22K Ohm impedance matching

Stoner

10.10.20 you might be able to advise here. As you know by applying impedance matching we attempt to make the output impedance of a source equal to the input impedance of the load.
I have built an external mic that can measure a range of low frequencies, it is a tiny panasonic electrc mic, its running through a small amp with adjustable gain etc. I have a standard headphone connection arrangement, i have it connected to my PC via the sound card. Anyway it has a 9V battery and it works, i have no issues wioth the circuit or the mic.

However I'm concerned that i might blow the bejesus out of my PC if i have an impedance matching issue. A resistance placed in the lead may help with this, there is one called a called a dubbing lead with a 22K Ohm resistance. But the thing is I dont know how to measure the load and input impedance to match them, any ideas as I'm making a second one for someone else and I dont want to blow their PC up either.
So how do I measure the impedance of the sound card and the output of my circuit?

DublinDilbert

Ummmm interesting problem......

There are 2 ways of thinking of impedances:-

1. In the low frequency / DC domain we would like to get the max power transfer, so we would generally like the impedance of the source and impedance of the receiver to match. As the same current will flow in both the source and receiver, if their resistances are equal they will both have the same power loss and max power will be transfered. If you draw up the circuit, and apply P = I^2 x R to the source and receiver with series resistances in each you'll see this working.
http://en.wikipedia.org/wiki/Maximum_power_theorem

2. In high frequency systems we like to terminate the transmission line in its characteristics impedance to prevent reflections due to miss matches, as these reflections due to miss match would degrade the signal at the receiver (destructive interference), this is exactly what happens if you connect more than one TV to an NTL signal without a booster / matching network. You only have to worry about terminating in the characteristic impedance of the transmission line when the wavelength of the signal being carried approaches 1/4 the length of the transmission line, hence usually only applies a high frequencies.
The key things are that transmission lines have a characteristic impedance, hence can't be measured with a simple DMM, you'll need an LCR meter to measure...
Typical characteristic impedances:-
75 Ohm termination for TV Aerial
50 Ohm for data networks
300 Ohm for PST networks



All that said, i think your audio setup falls into the first category above, as the frequency is so low, and your path is so short. You need to try to match the source and receiver impedances so you get max power transfer, hence max signal at the receiver... I would guess the device you can buy to match the impedances, will not just match them at DC (with simple resistors), but from say 50 -> 20KHz, hence will be some sort of LC network. I'd guess there'd be lots of matching circuits on the internet that would do what you need...

Stoner

thanks DublinDilbert,

I think you are correct it is option 1. I still need to get it sorted though

10-10-20

Sorry Stoner - I have to bow out on that question.
I'm more into the digital electronics than analogue.

Stoner

10-10-20 wrote: »

Sorry Stoner - I have to bow out on that question.
I'm more into the digital electronics than analogue.

ahh OK, it was worth a shot anyway........ thanks lads

DublinDilbert

Stoner wrote: »

ahh OK, it was worth a shot anyway........ thanks lads

I'm more into digital/embedded electronics too, but do a fair amount of hobby type work too.... I'd be surprised if maplin would not have something to match the impedances.

If you could find the matching network on the internet let me know and i'll check my stock of inductors / capacitors see if i have anything that would work for ya...


found this too:-
http://sound.westhost.com/impedanc.htm

RoundyMooney

I'm thinking some kind of matching transformer, but because your mic is homebrew, guessing the impedance without an LCR meter will be difficult. What IC did you use for the preamp?

It shouldn't be too difficult to find the impedance of the soundcard input, which is the other half of your quandary.

I'd be more worried about overloading the input tbh.

Michael Collins

Hi Stoner. Do you have a general schematic of your microphone circuit that you could post up? It would help a lot. The input impedance of your average soundcard is designed not to change too much between the frequencies of interest, so that simplifies it a bit. If the same is true of your microphone a simple resistive matching network would suffice.

Have you look on the net to see if you can find your card's output impedance?