How do you model an amp's FX loop?

guitarnerdswe said:
No that's not true. Having the effects after the amp vs the fx loop are much closer than you think. Having them in front of the amp is nothing like having them in the loop. Power amps on amps with fx loops really don't really produce audible distortion, if at all. They're aren't design to do that, and will sound like ass if you crank the master volume loud enough for that to happen.
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That’s not true. I ran the fox loop on my DC30 all the time with the Master volume up around 8-9. Sounded great!
 
boyce89976 said:
That’s not true. I ran the fox loop on my DC30 all the time with the Master volume up around 8-9. Sounded great!
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That's an amp that shouldn't really have an effects loop. AC30-style amps are not good candidates for a loop. It's like putting a loop on a plexi. Bad idea.

You have to differentiate between vintage and modern amp designs. Designs derivative of the AC30, plexis and other vintage amps with no master volume are very different than say a Marshall JVM, Mesa Mark V, Soldano SLO 100 etc. The former won't work with a loop, the later are designed around one, and have completely different circuits and design philosophy.
 
boyce89976 said:
Actually, that’s nothing like having effects in the loop and won’t sound the same. The loop is typically after the preamp and before the power amp, and will cause power amp clipping (dirty repeats).

The only difference being that the cab filters the effect placed in front of the cab.
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At least you are getting the filtering from the cab without the effects getting the smearing from the amp preamp.

Placing the effects before the amp or after the cab will sound even less like the placement of effects in an effects loop, which doesn't exist in the Fractal ecosystem.

Surely, you agree with that.
 
jamongrande said:
I'm still not 100% convinced on pre/post cabinet effects being the same. I thought there are components of the cab block that introduce non-linearities, such as the preamp section (this came up in a different thread). And then thinking about resonant filters in modulation effects, drives and filters in delays, and added harmonics in pitch-shifted sounds, the results before and after a cab always seem different to my ears (if subtly so).
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And you're right, the result is exactly the same only if there's no distortion/compression/modulation/pitch shifting invoved
 
boyce89976 said:
That’s not true. I ran the fox loop on my DC30 all the time with the Master volume up around 8-9. Sounded great!
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But how much dirt were you getting at that level?

If i take a triple rec and set the master to 10 but keep the channel volume down, im still not really working the power section at all.

As for dirty repeats, i think most if not all delay types have a drive parameter.
 
Budda said:
But how much dirt were you getting at that level?

If i take a triple rec and set the master to 10 but keep the channel volume down, im still not really working the power section at all.

As for dirty repeats, i think most if not all delay types have a drive parameter.
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Amp was breaking up pretty good, but not saturated. I have a good video somewhere... I’ll find it and post it.
 
guitarnerdswe said:
That's an amp that shouldn't really have an effects loop. AC30-style amps are not good candidates for a loop. It's like putting a loop on a plexi. Bad idea.

You have to differentiate between vintage and modern amp designs. Designs derivative of the AC30, plexis and other vintage amps with no master volume are very different than say a Marshall JVM, Mesa Mark V, Soldano SLO 100 etc. The former won't work with a loop, the later are designed around one, and have completely different circuits and design philosophy.
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In the case I was considering, it's a Bogner XTC, which was designed ground-up to have the loop at the point where the pre-amp channels tie together.
 
I just ran a little test. If the block adds any drive, which some time-based effects add, there won't be perfect phase cancellation. I split the signal from the amp into two separate chains that had exact duplicates of a cab block and a delay block. In their default state when set before or after the cab, there is perfect phase cancellation (any configuration). If you add drive to the delay (hand typed, same amount), the phase cancellation is imperfect when one is set before the cab and the other is set after the cab. I believe some delays adds some sort of harmonic distortion by default. Maybe other blocks too. I haven't checked.
The difference is probably negligible unless settings are extreme. I assume the configuration set after the cab block will be brighter due to the added harmonic distortion not being shaped by an impulse response
 
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Budda said:
My understanding is that effects after the cab is the same as in the loop.
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Not quite, but for most practical uses where there isn't a lot of power amp distortion this is true. In fact, usually after the amp is preferred, just it isn't possibly to run most real effects between the amp and cab so loops are used instead

boyce89976 said:
Man, just run them in front of the amp(s)... more like having them in the loop, that putting them behind the amp/cab.
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It depends what you're going for. If you have a lot of power amp distortion but no preamp distortion, then in front would be closer to the loop. If you have a clean power amp but lots of preamp distortion, then after the amp would be closer.

