High Impedance vs. Low Impedance...help me understand

[PharmRock]

So I am a bit embarrassed to ask this but I'm going with the old adage, "there's no such thing as a stupid question".  I understand resistance, capacitance, current, voltage.  But what I can't get my head wrapped around is impedance and load, particularly as it pertains to tube stages. 

For example, in another thread relating to my EF86-driven tone control, Tubeswell said, "Yes the EF86 stage has high output impedance, which means there will be high signal loss if the following load (tone stack) is a heavy load."

What exactly does this mean...impedance and load.  I've read the usual resources and also done some internet scouring but I can't find anything that explains it in a way I can understand.  My rudimentary understanding is that impedance is a combination of resistance and capacitance of a circuit, which makes sense that a typical TMB tone stack would have high impedance (at least my understanding).  How does this relate to the tube driving that circuit?

I know this is something I should know but I can't get my head around it. 

Thanks

<edit> I just located this: https://el34world.com/Forum/index.php?topic=28682.0 and reading up.  A little over my head but will try my best to comprehend.

[shooter]

here's a deep dive
Designing Single-Stage Inverting Feedback Amplifiers (aikenamps.com)


understanding the math makes understanding impedance easier. it's not much fun, but it's better than politics   

[PharmRock]

Thanks...I'll check it out.  Need to familiarize myself with terminology such as input/output impedance, impedance matching, etc.

[tubeswell]

Impedance = ‘resistance to AC’


A signal behaves in a similar way to AC - there are peaks and troughs in current (which we want to manipulate through the amp in order to make the signal louder and perhaps muddy things up a bit).


Analysing the signal path, we see there are locations in the amp where there is no signal- the ground return and the High VDC power supply rail. Between these 2 ‘no signal’ areas, the signal has to exist. The linkages between the ‘no signal areas’ and the ‘signal areas’ is compromised of various current pathways (through resistors, frequency filter caps and coupling caps (which have ‘high resistance’ to DC), and the tubes themselves). The ‘relative ease’ with which the signal ‘moves’ is related to the amount of impedance that these pathways provide.


Low impedance areas (such as each signal source) are supported by higher resistances between the signal areas and the ‘no signal areas’ (in the PS rail and the ground). A plate is a  (relatively) ‘low impedance signal source’ (and impedance is relative, which is why you get gain stages that can be higher in output impedance- like a pentode).


Whereas high impedance areas either have smaller amounts of resistance (I.e. through which the current in the signal ‘drains’ more easily towards the ‘no signal’ areas), or more current pathways through to the no signal areas (resistances in parallel = lower overall resistance). The higher impedance areas are where the signal load is higher/‘heavier’.


A useful analogy is a house spider web. If you poke the middle of the web (assuming you don’t wreck the web), it moves back and forth more easily than if you try and poke the edge of the web (where the web is anchored to solid structures). The more strands that the web has directly from the middle to where the web is anchored (‘parallel resistances’), the more difficult it will be to move the web back and forth. The shorter the strands, the more difficult it is to move the center of the web (‘smaller resistance’). The higher the tension in each strand, the more difficult it is to move the center (and the quicker the center will vibrate when plucked).

[pdf64]

… Need to familiarize myself with terminology such as … impedance matching …

In the real world, impedance matching finds little application outside of RF, transmission lines, and school physics class.
Impedance bridging is the norm between audio equipment, instruments and circuit stages.
But not between a valve power amp and the speaker; that defies categorisation 
https://en.m.wikipedia.org/wiki/Impedance_bridging

[HotBluePlates]

... in another thread relating to my EF86-driven tone control, Tubeswell said, "Yes the EF86 stage has high output impedance, which means there will be high signal loss if the following load (tone stack) is a heavy load."

What exactly does this mean...impedance and load.  ...

<edit> I just located this: https://el34world.com/Forum/index.php?topic=28682.0 and reading up.  ...

The whole issue is summed up in Table 1 on Page 2.

   -  Source Impedance is 100Ω.
   -  Power into the load (3rd Column) is highest when the load impedance is also 100Ω; it "matches" source impedance. 
   -  Voltage Across Load (4th Column, with unhelpful naming) is highest when load is many-times source impedance.

In a preamp, we do not attempt to deliver "power" from one stage into the next stage, we try to deliver "voltage."  So what helps is a Next-Stage that looks like "high resistance" compared to Previous-Stage.

...  I understand resistance, capacitance, current, voltage.  But what I can't get my head wrapped around is impedance ...

Do you know Ohm's Law?  Volts = Current x Resistance.

Use algebra to rearrange the terms:  Resistance = Volts / Current
   -  If "Volts" increases but "Current" stays the same, "Resistance" increases.  "High Resistance"
   -  If "Current" decreases while "Volts" stay the same, "Resistance" increases.  "High Resistance"

   -  If "Volts" decreases but "Current" stays the same, "Resistance" decreases.  "Low Resistance"
   -  If "Current" increases while "Volts" stay the same, "Resistance" decreases.  "Low Resistance"

   -  Replace the word "Resistance" with "Impedance" because they're the same when talking about AC.

