finally convinced me. Even with the gain cranked all the way up on the 1/8″ input mics, I couldn’t get the sound levels over -25dBFS. The recording was just too quiet to use. I cranked up the gain on the XLR input, and was able to get -12dBFS with the same subject.
Good news is the mics really do perform better with the 9.6v bias voltage David McGriffy’s circuit provides. So this is a win-win.
The lavalier mics were no problem to convert. I bought a stash of Neutrik XLR connectors when I started this whole investigation, so it was just a matter of lopping off the 1/8″ connectors and soldering up the XLRs with the resistor and capacitor from McGriffy’s circuit.
My SASS was another story. I really hate having things with cords that can’t be unplugged, so I wanted to connectorize everything and use extension cables. Only problem: I’m a beginner! So I had no idea how all the connectors worked.
After some Googling and image searching I learned that:
- XLR extension cables are gender-inspecific. One end is male, the other is female.
- Female XLR connectors are the ones with the latch. This is true of both panel and cable connectors. So female panel connectors have a latch, but male panel connectors don’t. (This confused me.)
- Neutrik makes a crapload of XLR connectors you can choose from. It’s worth looking them up in multiple catalogs to find out which series were developed to fix the bugs in previous series. Though it’s really hard to go wrong, so long as you get all the genders right. These things are built like tanks.
I picked up a pair of pre-built 10′ extension cables for a little over the price of the connectors themselves along with some male panel jacks to install in the SASS. Installation meant cutting into the back of my SASS, but it went quite smoothly and the results look (and sound!) nice. (Yeah, this is an infrared photo. Ironwood trees look like Dr. Seuss trees in the IR, so I just had to play.)
Meanwhile I figured it was finally time to solve the issue of wind protection. A few months back I learned I’m really REALLY bad at sewing fake fur. I did some reading since then, so I think I know what I did wrong. But rather than getting stalled on my own lack of sewing skill I ordered a pair of lavalier windscreens from Cat Ears. They fit over my oversized mic bodies, but they’re too small to go over a foam windscreen. I probably needed the larger ones. They do a decent job by themselves, but in wind over 15-20kts the mics still suffer from wind noise. Good enough to use the lavs as tree ears, but not enough to use them at the beach in solid wind.
Now I just need to solve the issue of wind protection for my SASS. Back to learning to sew fur…
In any case my gear and I are off the soldering bench and back out in the field. Finally. YAAAAAAAY!
Posted in Audio, Electronics, Engineering | Tagged: Audio, Cat Ears, EM172, Field Recording, Power, SASS, Sound, Wind Protection, Windjammer, XLR | 1 Comment »
Posted by Tom Benedict on 06/02/2016
In my previous post I wrote about the issue of powering the Primo EM172 microphone capsule. Quick summary: The 1/8″ stereo mic input jack on the Tascam DR-05, the Tascam DR-70D, and the Sony PCM-M10 supply a plug-in-power voltage of around 2.3-2.7v. The EM172 datasheet says it’s designed for 5-10v. My question was how this affects performance, and what my options are for doing something about it. I took a two-headed approach: The first was to buy a 9v microphone battery box from Church Audio. The second was to order all the bits from Mouser Electronics to convert my microphones to XLR plugs and 48v phantom power. Then I tried to calculate how each option should perform. Push comes to shove, no matter what I do I should get better performance out of my microphones.
Church Audio builds equipment to order, so delivery takes a while. (Translation: The battery box isn’t here yet.) The Mouser order came in a couple of days after I placed the order, so that’s where I started.
Very early on I killed one of a pair of EM172 capsules, so I built the survivor as a standalone mono mic. I decided this was a good donor mic for the XLR conversion. I gutted it, cut off the 1/8″ connector, and rebuilt it for phantom power according to Richard Lee’s write-up of David McGriffy’s circuit. In the process I learned one of the drawbacks of using a pressed/glued in housing: The only way to get the mic out of the housing is to destroy the housing.
(I’ve now done this to three microphones, and made three new housings for them. Want to know what the new housings look like? They look exactly like this one! And the next time I re-wire one of them I’ll have to saw it apart, too! Part of me wishes I’d made the new ones with screws, but these are so easy to make I just can’t justify it.)
Here’s a list of the differences between how I’ve been building mics up to this point and what I did for this one:
- The mic is wired differentially, and is now shielded end-to-end. I followed Richard Lee’s idea of heat-shrinking the mic capsule, wrapping that with metal tape that contacts the cable shield, then heat-shrinking that to insulate it. The shield is also tied to the XLR plug’s shell, so it’s living in its own little Faraday cage from the diaphragm to the plug.
- I sized the resistor in David McGriffy’s circuit so my DR-70D would provide 9-10v plug-in-power to the mic. (It wound up being 9.6v.) This puts it in the S/N sweet spot, according to Primo.
- It’s got an XLR plug instead of a 1/8″ plug, so I can only use it on my DR-70D now. (No more plugging it into my DR-05. Bummer.) But the DR-70D has four XLR inputs so I can use four of these at once.
As an initial test I plugged it into one of the XLR inputs on my DR-70D and plugged my twin EM172 lavaliers into the 1/8″ jack. Using the built-in plug-in-power for the lavaliers I set all channels to the same gain settings and gave it a listen. The XLR mic sounded great! The 1/8″ mics were almost inaudible. Next I pointed both mics at a compressor in one of our labs at work so I could measure the difference in signal. With the gains set the same I got -18dBFS on the XLR converted mic and -39dBFS on the 1/8″ jack mic. I was only supposed to get a 0.9dB boost in sensitivity out of this, so clearly I missed something in my calculations.
That’s when I learned that not only do the two types of inputs on the DR-70D supply different voltages and have different input impedances, they also have different gains! The 1/8″ input provides gains of +3dB, +11dB, +26dB, and +38dB. The XLR input provides gains of +21dB, +36dB, +51dB, and +63dB. So not only was I not comparing apples and oranges, I’m not even sure I was comparing two fruits!
At that point I tossed all my quantitative testing out the window. In the end all that matters is what the darned thing sounds like! I made a couple of side-by-side recordings with the gains set so the two tracks would have the same levels, but eventually gave up. The S/N was so much better with the converted microphone, there was no question which one won.
I have enough parts to convert the rest of my microphones, but I’m still holding out for the Church Audio battery box. The DR-70D is really too heavy to fly from a kite, and I still have aspirations of recording aerial soundscapes. So I have to leave at least some of my microphones on 1/8″ connectors. Meanwhile I’m looking at my SASS and trying to figure out where the XLR jacks will go.
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