Microphone Impedance - a mystery ?

[Musicmanager]

Hi
I've built a voice modulation kit for my grand kids for Christmas which I got from Velleman ref MK171
There is a PCB mounted electret microphone onboard which I'm guessing has an impedance around 300 Ohms. I want to use a handheld Mic via a jack socket and whilst I understand that a dynamic Mic doesn't need a power source, which is easily dealt with, the nearest impedance I can find is around 600 Ohms and I don't understand the implications of that other than I imagine the output level will be affected. Will it damaged the circuit ?

 

[MikeMl]

I would put a DC blocking capacitor (1uF) in series with the dynamic mic. This will prevent the DC current sourced by R3 and R6 normally used to bias the electret mic from back-driving the dynamic mic element.

The levels may or may not be comparable.

 

[Musicmanager]

Hi Mike
I think I sussed that one, there is a DC voltage at the electret which comes almost directly from the power source battery via a 4K7. If I remove that 4K7 that should solve that problem. I asked Velleman technical about that and they agreed with that. However, they also went to some lengths to emphasise the difference in impedance between the types of Mic and went on to suggest I test the circuit specified before I make any alteration, thus implying a problem will arise.

Should I ask you how you know ??

 

[MikeMl]

Mics do not have to be terminated with their nominal source resistance. The input impedance of the kit is about 2.3K (two 4.7Ks in parallel), close enough for a 600Ω mic...

 

[Musicmanager]

Thanks Mike
I have to apologise, I didn't realise you'd posted the schematic because I read for first reply in email, not the forum.
I'm curious, where is Big Springs Ranch ?
Thanks again, Stuart

 

[Nigel Goodwin]

Mics do not have to be terminated with their nominal source resistance.

In fact it's VERY important that they aren't, simply removing R6 is all that should need to be done, a normal '600' ohm mike should work fine then.

 

[MikeMl]

...
I'm curious, where is Big Springs Ranch ?

Near Prescott, AZ