Explain impedance?

[tadawson]

The key is that the input impedance should not excessively load the source. Passive inputs are taking signals directly from magnetic pickups, which have very little 'oomph' and are subject to loading and coloration. Active instruments have a powered amp driving the line, so high input impedance isn't mandatory, but typically won't hurt. About the only downside I can possibly see with too high an imput impedance is more noise, since external influences won't be loaded down in the signal. But then again, if the source impedance of the active pre is low(as it should be), that's not really a factor. This is also one of the reasons that passive/high impedance setups can be more subject to noise.

Why anyone would consider bias having anything to do with this, as an EE, I also don't comprehend. Impedance is a concept that exists in the absence of any signal . . .

For more detail than most likely want on impedance, try this:

Complex Impedance

 

[agedhorse]

The term active and passive with respect to level have become blurred with the introduction of high output Neo magnet based passive pickups, which can be hotter than traditional active pickups. This is why I abandoned impedance lowering in the active position, at the expense of less than 1/2dB of noise penalty.

 

[Wasnex]

It depends on the intent of the circuit. The way I do it is becoming more common, it's more flexible.

I would think the way you do it is probably better . I do have a few other amps were input impedance is independent of the pad setting. But think it's a bit unusual for a tube amp like the SA200.

 

[Clipped]

It's when you can’t...

Hold on.

Never mind.

 

[megafiddle]

A good example of low level, low impedance, high gain, is the balanced dynamic microphone signal.

Bass amp inputs are designed for bass instruments, which typically fall into two types: passive and active.

Passive bass circuits have an impedance that can be in the tens or hundreds of kohms, depending on frequency. To prevent loading, the amp input for these type signals are typically 500K or greater. The signal level is also often low, and so the gain is also high.

Active bass circuits typically have relatively low output impedances, and only require about 50K or greater at the amp input. These active outputs are also hotter due to amplification at the source, and so require less gain at the amp input.

Low impedance at the amp input itself does not reduce noise in practice; once a low impedance source is connected, the total impedance at the input drops to lower than the input alone. The source impedance, being much lower, is then the dominant factor, not the input impedance.

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[FEBass]

I read the OP as just trying to understand why high impedance didn't mean a lower volume. I mean just going by the words ... "High Impedance" sounds like the signal from the pickup has to go thru something that is more difficult to go thru. But in this case (a bass guitar pickup seeing the impedance at the input of an active device) the impedance in question is how difficult it is for the signal from the pickup to get thru to ground (shorting out that bass pickup to ground) so a higher impedance is less loss than a lower impedance would be. (less of the signal from the pickup being shorted out to ground).

I think, as long as this is accurate, this is the best answer for my level of understanding. I should have been clear in my original post that I'm just in the beginning stages of learning about electronics. I have a cursory understanding of the basic functions of components like resistors, tubes, and transistors. From this explanation, it sounds like my main misunderstanding was that I was thinking of impedance as the gatekeeper preventing part of the input signal from getting through to the preamp, so by that logic, a stronger signal would need a stronger gatekeeper to keep from overwhelming the preamp.

From the explanation above, my new understanding is that the impedance referenced in the brochure is actually the gatekeeper determining how much of the input signal "escapes" to ground instead of going through the preamp circuitry. In that case, stronger impedance means more of the signal goes through the preamp, which is what you want if the original signal is weaker.

Is that about right?

Also, thanks for the differentiation regarding resistance and impedance.

 

[agedhorse]

This world revolves around ratios, it would be helpful to learn Ohm's Law and how it applies to voltage dividers. The concept of a voltage divider is essential to understanding input and output impedances.

 

[FEBass]

This world revolves around ratios, it would be helpful to learn Ohm's Law and how it applies to voltage dividers. The concept of a voltage divider is essential to understanding input and output impedances.

I do know Ohm's law and I understand a voltage divider as being the voltage taken from between two resistors (R1 and R2). So...in this case, R1 would be the pickups and R2 would be the resistor to ground? And that would mean that a higher R2 value (insert Star Wars joke here) will result in a higher percentage of the the original voltage going into the preamp?

 

[agedhorse]

I do know Ohm's law and I understand a voltage divider as being the voltage taken from between two resistors (R1 and R2). So...in this case, R1 would be the pickups and R2 would be the resistor to ground? And that would mean that a higher R2 value (insert Star Wars joke here) will result in a higher percentage of the the original voltage going into the preamp?

For a given value of R1, but at some point the ratio becomes so high that any differences are too small to matter. This is why the ratio is important.

