Measurement mic/preamp questions

[Vikash]

Hi,

I need advice on how best to house my new WM-61A electrets. I've seen some people using long brass/plastic tubes to reduce reflections. Can anyone share their experiences on materials, lengths, thicknesses they used? Should I use a particular glue, or flexible sealant to fix the electret at the end of the wand/tube? Stuff anything else in the tube to reduce vibrations?

I think I've decided not to mod the electrets as everything I've read so far indicates that it merely increases its SPL handling capability within the same distortion levels. I will be using the mic to measure loudspeaker FR (for xover design) and don't think there is significant SPL involved to warrant the fiddly modification?

I've decided on the Wallin preamp, but don't know which version to build. I've heard that the only advantage v2 has is power consumption and v1 actually has less distortion and is easier to get parts for. Can anyone confirm?

[Centauri]

Hi,

I have WM-60 insert, but I think diameter is same. I used a 1/4" ID thin brass tube 12" long. The mic capsule was a push fit in the end, and the other end is soldered to the back (cable entry end) of a metal ('gold' plated) RCA plug. Standard RCA lead can then connect to preamp.

I used preamp1 simply because I had bits on hand - it works fine. I managed to cram the preamp into an old pro microphone casing, using the original on/off switch as the gain switch. A cover screwed on where the ball used to be houses the battery and output jack, and the original XLR connector I replaced with RCA to plug the wand into. Looks strange as microphone case is in reverse, but allows standard mic clip and stand to be used.

Cheers

[Vikash]

Hi Centauri,

Why brass? I've just bought a 6mm ID x 8mm OD aluminium tube. Off course the 6mm electret doesn't fit (why would it be that easy? ).

The other tube options available are iron and plastic but the alu looks nice.

Edit: Just re-read your post, and I think I see what you've done. You've used a female RCA connector on the end of the tube and used the tube to connect the ground from the electret to the RCA. So I guess there's just a single cable inside to connect the output signal right?

I also like the idea of being able to use a standard mic stand.

Did you do the electret mod?

[officeboy]

I made up two mics, with one I used a cheap computer boom mic and just replaced the cartridge and then drilled out the face of the mice to hold my WM-61A .

On my other mic, i had a shortish piece of a nice thick RCA cable that i just attached the mic to the cut end of, then i wrapped the whole thing in shrink tube, with a stiff wire added in. it lets me bend and flex the cable and it holds it's shape. I'll be using that one to hook up to my radio shack spl meter (after I get the proper mods done there)

[Vikash]

I've just been through the speakerbuilder.net description of how to mod the capsule and it appears a lot simpler than on SL's site. I'm still unsure of a couple of things, but I'm thinking that if it really is this simple, then I may as well do the mod. Here's a quick pic of the WM-61A. If someone could offer a definitive method of doing the mod, that would be great.

The SL pic shows the new connection to a tiny tab (which I can barely see on the actual capsule), and the speakerbuilder.net write-up says ...to cut the lead that connects the ground terminal to the case and dab a little silver solder to connect the other terminal to the case (where the (+) lead connects), and doesn't mention anything about shaving part of the aluminium surround, or copper ring.

Is it simply step 1, 2, 3a, then connect it to a preamp?

[Centauri]

Originally posted by Vikash
Why brass?

So I guess there's just a single cable inside to connect the output signal right?

Did you do the electret mod?

I used brass tube so that it could be soldered to the RCA shell for the earth. A little scraping on the inside was enough to get the insert to fit.

I used twisted cables to connect the mic insert to the RCA connector, and when the connector is screwed into the shell, the tube is the earthed for shielding.

My mic capsule is standard - seems to work ok for me so far.

Cheers

[Sheldon]

Originally posted by Vikash
I've just been through the speakerbuilder.net description of how to mod the capsule and it appears a lot simpler than on SL's site. I'm still unsure of a couple of things, but I'm thinking that if it really is this simple, then I may as well do the mod. Here's a quick pic of the WM-61A. If someone could offer a definitive method of doing the mod, that would be great.

The SL pic shows the new connection to a tiny tab (which I can barely see on the actual capsule), and the speakerbuilder.net write-up says ...to cut the lead that connects the ground terminal to the case and dab a little silver solder to connect the other terminal to the case (where the (+) lead connects), and doesn't mention anything about shaving part of the aluminium surround, or copper ring.

Is it simply step 1, 2, 3a, then connect it to a preamp?

I did one stock and one by your method 3a. They both work fine, but I haven't tried measuring distortion in either. You have to scrape away the one trace, but the other tiny little tab is ready to go. I just bent the end of the wire so that it soldered to the main contact with the end of the wire touching the little tab. Added a little solder there.

However, I don't see any reason why you can't just scrape away the original ground trace, solder the leads to the main terminals, then just connect a ground by physical contact to the outside surface of the electret. As long as you can maintain good contact with the surface of the capsule, this should work fine. If you run into problems, you can bridge the upper lead (as shown in your diagram) and the tiny trace.

Sheldon

[Centauri]

Update on my measuring mic details. I put some photos etc up on my page at http://members.optusnet.com.au/~ggoo...i/diy/mic.html


Cheers

[Vikash]

I've got my first mic finished and it looks good. It sure isn't easy working with those tiny capsules! I want to get a moded one working next (well I've already tried and cocked two capsules up so far).

To test it, I connected the mic to my sound card which which has a +20db gain on the mic input, and there was a lot of hiss (Hercules GT XP). Are these DIY premaps much better? I would like to use the mic for some home studio recordings as well, and don't know whether the Wallin preamp will be usable for such a task?

[Centauri]

Good job on the mic - the brass tube makes things easier.

I would think most of the hiss would be coming from the mic capsule electronics. The use of an external preamp (such as Eric's) will improve this as the capsule will have a higher operating voltage than what is provided by the sound card. Mods to the capsule itself can further reduce this, but the gain provided by these mods may not be fully realised if still being powered by the sound card mic preamp.

Cheers