Impedance Curves are my new Go-To for Tone Shaping

[mr_fender]

Measuring the impedance curve requires specific hardware (LCR meter), so it's unlikely that it would get added to any existing Fractal products.

Something like this would do it for you:

https://www.daytonaudio.com/product/1650/dats-v3-computer-based-audio-component-test-system.
Amazon product ASIN B07V821D2B

 

[FractalAudio]

Very interesting debate. Here's my two cents (and I might be very wrong), but as I understand it, the IC curve is not in fact static, but variable in a real life setup? Let's say you play thru a real Mesa rectifier & a 4x12 cab. Now when you chunk at higher volumes (palmmute on the lows), I would assume the resistance from the cab would change, as you are pushing the speakers/coils, thus compressing the air harder in the cab. Or if you downtune you gtr, you push other frequencies thru the cab when playing the lows, again making the speaker loading the power section of the real tube amp differently.

At least, that's how I perceive it, any thoughts?

That's not correct.

Air is very linear.

The impedance curve changes, but not for those reasons. It changes because the amount of voice coil in the magnet gap changes as the speaker moves. When the speaker moves one way more windings are in the gap and vice-versa. This changes the effective voice coil inductance.

Also, the low frequency resonance changes (slightly) as a function of displacement. This is due to the compliance of the suspension changing with displacement. Compliance also changes due to a change in Force Factor (B*l) which is a function of displacement.

There are no nonlinear effects due to "chunking at higher volumes". The cab does not have "resistance".

FWIW, the impedance curves in the Axe-Fx are not static. They are dynamic. You can vary the amount of impedance shift using the Speaker Compliance parameter.

 

[Jipps0525]

Of course you need a good IR to get you in the ballpark.

But the amount of shaping you can do with the impedance curve in the Amp Speaker page is astounding. Also Negative Feedback, which basically just throttles back the impedance curve....

I was dialing in a Morgan AC20 style tone and wanted more dynamic low end response, especially on mid frequency notes, like a 12th fret D. I wanted it to pop like Fender Deluxes do, for instance. It was already close, but I wanted a little more low end and adding bass EQ was making it muddy.

So I dove into impedance curves. I was using the 1x12 AC20 curve but ended up in the 1x12 Deluxe Reverb, Brit G12H75, or possibly the 1x12 Alnico Blue (strongest bass response, but gets muddy, oddly similar to how my real Alnico creams were).

The great part is that I play though amp/cab, and send FOH my IR. So since the impedance is in amp block, this affects both my amp/cab and FOH signals at the same time. I dialed in the tone through amp/cab, and it translated very very well to headphones.

Plus obviously the Impedance Curve can be tweaked in the speaker page as well. So if something isn't exactly what you want, fix it there.

I'm sure many already do this. But it was an epiphany for me.

So you would or wouldnt recommend trying to mess with the impedance curve with the factory cabs?

 

[JRod4928]

That's a great question. I want to say volume makes a difference because Air moving in a cab is still moving at speed of sound regardless of volume (amplitude)..... if the reflecting air waves are at the same resonant frequency as the speaker itself, it will amplify the speakers physical amplitude, this creating inductance and higher impedance? Maybe the affect from cab resonance isn't as pronounced as speaker resonance so it isn't a big deal? Cliffs post from 2013 doesn't discuss this much. Maybe avoiding confusion? Wondering if the ICs are just a snapshot at some volume, and if it changes by volume as a result of the cab contribution, as you said

That's not correct.

Air is very linear.

The impedance curve changes, but not for those reasons. It changes because the amount of voice coil in the magnet gap changes as the speaker moves. When the speaker moves one way more windings are in the gap and vice-versa. This changes the effective voice coil inductance.

Also, the low frequency resonance changes (slightly) as a function of displacement. This is due to the compliance of the suspension changing with displacement. Compliance changes due to a change in Force Factor (B*l) which is a function of displacement.

There are no nonlinear effects due to "chunking at higher volumes". The cab does not have "resistance".