 

[FEBass]

For a given value of R1, but at some point the ratio becomes so high that any differences are too small to matter. This is why the ratio is important.

I understand that, but am I labeling R1 (pickups?) and R2 (connection to ground?) correctly when I think about my new understanding of impedance as it relates to my original question?

 

[agedhorse]

I understand that, but am I labeling R1 (pickups?) and R2 (connection to ground?) correctly when I think about my new understanding of impedance as it relates to my original question?

Correct, the impedance ratio defines the losses. The ideal pick-up (which obviously doesn't exist would be R1 = 0 ohms) and the ideal amplifier would be R2 = infinite ohms (which also doesn't exist).

The problem with ideals, is that whenever considering only one variable in the absence of all other (maybe more important) parameters, you neglect how the system operates in the big picture. Yes, that's a common theme on many forums, but the big picture is more important than any one single spec.

 

[beans-on-toast]

High impedance typically goes with higher sensitivity.

View attachment 3370487

I believe this would be termed a self biasing circuit. The control grid is basically held at ground potential and bias is developed by the positive voltage drop across the cathode resistor that occurs when the tube conducts. More current means more positive voltage on the cathode...at some point the current will stabilize because of the bias voltage.

(edited and thanks @JKos ) When the 1M pot is adjusted for less resistance, more of the input signal is shunted to ground and the input impedance goes down.

Current flow from ground through the control grid is considered insignificant in this type of circuit, so I don't believe the input impedance has an effect on tube bias. You may be getting confused by grid leak bias which operates off different principles.

Your input gain schematic reminds me of this old Ampeg preamp. Note on the top right, an optional matching circuit for crystal and magnetic pickups. There were no input impedance standards at the time, there weren't standardized pickups.

 

[FEBass]

Thanks everyone! I think my basic question is answered, and now I also have a whole bunch of more technical information I can digest in the future as my electronics knowledge deepens! You folks know your stuff!

 

[Wasnex]

Your input gain schematic reminds me of this old Ampeg preamp. Note on the top right, an optional matching circuit for crystal and magnetic pickups. There were no input impedance standards at the time, there weren't standardized pickups.

The amp that circuit is from is rather quirky IMHO. The Input Level acts somewhat like a trimmer with about 15dB worth of range. Then the Volume, which comes between the tone controls and PI, also works as a variable high pass filter. So basically you set the Volume to get the desired bass response and then hope there's enough range in the Input Level control to set the output of the amp where you want it.

 

[beans-on-toast]

The amp that circuit is from is rather quirky IMHO. The Input Level acts somewhat like a trimmer with about 15dB worth of range. Then the Volume, which comes between the tone controls and PI, also works as a variable high pass filter. So basically you set the Volume to get the desired bass response and then hope there's enough range in the Input Level control to set the output of the amp where you want it.

The volume after the James tone stage was common in the early B-15 as well. It acted as a voltage divider, people often like to turn up these amps so the arrow would be near the top in the schematic. A gain makeup stage the cathode follower would have required an extra tube. Despite any quirkiness those were good sounding little amps.

When analyzing an circuit in a quick and dirty way, it helps to look at different settings to simplify the circuits. Low frequency response where caps are open, high frequency response where they can be shorted.

But it isn’t unusual for controls to be interactive. For an extreme example, see the tweed Fender 5E3 Deluxe circuit.

 

[Wasnex]

The volume after the James tone stage was common in the early B-15 as well. It acted as a voltage divider, people often like to turn up these amps so the arrow would be near the top in the schematic. A gain makeup stage the cathode follower would have required an extra tube. Despite any quirkiness those were good sounding little amps.

When analyzing an circuit in a quick and dirty way, it helps to look at different settings to simplify the circuits. Low frequency response where caps are open, high frequency response where they can be shorted.

But it isn’t unusual for controls to be interactive. For an extreme example, see the tweed Fender 5E3 Deluxe circuit.

Here's a page that analyzes/discusses the channel interaction of the 5E3. How the 5E3 Works

I have also noticed quite a bit of interaction between unused channels on my JMI era Vox AC30.

 

[ThisBass]

I would have thought the high sensitivity input would have lower impedance. Can anyone explain why the low sensitivity input has so much less resistance?

Cause the input is meant to be used with active pickups or a battery powered on board EQ (for example).
As the input resistance is of rather lowish number (about 10kOhm) there is some current running through the instrument cable connected between instrument and amplifier thus the "increased" current helps to keep noise floor small.

for example in telecommunication industry the standard 0dBm level equals 1mW into 600 Ohm input impedance.

edit,
the more power is transfered from transmitter to receiver the better the SNR will be.