Impedance for idiots????

[Gtr0]

Hi Everyone.

I've been trolling various forums for some time trying to find a place where I can reveal my ignorance and get thoughtful and direct answers rather then snarky comments. Thinking that I have found such a place at MEF:

I've been building and messing around with amps for over a decade and a half! (not in any hardcore fashion however) - And though I have learned a lot of great information, impedance has, and continues to, escape(d) my understanding.

Does anyone have an "idiots" source for understanding impedance. I have read several articles, but have not reached a lightbulb moment with it.

The topic comes up as I am learning how to draw load lines - drawing the AC LOAD LINE in order to calculate voltage gain requires that the "impedance of the following stage" be calculated. I realize that it is only a matter of adding grid leak and grid stop resistors, but I very much would like to understand this mysterious topic! Please and thank you!!!

[ric]

Hi Everyone.

I've been trolling various forums for some time trying to find a place where I can reveal my ignorance and get thoughtful and direct answers rather then snarky comments. Thinking that I have found such a place at MEF:

I've been building and messing around with amps for over a decade and a half! (not in any hardcore fashion however) - And though I have learned a lot of great information, impedance has, and continues to, escape(d) my understanding.

Does anyone have an "idiots" source for understanding impedance. I have read several articles, but have not reached a lightbulb moment with it.

The topic comes up as I am learning how to draw load lines - drawing the AC LOAD LINE in order to calculate voltage gain requires that the "impedance of the following stage" be calculated. I realize that it is only a matter of adding grid leak and grid stop resistors, but I very much would like to understand this mysterious topic! Please and thank you!!!

I believe impedance is AC resistance.

I'll want to follow your thread as this gets explained from the different experts here.


...these discussions, with varying opinions are really a good thing. -Jim Darr

[Chuck H]

Well I'm no expert, but it may be advantageous that I have a more mechanical, layman's understanding of impedance because I won't get all mathy about it...

Basically it's like ric said. Impedance is AC resistance. But if you're like I was when I was trying to put the pieces together that doesn't mean a whole lot at face value. For many amp circuits the resistance and the impedance will be the same when there is no significant resonant influence. Impedance, as it applies apart from resistance would have to do with capacitive and inductive circuits where there would be a resistance to AC that is different from that circuits affect on DC. This may not help a whole lot right now, but if you keep this in mind it will sink in fast as you go through future scenarios.

[eschertron]

For a layperson's understanding (like you, me, and Chuck) what you have sussed out is a good and fairly accurate 'first guess' at the loading behavior on a tube gain stage. The plate R in parallel with the tube itself is the DC load line, and adding any resistances to ground in parallel with those results in the signal, or AC, load line. In Merlin's book - a quick look at his website did not show this - he mentions two thumbrules that can be used for capactors:
1) for DC current, a cap is open (infinite resistance), and 2) for AC current, a cap is a short. This helps analyze the circuit following a stage's plate. For most coupling caps, treating them as a short and doing the math with just the resistors is about as accurate as you'd ever need.
If a tone stack (for example) follows the plate, using these rules can provide a 'decent' answer.
The truth is that with the combination of high- and low- pass filters, the impedance can vary wildly over the audio range. Really important for guitar amps? Not so much. But this is why you may see tone stacks driven by low-impedance sources, like a cathode follower stage. Better control over the signal (read that as lower signal distortion). Useful, but maybe not desirable enough to sacrifice a triode in a guitar amp. JM2C

[glebert]

My way of looking at it is that impedance can have a resistive part and a reactive part, the reactive part being made up inductances and capacitances. All of these things can occur as intended characteristics (eg. a resistor having resistance) or unintended (eg. a resistor having serial inductance). On a first pass cut (and maybe second) you will ignore the unintended impedance (but should always keep in mind that it is there).

The impedance of a resistance is independent of frequency, the impedance of a reactance is always highly dependent on frequency. For things like coupling capacitors the value is chosen such that it will have a high impedance for DC (i.e. blocking it) and will have low impedance at audio frequencies (i.e. passing it), but for the audio it is essentially a low impedance component (but will also have some small amount of serial resistance aka ESR). For circuits where the reactances do actually come into play it may be useful to think of the network as a filter (highpass, lowpass, bandpass) with knee frequencies that at defined by reactive and resistive components. For first order RC or RL type circuits the bandwidth is easy to calculate, for higher order it gets tougher (and why people make tonestack calculators). For these circuits the reactances are low impedance within their band and high impedance outside of the band.