FWIW, the impedance curves in the Axe-Fx are not static. They are dynamic. You can vary the amount of impedance shift using the Speaker Compliance parameter.

That's what I was hinting at, but explained poorly. Glad FAS weighed in, I think this is a cool discussion and figured his expertise would be helpful.

Here's @FractalAudio discussing speaker compliance: https://forum.fractalaudio.com/threads/axe-fx-iii-firmware-2-00-public-beta-2.144615/post-1711452

Cab's affect on Impedance Curve... and 'free air'.

Jay Mitchell on TGP also discusses this, and gives kudos to FAS for being able to model this.
https://www.thegearpage.net/board/i...ering-a-return-to-tubes.2272504/post-32922032

(Didn't know about all the Jay drama)

 

[JRod4928]

So you would or wouldnt recommend trying to mess with the impedance curve with the factory cabs?

I think it depends on your usage....

If you use a REAL cab with a Solid State power amp, I'd personally experiment with changing IC to an IC curve that more closely matches the Cab/Speaker that I'm using (ie: use a 1x12 IC if you play through a 1x12, even if the default IC for (say) the SOLO 100 Lead model is a 4x12 SOLO100 for instance).... This should adjust the virtual power amp to more closely behave as it would if it were connected to your actual cab....

However some may suggest to leave it alone because it deviates from how the amp, and model, was designed to operate. So, take my suggestion with a grain of salt.

In any case, if it doesn't sound or feel right, just use whatever works.

 

[Jipps0525]

I think it depends on your usage....

If you use a REAL cab with a Solid State power amp, I'd personally experiment with changing IC to an IC curve that more closely matches the Cab/Speaker that I'm using (ie: use a 1x12 IC if you play through a 1x12, even if the default IC for your amp is a 4x12).... This should adjust the virtual power amp to more closely behave as it would if it were connected to your actual cab....

In any case, if it doesn't sound or feel right, just use whatever works.

I just use my fm9, nothing external haha

 

[York Audio]

It’s cool that we’re talking about this. I’m learning that the IC plays a MASSIVE role in getting a model to sound closer to a real amp when A/B’ing them going through a live cabinet.

My dream wish is that the Axe could somehow measure our personal cabinets’ impedance curves and store them in User slots… or store them as “corrective” curves that could be used in parallel with a factory base curve like the LB-2. A friend of mine does that with the Suhr Reactive Load to match his personal cabinet and said the results were amazing.

 

[JRod4928]

I just use my fm9, nothing external haha

I'd leave the IC's as is, personally, but feel free to play with the settings if you want - maybe you'll find something you like.

The 'epiphany' I referred to in the OP was mainly in regard to adjusting IC's to improve my real amp/cab/AXE FX experience. Basically, the axe fx virtual power amp was set to experience a virtual impedance that was a mismatch with my actual cab. @FractalAudio had the foresight to know that this was an issue, so they made the option to set a global IC.... for anyone using headphones, this issue doesn't exist. For those using FRFR, I guess you could dial in an IC that matches the FRFR resonant frequency, but I'm not sure that's ideal - out of my area of experience.

 

[BBN]

Been messing quite a bit with ICs and finding is such a powerful parameter to getting the perfect 'feel' from a cab.
I would love to be able to 'save' my IC settings (create our own IC presets, so to speak).
That would allow us to use the Global feature and not have to copy the settings into each preset/amp.

 

[BBN]

It’s cool that we’re talking about this. I’m learning that the IC plays a MASSIVE role in getting a model to sound closer to a real amp when A/B’ing them going through a live cabinet.

My dream wish is that the Axe could somehow measure our personal cabinets’ impedance curves and store them in User slots… or store them as “corrective” curves that could be used in parallel with a factory base curve like the LB-2. A friend of mine does that with the Suhr Reactive Load to match his personal cabinet and said the results were amazing.

That would be awesome.

 

[Rasmus Toftlund]

That's not correct.

Air is very linear.

The impedance curve changes, but not for those reasons. It changes because the amount of voice coil in the magnet gap changes as the speaker moves. When the speaker moves one way more windings are in the gap and vice-versa. This changes the effective voice coil inductance.

