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Speaker Impedance Curve measurements

lorandkiss

New Member
Here are two of my cabinet SIC measurements in the format Cliff requested. It might be a good place for others to post their measurements too. I did these measurements early 2019, I felt back than that it's a missing thing for the accuracy of the AxFX III.

I attach also two measurements of my LoadBox. It's a ToneHound loadbox. I wasn't happy how it originally sounded so I modded it. Changed a coil and a capacitor so it would be closer sounding to my cabinets.

The measurements were done with REW v5.20, using the method described here:
roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html

The soundcard used is a RME Fireface802



The cabinets measured are:

- 2x12 Mesa Rectifier Horizontal (OS, not the compact)

- 2x12 Orange PPC212CB (closed back)

This thing is in its early stages, but I hope some day we'll have in our AxeFX a space to upload custom SIC and than use them in the Amp block. Some people might have non commercial cabs and maybe they would love to have their SIC inside their Axe.


2x12 Mesa Recto:
2x12 MesaRecto.jpg

2x12 Orange Closed Back:
2x12 Orange ClosedBack.jpg

Later edit:
I forgot to mention that my Orange cab has it's speakers re-wired in parallel. Originally they were is series giving 16Ohm, but I preferred to have them in parallel at 4Ohm.
 

Attachments

  • ToneHound Lite Modded.txt
    1.3 MB · Views: 15
  • ToneHound Lite Original.txt
    1.3 MB · Views: 8
  • 2x12 Orange PPC Closed Back.txt
    1.3 MB · Views: 9
  • 2x12 Recto OS.txt
    1.3 MB · Views: 15
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DLC86

Fractal Fanatic
I already sent mine to Cliff by PM, but I'll post it here for reference.

It is a 2x12 Fender Bassbreaker BB212 loaded with Fane F70 speakers.

BB212 NEW.JPG

During measurements I also did some tests just out of curiosity.
The cab has 2 rear ports that produce a huge resonance around 40Hz as you can see in the previous graph (low resonant frequency of the speakers is 70Hz in free air), so I pushed the cab back against a couch to shut the rear ports and make it basically a closed-back cab.
Then I took two measurements of the cab with only one speaker driven and another one with both driven, here you can see all 3 curves:

BB212 Closed Back.JPG

As you can see they're quite different from the first curve, the type of cabinet can have a huge influence on the speakers' impedance!
 

Attachments

  • BB212 F70.txt
    1.5 MB · Views: 9

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Here are two of my cabinet SIC measurements in the format Cliff requested. It might be a good place for others to post their measurements too. I did these measurements early 2019, I felt back than that it's a missing thing for the accuracy of the AxFX III.

I attach also two measurements of my LoadBox. It's a ToneHound loadbox. I wasn't happy how it originally sounded so I modded it. Changed a coil and a capacitor so it would be closer sounding to my cabinets.

The measurements were done with REW v5.20, using the method described here:
roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html

The soundcard used is a RME Fireface802



The cabinets measured are:

- 2x12 Mesa Rectifier Horizontal (OS, not the compact)

- 2x12 Orange PPC212CB (closed back)

This thing is in its early stages, but I hope some day we'll have in our AxeFX a space to upload custom SIC and than use them in the Amp block. Some people might have non commercial cabs and maybe they would love to have their SIC inside their Axe.


2x12 Mesa Recto:
View attachment 60252

2x12 Orange Closed Back:
View attachment 60253

Later edit:
I forgot to mention that my Orange cab has it's speakers re-wired in parallel. Originally they were is series giving 16Ohm, but I preferred to have them in parallel at 4Ohm.
Thanks for these but I'm leery of the measurements. The DC resistance for both is well above 4 ohms which indicates that something wasn't calibrated correctly. A 4-ohm cab should have a DC resistance around 3 to 3.5 ohms typically.
 

lorandkiss

New Member
No problem, I can redo it, this time with proper calibrations.
I remember now why I did those measurements, I wasn't happy how my loadbox "sounded", so I needed some reference curves. I was interested more in visualizing them so I could simulate my loadbox in Spice to obtain similar curve. Actually it helped, I made my loadbox to be in the ballpark of my cabs.
If you have any tips how to properly do the measurements/calibrations, you are more than welcome.
 

DLC86

Fractal Fanatic
No problem, I can redo it, this time with proper calibrations.
I remember now why I did those measurements, I wasn't happy how my loadbox "sounded", so I needed some reference curves. I was interested more in visualizing them so I could simulate my loadbox in Spice to obtain similar curve. Actually it helped, I made my loadbox to be in the ballpark of my cabs.
If you have any tips how to properly do the measurements/calibrations, you are more than welcome.
All you need to do is read here, specifically the parts regarding calibration and test leads resistance measurement.
https://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html
 

lorandkiss

New Member
I referenced to the same link in my OP. I was thinking more about some extra tips.
Anyway, I do not remember making any calibrations, as I mentioned, I was interested in the curve itself, not the numbers. The only thing I did was measuring the sense resistor, even if it is a high precision one, it turned out to be 99.5 Ohm.
I will try my best next time, it should be this week or so.
 

DLC86

Fractal Fanatic
For anyone who wants to measure the curve of his own cab, here's the schematic for a little box I built so that I could use standard speaker cables to take the measurements

Impedance Measurement Box.png

I suggest using speaker cables (or very short audio/instrument cables) also for the connections to interface inputs and headphone out cuz those have a much lower resistance than audio/instrument cables and this will lead to a more precise measurement.

I put the second optional speaker jack in case your cab is hard-wired for stereo and don't want to rewire it just for the measurement. If your cab is mono just use one jack.
 
