Residual charge on rear membrane altering polar pattern

[soapfoot]

On a recent thread, @kingkorg said something that interested me:

I've been struggling for the longest time with multi pattern mics because of remaining charges on capsules even when pattern is changed. Sometimes there is no path for the capsule to discharge depending on topology. There is good chance everyone has been using a wide (or hyper) cardioid while they were thinking they were using cardioid. And suddenly the day after the mic sounds different.

This makes complete sense, but honestly had never occurred to me before!

I have some follow-up questions:

• To determine whether a given mic would be susceptible to this, what would I look for in the schematic?
• Or, if someone has already done that work--are there any notable mics that exhibit this phenomenon to a particularly large degree?
• Which components would I look at to calculate the time constant of the discharge?

Thanks for any thoughts!

 

[dmp]

I can't think of any situation where this would be true. If there is a path to set the polarity, then there is a path to discharge it (unless it had a diode???)
In a typical situation, the polarity is set with a stiff voltage from 0-120v on the rear capsule (i.e. 100k pot), through a high value resistor (~100M to 1G). If the rear is set to 120v for example, and then the polarity voltage is decreased to 60v, the discharge path is back through the same high value resistor, 120v flowing to 60v, i=60/100M. There is no one way component (diode) in any schematic I have ever seen. It may take sometime to settle, but the time constant to go either way is comparable.

 

[kingkorg]

@dmp @soapfoot
Simplest example. Dual capsule. 0v at backplate, 60v at front diaphragm. Omni is formed by connecting back diaphragm to front via switch. You turn of the switch, want to go back to cardioid, the charge on the back diaphragm will remain there for a while. Not all 60v, but enough to skew the polar pattern. Measured and confirmed true. Discharge time will depend on number of factors including SPL that reaches the rear diaphragm. In any case something to keep in mind.

Edit:
Even though back and front diaphragms are no longer connected after omni to cardioid switch the audio signal will pass through the capsule itself as it's still a capacitor.

 

[kingkorg]

On a recent thread, @kingkorg said something that interested me:



This makes complete sense, but honestly had never occurred to me before!

I have some follow-up questions:

• To determine whether a given mic would be susceptible to this, what would I look for in the schematic?
• Or, if someone has already done that work--are there any notable mics that exhibit this phenomenon to a particularly large degree?
• Which components would I look at to calculate the time constant of the discharge?

Thanks for any thoughts!

Easiest fix is to turn off mic power source before changing pattern, flip through the patterns wait in each position for everything to discharge and flip to desired pattern before turning the mic on.

 

[RuudNL]

I had this problem before. Solved it by adding a 1 G.ohm resistor to the rear membrane connection. (And ground)
Since the polarisation voltage had a relative low source impedance, the influence of the 1 G.ohm resistor could be neglected, but it discharged the rear membrane when it was switched off. ('Cardioid only')

 

[dmp]

This is a good point. I can see this being a design compromise 50-100 yrs ago with prohibitive cost / availability of 100M-1G resistors, but today? Good info to have out there to anyone cloning the U47. Poctop's EF47 is an example - should add the 1G to the rear capsule.