# Audio Impedance

Discussion in 'General Electronics Chat' started by electronewb, Sep 23, 2012.

1. ### electronewb Thread Starter Member

Apr 24, 2012
260
3
What is audio impedance what to use to audio match between 2 circuits?

Does a transformer can be used to match impedance?

As I understand it a circuit (source) would be 600Ohms and the (load) circuit would be 1500 Ohms that is impedance mismatch. But what determines the 600Ohm in the first circuit? The last resistor in the circuit would be a 600Ohms resistor?

And which one of those 2 circuits would be high impedance and which one would be low impedance?

2. ### liquidair Well-Known Member

Oct 1, 2009
92
5
Impedance matching is when you have say a 600 ohm source and it drives a 600 ohm load. It is only critical when you are concerned about extracting maximum power from a system. This is why we match speakers to an amp. A low impedance source driving a high impedance load (such as 10k in audio) transfers maximum voltage.

Transformers can match impedances yes and is common for audio.

600 ohms is really a standby from back in the tube days as it is the rough impedance output impedance of a cathode follower circuit. It's not necessarily the last value in the circuit, although in say an op amp circuit you would use a resistor to make the source 600 ohms.

There's no real cross over point from low or high impedance, it depends on the application, but in audio we'd prolly say anything over 10k is considered a high impedance.

3. ### Dyslexicbloke Well-Known Member

Sep 4, 2010
546
34
Those are odd values for any standard audio circuit, what are you trying to do?

And yes you can match with a transformer but you are going to need much more info than that to do it properly.

Impedance is resistance but its much more complex than that when a wide range of frequencies are in play. Its basically resistance at a given frequency.
see For a relatively simple description. :-
http://sound.westhost.com/impedanc.htm

To give you a real world example of things looking wrong and working beautifully.
Line level is usually described as having 75 Ohm characteristic impedance.
In fact if you pull apart a simple amp you will almost certainly find a 75Ohm resistor between the input and ground.

Twisted pair telephone cable is generally considered to be between 100 and 140 Ohm, again characteristic impedance as opposed to something you can measure with a multy-meter.

Many modems, designed to work over telephone lines use 600/600 isolation transformers and I only mention that because the very same 600Ohm transformers make ideal baluns for sending line level audio over twisted pair.

Like I said, anything but clear or simple.

4. ### crutschow Expert

Mar 14, 2008
20,876
5,929
Actually you do not match the speaker impedance to the amp impedance. A modern solid-state audio amp has an effective output impedance of much less than an ohm. The speaker impedance is just that for which the amplifier delivers its rated power. The amplifier may actually deliver more power at other than its rated impedance but that could possibly overheat or damage the amp.

5. ### liquidair Well-Known Member

Oct 1, 2009
92
5
Correct. I meant in tube circuits but forgot to clarify.

6. ### crutschow Expert

Mar 14, 2008
20,876
5,929
Well, I remember tube circuits, but probably not too many in this forum do...

7. ### Audioguru Expert

Dec 20, 2007
10,743
1,202
No we don't anymore.
60 years ago vacuum tube amplifiers used an output transformer to match the fairly high impedance of the vacuum tubes to the low impedance speaker.

Modern solid-state amplifiers do not have an output transformer and the output impedance of the amplifier is extremely low, 0.04 ohms or less.
The low impedance damps the resonances of a speaker and is called "a good damping factor".

EDIT: Sorry to repeat but I did not see the previous replies.
I guess some old geezers still use an old vacuum tube amplifier for their geetar.

8. ### liquidair Well-Known Member

Oct 1, 2009
92
5
Hehe...Watch it guys, I'm only in my early 30's!