diy mics - big fun for all!

[leigh]

Alright, y'all, I've been getting heavy into researching making some more mics. As I mentioned here before, I've already built the infamous Tape Op omnis from issue 16. They work great on some stuff, but I've had trouble close micing drums with 'em - they're not quite Earthworks, you know.

So I'm gearing up for round 2 of mic building. The big differences will be a different circuit, to allow phantom power, and the Linkwitz capsule mod. I'm putting the call out to see if anyone else with the requisite curiosity and soldering iron wants to join me in this endeavor, in a remote sense.

My first question I need answered: the circuits I linked to above use a 12V zener diode to drop the phatom power voltage from 48V to something lower. (Presumably, 12V.) However, the Panasonic WM-60A is rated with a maximum operating voltage of 10V. Problem or not?

Leigh

[aurelialuz]

[]

[leigh]

Motherfucker - I was just finishing writing this response when my broswer crashed and lost it. Here goes the resurrection:

On the spec sheet for the WM-60A it does state that 10V is the max. You're probably right that 12V would be OK, but if I can find a 9V zener that would be safer. However, more volts = more headroom. I don't know if the RShack PZM used the same capsule, but I ran mine with a pair of mini 6V lithium cells for years, with no problems.

The trace-cutting capsule mod looks quite do-able, and I'm fairly certain that all the pro-audio products that use this capsule (and there are a few) use this mod. There's another mod listed on that page with the circuit, which involves removing the felt and enlarging the hole. I found stories of people trying this with a razor and a miniature lathe arrangement, but it looks tricky. The page with the circuit claims this can help tame the "phase contortion around 4-6kHz". Looking at the frequency plot on the spec sheet, you can see they claim ruler flatness right up to 5K, and then it gets wavy. So it seems like it would be a great idea, but that diaphragm is only about 1mm inside the hole. One tiny slip and your capsule is trashed.

I won't get started on this until mid-June (busy/out of town until then) but I would like to share ideas and results with as many people as possible. So thanks for responding, Alex.

knowledge is power,
Leigh

[edanderson]

scott dorsey wrote a great article that about building electret capsule mics. it was in (i think) the october 2001 issue of recording magazine. it has info on most of the possible configurations, including the "schoeps style" active balancing circuit that runs off of phantom power (very similar to what you've linked to). you really should try to hunt the article down at the library, or buy the backissue.

there is also a DIY mic forum at yahoo groups that has a bunch of people who have done the linkwitz and other mods. i've been thinking that it is about time for a revised version of the tapeop article.

ed

[aurelialuz]

[]

[phat-ass]

I'm in, I think I can make time. Not this week, though (or maybe). I've got the parts already.

phat

[leigh]

scott dorsey wrote a great article that about building electret capsule mics. it was in (i think) the october 2001 issue of recording magazine. it has info on most of the possible configurations, including the "schoeps style" active balancing circuit that runs off of phantom power (very similar to what you've linked to). you really should try to hunt the article down at the library, or buy the backissue.

Hey Ed,

Thanks for the tip. I actually just stumbled across that very article last week in the public library archives. It does have some good explanations of how the electret capsules work and what they require, electronically. The active balancing circuit is included as well, and it's very similar to the circuit he uses in his Shanghai mic mod article.

I'm not sure if I would require the active balancing circuit or not. Both are included on the page with the circuits that I mentioned before. It claims that the active balancing one, the one with two transistors, has a lower output impedance, "thereby allowing longer lines to be driven." If that's the only advantage then I don't care, as it's just for studio use with roughly 20' cable runs.

Mainly, my goal in finding a new circuit (I mean "new" beyond the Tape Op omnis) is because I would like to run these mics with phantom power. The battery box isn't that great a hassle, it's more that phantom power will (reportedly) damage the Tape Op circuit, and I'd like to be able to use my board's phantom on other mics at the same time.

Leigh

[mandalamala]

Count me in. Unfortunately I can't offer much help as I've not done a project like this before. I do, though, have copious ammounts of curiosity.

[leigh]

Alright, I'm back in town, and working on this project again.

