PL-IR reactive load evaluation vs speaker cab load

Stage & Studio Power Load PL-IR
,
1

I’ve owned this PL-IR unit for a while now and am just now evaluating how the reactive load compares to a real speaker cab (my D412 F70).

Bottom line: the PL-IR’s load in “warm/edge” gets close to the response of my D412, but the difference is discernible. Some further adjustment of the load impedance values could maybe get it from “close” to “magic?” Those are my thoughts for future iterations.

The experiment: I recorded 10 samples of me playing the same thing through the same amp (my D60) with the same guitar with the same settings, into the PL-IR and recorded both the analog cab out and the IR out (bank 2 preset 3), while only changing the following things:
Sample 1: used the “cab-through” of the PL-IR into my D412 cab
Samples 2-10: disconnected the “cab-through” and used the PL-IR load, and went through all 9 setting combinations of the reactive load switches.

I should have used my looper pedal for this but I’m too lazy. The intent was to figure out which setting of the reactive load switches best emulates the response of the cab-through with the D412.

It turns out that “edge” is the best option for the top-end, but it doesn’t quite get it across the finish line. The real cab’s load resulted in more “edginess” than the PL-IR’s load could afford.

“Warm” was the best option for the low-end. “Deep” was kind of muddy in comparison. Still a very useful setting if you adjust the amp or use post EQ, but in comparison to the real cab’s response, the PL-IR’s load presented more low-mids in both settings. Interestingly, the real cab’s load response sounds “deeper” in the bottom register. So maybe the resonance peak of the real cab is at a lower frequency that that of the PL-IR’s load?

Speaking of the low-end, the real cabinet’s load gave the sound a little more liveliness and bounce as a result compared to the PL-IR’s load, while still staying clear. it’s most noticeable on those chunky palm mutes.

edit: almost forgot another difference: the cab-through version ended up coming in at around 1.5 dB higher than the version with the reactive load. Not sure why. I did apply a pad in post to make it a more fair comparison.

Here’s the two samples. I probably should have tuned my guitar and played on time, oh well. I wasn’t intending to post these, it was an experiment for my own purposes, but then I thought that it was useful information to share. The analog cab is on the left channel and the digital cab on the right. They channels are time aligned, but there’s still a bit of phasing if you’re playing it back in mono or with a small stereo field. There is a tiny amount of post-EQ and also a touch of reverb, just to make it closer to something you’d actually put in a mix for a fair comparison.

With the cab-through (D412):

With the PL-IR’s load in warm/edge:

Just for giggles, here’s the PL-IR in flat/flat. I’m not sure why you would ever use this setting. It makes sense for the Power station, but I don’t know why I would want it for the PL-IR:

So, why is it potentially important for the PL-IR’s load to exactly match the actual cab’s? I want to get started with a workflow where I use the PL-IR in “cab-through” mode along with microphones on the cab to record those along with the cab-less signal and make some IRs. Last album I did the mic setup was ridiculous: four close mics (two on each cab) and two far-field mics. It took a long time to set-up and even longer to tweak into exactly the right position. Next time I do this I’d like to be able to capture the setup as six IRs for use in the future. Okay, so I set-up the mics, use the cab-through, and make the IRs. Now, I can disconnect the cab-through and silently record the PL-IR output with the reactive load, and either load my IRs into the PL-IR and/or record the cab-less output and convolve with my IRs in the DAW. Now, if the reactive load matched my cabinet’s load exactly, I would expect a very close match in the “silent” recording vs. what I would get with the mics. But since there is a difference, that difference will manifest in the silent version compared to the version I’d get in the room, and there is not a clear way to “undo it” in post except do some careful EQing with my ears. Anyway, just thought I’d give some feedback on what could turn this from a really great product into a magical one. Thanks for listening.

…

I don’t want to make a separate thread for this, but I’m also curious if anyone else here is making their own IRs. I’ve never really been super happy with any of the deconvolution software I’ve found before so I spent a few hours yesterday rolling my own matlab script to do it and the results were very good. If anyone wants the script I’ll put it on github or something with instructions. Maybe I’ll get around to porting it to a real program someday.

