High Frequency Smoothing/roll off in SDC microphones

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Potato Cakes

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Hello, everyone,

I'm fixing up a pair of MXL 993s for a friend of mine and swapping out some capacitors per a simple mod posted here many moons ago. It sounds pretty good after the fact but the HF still has a bit of that classic cheap Chinese mic harshness I know how to tweak HF with tube and LDC mics, but I was not certain where around the FET in an SDC mic I would need to add a capacitor as there is no backplate or grid. Or does this need a simple RC filter after the Hi-Z section?

Thanks!

Paul
 

Potato Cakes

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I don't have a schematic, unfortunately. I drew one out up to the 10dB pad. I messed up the source and drain pinout so out of my laziness I just swapped the labels.
 

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Potato Cakes

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My bigger question is where would one look to add a low pass to SDCs in general? I have been doing searches but haven't found this information yet.

Thanks!

Paul
 

Potato Cakes

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That seemed to do it, but I had to use a higher value than expected (based on experience with other circuits) before I started noticing anything happening. I wound up with 0.22uF being the value that gave me a nice balance between roll off and still having enough presence. I'll see what my friend thinks. He bought the pair for about $90, so I think he'll be happy with any improvement.

Thanks!

Paul
 

RuudNL

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The limitation is that you only can get a (theoretical) HF roll-off of max. 6 dB.
At low(er) frequencies the FET works as a phase splitter (differential gain 6 dB), at high frequencies the FET becomes a source follower (gain 0 dB).
0.22 uF seems a pretty high value to me, but if it works, it works!
 
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