MCA SP1 - Modded to LD Capsule - Speak too loud; the capsule quits until reset

[777funk]

I have an SP1 mic that I pulled the little MXL603 sized capsule out of and put in a full sized one. IT's never worked right. A pop of wind from a P sound hits the capsule and it's out of commission for a bit.

I'm guessing the polarization voltage is too high and the mylar is sticking to the baffle, but that's just a guess. Why would this happen in just changing from the small diaphragm to the new large diaphragm capsule?

How do I lower the voltage if this is infact the problem?

 

[777funk]

Maybe I have it wired incorrectly???

 

[RuudNL]

Did you measure the polarisation voltage?
In most SP-1 microphones I have seen the polarisation voltage wasn't much higher than 43 Volts, so that should not be the problem. Maybe the capsule itself has a problem? Does everything work as it should with the original capsule?

 

[777funk]

I read quite a long time ago (my memory may not be completely correct here! lol) that it's not possible to measure polarization voltage with normal tools (multimeter, oscilloscope) is this correct?

EDIT: side note, I measured the phantom power and get 47VDC before the mic is plugged in. After it's plugged it drops the voltage at the XLR to around 28VDC. Maybe this is normal?

 

[Khron]

You indeed can't measure the polarization voltage at the capsule, but since that's applied through one of the 1Gohm resistors, you can at least try to probe the other end of the respective 1Gohm resistor.

Considering the phantom power is supplied through 6.8Kohm resistors, the voltage dropping under load at that point is perfectly normal - 19V dropped across 6.8Kohm works out to 2.8mA (possibly per leg..?).

I read quite a long time ago (my memory may not be completely correct here! lol) that it's not possible to measure polarization voltage with normal tools (multimeter, oscilloscope) is this correct?

EDIT: side note, I measured the phantom power and get 47VDC before the mic is plugged in. After it's plugged it drops the voltage at the XLR to around 28VDC. Maybe this is normal?

 

[JohnRoberts]

I am not the microphone guy here but is it possible a high Z bias resistor is bad solder, or open?

JR

 

[RuudNL]

There is also a 1M.ohm resistor at the output of the DC/DC converter.
So: measure before the 1 M.ohm resistor!

 

[Bo Deadly]

That soldering job is very suspect. Usually a little sloppy soldering doesn't matter too much. But when you're soldering a capsule it matters a LOT.

The reason is that the impedance of that network is on the order of 1G. That's 1 giga-ohms or 1000 Mega ohms. Why is that important? Because if there is any path for current two leak out somewhere, you will get a significant voltage drop. Even a little spec of dust or, more likely, a little bit of solder flux in the wrong place and that can conduct enough current to cause a V drop across 1G. Think about ohms-law: V = I * R. So if R is 1G, that means even 0.1 micro amps of leakage would cause a voltage drop of 100 volts (which considering you only have 48V means it will drop to 0 volts but you'll loose output long before that).

So you should really read up about soldering a mic capsule. Usual they "float" or "fly" the gate of the transistor so that it's not even touching the PCB. You'll need to use a good thoroughly cleaned pencil tip soldering iron. And then you'll have to very carefully clean away any excess flux with 99% isopropyl alcohol.

 

[777funk]

It was pulled off and put back on sloppily (wouldn't leave it soldered like that) for checking with the factory capsule and switching the polarity of the capsule that's on there. It works the same (not well) as when the soldering was good.

IT seems like the factory small capsule works fine. I'm going to try switching to the other side of the new capsule and see if that fixes the problem.