Measurement mic & preamp sensitivity

[mitchy_g]

Hi EEVblog folks,

Beginner here and I'm putting a stop to my lurking!

I have a measurement mic and preamp combo with an "open circuit sensitivity" of 50mV/Pa.

Today I put together the constant current supply (4mA) and starting measuring the sound waves with my scope.

Constant current source schematic here (http://sound.westhost.com/project134.htm). Using the JFET variant.

Everything looks fine I can see the mic is getting bang on 4mA, when I speak into the mic I can see my voice etc.

Now onto measuring!


With my sound level calibrator set to 94dB (1Pa) I was seeing 5.0mV (RMS)

With my sound level calibrator set to 114dB (10Pa) I was seeing 44.8mV (RMS)

The results are coming back pretty much 10 times lower than the spec for "open circuit sensitivity".

Am I right to think that the voltages are much lower due to the mic/preamp driving the scope input (1meg ohm) instead of a open circuit?

Wouldn't it be better if they quoted voltages at a know circuit impedance?

Is it inevitable that I'm going to have to build more amplifying circuitry? (I though the sensitivity of this mic was reasonably high and I'd be able to sneak it straight into an ADC)


Let me know if you need anymore mic specs or info.

Thanks

mitchy_g

[w2aew]

Is your scope vertical scale set to expect a 10x probe attached?

[mitchy_g]

I'm at work so I cant check for sure, but just with some quick research I found;

Standard Rigol DS2072 comes with RP3300 probes. It is detailed as a 10:1 (http://www.rigol.com/prodserv/Digital%20Oscilloscopes/attachment/)

The last memory I have of the probe settings was that it was set to "1x"......

Looks like this could just be a trap for young players. I'll be back to report in detail!

Mitch

[mitchy_g]

Yup, I changed it to 10x and the voltage readings were correct.

I first checked it against my multimeter.

Sigh

Thanks