Impedance Matching

maxzimum · 9th April 2009, 10:07 AM

How important is it to match impedance when choosing a Preamp / Power Amp combination? What is the ideal match?

Also, does input/output impedance have any bearing with overall gain?

Thanks.

AndrewT · 9th April 2009, 10:19 AM

it depends.

Sorry, but it depends on how the receiver stage has been designed.
A non inverting stage usually likes to see a low source impedance/resistance. It also usually expects the source resistance to be nearly constant.

The gain of the non inverting receiver is independant of the source resistance/impedance. However there is an interaction between Rs and Zin which attenuates the input voltage slightly.

An inverting amplifier uses the series input resistance/impedance to set the gain of the NFB loop.
Any added resistance at the input will affect the amplifier gain.

Using a ratio of 1:10 for Rs:Rin attenuates most of the errors, but 1:20 is very slightly better.

eg.
Rin=100k allows Rs <=5k. This allows a <=20k passive pot to work effectively.

Watch for capacitance. It will interact with these higher impedances to act as a low pass filter.

A low Rs (<=200r) usually indicates universal compatibility, but do check that the source can drive low impedances.

darkfenriz · 9th April 2009, 02:03 PM

It's a completely non-issue as far as the proportion is more than 1:100.
Order of 1:10 000 is not uncommon at all, so just forget it.

9th April 2009, 10:07 AM	#1
maxzimum diyAudio Member Join Date: Feb 2005 Location: India	Impedance Matching How important is it to match impedance when choosing a Preamp / Power Amp combination? What is the ideal match? Also, does input/output impedance have any bearing with overall gain? Thanks.

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9th April 2009, 10:19 AM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	it depends. Sorry, but it depends on how the receiver stage has been designed. A non inverting stage usually likes to see a low source impedance/resistance. It also usually expects the source resistance to be nearly constant. The gain of the non inverting receiver is independant of the source resistance/impedance. However there is an interaction between Rs and Zin which attenuates the input voltage slightly. An inverting amplifier uses the series input resistance/impedance to set the gain of the NFB loop. Any added resistance at the input will affect the amplifier gain. Using a ratio of 1:10 for Rs:Rin attenuates most of the errors, but 1:20 is very slightly better. eg. Rin=100k allows Rs <=5k. This allows a <=20k passive pot to work effectively. Watch for capacitance. It will interact with these higher impedances to act as a low pass filter. A low Rs (<=200r) usually indicates universal compatibility, but do check that the source can drive low impedances.

9th April 2009, 02:03 PM	#3
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	It's a completely non-issue as far as the proportion is more than 1:100. Order of 1:10 000 is not uncommon at all, so just forget it.