I usually try to keep the technical information pretty high-level on this blog. But I think in this case it's helpful to maybe go down one level.
Guitarists quickly comes to grips with the idea that every speaker cabinet has an impedance and that you need to match an amp's impedance to the cab it's driving.
But what exactly is impedance?
Impedance is the resistance of the speaker to current flow. In other words, how much the speaker "pushes back" on the amp. Since impedance is a measure of how much the speaker resists current, the lower the impedance in ohms, the more power the speaker will draw from your amp.
As you know, every speaker has a rated impedance, which would seem to mean that speakers have a fixed impedance like we all have a fixed number of fingers. But that isn't the case. Speaker impedance is variable like the number of guitars a player has, which can vary from day to day and paycheck to paycheck. A speaker's impedance rating is actually a "nominal" rating – it's a calculated approximation. In reality, a speaker's impedance varies by the frequencies it is reproducing. As you play your guitar, the speaker's impedance is constantly changing in response to the frequencies present in audio you're feeding to it.
A more accurate representation of speaker impedance is the "impedance curve" – a chart like the one above that shows how the impedance of a speaker (in this case, a Celestion G12H Heritage) varies over the audio frequency range.
There are two main factors involved in this varying impedance:
- Every speaker has a resonance frequency where there is a very large spike in impedance. On the left side of the chart at the top of this post, you can see the resonance frequency of the G12H. It is the steep mountain in the curve centered at around 55Hz.
- The voice coil's inductance increases at higher frequencies which also causes the impedance to go up. In the chart above, that slope going up on the right-hand side shows the impedance climbing at the higher frequencies.
In addition, impedance also varies based on the cabinet that the speaker is put in. So the chart looks different when you mount that G12H in an open-back 1x12, and different again when you put it in a closed-back 4x12.
So, bottom line, impedance is actually a much more complicated phenomenon than you'd guess from merely looking at a speaker's impedance rating. More importantly, the impedance curve of a speaker and cab affects the tone and response of an amp a great deal. It's one of the factors that make speakers and cabs sound so much different from each other.
Resistive vs Reactive Loads
In electronics, a "load" is a part of a circuit that dissipates power and does some work. In a guitar amp, the load is the speaker. It's using the power from the tube amp to move the voice coil and produce sound. It's not a 100% efficient system so some of the power generates heat. The TAE replaces the speaker by acting as a substitute load for the amp, dissipating almost all of the power as heat. There are two types of loads for this kind of thing: resistive and reactive.
Resistive loads, like the Weber MASS or the Dr. Z Airbrake, have a fixed impedance that doesn't vary by frequency – it would be a flat line if depicted in an impedance curve. A flat impedance curve doesn't look much like a real speaker's impedance curve. This is why the more you attenuate an amp with a resistive load, the more the tone degrades. The resistive load makes the amp sound and respond very differently from a real speaker.
And that's the innovation of the reactive load, such as the Boss Tube Amp Expander or the Suhr Reactive Load. A reactive load presents a more realistic impedance curve that varies by frequency like a real speaker and cabinet. And that seriously changes the sound of the amp. Consequently most players agree that reactive loads sound a lot better than resistive loads
Resonance-Z & Presence-Z
So, reactive loads are a very good thing. They're much better at preserving the sound and feel of an amp. As we talked about already, every speaker has a unique impedance curve, and the enclosure you put that speaker in changes that curve as well. Most reactive loads have a single impedance curve so they can only emulate one speaker/cab configuration. Typically the manufacturer designs the reactive load to have an impedance curve that emulates their concept of the "ideal" speaker/cab combination, or represents a good all-around compromise that will work with a wide variety of amplifiers.
The Tube Amp Expander's Resonance-Z and Presence-Z controls allow you to adjust the impedance curve of TAE's reactive load. You can adjust the impedance at the resonance frequency and at the high-end frequencies where voice coil inductance would rise with impedance on a real speaker. These are the areas in the impedance curve that speakers will show the most variance from one another.
The Resonance-Z and Presence-Z controls allow you to tune the impedance curve of the TAE's reactive load to better match particular speaker/cab combinations, and to sculpt your overall tone.
Tips for Settings
Unfortunately, there's no information source that I'm aware that tells you the impedance curve of common guitar speaker/cabinet combinations, unfortunately. Manufacturers sometimes publish impedance curves for speakers, but I've never seen them for guitar cabinets. So at the end of the day, your ears will need to guide you.
On the last page of the the user manual, Boss provides some recommended settings to use with the IRs built into the TAE. This make sense because you want to tune the impedance curve to respond the way that the selected cabinet would respond. These are good settings, but you should also feel free to vary from them if you like the tone better with different settings. It won't cause any harm and at the end of the day, you want to optimize around what sounds best.
How should you set the Resonance-Z and Presence-Z controls when you're using the TAE as an attenuator into a real speaker cabinet? How should you set them when you're using a third party IR? In both cases, this is what I suggest: From Boss' recommend settings, choose with the one that is closest to the cabinet or IR you're actually using. This will theoretically produce a sound closest to your actual cab or IR. That works as a starting point. From there, play with the settings to see if any of them improve the tone.