Or something.

[SoulFetish]

Hi Everyone.

I've been trolling various forums for some time trying to find a place where I can reveal my ignorance and get thoughtful and direct answers rather then snarky comments. Thinking that I have found such a place at MEF:

I've been building and messing around with amps for over a decade and a half! (not in any hardcore fashion however) - And though I have learned a lot of great information, impedance has, and continues to, escape(d) my understanding.

Does anyone have an "idiots" source for understanding impedance. I have read several articles, but have not reached a lightbulb moment with it.

The topic comes up as I am learning how to draw load lines - drawing the AC LOAD LINE in order to calculate voltage gain requires that the "impedance of the following stage" be calculated. I realize that it is only a matter of adding grid leak and grid stop resistors, but I very much would like to understand this mysterious topic! Please and thank you!!!

The term impedance is often used in many different scenarios (particularly in audio), so it can be difficult to derive a definition out of the context in which it is used. One reason is that it assumes a certain understanding of the reader, or there is common industry standard data which may be left out (again, probably assuming the reader has a understanding of how the data is calculated)
The simplest definition is that impedance is the resistance to AC, but lets go a little beyond that.
Here is a quick and dirty explanation of impedance: Impedance is actually a vector (or maybe think about it as a kind of coordinates). What I mean by that is impedance will always include a measurement of resistance (which is the same at all frequencies and no phase shift occures), measurement of reactance (which increases or decreases linearly with frequency in which a 90˚ phase shift occures), and will be measured at a specific frequency. So, on a graph you plot out impedance as the combined effects of resistance (R - along the x axis) and reactance (X - along the y axis), and the straight line through the junction point will show the shift in phase.
.....does that make sense?

maybe, one of the real mathematicians or engineers can explain it better.

correction: to clarify, the angle between the +X axis and hypotenuse shows the phase angle. (The length of the hypotenuse also shows the magnitude of the impedance as well, but the above is probably enough to get you going.)

[ric]

Well, i don't know how Gtr0 feels about these answers yet, but it's been helping me get a better handle on what impedance is.

Seeing it from the different angles of each individual's understanding of it.

[Old Tele man]

Trig (Pythagorian) Analogy: Impedance (Z) is the "5" in the 3-4-5 right triangle relationship...like this:

5 = SQRT[ 3^2 + 4^2 ]

Z = SQRT[ R^2 + (XC-XL)^2 ]

5 = hypotenuse = Impedance (Z) = Total opposition; vector sum of DC-resistance (R) and AC-reactance (XL,XC), has phase angle...Ohms
4 = Z-axis = Reactance (XL↑ or XC↓) = AC-opposition, Inductive (XL↑) or Capacitive (XC↓); varies with frequency, has "+↑/-↓ phase"...Ohms
3 = X-axis = Resistance (R) = DC-opposition; what your Volt-Ohm-Meter measures...Ohms

[Gtr0]

Definitely helpful. As reluctant as I have been to start this topic, I am glad I did. I'm sure it has been talked about at some point, but "impedance" is a pretty generic word, so search results vary wildly.

My understanding, as been said I think, that impedance consists of two parts... straight up resistance (which I know and love) and ac voltage/phase... the latter is obviously the part I am quite unsure of. BUT, from all of my reading of input impedance, tube stage output impedance, drawing AC load lines, and learning how to create Thevenin equivalents, I am drawing the conclusion that the only part that is constantly necessary to know is the resistance part - in which case it is only a matter of terminology - that is until you get to chokes and output transformers.

Is this a fair assessment?

The desire to learn more of impedance extends from the desire to draw and calculate load lines (as well as ac load lines). Much of my line load learning has come from Rob Robinette's site where he says that calculating the ac load line includes determining the impedance of the following stage. In this case he uses v2b of a fender showman if I recall correctly. However, when you have something high gain, like this schem (which I took as an example from google searching "high gain schematic"), you see after v2a there is a 470kΩ Rs and a 1MΩ Rg... is the intent to add these together for that stage's impedance resulting in 1.47MΩ?

[SoulFetish]

Definitely helpful. As reluctant as I have been to start this topic, I am glad I did. I'm sure it has been talked about at some point, but "impedance" is a pretty generic word, so search results vary wildly.

