Bizzare Yet Interesting Legend Amp Model (Possible) Issue

[AndrewJay]

This is kind of a bizarre sound that I found a while back but since I don't normally use this amp model, I just moved on.
When using either of the Legend models in combination with the Rumble speaker impedance curve, you get this strange
low frequency lo-fi kind of sound underneath the original sound. I included a preset to demonstrate this. So here is the breakdown:

Legend>2x12 Tx Star curve>Recto IR=No issue
Legend>Rumble curve>Recto IR=Issue
Legend>2x12 Tx Star curve>Rumble IR=No Issue
Legend>Rumble curve>Rumble IR=Issue

If you lower the LF Res Freq the effect gets more pronounced. Also, the effect can be heard whether through my monitors, headphones, or power amp and cabs. Of course, I can always just not use this combination, and in general I don't.
But I am wondering if it is coming from the Rumble impedance curve. That I do use and modify because I like to use EV IRs as well as my
real 1x12 thiele EV cabs. Out of all the amp models I've tried, all of which I like to use with the Rumble curve, the Legend models are the only
ones I've found that do this. Who knows, this may be an inspiring sound for someone out there, but for me, it's just interesting.

The Volume block source is tied to Pedal 1 btw.

 

[AndrewJay]

I'll record a sample tomorrow, but has any one else experienced this with this combination or any other? Just wondering. Thank you in advance either way.

 

[Ocnor]

I had a similar issue with the Legend 100 when I tried to use the 4x12 Hipower impedance curve. It would start out with a low rumble and then build in volume like a feedback loop. The volume would just keep building making a horrendous noise until I changed the impedance curve.

 

[FractalAudio]

So I looked into this. I'm pretty sure the real amp would do the same thing. What is happening is the power amp is oscillating at that very low frequency cabinet resonance. This occurs because the Legacy has so much negative feedback (12 dB, which is the highest of any amp I've ever modeled). Combined with the smallish output transformer there is a loss of phase margin at low frequencies which causes the power amp to become unstable.

The Rumble cab is unique in that it is ported and has two different types of speakers in it. This causes a double resonance and one of the resonances is at a very low frequency. For typical power amps with reasonable amounts of feedback this isn't an issue.

You can stabilize the power amp by reducing the Transformer LF. This will improve the phase margin and increase stability. Another solution is to reduce Cabinet Resonance but that will change the tone a bit.

Whenever you have negative feedback you run the risk of instability. Highpass and lowpass filters introduce phase shift which reduces phase margin. Combine that with a strong resonance and the system can become unstable.

 

[AndrewJay]

So I looked into this. I'm pretty sure the real amp would do the same thing. What is happening is the power amp is oscillating at that very low frequency cabinet resonance. This occurs because the Legacy has so much negative feedback (12 dB, which is the highest of any amp I've ever modeled). Combined with the smallish output transformer there is a loss of phase margin at low frequencies which causes the power amp to become unstable.

The Rumble cab is unique in that it is ported and has two different types of speakers in it. This causes a double resonance and one of the resonances is at a very low frequency. For typical power amps with reasonable amounts of feedback this isn't an issue.

You can stabilize the power amp by reducing the Transformer LF. This will improve the phase margin and increase stability. Another solution is to reduce Cabinet Resonance but that will change the tone a bit.

Whenever you have negative feedback you run the risk of instability. Highpass and lowpass filters introduce phase shift which reduces phase margin. Combine that with a strong resonance and the system can become unstable.

Thank you very much for looking into this. I am quite relieved that it is not the Rumble Impedance curve, seeing as I very much enjoy it. With adjustments it works really well with my real 1x12 EV cabs.
And a quick follow up question to your point about the phase margin. On older amps, especially the Marshall and Fender models, they are exhibiting more of that vintage "swirl" on sustained notes, especially on the lower notes and very much so on the low E twelfth fret with Cygnus. The notes will sustain and then it sounds like there is a phaser on the note. Is this essentially the same or similar principal at work? That there is something causing the system to become unstable? I figure that this would also happen on the real amps, so is this just a by product of those early designs? Thank you in advance.

 

[FractalAudio]

Thank you very much for looking into this. I am quite relieved that it is not the Rumble Impedance curve, seeing as I very much enjoy it. With adjustments it works really well with my real 1x12 EV cabs.
And a quick follow up question to your point about the phase margin. On older amps, especially the Marshall and Fender models, they are exhibiting more of that vintage "swirl" on sustained notes, especially on the lower notes and very much so on the low E twelfth fret with Cygnus. The notes will sustain and then it sounds like there is a phaser on the note. Is this essentially the same or similar principal at work? That there is something causing the system to become unstable? I figure that this would also happen on the real amps, so is this just a by product of those early designs? Thank you in advance.

Swirl is a different phenomenon. Mostly caused by bias shift. I like it, makes an amp sound more alive and less sterile.

 

[AndrewJay]

Swirl is a different phenomenon. Mostly caused by bias shift. I like it, makes an amp sound more alive and less sterile.

Thank you for your response. All of this engineering is incredibly fascinating. And it all leads to people creating music, it's wonderful. While in my music that swirl is not as desirable, I can see how musicians would find it inspiring. Either way, having the amp models force me to react to their needs is something no other modeler can boast about and I just find that moving.

 

[yyz67]

Swirl is a different phenomenon. Mostly caused by bias shift. I like it, makes an amp sound more alive and less sterile.

Cool. What exposed user parameters, if any, could be used to either accentuate or attenuate that effect?

 

[LiamH]

I'll take a wild guess at Bias Excursion in the power amp, but I made that guess without looking at anything in the Axe FX. Power amp sag might help too.

Liam