zedeloc said:
I just ran a little test. If the block adds any drive, which some time-based effects add, there won't be perfect phase cancellation. I split the signal from the amp into two separate chains that had exact duplicates of a cab block and a delay block. In their default state when set before or after the cab, there is perfect phase cancellation (any configuration). If you add drive to the delay (hand typed, same amount), the phase cancellation is imperfect when one is set before the cab and the other is set after the cab. I believe some delays adds some sort of harmonic distortion by default. Maybe other blocks too. I haven't checked.
The difference is probably negligible unless settings are extreme. I assume the configuration set after the cab block will be brighter due to the added harmonic distortion not being shaped by an impulse response
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That's a good point. Really it's time based effects which are the same before an after. If your delay repeats then gets EQ'd, or gets EQ'd then repeats the EQ'd signal the result is the same. But distortion is introducing new harmonic content based on the input. So EQing the input (by running it through the cab) will change the distorted result. And putting the cab after will run the new distorted content through the cab EQ.
 
DLC86 said:
And you're right, the result is exactly the same only if there's no distortion/compression/modulation/pitch shifting invoved
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True - at least within the limits of the math algorithms being used.

For the FX that I normally run in my loop, the majority are time-based except for:

Compression
EQ (this is already in the amp block, so probably won't use it).
Phase shifter

I'm guessing that putting them between the amp and cabinet will be close enough not to matter.
 
Budda said:
But how much dirt were you getting at that level?

If i take a triple rec and set the master to 10 but keep the channel volume down, im still not really working the power section at all.

As for dirty repeats, i think most if not all delay types have a drive parameter.
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Here's a couple videos where I was running the fx loop on my Matchless. Amp was set at the "edge of breakup" under normal strumming, and would get fairly gritty when digging in. At the time, I was running a Morning Glory as a first gain stage and JHS808 as the 2nd/lead stage. Reverb and delay are in the loop. Both of these are from my church.

Here's the first one:


Here's another one:

guitarnerdswe said:
That's an amp that shouldn't really have an effects loop. AC30-style amps are not good candidates for a loop. It's like putting a loop on a plexi. Bad idea.

You have to differentiate between vintage and modern amp designs. Designs derivative of the AC30, plexis and other vintage amps with no master volume are very different than say a Marshall JVM, Mesa Mark V, Soldano SLO 100 etc. The former won't work with a loop, the later are designed around one, and have completely different circuits and design philosophy.
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I don't disagree with that. It's not a great fx loop, and posed some problems. Most notably, it was so hot from the send that it clipped the input of the H9 I had at the time. I have to put a -6dB pad on my junction box to make it work. Other than that, though, it was pretty good. The loop is pre-PI as far as I know, so likely most of the dirty repeats is probably a combination of the PI and power amp breaking up rather than just power amp distortion.
dr bonkers said:
At least you are getting the filtering from the cab without the effects getting the smearing from the amp preamp.

Placing the effects before the amp or after the cab will sound even less like the placement of effects in an effects loop, which doesn't exist in the Fractal ecosystem.

Surely, you agree with that.
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I guess this is where preference comes in. I prefer that smearing... in the real world, either from running fx in front of the amp, or in the fx loop. That doesn't work on some amps, but I've never had one of those amps.

I agree that placing fx after the amp in the Axe III won't sound like fx in the fx loop of a real amp. I would say having the fx in front of the amp can get you closer to the sound of fx in the loop than any other placement in the Axe III world.
 
FWIW and IIRC, the main reasons why there is no effects loop is (but correct me if I'm wrong):
  1. Amp modeling is done on its own DSP due to oversampling and CPU intense algorithms within the entire preamp/amp/speaker circuit
  2. Effects are run on the other processor so sample conversion and I/O might be too CPU intensive
  3. There may be non-linear interactions between the preamp and the power amp
My guess is that if the amp block ran at normal sample rates internally and the preamp and amp were separable, it would probably be doable.
 
zedeloc said:
I don't think there's a way. I could be wrong. But I recall reading that FAS avoids any sort of separating of the power amp section because there are a lot of proprietary algorithms going on that could be analyzed when isolated. BUT you can always plop a few blocks between the amp and cab block so the effects sound like they're being sent through a traditional cab.
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This is my approach for a few things here and there, particularly when I want it to sound more like amp reverb, or for the complex 6G4/6G12 'harmonic vibrato'....
 
Joe Bfstplk said:
Depends on the effect.
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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.
 
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