DIY KM84 Continuity Between Pins 2 and 3 Unbalanced Mic?

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Scrappersa

Well-known member
Joined
Jul 17, 2011
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150
I'm having a bit of an odd issue that I was hoping somebody might be able to help out with.

I have a home-brewed KM84 board that seems to be getting continuity on the XLR between pins 2 and 3- but only when the transformer is soldered into the PCB. I have a few Cinemag 5:1 and 7:1 transformers that I've tried in the circuit and they all behave the same way. The XLR is an Amphenol that's soldered directly to the PCB. I have three drastic revisions to this board and they all do the same thing.

The Cinemag transformer leads do not have continuity on their own. The XLR pins do not have continuity without the transformer soldered in. If I desolder a single lead (any lead) from the PCB, the continuity disappears.

When the transformer is soldered to the PCB, XLR pins 2 and 3 have continuity.

I'm wondering if I've missed something in the schematic that is causing this. My understanding is that with pin 2 and 3 continuity, the microphone is unbalanced so I'm losing about 6dB of output.
 

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There should be continuity without the transformer through R9 and R10 also

Why so high C3 & C4? Do you tried the original values?
 
In the original KM84 C9 and C10 are absent.
With such high values there is a risk of resonance in the audible frequency range with the inductance of the secundary winding of the transformer.
 

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For continuity, I've checked a real KM84 on pins 2 and 3 and there isn't any. This is what sparked my concern.

Edit: I'm confusing myself. C3 is higher to allow more bass since I'm using a larger capsule.

C4 seems normal to me based on some of the other schematics out there (Poctop's and Woller's).

C9 and C10 I believe are meant to be shunt caps. What's your recommendation here? I've been getting some cell phone interference. Ferrite beads don't seem to be helping. I see Poctop does his RF filtering with a set of 330pf caps connecting pin 2 and 3 and 47uh inductors between the transformer and pin 2 and 3.
 
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