My understanding, as been said I think, that impedance consists of two parts... straight up resistance (which I know and love) and ac voltage/phase... the latter is obviously the part I am quite unsure of. BUT, from all of my reading of input impedance, tube stage output impedance, drawing AC load lines, and learning how to create Thevenin equivalents, I am drawing the conclusion that the only part that is constantly necessary to know is the resistance part - in which case it is only a matter of terminology - that is until you get to chokes and output transformers.

Is this a fair assessment?

The desire to learn more of impedance extends from the desire to draw and calculate load lines (as well as ac load lines). Much of my line load learning has come from Rob Robinette's site where he says that calculating the ac load line includes determining the impedance of the following stage. In this case he uses v2b of a fender showman if I recall correctly. However, when you have something high gain, like this schem (which I took as an example from google searching "high gain schematic"), you see after v2a there is a 470kΩ Rs and a 1MΩ Rg... is the intent to add these together for that stage's impedance resulting in 1.47MΩ?

Speaking of Rob, where the hell's he been?? I haven't seen him drop by here in a while.

[Jazz P Bass]

In it's simplest form, to me, the term 'impedance' is useful when going from one circuit (or device) to another.

There must be a match.

The first circuit (device) has a certain output impedance.

The circuit (device) that it is connected to must meet that impedance.

If not, it will either be overdriven or it will load down the previous circuit.

[the fatch]

In it's simplest form, to me, the term 'impedance' is useful when going from one circuit (or device) to another.

There must be a match.

The first circuit (device) has a certain output impedance.

The circuit (device) that it is connected to must meet that impedance.

If not, it will either be overdriven or it will load down the previous circuit.

For maximum power transfer, the source impedance should be equal to the destination impedance (known as impedance matching). An output transformer converts the high impedance (high voltage, low current) of the output tube(s) to match the low impedance (low voltage, high current) of a speaker. For maximum voltage transfer, the source impedance should be low compared to the destination impedance (impedance bridging). The source impedance and the destination impedance form an AC voltage divider with the output signal taken from the junction. The low output impedance of a cathode follower is often used to drive the high impedance of a tone stack for minimum signal loss.

[pdf64]

Just to note that impedance matching doesn't apply to regular tube guitar output stages, ie the design intended load impedance is a rather different thing to its output impedance.
This is mainly due to their output devices not having a simple, linear relationship between plate current and plate voltage.

Outside of RF / transmission lines, are there any other real world applications using impedance matching?

[bbsailor]

Just to note that impedance matching doesn't apply to regular tube guitar output stages, ie the design intended load impedance is a rather different thing to its output impedance.
This is mainly due to their output devices not having a simple, linear relationship between plate current and plate voltage.

Outside of RF / transmission lines, are there any other real world applications using impedance matching?

Consider the cable carrying the output of the guitar to the amp input as being the transmission line. However, this transmission line is feedings 1Meg ohm input impedance that tends to alter the sound of guitar pickups with the typical 30pf capacitance per foot of cable. Low impedance microphones rated at 150 Ohms have actual impedance of from 150 Ohms to 250 Ohms and typically feed an XLR mic input of about 2400 Ohms. At the low level of impedance any capacitance in the mic cable has very minimal audio effect and mic cables can be 100 or more feet long and still have minimal audio effect. Consider the cable connecting devices together as the transmission line and consider the required transmission line impedance to be used at various distances and noise environments to drive your output and input impedance decisions.

High impedance passive devices like guitar pickups need low noise ways get the signal to the amp such as using an active buffer amp in the guitar which isolates the pickup from the capacitance of the typical guitar cable and would allow the use of long guitar coax cables with minimal audio effect.

Joseph J. Rogowski

[SoulFetish]

For maximum power transfer, the source impedance should be equal to the destination impedance (known as impedance matching). An output transformer converts the high impedance (high voltage, low current) of the output tube(s) to match the low impedance (low voltage, high current) of a speaker. For maximum voltage transfer, the source impedance should be low compared to the destination impedance (impedance bridging). The source impedance and the destination impedance form an AC voltage divider with the output signal taken from the junction. The low output impedance of a cathode follower is often used to drive the high impedance of a tone stack for minimum signal loss.

^This is an important point (and provides the answer to pdf’s question). In audio, your not concerned with impedance matching, because maximum, efficient power transfers is not a concern (outside of a few important cases, ie.driving a speaker load). It’s the signal voltage that are the concern in interstage coupling, as mentioned above.