Explain impedance?

[FEBass]

I was looking at the manual for a tech 21 amp, and it says "INPUT JACK Faithful to the original designs of traditional tube amplifiers, this piezo-friendly 1/4" Input is high impedance 4.7 MΩ. The typically additional low sensitivity input, however, is not needed due to the Pad switch. When the Pad switch is engaged, the input changes to low impedance, approx 10 KΩ, which helps decrease line noise interference and protects the pre-amp from being overloaded by high output, active pickups."

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?

 

[bucephylus]

I was looking at the manual for a tech 21 amp, and it says "INPUT JACK Faithful to the original designs of traditional tube amplifiers, this piezo-friendly 1/4" Input is high impedance 4.7 MΩ. The typically additional low sensitivity input, however, is not needed due to the Pad switch. When the Pad switch is engaged, the input changes to low impedance, approx 10 KΩ, which helps decrease line noise interference and protects the pre-amp from being overloaded by high output, active pickups."

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?

@agedhorse may have a better explanation; but, FWIW, here is my understanding.

Signal amplification involves a small signal (in this case, the bass output) affecting a larger DC bias through an amplification device - either a vacuum triode or a transistor. The DC bias is set by a power supply, and the input level is set by a bias resistance (impedance), in the simplest model. What you don’t want is the bias resistance to have such a low value that it participates in the input circuit / device that is driving the amplification stage. So, you typically want the input impedance of the amplification bias to be two or three orders of magnitude larger than the impedance of the device that is feeding into it.

That’s all the spec description is saying, that their amp adjusts the input bias when the pad gets engaged.

Hope that makes sense.

 

[Wasnex]

I was looking at the manual for a tech 21 amp, and it says "INPUT JACK Faithful to the original designs of traditional tube amplifiers, this piezo-friendly 1/4" Input is high impedance 4.7 MΩ. The typically additional low sensitivity input, however, is not needed due to the Pad switch. When the Pad switch is engaged, the input changes to low impedance, approx 10 KΩ, which helps decrease line noise interference and protects the pre-amp from being overloaded by high output, active pickups."

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?

High impedance typically goes with higher sensitivity.

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.

 

[agedhorse]

A simple explanation:

Gain and impedance are not terribly related in technology. There are plenty of examples of low impedance high gain circuits as well as high impedance low gain circuits. Therefore each of the terms needs to be considered independently.

Gain: (in terms of this discussion) is simply the increase in output voltage compared with the input voltage to a circuit. The higher the gain, the higher the ratio of output to input voltage. That's it.

Input Impedance: (in terms of this discussion) is simply the load that the circuit places on the source.

Impedance is simply the AC equivalent of DC resistance, the reason for the different terms is that the resistance of some elements (specifically capacitors and inductors) changes with frequency, therefore impedance itself is not a single number but a set of numbers that vary with frequency. Since DC has no frequency component, there is just a single number. Impedance in math terms consists of the fixed DC resistance plus the varying AC component, Z = (R +jX) in Cartesian form.

 

[walterw]

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?

because it's not "resistance".

it might make more sense to think of the high impedance input as "high impedance capable", that is, able to take a more "delicate" high impedance signal without a bunch of loss.

the low impedance setting means that it needs to get a lower impedance signal or there will be loss.

the tradeoff is that typically low impedance signals are louder and "stronger", so they might be too much for the high impedance input and clip stuff.

 

[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.

Fixed that for you.

- John

 

[bucephylus]

Input Impedance: (in terms of this discussion) is simply the load that the circuit places on the source.

Andy, that is strictly correct; but, also that explanation doesn’t really clarify for a non-engineer what the interaction is, nor the importance of the input impedance.

And, in fact, the whole concept of input impedance is somewhat counter intuitive. What I mean is that higher input impedance within the amp actually allows more sensitive signal input; since lower input signals usually indicate higher device impedance. From your explanation, a non-engineer might rightly wonder why a smaller signal level source would benefit by seeing a higher input impedance.

The answer lies in which part of the the overall circuit / signal actually gets amplified. Really small amplitude signals “see, interact, and respond” to lower input impedances more than higher input impedances. So, higher DUT (device under test, in this case your bass pickup) impedances need higher input impedances for the amplification to discriminate the source vs simply amplifying its own internal impedance. I had to struggle with this in a past life trying to characterize high performance, low dielectric constant / dissipation factor materials; which have high impedances. Had to have really high input impedances in the instrumention input to avoid measuring the instrument. You generally want a couple of orders magnitude larger input impedance than the DUT impedance.

Sorry for TL;DR. It’s a non-trivial concept to digest.

 

[DigitalMan]

@agedhorse may have a better explanation; but, FWIW, here is my understanding.

