Impedance at a Glance...

[JohnRoberts]

Hmmm.... In "principal" wouldn't you want to transfer as much "power" (as opposed to voltage or current) as possible from the source to the next stage to preserve as much of the s/n ratio as possible at the final output? I realize that there are lots of other reasons not to do this. I am thinking of a giant tone arm/stylus playing a huge record with an output z of 8 ohms. Wouldn't you get a better s/n ratio by hooking it directly to an 8 ohm speaker than any kind of amp with a higher input impedance? ( Ignoring stylus loading and lots of other stuff).

No... I'm not sure how to answer without repeating myself. Modern interfaces are about maximizing signal voltage transfer, not maximum power transfer.

Old school transformer I/O did indeed look for max power transfer (when source impedance and input impedance are equal.) Modern interfaces use what is called "bridging" terminations where input impedance is 10x or more the source impedance. (Not to be confused with bridged power amp outputs, something completely different).

JR

PS; EIN (equivalent input noise ) is generally characterized as a noise voltage (or noise voltage and noise current) not a noise power. Best S/N is realized from most signal voltage vs least noise voltage.

 

[Rob Storms]

No... I'm not sure how to answer without repeating myself. Modern interfaces are about maximizing signal voltage transfer, not maximum power transfer.

Old school transformer I/O did indeed look for max power transfer (when source impedance and input impedance are equal.) Modern interfaces use what is called "bridging" terminations where input impedance is 10x or more the source impedance. (Not to be confused with bridged power amp outputs, something completely different).

JR

PS; EIN (equivalent input noise ) is generally characterized as a noise voltage (or noise voltage and noise current) not a noise power. Best S/N is realized from most signal voltage vs least noise voltage.

John thanks for the reply and indulging me. I believe I did understand your previous but to beat this some more....Using a far fetched and strictly " in principle" example, the louder the mic ( voltage at the pre input) the less gain you need from the mic pre and the less noise it will contribute. "In principle" you could use an ideal transformer ( ignoring for the moment all the terrible things the real ones do) between mic and pre to increase the mic voltage as high as possible ( ignoring supply rail limits and much else). The max voltage at the pre will be realized when the transformer turns ratio makes the mic output z equal the reflected input z of the pre ( whatever that is) and thus you are transferring max power to the pre. I have had several concussions......

 

[Tubetec]

I found this video yesterday, even though the question in this thread seems more in the direction of solid state impedences, I felt the video might be useful to people interested in tube/transformer output impedence . Uncle Doug can be a little pedantic in his delivery at times but I'm sure its all for the purposes of helping people understand better and keep safe.
Its good though as his method requires only two multimeters and he uses an oscilloscope but REW would be even better for finding the onset of clipping precisely . A step down transformer is used as an AC signal source .

youtube.com/watch?v=5jUitplchok

Uncle Dougs video about the Eurotubes bias probe down the page is also worth looking at for tube heads .

 

[ruffrecords]

Hmmm.... In "principal" wouldn't you want to transfer as much "power" (as opposed to voltage or current) as possible from the source to the next stage to preserve as much of the s/n ratio as possible at the final output? x

I remember learning the derivation of the maximum power transfer theory in the first year of university but I never understood its importance. Bottom line is the maximum power is transferred when the load impedance equals the source impedance but there are lots of problems with this. First of all, half the power is dissipated in the source resistance so your big tone arm is going to get quite warm. Second, if you loaded most amplifiers with their source resistance they would clip at a very low output level.

The problem with the maximum power transfer theorem is it considers things from the point of view of the source not the load. When transferring power, most of the time we have a defined load. Ir you restate the maximum power transfer theorem to be "what source resistance do you need to transfer the maximum power into a given vlaue load then the answer is always a source impedance of zero ohms. Anything less creates a pot divider with the load, hence lowers the voltage across the load and therefore the power in the load. Which is why we always transfer these days from low source impedances to high load impedances.

Cheers

Ian

 

[Rob Storms]

I remember learning the derivation of the maximum power transfer theory in the first year of university but I never understood its importance. Bottom line is the maximum power is transferred when the load impedance equals the source impedance but there are lots of problems with this. First of all, half the power is dissipated in the source resistance so your big tone arm is going to get quite warm. Second, if you loaded most amplifiers with their source resistance they would clip at a very low output level.

The problem with the maximum power transfer theorem is it considers things from the point of view of the source not the load. When transferring power, most of the time we have a defined load. Ir you restate the maximum power transfer theorem to be "what source resistance do you need to transfer the maximum power into a given vlaue load then the answer is always a source impedance of zero ohms. Anything less creates a pot divider with the load, hence lowers the voltage across the load and therefore the power in the load. Which is why we always transfer these days from low source impedances to high load impedances.

Cheers

Ian

Thanks much for your input Ian! My tone arm might get warm but my interest there was in max S/N through the entire audio chain. Power amps do indead depart from their low output impedance once the current limit ( and other things) is exceeded. But lots of sources dont depart in that way. My guess is that most device connections employ low output z and hi input z is goof proof hookup and lots of fan out ability. Once the signal level is high enough that noise contribution is not a big issue it makes sense too. I am no authority, just thinking out loud. Thanks again!

 

[JohnRoberts]

Thanks much for your input Ian! My tone arm might get warm but my interest there was in max S/N through the entire audio chain.

Maximizing S/N through an entire audio chain is a slightly different calculus than maximizing interface S/N. Of course the (signal) chain is only as clean as the weakest link. Noise at every stage is cumulative but noise floor makes a less significant contribution after input signals are already boosted to healthy nominal levels. For this reason it is good practice to put significant gain in first stage so it doesn't amplify noise from later stages along with the input signal.

JR

Power amps do indead depart from their low output impedance once the current limit ( and other things) is exceeded. But lots of sources dont depart in that way. My guess is that most device connections employ low output z and hi input z is goof proof hookup and lots of fan out ability. Once the signal level is high enough that noise contribution is not a big issue it makes sense too. I am no authority, just thinking out loud. Thanks again!