How do you model an amp's FX loop?

Reverb (the Axe-FX reverb block at least) doesn't do this. That's the error in your thinking. It's all based on delaying and EQing what's already present.
That makes sense that the onboard reverbs can only work with what they are given. I run an analog pedalboard so my tendency is to come at it from that standpoint, which should be theoretically different, right?

I wish they would build an Axe version with just amps and cabs. I'd buy two and run 4 amps in stereo all the time! ;)
 
So check me here (haven't tried this but can tonight):
  • With a reverb between the amp and the cab, the cab is filtering the dry signal and the output of the reverb
  • With the reverb after the cab, the cab is filtering the dry signal before the input of the reverb

Intuitively, there is a difference there. In the first scenario, the reverb's response is limited by the cab and will only reproduce frequencies relative to the cabs frequency response. Whereas in the 2nd scenario, the reverb's response is in theory full frequency, only limited by the cab's response, and whatever overtones/harmonics the reverb creates.

That's how I think about it... curious of your thoughts.
Reverb is inherently going to react differently and sustain differently depending on input drive. The signal before the cab will have more highs and lows due to not being filtered by the cab. This makes it react differently than it does after the cab. It's a small difference, but noticeable, at least to me.
 
That makes sense that the onboard reverbs can only work with what they are given. I run an analog pedalboard so my tendency is to come at it from that standpoint, which should be theoretically different, right?

Not automatically just because it's pedals. It depends if there are any non-LTI elements in the things you're theoretically changing the order of.

Reverb is inherently going to react differently and sustain differently depending on input drive. The signal before the cab will have more highs and lows due to not being filtered by the cab. This makes it react differently than it does after the cab. It's a small difference, but noticeable, at least to me.

This is incorrect if there's no distortion happening in the cab block (preamp sim) and (technically although this should matter much less) no modulation in the reverb, and stereo panning is maintained in both examples. For a stereo reverb type you'd need a stereo cab for the cab-last part of the test.
 
This is incorrect if there's no distortion happening in the cab block (preamp sim) and (technically although this should matter much less) no modulation in the reverb, and stereo panning is maintained in both examples. For a stereo reverb type you'd need a stereo cab for the cab-last part of the test.
Almost all of the reverb types I've looked deeper into have at least a little modulation built in straight out of the box as a standard setting.
Recording Studio C, for example, has 20% at a 0.2Hz rate.
Medium Plate has 25% at 0.3Hz.
Deluxe Spring Reverb has 0% modulation.
It makes a subtle difference whether the filtering happens before it or after it.

Here's a test with the Deluxe Spring Reverb:


Audio is the difference between the pre-cab reverb and post-cab reverb. The Looper placed directly after the Amp block was used to ensure the audio used was identical. A zero-gain Gain block with the phase inverted is applied to the pre-cab reverb channel to cause it to cancel common audio elements in the two tracks. The two clips were lined up so that they are sample-accurate in the timeline. Cab block has two cabs in either channel, panned hard left and hard right, so that the stereo-ness of the reverb is preserved in the pre-cab audio.

The preset used (sorry, uses OH 4x10 Super cab, not attached) - per usual, use 'save-as' feature to download.
 
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Audio is the difference between the pre-cab reverb and post-cab reverb. The Looper placed directly after the Amp block was used to ensure the audio used was identical.
There's nothing to ensure the modulation sweep is aligned from one take to the next, so this doesn't actually demonstrate how the order itself makes a difference. You could remove the cab block and record two passes through reverb only and still get different results.

It might be possible to align the sweeps of two reverb blocks with the same settings by powering on to that preset, but that's a guess. They might also initialize and run at different times instead of one waiting for the other.

The real difference with modulation will be subtle if not using extreme modulation settings. For example, when a 440 Hz tone gets pitched up/down to something like 441 & 439 Hz, the cab's response at those new frequencies matters when it's after reverb. But it's probably not drastically different from the response at 440 Hz.
 
There's nothing to ensure the modulation sweep is aligned from one take to the next, so this doesn't actually demonstrate how the order itself makes a difference. You could remove the cab block and record two passes through reverb only and still get different results.

It might be possible to align the sweeps of two reverb blocks with the same settings by powering on to that preset, but that's a guess. They might also initialize and run at different times instead of one waiting for the other.

The real difference with modulation will be subtle if not using extreme modulation settings. For example, when a 440 Hz tone gets pitched up/down to something like 441 & 439 Hz, the cab's response at those new frequencies matters when it's after reverb. But it's probably not drastically different from the response at 440 Hz.
The sample was using the Deluxe Spring Reverb, which, at stock settings, has the modulation zeroed out. I left all settings at stock values.
 
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The real difference with modulation will be subtle if not using extreme modulation settings. For example, when a 440 Hz tone gets pitched up/down to something like 441 & 439 Hz, the cab's response at those new frequencies matters when it's after reverb. But it's probably not drastically different from the response at 440 Hz.
Once the drums come in will it really matter?
🙃
 
Once the drums come in will it really matter?
🙃
Probably not, but if we didn't care about the minutae of our sound, a Peavey Bandit would be fine, right?