Signal amplification involves a small signal (in this case, the bass output) affecting a larger DC bias through an amplification device - either a vacuum triode or a transistor. The DC bias is set by a power supply, and the input level is set by a bias resistance (impedance), in the simplest model. What you don’t want is the bias resistance to have such a low value that it participates in the input circuit / device that is driving the amplification stage. So, you typically want the input impedance of the amplification bias to be two or three orders of magnitude larger than the impedance of the device that is feeding into it.

That’s all the spec description is saying, that their amp adjusts the input bias when the pad gets engaged.

Hope that makes sense.

Using a bias to explain an impedance is like using dark matter to explain gravity.

 

[agedhorse]

Correct, I glossed over why source and load impedances are chosen the way they are. This is much more difficult than the concept of impedance itself because there are rules and exceptions, where the exceptions are realy just different rules.

As a generality (these days, it wasn't always true), the load on a source should be 10 times the source's impedance to reduce interactions between the two. To clarify,. when I say 10 times the source impedance, that means the number is 10x higher but the action of the load itself is 1/10th.

When designing bass amps for example, impedance interactions can be used as a tool to generate or create non-linear voicings and interesting interactions that can work towards a tone or texture goal. This starts getting very complicated because there are a lot of interactive tradeoffs involved.

 

[bucephylus]

Using a bias to explain an impedance is like using dark matter to explain gravity.

OK, well since we can’t explain gravity yet, I’m not exactly sure how to interpret your post.

Using a bias to explain the ROLE of the biasing impedance makes sense to me. If you have a better way to explain it, I’m all ears.

 

[DigitalMan]

OK, well since we can’t explain gravity yet, I’m not exactly sure how to interpret your post.

Using a bias to explain the ROLE of the biasing impedance makes sense to me. If you have a better way to explain it, I’m all ears.

I was basically saying that bias is something a majority of folks around here know absolutely nothing about, whereas impedance is at least understood by most at a simplistic level. In other words, it’s like using trigonometry to explain arithmetic.

 

[AndyPanda]

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).

 

[bucephylus]

I was basically saying that bias is something a majority of folks around here know absolutely nothing about, whereas impedance is at least understood by most at a simplistic level. In other words, it’s like using trigonometry to explain arithmetic.

Gotcha. Point taken.

 

[agedhorse]

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).

Independent of impedance, the gain is also lower on the active or lower impedance input.

Important clarification here, the input impedance does not have to be lower on the active input. For example, the input impedance does not change on the Subway amps when switched between active and passive.

 

[Wasnex]

Independent of impedance, the gain is also lower on the active or lower impedance input.

Important clarification here, the input impedance does not have to be lower on the active input. For example, the input impedance does not change on the Subway amps when switched between active and passive.

That a bit unusual in my experience, but not totally unheard of. For example, the Sadowsky SA200 maintains a 1 meg input impedance regardless of whether the -6dB pad is engaged or not. Of course the pad is implemented in a totally different way on this amp.

 

[Smooth_bass88]

here we go again

 

[Russell L]

I know how to use my equipment, but when it comes to stuff like this I am lost trying to understand it. I read this thread and can't understand it. I'm the same way about computers. When I read the definitions I don't even understand what is being said.

Very frustrating for me, always. It's why I envy you cats who know this stuff so well.

 

[agedhorse]

That a bit unusual in my experience, but not totally unheard of. For example, the Sadowsky SA200 maintains a 1 meg input impedance regardless of whether the -6dB pad is engaged or not. Of course the pad is implemented in a totally different way on this amp.

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

 

[seamonkey]

I was looking at the manual for a tech 21 amp, and it says "INPUT JACK Faithful to the original designs of traditional tube amplifiers, this piezo-friendly 1/4" Input is high impedance 4.7 MΩ. The typically additional low sensitivity input, however, is not needed due to the Pad switch. When the Pad switch is engaged, the input changes to low impedance, approx 10 KΩ, which helps decrease line noise interference and protects the pre-amp from being overloaded by high output, active pickups."

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?

Tech-21 probably uses FETs and their own design as they know how to design pre-amps and inputs.

It's probably like:
Link Removed

Adjust the resistor values and circuit gain to match.
With the switch up as shown it in High sensitivity mode, the input impedance is 1M. 1M is much higher than 68k to the grid so the grid gets to see nearly all the input voltage.
"Pad" switch On would be Down in this case. It sets the input impedance to two 68k resistors in series, and the grid sees the middle of the voltage of these resistors.

When you think about it "High Sensitivity" means passing through as much voltage from the source as possible.

 

[Rick James]

There are two questions here. One is why an amp active input has lower impedance than a passive input. That's because the output of an active bass is lower impedance than the output of a passive bass. The higher the output impedance of the bass the higher the input impedance of the amp must be to prevent signal losses. The other question is why the active input has less gain than the passive input. That's because an active bass output is higher level than a passive bass output, so it doesn't need as much gain from the amp. This is why your favorite band should be K.I.S.S.