Mic capsule polarization question

[milesjackson]

Hello all,

I'm going to build a tube mic based on ioaudio's mk7 schematic, but I have a question.  I've decided I only need a cardioid pattern on the mic.  When I look at designs like Gyraf's G7 and ioaudio's mk7, I see that the back capsule is polarized at the same voltage as the backplate to generate the cardioid pattern.  However, in the U47 schematic, the cardioid pattern is generated by simply removing the back capsule from the circuit (switch open).  I prefer simple in circuit design whenever possible, so I'm leaning toward the U47 approach (just leaving the back capsule unconnected).  Is there any advantage to the G7/MK7 approach of polarizing the back capsule at the same voltage as the backplate?

Miles

 

[gyraf]

If you don't need remote controlled pattern switching, the best and simplest would be to simply disconnect back membrane.

Jakob E.

 

[gemini86]

agreed...but seems like a waste of half a capsule to me... just saying.

 

[pasarski]

Disconnecting the back membrane makes the output and S/N noise ratio of the capsule a little better. A neat solution people have done is to make U47 style switch with a (high insulation resistance) relay.

Though making the mic cardioid only is not total waste of the capsule because the back membrane tends to keep in better shape, at least in vocal use, and after some decades you can flip it over.

 

[mad.ax]

I haven't tested this myself yet, but some people says that 'True Cardioid' (disconnected back membrane) sounds better. That's why Oliver Archut implemented a relay to switch of the back membrane in the CS1.

Axel

 

[milesjackson]

Thanks to all for the guidance and suggestions.  I'll take a look at the CS1 schematic.

 

[gemini86]

Though making the mic cardioid only is not total waste of the capsule because the back membrane tends to keep in better shape, at least in vocal use, and after some decades you can flip it over.

good point, never thought of it that way...

 

[Rossi]

I haven't tested this myself yet, but some people says that 'True Cardioid' (disconnected back membrane) sounds better. That's why Oliver Archut implemented a relay to switch of the back membrane in the CS1.

Axel

It sounds the same, but disconnecting the rear diaphragm for cardioid mode improves S/N by about 3 dB.