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Poxican 18th September 2012 02:26 PM

Mic capsules distorting at low level in binaural dummy head? Resistor help please?
Hi folks.

So I've built a binaural dummy-head mic featuring the Panasonic WM61a capsules. The problem is that the mic capsules seem to be distorting, apparently caused by upper-mid frequencies. Here's a very short demo showing what I mean.

I performed the linkwitz mod on the capsules as the idea is to use the head at gigs, and built a pair of phantom-powered, phase inverting circuits, one for each mic. I used this circuit:

The only difference is that Terminal 3 on the capsule goes to ground as per the advice of Bill Wall.

However, as I said the capsules are distorting easily, at low input settings, with human voices seeming to break up very easily. Both are doing it the same way. So I was wondering if anyone might have any idea what might be going on here? I have another head with the same capsules, unmodified, and that can handle much higher levels than these, even though the Linkwitz mod is meant to raise the SPL.

If anyone's got ANY ideas I'll be incredibly grateful, I'm really not sure what to do here now.


Mooly 18th September 2012 02:37 PM

Nate... I'm not expert at all on mic techniques and circuitry but I would suggest at least checking that the capsule is operating with the correct voltage according to the data sheet. Perhaps the capules you are using don't behave as expected when modified away from their original design, that's something I have no experience of.

Also, I think this might get more exposure in the "analog source" section. If you want it moving just ask :)

Edit... a scope check might reveal where the distortion originates. ie mic or following stage.

Poxican 18th September 2012 03:06 PM

Cheers Mooly, I wasn't sure where best to put this, if you wouldn't mind moving it to where you think it should be that'd be excellent.

The electronics aspect of this project was admittedly a little over my head, so Bill Wall really helped me through it a fair bit via email. He assured me the voltage output was correct, but perhaps that's at fault. I'll take a look with the multimeter and see what voltage is being put across the cap. Would it be across terminals 1 and 2 I'd want to test?

I'm not sure what a scope test is so I'll look into that!

scott wurcer 18th September 2012 03:48 PM

Are you sure of the pin connections? The capsule comes with the gate (can) connected to the source. If pin 2 is the source it can not be the most positive pin because the FET used in these capsules is not reversible. Try 1 on top 2 on bottom and 3 ground.

Poxican 18th September 2012 04:34 PM

So I put the multimeter to the thing and:
- With the mic connected, I put the probes across 2 and 3 and it reads 6v.
- I don't know if this is of note, and I'm probably betraying my lack of electronic knowledge, but with the microphone disconnected, it reads 46v across 2 and 3.

Scott, my understanding was that the Linkwitz mod makes 2 hot, so I thought I'd connected this properly. Bill's advice was to connect it in this way, I'll try reversing them and see what happens.

As I say, I bit off a little too much doing this project, I'm great with the building aspect, not so much with the electronics, so I kind of feel like I'm bumbling my way through this stage of it!!

Mooly 18th September 2012 05:52 PM

Moved to Analogue Source :)

The 46 volts with the mic disconnected sounds OK as that will be the phantom PSU voltage. With the mic in circuit and drawing its rated current the voltage settles to around the 6v you measure. That to me sounds OK.

Scott is a real expert around here on these kind of FET related problems so well worth trying his suggestions.

scott wurcer 18th September 2012 06:04 PM

1 Attachment(s)
Looks like in copying things often there was a switch. In drawing below I see your same images of the cut but if you look closely they went with a simple direct connection to an XLR. The numbers in the little circle are the capsule pins (1 is hot) and the 2 here refered to XLR pin 2.

Note: pin 3 must go to an AC ground. In the original pin 1 was connected to the battery so it did not matter but you must ground pin 3.

Poxican 19th September 2012 10:33 AM

Hmm I swapped the connections around on one of the capsules and it seems to have sorted it.. going to solder it up properly this morning and see how it goes. Looking at all the wiring diagrams for the mod I can see what you mean, it does look like I did it back to front!

Is there any chance I'll have damaged the capsule by having it wired back to front?

Pano 19th September 2012 02:02 PM

I hope you get this going, it's a very cool project.

Poxican 19th September 2012 02:41 PM

That seems to have done the trick. I've shouted as loud as I dare, and turned my monitors as loud as they physically go, and can't get it to distort. I'm heading into Trafalgar Square on Saturday, so I'll set it up in the middle of the crowd and do some recording with the H4N.

But I don't think it's premature to say thankyou guys, particuarly Scott obviously. I don't think I'd have found that fault, and may well have ended up rebuilding the entire circuit incorrectly again to try and solve it. So thanks very much.

I'll report back in a few days with some recordings. :D

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