Also, the low frequency resonance changes (slightly) as a function of displacement. This is due to the compliance of the suspension changing with displacement. Compliance also changes due to a change in Force Factor (B*l) which is a function of displacement.

There are no nonlinear effects due to "chunking at higher volumes". The cab does not have "resistance".

FWIW, the impedance curves in the Axe-Fx are not static. They are dynamic. You can vary the amount of impedance shift using the Speaker Compliance parameter.

Brilliant. Thank you for clearing that up.

 

[Chris Hurley]

Of course you need a good IR to get you in the ballpark.

But the amount of shaping you can do with the impedance curve in the Amp Speaker page is astounding. Also Negative Feedback, which basically just throttles back the impedance curve....

I was dialing in a Morgan AC20 style tone and wanted more dynamic low end response, especially on mid frequency notes, like a 12th fret D. I wanted it to pop like Fender Deluxes do, for instance. It was already close, but I wanted a little more low end and adding bass EQ was making it muddy.

So I dove into impedance curves. I was using the 1x12 AC20 curve but ended up in the 1x12 Deluxe Reverb, Brit G12H75, or possibly the 1x12 Alnico Blue (strongest bass response, but gets muddy, oddly similar to how my real Alnico creams were).

The great part is that I play though amp/cab, and send FOH my IR. So since the impedance is in amp block, this affects both my amp/cab and FOH signals at the same time. I dialed in the tone through amp/cab, and it translated very very well to headphones.

Plus obviously the Impedance Curve can be tweaked in the speaker page as well. So if something isn't exactly what you want, fix it there.

I'm sure many already do this. But it was an epiphany for me.

In a way, its better than EQ- I've found reducing the high frequency resonance cuts harshness that I have a hard time matching with EQ...

 

[JRod4928]

In a way, its better than EQ- I've found reducing the high frequency resonance cuts harshness that I have a hard time matching with EQ...

If I understand properly, reducing HF Resonance reduces power amp high frequency clipping - so harshness should improve - so that's cool. Side note - I think reducing the Presence knob does the same thing (edit: and for LF Resonance, Depth knob)

 

[Stratoblaster]

You can vary the amount of impedance shift using the Speaker Compliance parameter.

Does the Speaker Compliance parameter still have that shift if the Speaker Compression is at 0.00/disabled?

 

[Chris Hurley]

If I understand properly, reducing HF Resonance reduces power amp high frequency clipping - so harshness should improve - so that's cool. Side note - I think reducing the Presence knob does the same thing (edit: and for LF Resonance, Depth knob)

I was thinking about that today- whether or not they are the same thing. Presence and Depth are reducing the amount of negative feedback that reduces gain and distortion from the power amp. That's definitely in the ballpark of the same thing as the impedance curve.

They are not aligned in terms of the frequencies that they affect, however.

 

[State of Epicicity]

The inclusion of the sound deadening material in my Mesa 1x12 Widebody makes me think there are details that top manufacturers like Mesa are aware of, that others aren't.

I would totally call or write them and ask why that’s there. You might actually get a cool explanation!

That's not correct.

Air is very linear.

The impedance curve changes, but not for those reasons. It changes because the amount of voice coil in the magnet gap changes as the speaker moves. When the speaker moves one way more windings are in the gap and vice-versa. This changes the effective voice coil inductance.

Also, the low frequency resonance changes (slightly) as a function of displacement. This is due to the compliance of the suspension changing with displacement. Compliance also changes due to a change in Force Factor (B*l) which is a function of displacement.

There are no nonlinear effects due to "chunking at higher volumes". The cab does not have "resistance".

FWIW, the impedance curves in the Axe-Fx are not static. They are dynamic. You can vary the amount of impedance shift using the Speaker Compliance parameter.

Dude, thank you for this. I had no idea this was a dynamic parameter. If chugging doesn’t really exploit the dynamic nature of the curve, are there any particularly good kinds of licks or riffs to play or amps at particular settings where you can learn how to spot how to hear the dynamic effect of it? Sorry if my question might be off base.