Last edited:

DLC86

Fractal Fanatic
Another tip to set levels correctly:

As Cliff told me in the beta FW thread, if we use the axe fx as the interface we should keep the output level of the headphone out below 1Vrms to avoid distortion.
To do so simply set "sweep level" in REW settings to -20dB and turn out 1 knob on the axe front panel all the way up.
 

lorandkiss

New Member
I suggest using speaker cables (or very short audio/instrument cables) also for the connections to interface inputs and headphone out cuz those have a much lower resistance than audio/instrument cables and this will lead to a more precise measurement.
There's also a method to compensate for resistance of the test leads.
As described in the REW's tutorial:
When measuring very low impedances the resistance of the test leads may become significant. To measure this, first calibrate the rig as above, then make a measurement with the leads shorted together at the point the load is attached (set the RLEADS value to zero before making the measurement). The measurement should be fairly uniform, perhaps showing variation at very low frequencies depending on the low frequency limitations of the headphone drive stage. If the result is more than one or two tenths of an ohm, check that the connections are tight and the leads are not too flimsy. Enter the resistance from the flat part of the measurement into the RLEADS box on the measurement panel.

Another must, you have to measure (I do not know yet how, because it has a coupling capacitor) or get it from from your soundcard's specs the impedance of the soundcard's input. (Mine is 8k unbalanced, REW defaults to 20k)

Also, it would be welcome if Cliff would advice us what length the measurement should be (128k, 256k, 512k, 1M, 2M, 4M).
 
Last edited:

DLC86

Fractal Fanatic
There's also a method to compensate for resistance of the test leads.
As described in the REW's tutorial:
Yes, but that compensation doesn't take into account other side-effects of higher resistance it seems, in my first attempt I used audio cables and even with the correct RLeads there was high frequency roll-off compared to the measurements done with speaker cables.

Another must, you have to measure (I do not know yet how, because it has a coupling capacitor) or get it from from your soundcard's specs the impedance of the soundcard's input. (Mine is 8k unbalanced, REW defaults to 20k)
Didn't see there's also a setting for that in REW, where is it?

EDIT: nevermind, i saw I had an older version of REW installed that still didn't have that setting, I'll try to repeat the measurement to see if it makes a substantial difference.

Also, it would be welcome if Cliff would advice us what length the measurement should be (128k, 256k, 512k, 1M, 2M, 4M).
I just used the higher setting but I don't think that makes a noticeable difference in our case.
 
Last edited:

Joe Bfstplk

Axe-Master
Yes, but that compensation doesn't take into account other side-effects of higher resistance it seems, in my first attempt I used audio cables and even with the correct RLeads there was high frequency roll-off compared to the measurements done with speaker cables.
Audio cables are shielded, and unless they were XLR, signal ground is the shield around the central hit conductor, which means you effectively have a capacitor running from hot to ground, and rolling off the top end at some frequency.
 

DLC86

Fractal Fanatic
Audio cables are shielded, and unless they were XLR, signal ground is the shield around the central hit conductor, which means you effectively have a capacitor running from hot to ground, and rolling off the top end at some frequency.
Yep, the high frequency roll of is caused by both resistance and capacitance of the cable, it's an RC filter after all.
Speaker cables have both lower resistance and lower capacitance.
 

DLC86

Fractal Fanatic
I'll try to repeat the measurement to see if it makes a substantial difference.
I just verified this and between the default 20k setting and the proper 1M (for the axe inputs) the difference is a whopping 0.01 ohm at DC and 0.1 ohm max across the spectrum, the curves look identical except for this slight shift.

So I'd say it's not the most important calibration setting and there's a reason it wasn't there till the previous version. But since it's there now there's no reason to not set it right.
 

lorandkiss

New Member
Back from the studio. Been there doing calibrations, measurements, hot water repair, mounting a wall clock and so.
This time (I think) I did it properly, did a system calibration, lead calibration. I kept the headphone output under 1V.
The graphics and values are different but not in a drastic way.
  • Sense Resistor: 98.7 Ohm
  • Lead resistance: 0.160 Ohm
  • Input Impedance: 8000 Ohm
  • Calibration factor: 1.0218
  • 2x12 Mesa Recto DC Resistance: 3.7 Ohm
  • 2x12 Orange PPC CB DC Resistance: 4~4.1 Ohm

2x12 Mesa Recto:
2x12 Mesa Recto.jpg

2x12 Orange PPC CB:
2x12 Orange PPC CB.jpg


ToneHound Lite Modded Loadbox:
ToneHound Lite Modded.jpg

In the leads measurement I didn't witness any HF roll off because of the low-ish input impedance (8k).
Based on the measurements I made (at least 20-30 on each cab) I highly advise everybody not to take for granted the LF resonance, just adjust it confidently to taste. I mean this for every SIC in the AxeFX. I don't want to get into details but a given SIC is just like an IR. It gives you the SIC of a given cabinet in a specific position in a specific room/place. You move the mic, you have a different IR, you move the cab, you have a different SIC.
This time I measured my cabs in the control room, in the hallway, in the rehearsal room, in the vocal booth, in the corner, in the middle, back to the wall, away from the wall, facing the wall , with acoustic absorption panel in front of them, without it. The measurements posted are what I THINK are the most relevant.
 

Attachments

  • 2x12 Mesa Recto.txt
    1.3 MB · Views: 10
  • 2x12 Orange PPC CB.txt
    1.3 MB · Views: 1
  • ToneHound Lite Modded.txt
    1.3 MB · Views: 1
Last edited:
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