Don't overlook the importance of these controls.
It is often said that one of the best mods to an amp is to try new speakers. This is for two reasons: first is the obvious tonal change. Second, and oft overlooked, is the different load the speaker puts on your amp.
A tube amp is actually a transconductance amplifier, meaning that it varies the output current in response to a change in input voltage. So the actual voltage at the speaker is a function of the speaker's impedance. A solid-state amp, OTOH, is usually nearly independent of speaker impedance.
The net result of all this is that the amp's voicing will change with the speaker impedance. The Axe-Fx allows you to alter the speaker impedance. The default values are consistent with the speaker most commonly used with the amp being modeled.
Here are two Jensen speakers:
The top trace in each is the frequency response, the bottom trace is the impedance (scale on right).
They are both 8-ohm, 12" speakers. However the speaker impedances are quite different. In the first the low-frequency resonance is around 100 Hz. In the second that resonance is around 80 Hz. In the first, the impedance is roughly 16 ohms at 2 kHz, in the second it's around 12 ohms. The Q of the LF resonance on the first is noticeably lower (broader response).
Adjusting the low-frequency resonance is easy. You simply move the frequency to match and adjust the amount and Q as desired.
High-frequency resonance is a little more tricky. You'll notice in the Axe-Fx that the curve does not look like the graphs. The graphs increase smoothly whereas the Axe-Fx has a peaking response. That's because the speaker impedance reacts with the transformer and power tubes to form a higher-order network. The general rule-of-thumb is to use your ears. Lower the frequency and Q to get more midrange. Increase them to get more "chime".
These controls are so powerful that I made a separate GUI page and graph for them. Don't overlook their importance in your tone-shaping quests. I always start with these before resorting to the graphic EQ or EQ blocks (in fact I almost never use any EQ). If you have some favorite speakers you can try to find published impedance data to help match the response. I use a device called a "Woofer Tester" to measure the impedance of speakers. About a $100. Probably not something for the casual user though.
It is often said that one of the best mods to an amp is to try new speakers. This is for two reasons: first is the obvious tonal change. Second, and oft overlooked, is the different load the speaker puts on your amp.
A tube amp is actually a transconductance amplifier, meaning that it varies the output current in response to a change in input voltage. So the actual voltage at the speaker is a function of the speaker's impedance. A solid-state amp, OTOH, is usually nearly independent of speaker impedance.
The net result of all this is that the amp's voicing will change with the speaker impedance. The Axe-Fx allows you to alter the speaker impedance. The default values are consistent with the speaker most commonly used with the amp being modeled.
Here are two Jensen speakers:
The top trace in each is the frequency response, the bottom trace is the impedance (scale on right).
They are both 8-ohm, 12" speakers. However the speaker impedances are quite different. In the first the low-frequency resonance is around 100 Hz. In the second that resonance is around 80 Hz. In the first, the impedance is roughly 16 ohms at 2 kHz, in the second it's around 12 ohms. The Q of the LF resonance on the first is noticeably lower (broader response).
Adjusting the low-frequency resonance is easy. You simply move the frequency to match and adjust the amount and Q as desired.
High-frequency resonance is a little more tricky. You'll notice in the Axe-Fx that the curve does not look like the graphs. The graphs increase smoothly whereas the Axe-Fx has a peaking response. That's because the speaker impedance reacts with the transformer and power tubes to form a higher-order network. The general rule-of-thumb is to use your ears. Lower the frequency and Q to get more midrange. Increase them to get more "chime".
These controls are so powerful that I made a separate GUI page and graph for them. Don't overlook their importance in your tone-shaping quests. I always start with these before resorting to the graphic EQ or EQ blocks (in fact I almost never use any EQ). If you have some favorite speakers you can try to find published impedance data to help match the response. I use a device called a "Woofer Tester" to measure the impedance of speakers. About a $100. Probably not something for the casual user though.