The View Up Here

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Archive for September, 2017

LDC Alice – Take Two

Posted by Tom Benedict on 16/09/2017

I decided to build another set of Alice microphones along the lines of Jules Ryckebusch’s Instructable, using the circuit originated by Scott Helmke, modified by Jules, and put in PCB form by Homero Leal. For these, though, I made a couple of changes:

When I built my first Alice microphone, I’d intended to use it for field recording. Unfortunately I found that the very qualities that give a vocal mic a lot of clarity and presence wind up making it extremely sensitive to the sound of wind in the treetops. It did a beautiful job, but was overly sensitive to wind hiss.

Homero came to my rescue and told me about a document written by another member of the Yahoo! micbuilder forum, Ricardo Lee, describing a modification I could make to the Alice circuit to tame the microphone’s presence peak. I rolled this change on my first Alice and on the Mid-Side Alice I built shortly after. The modification worked, and tamed the microphone’s over-sensitivity to wind hiss. I eventually put the modification on a switch so I could choose how the mic was voiced.

For this set of Alice mics I put those components on the PCB with a dedicated set of pads to wire in the switch, visible just to the left of the filter cap on the back of the board.

New PCBs

The next change I made was to use the Transound TSB-25AX capsule rather than the TSB-2555B capsule I’d used in my first Alice. From the datasheets on the Transound web page the two should behave almost identically, with the S/N performance of the 25AX just barely edging out that of the 2555B. To my ear the two sound identical.

Probably the biggest change was how I mounted the capsules. When I built my first Alice I didn’t own a 3D printer and still thought entirely in terms of subtractive machining (taking a chunk of stuff and cutting away whatever doesn’t look like what you want). This time I started over and designed a completely new saddle and post with 3D printing in mind. I wanted at least rudimentary shock mounting and a cleaner run for the wires.

Shock-Isolated Capsule Mount

I tried a number of approaches on shock mounting, but most of them strayed from something else I wanted out of this design: I wanted it to be something anyone could print and use. So I stuck with easily accessible materials for the shock mount itself, in the form of anti-vibration servo grommets available at practically any hobby shop.

Shock-Isolated Capsule Mount with Grommets

Shock-Isolated Capsule Mount Grommets Installed

As with all of the capsule saddles I’ve designed thus far, the capsule is attached using E6000 adhesive. This stuff is at least somewhat compliant, extremely sticky, and can be removed after the fact by passing a fine wire between the capsule and saddle. So far I haven’t needed to remove one, though.

I printed the final parts using Proto-Pasta Carbon Fiber PLA filament. It’s not the cheapest filament out there so I try to save it for lightweight structural parts, but it just looks so danged good, I couldn’t resist.

TSB-25AX Installed Front Quarter View

One of the things I like about 3D printing is that you can add features that are difficult to get any other way, except possibly by casting. In the case of this saddle and post, a 3mm tunnel runs from the back facet of the post down the center to take the capsule wires to the PCB inside the mic body. The tunnel has a 6mm radius curve inside the post, which makes wire installation easier.

TSB-25AX Installed Rear Quarter View

The final change I made was to switch from the admittedly gaudy BM-800 mic bodies I’d used on the previous two microphones to a slightly less gaudy un-branded BM-700 body I picked up off of Ebay for under $20USD. This had the added benefit of switching to a different headbasket geometry that doesn’t contribute as much ringing to the handling noise.

Here are all the bits and pieces going together. (I haven’t installed the voicing switch on this mic yet.)

Microphone Innards Front

Microphone Innards Rear

And a closer look at the wire routing, which is a lot more satisfying than the routing on my previous mics:

Headbasket Innards Rear

Here’s what it looks like closed up and ready to roll:

Microphone Front

For anyone else building their own condenser microphones, I’ve made this and a saddle for the TSB-2555B capsule available on Shapeways and Thingiverse. Be sure to print the right one for the capsule you’re using:

TSB-25AXZ3 Shapeways

TSB-25AXZ3 Thingiverse

TSB-2555BXZ3 Shapeways

TSB-2555BZ3 Thingiverse

 

Happy recording!

Tom

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