LDC capsule replacement question

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tgs

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Hi all,
Total beginner question here, feel free to ignore if you think it's too obvious. I've searched around a bit for info but I haven't found anything that seemed very clear. Might have looked in the wrong places.

Anyway. I have a couple of old ADK A51 mics, very early model. Just cardioid pattern, no switches or anything. I recently bought two capsules that I want to use in these. These capsules have separate connectors for front and back membrane. If I understand it correctly: if I connect both ++ and --, I should get cardioid response, and shorting back membrane to ground should make it omni. Is that correct? And what would happen if I flip the polarity of the back plate?

First mic mod for me, looking forward to messing around a bit with it!
 

RuudNL

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For cardioid only, you only have to connect the front membrane and the backplate.
Leave the rear membrane wire unconnected.
(If you connect the front- and rear membrane together, you get an omni microphone.)
 

tgs

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I don't have schematics for it unfortunately. I have checked before though about the capsule I got (Advanced Audio AK67) and from what I know it's possible to replace without further modification (if necessary).

The AK67 has two red and two blue wires though. Two separate backplates?

EDIT: Dual backplate indeed.
 

RuudNL

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For cardioid you only have to connect the 'front' backplate.
If you want omni, connect the two backplate wires together. (And the two membrane wires of course.)
 

Gus

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A search found the following


 

tgs

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Thanks! Will look into this soon. I'll see if I can find the revision as well, the grille on mine is different so maybe a later one.

Any way to get fig-8 as well?
 

RuudNL

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In that case you should connect the rear membrane wire to the front backplate and the rear backplate to the front membrane, so that they have opposite phase. Never done this, but it should work.

Edit: Now I think about this, I am not so sure if this will work...
The phase of both capsule halves should be opposite, but I am afraid that you will get an omni microphone, even after connecting the membranes and backplates 'crossed'!
 
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tgs

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In that case you should connect the rear membrane wire to the front backplate and the rear backplate to the front membrane, so that they have opposite phase. Never done this, but it should work.
Will try. :) I'll have to think a bit about how to sort this out with a 3-position toggle switch
 

tgs

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Maybe it is better to have a look at the way AKG did the pattern switching, in the C414.
To me it seems that the difference between omni and fig-8 in that schematic is the reversed polarity, and cardioid just shorts the back membrane to ground. That is seem through morning eyes though...
 
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