Impedance woes...

[Mickj83]

I was at Shure's webpage reading up on some stuff. ( http://www.shure.com/app-impedance.html ). I always thought that impedance matching was important. But they say here that the load should have a greater impedance than the source. These are the guys at Shure so I am not gonna argue. But that brings up some important questions. What is the input impedance on a guitar amp? What is the output impedance of a guitar? In addition to that... If an amp head has a 4ohm Output impedance does that mean that the cab should be greater? I was told that is was important to match so the head can transfer all its power to the speaker. For the love of god can someone clear all this up? Thanks and praise in advance!!!

[Emeric]

Hi,

Shure is relating this more specifically to microphone and mixer technology (understandably) where the input signal is of very low signal voltage and current (at the input stage.) So, the load impedance (Z)being 10X that of the source Z rule certainly makes sense, for Maximum Voltage Transfer.

For guitar amps, the input Z is High (could be as high as 1 Meg Ohm,), so it's safe to say your guitar output or it's source Z is going to have to be low for things to work correctly.

So, this 10X rule applies well at the input stage of the amplifier, whether it be at the preamp of a mixer or the preamp of a guitar amplifier.

Now as for the final output stage of your guitar amp, when your speakers are recieving signal, this is where I feel Maximum Power Transfer is a more relevant term (I may be wrong, it's been a while since I've done any textbook amp design). Your working with much higher current, voltage and the resulting power. So.. should your speakers Z match the output Z of your amp - yes, ideally. The output Z of your amp will vary with frequency though, so it will not always have a constant output Z of 4 Ohms. It will fluctuate, how much depends on the amp design, but usually more than 25% or so is unacceptable.

Hope this helps.

Emeric




[This message has been edited by Emeric (edited 06-24-1999).]

[Mickj83]

That cleared everything up thanks.