Here's what I've got in mind. Details that still need help are highlighted with an asterisk (*):

ABSTRACT

Build a 6mm omni mic. The mic will consist of 2 units, the capsule mounted in an XLR connector, and the output/balancing circuit, built into a seperate box. The circuit will work off phantom power and provide a balanced output. Many tricks will be used to get the capsule to sound as good as possible.

THE CAPSULE

Panasonic WM-60AY, Digikey part P9959-ND

May attempt some crude stereo pair matching.

This capsule will be modded in two ways:

1. Reconfigure the output pins so that the internal FET runs as a source follower. This will improve the SPL handling of the mic, raising it from roughly 105 dB to 140 dB (these figures are hearsay!). See the Linkwitz diagram for the details.

* Should we go with the 2-wire or 3-wire config? Note the difference in the polarity of the supply with the 2-wire version.

2. Remove the entire front face of the capsule. This will improve the high frequency response, hopefully smoothing out that 6K wobble seen on the capsule's data sheet. Requires a steady hand and a file/razor blade. Has been done successfully, but not by me yet.

Once the mods are done, the capsule will be mounted at the end of a 1/4" dowel, or inside a 1/4" pipe, about 6 inches long.


THE CIRCUIT

The circuit will be based one of the two Hicks circuits, labeled 'Method 1' and 'Method 2'. I'm leaning towards the second one, which is the same Schoeps style circuit that Scott Dorsey describes in his Sept 2001 Recording Magazine article. This uses a pair of transistors to drive the balanced output.

* Is Hicks' first circuit just as good for this application?

* What changes need to made in these circuits to use them with the modded capsule (with the internal FET operating as a source follower)?



Thanks to all who are joining in this endeavor. Anyone is welcome to join, but please take some time to read through the above information and links carefully. I would like this collaboration to run efficiently, and not get bogged down with redundant questions.

Cheers,
Leigh

[aurelialuz]

[]

[leigh]

Yeah, the triangle does mean op-amp, but ignore it for this... think of it as "here's where your mic pre goes".

I didn't think of trying different PNP's. I'd hope that the differences between decent quality transistors would be minimal. In his circuits of this type, Dorsey recommends the 2N5087.

Do you really play a harmonium live? In clubs?

Leigh


PS - As for my earlier concern about the 12V zener, it's fine. As I understand it, the voltage that appears at the capsule is more like 4V, due to the resistors in series acting as a voltage divider.

[aurelialuz]

[]

[leigh]

I haven't gotten any parts yet. I agree that the design with the pair of transistors would provide a better output impedence, which would be my choice if I was going to bring these mics into clubs. However, for the studio, that may not be necessary.

I haven't done the mod at all yet, and I'd guess that using a modded capsule with the original TO electronics would provide most of the sonic improvement. My desire for different electronics is mostly about being able to run the mics with phantom power. It's not that I think a battery box is too much of a pain - it's that phantom power will supposedly ruin the TO mics (that's what it says in the article).

Anybody else out there want to weigh in on the 2 different circuits? Or shed some light on how they'd need to be modified to use the Linkwitz-modded capsules?

Another thing that needs a bit more design work is the capsule mounting. My ideas so far only include a male XLR and a length of 1/4" pipe. However, if that pipe is more than a couple inches long, I don't think the rubber boot of the XLR will support it well.

So let's get some ideas going!

Leigh

[gyraf]

[quote="aurelialuz"]i thought the capsules were rated to 12v...? alex[/quote]

Remember that there's a voltage drop over the resistor feeding the actual capsule. Depending on capsule current consumption and resistor value, you'll get signifikantly less than 12V at the capsule.

For symmetrical clipping, I think you'd want to have about half the supply voltage appearing at the actual capsule.

But what about REAL condenser microphone DIY?
http://www.americanproaudio.net/misc/mic/index.html

Jakob E.

[leigh]

For symmetrical clipping, I think you'd want to have about half the supply voltage appearing at the actual capsule.

?? We're trying to avoid any clipping. ??

But what about REAL condenser microphone DIY?
http://www.americanproaudio.net/misc/mic/index.html

Have you tackled any of these projects?

Leigh