   -  Plug any concrete numbers you choose for "Volts" and "Current" in the equation above to prove the "Rules" to yourself.

So "high impedance circuits" tend to have higher voltage present, and smaller currents.  Or maybe smaller current for a given voltage.

We're concerned about delivering signal voltages from one stage to the next in a vacuum tube preamp.  Having high "resistance/impedance" means we can generate those signal voltages with less current.  Which is good because our preamp tubes are unable to deliver "a lot of current"... they're "high-impedance devices."


The tube's internal impedance (described as "internal plate resistance") is part of what determines the "source impedance" a tube stage has.  A triode has a lower internal plate resistance than a pentode, and can deliver somewhat more current, more easily.

The "current" doesn't matter in & of itself, because the next stage doesn't demand a current-input (the way a transistor would).

What does matter is the triode represents a smaller "source impedance" and so can sustain its output voltage into a smaller load impedance than a pentode could.  It's like that Table 1 source impedance was 10Ω instead of 100Ω.  Or for a triode, like 38kΩ instead of a pentode's 240kΩ.


If a load impedance is small, it is like a "small resistance."  Current = Volts / Resistance.  So "smaller load impedance" ---> "higher load current."

"High Current" is colloquially described as a "heavy load."  Which also implies smaller resistance/impedance.

All the above is why we say, "a high impedance stage has a hard time driving a heavy load (small load impedance)."

[shooter]

got a question, i used to know the answer to 


does the mis-match alter frequencies, (Tone), or just amplitudes?

[dwinstonwood]

does the mis-match alter frequencies, (Tone), or just amplitudes?

Good question. I guess I thought that the coupling cap and the next tube's grid leak resistor created a high-pass filter, and that is what messes with frequencies. 

You see the "10X rule" mentioned a lot on the internet. So, if a 12AX7 has an output impedance of 39K, then the next tube (or, tone stack) should have an impedance of at least 390K-ish? A typical 1M grid leak would then ensure good bridging, yes?

Now, assuming I have a rudimentary grasp of that, is there an easy way to calculate the load impedance of a tone stack? I often see tone stacks described as being a "heavy load." HBP, from your previous post I take that to mean that they present a smallish resistance/impedance?

Thanks.

[tubeswell]

load impedance of a tone stack? ... "heavy load." ... present a smallish resistance

Yes. Small resistance between signal source and ground (and/or HT) = a 'heavy' load (i.e. signal suffers from high signal impedance)


10k source, 100k load = 100k/(10k + 100k) = 9% signal attenuation

10k source, 10k load = 10k/(10k + 10k) = 50% signal attenuation

100k source, 10k load = 10k/(100k + 10k) = 91% signal attenuation


To use the spider web analogy, taking a big diameter source web (with long radial strands*) and a stick connecting from the centre of that** to the centre of another smaller diameter load web (with short radial strands). You push the middle of the 'source web' harder*** (than you would otherwise) because the 'load web' acts as a brake on the 'back and forth' movement**** of the source web. (Whereas, if you swapped the webs around, the effort is going into pushing the source web and the load web doesn't absorb as much overall energy. That is to say, there is always 'some' signal impedance at the source, but if its lower than the load impedance, more of the signal strength will be preserved. (And if the load impedance is lower than the source impedance, more signal strength will be lost across the load). That's how I sees it anyway. YMMV

*i.e. higher DC resistances
**the coupling mechanism
***supplying the source with more energy/current
****lower voltage swings

[acheld]

Interesting discussion.   I have always had a rough time understanding source impedance    If something is a source for voltage and current, does it really have impedance?   It certainly does have the ability to drive a load with a certain voltage and current, but it (the source) will be either voltage limited, current limited, or both.

So, to me, and probably me only, a source impedance is what the source "expects" to see as a load; as an example, my lawn mower battery is able to supply 2 amps at 12VDC for an hour, but if you reduce the load (eg resistance) so as to increase the current flow, the battery is unable to supply the electrons and voltage falls.

So, source impedance is a design parameter that is looking for what the source needs to supply to maintain "expected" (or design) current and voltage.

I hope this is not too far off . . .

[PRR]

You know how a power saw, or amp, is weak at the far end of a long extension power cord? That's a form of source impedance. Not that the cord "expects" any specific load, just that it is not an infinite power source.

[PharmRock]

Thanks everyone for the responses and explanations.  The analogies and step-by-step breakdown are very helpful.  Just learning the association between load and current makes things a lot more clear, particularly the tone stack interaction....that's what always through me off when trying to understand this.  I probably learned more on this subject by reading your posts than I have in all the other sources combined.  Very practical.
thanks again. 

[pdf64]

… if you reduce the load (eg resistance) so as to increase the current flow, the battery is unable to supply the electrons and voltage falls.
…

A heavier load requires more power to work it. For a given voltage, that means more current. To get more current, the load resistance / impedance must be reduced.
Reducing the load would normally mean that the load resistance / impedance went up.
So ‘load’ and ‘load resistance’ are kinda the inverse (?) of each other.