# Impedance matching?



## Jason Brent (Mar 28, 2009)

OK toob gurus...

Doing some late night lurking around, and I found yet another subject I'm clueless about (I know add it to the list, right?:snaggletooth

It seems one needs to be careful in pairing tubes and SS equipment. What happens if you have an impedance mismatch? The basic rule of thumb I found says that if using a tube pre and ss amp, the amp should have an input impedance at least 10x the output impedance of the pre.....I think I'm ok there, just barely. But it got me to thinking, do I have a less than ideal equipment pairing?


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## HAL (Sep 22, 2009)

With a tube preamp driving a SS PA, the impedance mismatch will affect the low frequency response of the system. The tube preamp will typically have an output coupling capacitor. This forms a high pass filter for the system, that depending on the two impedances, sets the F-3dB corner frequency. 

This also happens with line level tube output stages and SS preamp input stages. This happens anytime an output coupling capacitor is in the previous stage.


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## lanceb1b (Nov 30, 2008)

Jason said:


> OK toob gurus...
> 
> Doing some late night lurking around, and I found yet another subject I'm clueless about (I know add it to the list, right?:snaggletooth
> 
> It seems one needs to be careful in pairing tubes and SS equipment. What happens if you have an impedance mismatch? The basic rule of thumb I found says that if using a tube pre and ss amp, the amp should have an input impedance at least 10x the output impedance of the pre.....I think I'm ok there, just barely. But it got me to thinking, do I have a less than ideal equipment pairing?



What is a standard output Z of a "Tube-Pre"?


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## Jason Brent (Mar 28, 2009)

Lanceb1b said:


> What is a standard output Z of a "Tube-Pre"?


Is "Z" the symbol for impedance? not quite sure what your asking.


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## lanceb1b (Nov 30, 2008)

Jason said:


> Is "Z" the symbol for impedance? not quite sure what your asking.


Yes what is the out put impedance?


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## Jason Brent (Mar 28, 2009)

For my pre it is 450 ohms. As you go up in the Rogue line, it gets lower. And then there's something on their top two offerings called unity gain that I'm not sure what it does..

So if there's an impedance mismatch, my FR will be off??


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## lanceb1b (Nov 30, 2008)

This place explains it all

http://www.tape.com/resource/impedance.html


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## HAL (Sep 22, 2009)

Lance has a very good site with info.

Here is another way to look at the issue. Here is an simplified circuit for the preamp to PA interface in picture 1 in a circuit modeling program. The frequency response of this circuit is in picture 2. The F-3dB point is ~20Hz. This would limit low frequency response of the system as it is still down 1dB at 40Hz. 

The full answer mostly depends on the output coupling capacitor in the tube preamp and the input impedance of the PA.


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