The thing that I noticed in the diff was that the reverb part of the signal cancelled better than the dry part. I lined both clips up by the initial sample of the initial attack of the guitar sound, so that the audio would be as in-synch as it could possibly be. It's dead on, within 1/48000 of a second or so.
 
The sample was using the Deluxe Spring Reverb, which, at stock settings, has the modulation zeroed out. I left all settings at stock values.
I missed that in the video, sorry. I'm thinking the "Boiiinnng" parameter might involve a separate type of modulation. Maybe try again with that at 0 with cab-reverb and reverb-cab paths right in the preset, plus a filter inverting one?

On the Axe-FX II any reverb type will cancel almost completely when no modulation is used, leaving some noise around -90 dB.
 
I missed that in the video, sorry. I'm thinking the "Boiiinnng" parameter might involve a separate type of modulation. Maybe try again with that at 0 with cab-reverb and reverb-cab paths right in the preset, plus a filter inverting one?

On the Axe-FX II any reverb type will cancel almost completely when no modulation is used, leaving some noise around -90 dB.
Will give it a try with all else held constant.

i wonder.... Perhaps two separate signal paths, one with cab --> reverb and the other with reverb --> cab, going out two separate outputs simultaneously will eliminate the possible sub 1/48000 second differences and make it cancel more effectively when diffed in the DAW.
 
Will give it a try with all else held constant.

i wonder.... Perhaps two separate signal paths, one with cab --> reverb and the other with reverb --> cab, going out two separate outputs simultaneously will eliminate the possible sub 1/48000 second differences and make it cancel more effectively when diffed in the DAW.
You don't even really need the separate outputs & recorded tracks. Just put a null filter in one path with phase reverse = both, bypass mode = mute, and see if engaging it cancels to silence.
 
Late to the party, and always an "effects loop luddite" as well as a "channel switching denier" in the past ;), but my first thought was this:

Effects loops were a bit of a compromise when first introduced. People had been recording amps with pretty high gain, and had then introduced a lot of "high in the mix" time-based and modulation effects in the recording studio. Trouble was, if introduced at the front end of a high gain amp, the non-linearity introduced by compression and distortion in the clipped preamp meant that the time based and modulation effects sounded nothing like as natural. So to get the effects a bit more linear, effects loops were able to kind of emulate what people were doing in the studio. If you were rich (or lucky enough to have loaned gear in my case) you could run a rack preamp into stereo effects, a stereo power amp and 2 cabs, and it sounded pretty good. Although the power amp might be clipping, it's seldom all that clipping all that hard.

So when I got my first Axe FX II, I had a similar thought to FullThrottle64. I wanted to model a Fender Deluxe Reverb, but I wanted the reverb to be introduced just where it is in a real Deluxe Reverb, i.e. I wanted it to be in the same gain stage of the amp. But you can't do that. So the best compromise for my first listening test was to put a spring reverb between the amp and cab block. Those days seem a long time ago now, because authentically reproducing amps I have loved no longer involves that sort of detail. Joe Bfstplk's video reminded me about this, there is only a subtle difference, but to me most of those effects sound better after the cab (and let's remember we are talking Fractal, so they don't have to go through the same cab as the main signal, or even through a cab at all for that matter).

So to my mind, this could be considered a minor inadequacy of the Axe FX, however to me it kind of removes a compromise that I never fully bought back in the 1990s. I can have some "pre" effects, an amp or 2, a cab (or 8!), and then pretty much whatever studio effects I feel like throwing in as "post" stereo effects, without ever having to worry that power amp non-linearity or distortion might make them sound a little bit weird. Nowadays when I set up a Fender Twin Reverb model, I don't even feel duty bound to use spring reverb in the reverb block :D. Fractal can definitely change your way of thinking.

Liam
 
You don't even really need the separate outputs & recorded tracks. Just put a null filter in one path with phase reverse = both, bypass mode = mute, and see if engaging it cancels to silence.
That simplifies the test quite a bit, thanks!

With Boiinnnng! set to stock setting (38%) there is a minute amount of reverb signal that doesn't cancel. Set to 0%, again just a minute amount of reverb signal is not cancelled. Neither registers on the -40dB output meters. Will record and see what kind of level it is hitting....
 
That simplifies the test quite a bit, thanks!

With Boiinnnng! set to stock setting (38%) there is a minute amount of reverb signal that doesn't cancel. Set to 0%, again just a minute amount of reverb signal is not cancelled. Neither registers on the -40dB output meters. Will record and see what kind of level it is hitting....
Found a difference in the mic alignment of a few mm, fixed it, now cancelling much better, but still not fully. The difference is minute enough that I will chalk it up to being pretty much inaudible. Switched to a Hallway reverb. Much different result. The modulation FUBARs the cancellation, which would mean either the modulation LFOs are not in synch, or there is a really easily audible difference....
 
To loop things back to the OP, while taking out all modulation and non-linearities can produce similar results between fx->cab and cab->fx, why take out all of the "flavoring" that makes those blocks interesting and dynamic (seeming more realistic?)?

Now whether someone prefers one or the other... I put stereo cabs after fx because of a parallel path outputing to a real cab, but I frequently like the cab-.fx sound better when it involves interesting reverbs and pitch fx.
 
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