High vs. Low Impedance, and why I think the general terminology is wrong

[JakeAC5253]

I am sure that as engineers we are all familiar with the terms high impedance, and low impedance. These words are usually used to describe a signal, and the load level that is needed by the signal. You will often see these words used to describe the input section of a piece of outboard gear, which is supposed to quickly tell you something about the input characteristics of that piece of gear. The general rule of thumb is that the higher the impedance at the input, the more accurate the tone will be, while low impedance inputs tend to be the more coloring of the two.

Here is why I do not understand the common terminology. I believe that instead of terming these inputs as high vs. low impedance, we should instead call them high vs. low resistance, or high vs. low potential. The word resistance is a straightforward word, a resistor impedes everything through it in an even fashion (the ideal resistor). The word impedance is resistance with phase angle and time constant. In short, impedance is the resistance to AC flow, and in audio, AC is our throughput signal. So it would follow that to impede the AC signal would be to change the sound from its natural state. When the parallel resistor in the input of a circuit is made larger, the input resistance or input potential difference grows higher, but I believe that the input "impedance" drops because it is providing a more sufficient load for the AC and will color it less assuming a low line capacitance. To overstate my point **The throughput signal is not colored by the input stage as input resistance increases, and thus I believe that as a result the input impedance decreases**

It very simply doesn't make sense. Lots of companies advertise their products as "Ultra high impedance input!!!" and it is supposed to connote a positive thing. Being that our field is audio, we are supposed to grab our wallets and say "oh yes! my audio signal is going to be decimated by this piece of gear!" We should instead describe the inputs of more transparent pieces of gear by saying that they are Ultra Low impedance instead, because to impede the AC signal is to impede its flow in at least one range of the spectrum, which would change the sound.

 

[John_C]

I am sure that as engineers we are all familiar with the terms high impedance, and low impedance. These words are usually used to describe a signal, and the load level that is needed by the signal. You will often see these words used to describe the input section of a piece of outboard gear, which is supposed to quickly tell you something about the input characteristics of that piece of gear. The general rule of thumb is that the higher the impedance at the input, the more accurate the tone will be, while low impedance inputs tend to be the more coloring of the two.

Here is why I do not understand the common terminology. I believe that instead of terming these inputs as high vs. low impedance, we should instead call them high vs. low resistance, or high vs. low potential. The word resistance is a straightforward word, a resistor impedes everything through it in an even fashion (the ideal resistor). The word impedance is resistance with phase angle and time constant. In short, impedance is the resistance to AC flow, and in audio, AC is our throughput signal. So it would follow that to impede the AC signal would be to change the sound from its natural state. When the parallel resistor in the input of a circuit is made larger, the input resistance or input potential difference grows higher, but I believe that the input "impedance" drops because it is providing a more sufficient load for the AC and will color it less assuming a low line capacitance. To overstate my point **The throughput signal is not colored by the input stage as input resistance increases, and thus I believe that as a result the input impedance decreases**

It very simply doesn't make sense. Lots of companies advertise their products as "Ultra high impedance input!!!" and it is supposed to connote a positive thing. Being that our field is audio, we are supposed to grab our wallets and say "oh yes! my audio signal is going to be decimated by this piece of gear!" We should instead describe the inputs of more transparent pieces of gear by saying that they are Ultra Low impedance instead, because to impede the AC signal is to impede its flow in at least one range of the spectrum, which would change the sound.

urmm.......either i'm getting confused, or you're misunderstanding things a little

Impedance is the complete representation of a component's opposition to current, resistance is the simplification of this term to only describe DC situations.

The potential divider:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/voldiv.gif (not using image tags as file has no background)
As you can see, increasing the load resistance (or impedance) INCREASES the voltage across the output. As audio input circuits are (as far as i know) voltage driven not current driven, this means you get more "signal" if you have a higher impedance. The amount of CURRENT that flows through the input to ground does indeed decrease when you increase the impedance, but that's not important, the voltage is what matters.

