The View Up Here

Random scribblings about kites, photography, machining, and anything else

Archive for May, 2017

Alice Microphone – Switchable Voicing

Posted by Tom Benedict on 10/05/2017

Back when I built my first Alice microphone I intended to use it for field recording. I took it out numerous times, but compared to other microphones I already had it always seemed noisy. Some testing indicated it wasn’t noise, just over-sensitivity to the higher frequencies generated by even the slightest wind in trees. Homero Leal pointed out that Ricardo Lee had documented an EQ modification to the Alice circuit on the Yahoo! micbuilders forum that could tame some of the high end sensitivity, so shortly after that I modified both of my Alice mics.

A couple of months afterward I started looking into voice acting. I have no aspirations of changing careers at this point, but I’ve been reading to my kids for close to eighteen years now, and one by one they’re starting to leave the nest. By the end of the summer the only way I’ll be able to read to my oldest is if I record each session and email the file.

To be fair the style of reading I do is closer to voice acting than straight reading. I like to give each character a unique voice. I like to read each book as if it was an old-school radio drama. So I knew my foray into voice acting would wind up being more than just figuring out how to record the sound of my voice. I wanted to become a better voice actor.

But yeah, I also wanted to learn how to record my sessions to the best of my ability. The more I poked into the technical requirements, the more I realized the Alice microphones make really nice voice mics. And the more I played with my TSB-2555B Alice, the more I realized I might want to disable the HF EQ mod for certain voices.

So I asked Ricardo if I could wire his HF EQ mod through a switch, making it an option rather than a default. He said the circuit isn’t particularly noise sensitive, and that he didn’t think there would be any problems. In the end I picked a switch that let me have three configurations: no HF EQ, Ricardo’s HF EQ to make the mic sound more like a U87, and one more that I haven’t committed to yet. (I’ll figure out how to wire that once I figure out which voices I need to EQ for.)

The fun part about adding the switch to the BM-800 Alice was figuring out how to mount it. I went back to the 3D CAD model I made of the mic, added the switch, and found I could fit some aluminum angle across the mounting holes on the opposite side from the PCB.

The only problem with that plan was that the aluminum angle had to fit inside the body tube of the mic. The top needed to be round, and for it to blend with the rest of the microphone I needed the sides to be tapered, just like the Alice PCB.

There are plenty of ways to make profiles like this, especially if you employ the aid of a CNC machine tool. But part of the joy of making things, for me, is spending time with the tools, hands on the handwheels, coming up with ways to get exactly what you want.

Switchable HF EQ

I wound up machining a lathe fixture to cut the profile. The fixture mimicked the internal frame of the BM-800 microphone, complete with screw holes and taper. Once I’d drilled the holes in the aluminium angle on the mill, I bolted it to the fixture and chucked it up in the lathe with the compound slide set over the necessary 2.3 degrees. It worked like a charm.

The switch isn’t the most convenient thing in the world since I have to take the body tube off the mic to change its position, and since the M-S Alice uses both sides of the frame it’s not something I can apply to that mic in its current form. But for now I’m not planning to use the M-S Alice for voice, and changing the mic’s characteristics isn’t something I’d want to switch on the fly, anyway. I think it’ll work out ok.

I’m still working through the nightmarish acoustics in my house, but with the help of a number of voice actors who have been kind enough to answer my barrage of questions, I think I’ll eventually get that sorted as well.

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Weatherproof Recording Box

Posted by Tom Benedict on 07/05/2017

In earlier posts I’ve mentioned that I do most of my field recording by dropping off my gear and recovering it later. This works great for a lot of reasons, not the least of which is that I make noise constantly. By using the drop-and-recover technique, I can get as far away from my mics as possible.

Another advantage of this technique is that I’ve found it takes between fifteen to forty five minutes for wildlife to return to their normal behavior after they’ve been disturbed. Since I can record for upwards of 24 hours using this technique, this means I get lots of time in each recording during which I know the animals I’m hearing aren’t stressed.

The big drawback to this technique, though, is that when something goes wrong I’m not there to deal with it. The first time I left my gear out overnight it rained. I’d planned for it by draping my recorder with a plastic bag, but it was a stop-gap measure at best. I wanted something more permanent.

Months ago the nearby Costco began selling ammo boxes. My wife picked one up for me, figuring it would be a good starting point for a weatherproof recording box. I hemmed and hawed about what I wanted the box to do and how I wanted it to do it, and finally came up with a plan.

Weatherproof Recording Box

I wanted the box to be able to hold my recorder and an external battery pack, and provide weatherproof XLR connectors I could plug mics into without risking the integrity of the weather seal. I came up with a couple of pencil sketch designs, but none of my plans beat a piece of equipment that already existed: the weatherproof outlet cover.

Four Channels

I had to modify the outlet cover so it had an opening large enough for the four XLR panel connectors. I did the work on the Bridgeport, but similar results could be had by using any number of other techniques such as saw and file, Dremel or Foredom, etc.

The panel connectors are standard Neutrik parts. I picked up the rainbow colored grommets (also Neutrik parts) to provide additional weather sealing and to let me color coordinate the connectors on the inside.

Weatherproof Routing

Cables route in through the slots at the base of the box (in this case I only opened up one, though I’ll probably have to open up the other once I start recording four channel sound.)