Also @yek, could Cliff’s response here go in the Tech Notes section too, along with the wiki?

 

[sprint]

Also @sprint , I also have Mesa 112 Widebody Closed Back (ported) cab. Inside of mine, the walls of the cab are covered with synthetic cotton type sound-deadening material. I was surprised when I saw this, but I haven't been inside many high quality cabs before. Does yours have this, and anyone know if it's typical?

hey - yes, mine have the sound deadening material in them.

fyi - when I bought mine, there was a V30 option for them - the guy talked me out of special ordering 2 widebodys with v30s saying the C90 ones sounded better and were in stock. So I got those and for the next like 8 years didn't like them. Bought + sold a couple of amp heads thinking that was the issue but about a year ago realized, mostly from IR auditioning, that a lot of guitar speakers have this "loose coily sounding whistlely" thing going on that I hate - and that's what the Mesa Black Shadows have a lot of to my ear. V30s didn't seem to have this so I bought 2 V30s and installed those in my Mesa Wide body cabs last summer - big difference - so much better to my ear - should of stuck to my guns and got the V30s option at the start but I didn't know what I was doing at the time. Am building a 212 open back to house the C90s in a last ditch effort to save them from Long and McQuade trade-in.

 

[RDeraz]

The inclusion of the sound deadening material in my Mesa 1x12 Widebody makes me think there are details that top manufacturers like Mesa are aware of, that others aren't.... and makes me NOT want to change out the C90 speaker that it's loaded with because I'm assuming there's some intention behind Mesa selecting that speaker for this cab.

My guess would be to get rid of standing waves in the cab. Or to kill a resonant freq. in the cab.

 

[JRod4928]

hey - yes, mine have the sound deadening material in them.

fyi - when I bought mine, there was a V30 option for them - the guy talked me out of special ordering 2 widebodys with v30s saying the C90 ones sounded better and were in stock. So I got those and for the next like 8 years didn't like them. Bought + sold a couple of amp heads thinking that was the issue but about a year ago realized, mostly from IR auditioning, that a lot of guitar speakers have this "loose coily sounding whistlely" thing going on that I hate - and that's what the Mesa Black Shadows have a lot of to my ear. V30s didn't seem to have this so I bought 2 V30s and installed those in my Mesa Wide body cabs last summer - big difference - so much better to my ear - should of stuck to my guns and got the V30s option at the start but I didn't know what I was doing at the time. Am building a 212 open back to house the C90s in a last ditch effort to save them from Long and McQuade trade-in.

I can't say I have heard the same thing you're hearing with the C90's, but I'm glad you ended up with your preferred speakers. I have more experience with changing speakers than I do cabs, From greenbacks to creambacks to Alnico Gold, Creams, etc... they all have their strengths. But for what I want this 1x12 to do (which is sound huge and provide strong punch for rock tones) - the cab and the C90 do really well..... for an open back Fender/Vox style cab, I'd probably go with Alnico. They're sweet.

 

[sprint]

I can't say I have heard the same thing you're hearing with the C90's, but I'm glad you ended up with your preferred speakers. I have more experience with changing speakers than I do cabs, From greenbacks to creambacks to Alnico Gold, Creams, etc... they all have their strengths. But for what I want this 1x12 to do (which is sound huge and provide strong punch for rock tones) - the cab and the C90 do really well..... for an open back Fender/Vox style cab, I'd probably go with Alnico. They're sweet.

I'm really a newb at speaker swaps, and really I don't much want to go there - just wanted at least one pair of real guitar cabs that sounded right to me. Speaker choice is very subjective - many rave about the C90s - I know they are a good quality speaker (actually they are the more pricey speaker compared to v30).

Am planning another mad experiment n try some Celestion F12-X200 FRs in there since the dimensions of the Mesa Wide body including size and position of the port are quite similar to Celestion's cab spec for the x200s.

I'm not expecting much from the C90s in an open back cab as they are probably not meant for that - we'll see