High impedance is the correct term. I think you're just getting confused between some wooly definition of "signal" and real physical quantities i.e. voltage and current.

edit: There is a genuine problem with the use of impedance in audio circuits however. Normally, manufacturers give a single real number for the impedance, which is normally the impedance as measured at 1khz. This covers up the possibility that the impedance could have a significant dependence on frequency. A 1MOhm input sounds great, but it could in fact be effectively a narrow bandpass filter centred at 1khz and you'd be none the wiser from the 1MOhm figure.

 

[JakeAC5253]

Exactly! I see what you mean, though I am not sure where the disconnect is. As the load resistor approaches a resistance of infinity, the input circuit goes closer and closer to being fully transparent to the input voltage because less and less voltage and current is bled to ground (or to the negative input) through the load resistor. As the load resistor approaches zero, the input circuit turns more and more "off". The circuit whose resistance is approaching infinity will have less coloration because to the AC signal, the short path is becoming more and more invisible, which is why I believe that it should be called low impedance because the AC characteristics of voltage and current are closer to what the source is actually putting out and not colored by the input circuit.

I think we may be describing the same thing using different words.

 

[John_C]

Yes, I see what you mean, though I am not sure where the disconnect is. As the load resistor approaches a resistance of infinity, the input circuit goes closer and closer to being fully transparent to the input voltage because less and less voltage and current is bled to ground (or to the negative input) through the load resistor. As the load resistor approaches zero, the input circuit turns more and more "off". The circuit whose resistance is approaching infinity will have less coloration because to the AC signal the short path is becoming more and more invisible, which is why I believe that it should be called low impedance because the AC characteristics of voltage and current are closer to what the source is actually putting out and not colored by the input circuit.

I think we may be describing the same thing using different words.

oh ok, i see what you mean. I agree, high impedance sounds like a bad thing unless you understand that it's referring to the resistors in a simple voltage divider.
However, the alternative would be a dual (and opposing) definition, which would only lead to more confusion.

 

[JakeAC5253]

edit: There is a genuine problem with the use of impedance in audio circuits however. Normally, manufacturers give a single real number for the impedance, which is normally the impedance as measured at 1khz. This covers up the possibility that the impedance could have a significant dependence on frequency. A 1MOhm input sounds great, but it could in fact be effectively a narrow bandpass filter centred at 1khz and you'd be none the wiser from the 1MOhm figure.

Yes, this is the other aspect of my thought which I may not have stated in the original post. The aspects of line capacitance wreak havoc with extremely high resistance inputs to roll off a significant amount of high end. Which to me, would make it a high impedance. Furthermore, with circuits that require input capacitors in series with the input, if the capacitor is too small or the series resistor too small, it could roll off low end in much the same way. Which to me, would again be a high impedance. I guess I am using the term impedance to generally describe the level of coloration. Much the same way that manufacturers do, though they approach it very one sided, like you pointed out.

Input resistance needs to be carefully chosen along with the input capacitance and source characteristics in order to achieve a non-coloring input circuit. I do believe that a non-coloring input should be termed low impedance regardless of the parallel resistors actual numerical value though. I think that this is what I am getting at. The way it is done now is pointless and misguided to me.

 

[TheWinterSnow]

As you can see, increasing the load resistance (or impedance) INCREASES the voltage across the output. As audio input circuits are (as far as i know) voltage driven not current driven, this means you get more "signal" if you have a higher impedance.

Transistors are current controlled, vacuum tubes (valves) are voltage controlled.

Impedance, the combination of capacitive reactance, inductive reactance and resistance, is the total opposition to current at both AC and DC whereas resistance is the opposition of current at DC.

Input and Output Impedances are the impedances between the signal and ground. Due to the way that valves and transistors operate, the signal will have to come into electrical contact with ground, the higher that impedance is, the less signal loss will take place. Signal loss is not always bad, in cascading gain stages the signal needs to be attenuated so by loosing some of the signal you can condition for the new signal to go to its next destination. Now I can't stress this enough, the signal loss is generally linear and wideband throughout gain stages because they are almost always resistors. Where there are filters (caps, inductors) the designer has designed specifically for the signal at a particular frequency to be attenuated. This is achieved however by placing the filter in series with the load which actually increases the output impedance of the previous stage, reducing its ability to supply current to the load therefor attenuating the signal.

Input Impedances therefor need to be high to prevent signal loss, and output impednaces need to be low so that they can provide adequate current to the load.