2

Hey Choalla,

I just wanted to say wow… I have not managed to sink my teeth into this properly yet… but I will.

Regards,

Dan

3

I had the inspired idea of trying to capture the transfer function from the PLIR output when using the internal load to the same output when using the cab-thru, hooked up to my Deliverance 412 F70.

Here’s what I did:

  • Recorded a DI guitar signal into my DAW
  • Routed the DI track in my DAW to an analog output
  • Connected the analog output to my pedal board hooked up to my amp (here is where I would use a DI box if I had one in order to get the isolation etc. But it worked okay regardless).
  • Connected the amp to my PLIR, and hooked up the cab-thru to my speaker cab
  • Turned the amp on and adjusted the MV (relatively low volume), and adjusted the analog output DI signal volume
  • Recorded the PLIR output signal when playing back the DI signal from my DAW.
  • Turned the amp off, disconnected the cab-thru, set the reactive load switches to EDGE/DEEP
  • Turned the amp on, recorded the PLIR output signal when playing back the DI signal from my DAW

This gives me two signals: the first is the “output” I want, and the second is the “input”. I ran a short segment (5 seconds worth) of those two through my deconvolution script I hinted at above to get the transfer function.

Here’s the frequency response of said transfer function:

image
image1060×832 59.9 KB

Interpretation: when using the cab-thru vs the internal load, there is an overall slight increase in volume, a pronounced high frequency boost above 3 kHz, a sharp boost of 5 or 6 dB at 80 Hz, and a broad attenuation around 250 Hz. Plus some other minor things.

Performance: I took the IR I generated from this experiment and applied it to the full recording I obtained from the PLIR using the internal load. This basically passed a null-test with the signal using the cab-thru. The error-to-signal ratio of the null-test was like -34 dB.

What is the significance of this? I have showed that I can duplicate a cab-thru recording environment completely silently.

What can I do next?

  • I can chain this with the speaker/mic IR generation I hinted at previously to create a full speaker/mic silent emulation using the PLIR which I think should pass a null-test (at low volume). I’m not aware of this being demonstrated by anyone else.
  • I accidentally created these transfer functions using the analog cab output from the PLIR instead of the raw output. The result I believe is the extra noisiness in the transfer function at high frequencies. I’ll re-create this experiment with the analog cab sim turned off.
  • Recreate the experiment with different reactive load switch settings
  • Recreate the experiment for different amplifiers. Different amplifiers have different source impedances and will react to load impedance changes in different ways. This experiment was done with my Deliverance 60 (gen 1)
  • I keep hinting at “low volume.” As we know, a tube amp (or any real amp) will have a non-linear relationship with its load as the power output increases. At low volumes the relationship is linear hence we can describe its transfer function in linear terms. I do not expect that the transfer function I modeled here will hold up at high volumes (the null test would not pass). The only way to silently emulate the cab-thru scenario at moderately-loud volumes (where the tubes/output tranny are non-linear but the loudspeakers themselves are not saturated) would be to construct a physical reactive load which models the physical speakers’ impedance, in situ. I have an experiment in mind where I can measure the loudspeakers impedance in situ while playing. More to come.

Edit: addendum:

I tried applying the transfer function above to some recordings I had made with the PLIR at moderate/loud MV settings on my amp and, while I wouldn’t expect them to pass a hypothetical null test vs recording with cab-thru, they still sound much more “alive” and realistic than without. So that’s a major win! And gives credence to the use case of “use the PLIR internal load to give you the gross reactance/dynamics and then get that last bit using digital means.” You could even take the IR generated from these means, and compose it into an existing speaker/mic IR which you then load into the PLIR itself.

I was additionally curious about those minor spikes in the transfer function a little above 500 Hz and 1 kHz. Obviously some kind of resonance in the loudspeaker/cabinet. So I went and looked at the F70 datasheet… lo and behold, you can see some spiking in the impedance at conspicuously close frequencies. https://www.fane-acoustics.com/view-product/Ascension-F70
Shifts in the resonances can be ascribed to either the “G” differences Steve sometimes talks about, or maybe the enclosure itself.