how high is high impedance

[Leclerc]

Ignoramus is setting out to dabble in electronics. Comes up against a requirement that headphones (for a certain application) need to be "high impedance". Has no idea what constitutes high impedance. Can anyone help?

 

[debodine]

Going on memory of long ago, but I believe headphones are 600 ohm impedance (high impedance) versus speakers which are 4 or 8 ohm (low impedance) or other headphones that vary from 50 to 150 ohms (low impedance).

This memory is from fifteen years ago, so it may be out of date for modern headphones.

 

[VEBill]

If your 'certain application' is a crystal radio, then high impedance can mean the crystal earphones that are (I believe) many meg-ohms.

 

[Skogsgurra]

If it is a crystal radio, then I have an exact figure for you: The old black "earphones" always had 2000 ohms stamped on them. But remember that that was 1920 technology. I doubt that it is something that you could run into today. VE1BLL has a more likely figure.

BTW: What IS the application?

Gunnar Englund

http://www.gke.org

 

[VEBill]

I thought that those old fashioned black headphones (zero padding) were coil-based; I agree with 2k ohms as that figure sounds very familiar. I have a set kicking around somewhere. They didn't work very well on crystal radios as compared to the much higher Z crystal earphone (single ear).

 

[Skogsgurra]

Yes, they had a coil in them. And a permanent magnet as well. The diaphragm (we used to call it a membrane?) was thin iron plate. I can very well understand that a newer crystal thing is more sensitive.

The 2000 ohms was probably chosen A) because that was about as high as you could get with a reasonable amount of wire and B) because that happend to be a good impedance match. I think(?).

I have seen some lab note books from that time. Those guys did it down to the fourth decimal. Without little calculators or computers. So I do not think that the impedance was chosen haphazardly.

Gunnar Englund

http://www.gke.org

 

[Leclerc]

thanks, gents.
just in case it makes any difference to advice, the exact application is this. I have built (from kit) a small circuit to learn about/develop microcontrollers. outputs are flashing LEDs, LCD display("hello world") and high impedance headset.

 

[Skogsgurra]

Then anything above 100 ohms will probably do. But not 4 or 8 ohms.

Gunnar Englund

http://www.gke.org

 

[Noway2]

A general rule of thumb is to make sure that the load (headset in this case)has an impedance that is at least 10x the the impedance of the source driving it.

If the kit uses any sort of amp output, this would most likely represent a low impedance and the use of anything about 100 ohms would be appropriate as Skogsgurra suggests.