It Closes!

Once the cover is closed the only way water can get into the connector area is to come in through the slot at the bottom of the box. It won’t stop immersion or flooding, but it stops even the horizontal rain we get here in town. (I had my kids spray the box aggressively with the garden hose before I trusted it to hold my equipment. Everything stayed nice and dry!)

Color Coordination

I lined the inside of the box with foam that I covered with leftover head liner material I had from when I replaced the head liner in my Civic. The foam panels are removable in case I change my mind or need more space inside, but for now they provide some measure of cushioning and no small amount of insulation for when conditions turn cold at night.

The inside connectors are also standard parts from Neutrik with boots that match the colors of the panel grommets. Unfortunately connector lengths caught up with me.

The Tight Fit

When I started this design I hadn’t planned on lining the box with foam. That came later once I realized how much my gear would rattle around if I did any hiking with the gear inside.

But once the foam was installed, the full-sized Neutrik connectors poked out so far I could only plug things in on one side of my recorder. Since the fourth channel is located on the opposite side from the other three on the DR-70D, this means I can’t actually plug in all four channels yet. At some point down the road I’ll replace all four with low profile right angle connectors from Cable Techniques, but for now I get two (well… three) channel sound.

I’d intended to build this box in time for the 2017 International Dawn Chorus Day. Unfortunately I missed by hours. IDCD 2017 happened at dawn this morning, and I just finished the box an hour ago.

Ah well…

Meanwhile I’ve got a setup that will let me do drop-and-recover recording, rain or shine, and keep my gear safe, sound, and dry.

UPDATE: I finally had the chance to answer one question I had about this design: With the foam lining, would I run the risk of overheating the gear in the box?

The answer is a qualified no. I put my recorder and my 10000mAh battery pack in the box with a temperature probe and closed the whole thing up. Ambient temperature was 21C. The temperature in the box rose steadily for the first hour, then began to roll off. After seven hours the temperature reached 32.5C with clear signs it would asymptote at or below 34C. That’s a delta of around 13C.

The DR-70D operating range is 0C – 40C, according to the Tascam web site. When I originally ran this test I thought this meant I was marginally safe. I’m pretty sure I was wrong. In the test I was measuring the air temperature inside the box, not the temperature of the electronics, which were considerably warmer than the air temp.

I came up with a fix for this and the connector issue, which I describe in the second part of this article: Weatherproof Recording Box – Part 2.

Tom

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Another Stab at Contact Mics

Posted by Tom Benedict on 07/05/2017

Several months ago I ran a bunch of tests on a contact mic to try to figure out the best (ok, best of several options) way to interface them to my recorder. It wasn’t the most exhaustive test, but it at least pointed me in a good direction.

Specifically I wanted to find which configuration had the best low frequency response. To that end the mic under test was a 35mm piezo element stuck to a subwoofer, and the tests consisted of running a frequency sweep through the subwoofer while I recorded the output of the contact mic.

I tried four configurations: 1 – direct TS plug-in, with all the high pass filtering that leads to. 2 – HOSA MIT-129 impedance-matching transformer. 3 – Alex Rice impedance matching preamp. 4 – Stompville impedance matching preamp.

It wasn’t the best subwoofer in the world so the data was lumpy, but a couple of things came through pretty clearly: Not surprisingly, the direct-plugin lost some of the low-end. The Alex Rice preamp is known to have noise issues, which also came out in the tests. As far as signal to noise went, the HOSA MIT-129 and the Stompville preamp were very close across the range of the test (10Hz-1kHz), but the Stompville had higher overall signal and required less gain out of the recorder.

My point in running these tests wasn’t to prove once and for all which setup was the best. It was just to get some direction for how to build my next round of contact mics. I decided to go with the preamp from Stompville and run with it.

Before going into just how I ran with it, here’s some background on the Stompville preamp: SmudgerD started the same place I did, with the Alex Rice preamp, but instead of just being frustrated with the noise floor, decided to redesign it, and then further refine it. The result is a very well thought out design. Even better, SmudgerD packed the whole thing onto a tiny PCB that fits on the back of a Neutrik XLR panel jack.

I bought two.

(But I wish I’d bought four.)

Then I stuck them in a box.

Contact Mic Preamp Box

The bottom of the box is tapped 1/4″-20 so I can attach it to the top of my DR-70D using the camera mount thumbscrew. Two of the Stompville preamps face out one side, and as soon as I buy another two they will face out the other side. The third face has a row of mini-XLR connectors for plugging in the contact mics themselves.

I was turned on to the mini-XLR connectors by Mo, who shared them in a comment on my EM-172 and XLR Plug article. I have to say these things are the cat’s pajamas. They’re small, they’re inexpensive, they have a positive lock, and they have the neat feature of having pin 1 make contact before the other pins. This ensures ground makes contact before the signal pins. I love ’em.

Unfortunately, right after I finished the box I was drawn into other things. So I only have two contact mics built so far. The first is the one that’s still attached to the subwoofer. The other is a prototype I built to test the preamp box in the field.

Prototype Contact Mic

It’s ugly, but it works remarkably well as long as you don’t apply any pressure on the center of the piezo. (This introduces a large DC offset.) The next rev, of which I’m building four, will be fully enclosed.

Just for grins I took the whole mess in to work to play.

 

– Tom

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