Speaker Resonance Controls

FractalAudio

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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:
p12n-nb_speakerg.jpg

mod12-50_speakerg.jpg


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.
 
Being ignorant of all things concering resonance, I am under the assumption that this feature is useless to those of us running through, say, Adam A7 monitors.

Or is there reason to hurt my brain trying to understand?
 
I've been kind of shy in asking about this. Thanx for taking the time to explain. I still don't really know what I'd be listening for. I've been timid and have avoided tweaking these parameters so far.

Let's see if I understand you properly... Are you saying that these knobs will allow me to manipulate the 'virtual' impedance of the speaker load in the power section? How does this differ from the transformer match? (another parameter I've delayed getting to know). They both seem to deal with the way the power section interacts with speakers.


I'm a high gain djent djent meedley meedely meedely type player...

Sent from my iPod.
 
I've been kind of shy in asking about this. Thanx for taking the time to explain. I still don't really know what I'd be listening for. I've been timid and have avoided tweaking these parameters so far.

Let's see if I understand you properly... Are you saying that these knobs will allow me to manipulate the 'virtual' impedance of the speaker load in the power section? How does this differ from the transformer match? (another parameter I've delayed getting to know). They both seem to deal with the way the power section interacts with speakers.


I'm a high gain djent djent meedley meedely meedely type player...

Sent from my iPod.

Yes, the speaker resonance stuff changes the virtual impedance seen by the virtual power amp.

Transformer matching sets how much impedance is reflected to the tubes. The higher the matching, the greater the impedance seen by the tubes. IOW, it's the OT primary impedance. Most amps are usually well matched or slightly under-matched. Some amps, like Trainwrecks, are over-matched. The primary impedance is quite high. This causes the power tubes to go into clipping easier.
 
Actually I would say that as you are not commited to a particular speaker but have the chance to elect from the list of virtual ones, it´s even more useful, isn´t it?
 
Thanks for the lesson.

Maybe you could do one a week in a separate forum called "Axe Inner Workings" or something like that. I will try out the speaker resonance with confidence of outcome.
I too believe that some tutorials from Mr Chase would be great. Short lessons like this one, not necessarily at a weekly pace :)
Thanks.
 
Great lesson! Thank You!

It would be really nice (I know it would be time consuming) to see detailed practical explanation like this on other important features especially on the advanced sections.

Are the impedance values available for the current Cabs in Axe-FX (II)?

If the impedance info is not provided with an IR, can I still measure it somehow?
 
Thank you for the explanation and sometimes you could explain more stuff and tips to optimize the better use of the AXE FX.
 
(I hesitate to post this, as I am afraid I have not been able to express this properly, and/or without appearing insulting; neither are my intention)
I am curious to know why speaker RF is an Amp Block parameter rather than Cab Block?
Since *resonant frequency* is solely a *characteristic* of the speaker PLUS cabinet being used (many graphs are free-standing curves?), the logical placement would appear to be the Cab Block, but the interactive nature would demand a(n) "(interdependent, interactive) link" with the amp it is used with. Thus, Amp Block? Could the Amp Block not "hook" the Cab Block parameter(s) (and vice versa)?

I ask because with the (inherent) *static* nature of a speaker + cab IR, it would also seem logical, then, to have a base *resonance|impedance curve* for each Cab choice. I do realize this is not usually possible, and that, especially with various settings, the SRF will be far less impactful, but still, I am (ever) curious. The Amp Block does not currently allow one to (simply) load a speaker's RF curve when one changes choices in the Cab Block, and I have been unable to figure a manner of achieving this... as minimal an impact this may have.
 
Oh, duh. I just thought of one scenario that demands (for the most part) the Amp Block have speaker RF: external amp + cab use, when the power amp part of the Amp Block is used. Since the external cab/speaker(s) cannot *address* the Axe itself, the Axe is providing this facet via speaker RF. Regardless, I would still love ot be able to defeat the Amp Block speaker RF and then have a Cab Block speaker RF available. Wish list I suppose.. or is this a "will never happen" item?
 
Great info. :) Hopefully someone will compile a list of settings from various speakers. I'd also imagine this could be a great tool to tweek when creating your own ir's .
 
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