Helix Amp Models Producing DC

MirrorProfiles

MirrorProfiles

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I used ChatGPT to tidy up what I wrote and make this post a bit easier to digest, hope that's not a problem (I think its easier to read and understand this way but some of the tone of what I was saying is very slightly altered due to being made more concise):

Description of the Issue:

While exploring some of the newer Bogner XTC amp models in the Helix platform, I discovered unusual behaviour that doesn't align with how the actual amp should perform. Using an FFT display (Pro-Q), I noticed two types of noise:
  1. Broadband noise (ripple) – common and intentional to Line 6 modelling, can be turned off.
  2. DC offset – visible as energy at the far left of the FFT (essentially 0Hz).
Unlike ripple, which can be disabled, this DC offset remains, and it’s present in the output of certain amp models even with ripple turned off. You can see that behaviour in action here, across various settings. Pay attention to the input and output meters - all of this noise and DC is produced when there is no input at all:



Why This Is a Problem:
  • DC has no place in an audio signal. In a real amplifier, DC is filtered out by design (e.g., by capacitors and the output transformer). Driving a speaker with DC would move it in one direction and cause it to stay there, preventing it from functioning properly.
  • Audio modelling consequences: If the DC offset is present before nonlinear processes like saturation or compression, it could cause:
    • Asymmetric or unintended distortion
    • Incorrect dynamic behaviour in the modelling
    • Audibly, it may contribute to clicks and pops when switching amps/settings – likely caused by sudden DC shifts moving the speaker cone abruptly.
  • Confirmed in multiple models: Beyond the Bogner XTC, I’ve confirmed the issue in various other Helix amp models, including several Marshalls, Mesa models, Cartographer, and others.
  • Power amp related: This appears tied to the power amp stage in the modelling; preamp-only models don’t show this behaviour.
Why It Should Be Fixed:
  • Realism: DC offset would not be present in a properly functioning tube amp output. Its inclusion contradicts the goal of realistic amp modelling.
  • Performance impact: The DC may be causing audible artefacts (clicks, pops) during amp changes and might affect tone in unpredictable ways.
  • User experience: Even though DC can be filtered after the fact, the root cause should be addressed to ensure proper, consistent behaviour in the model itself.
  • Legacy product support: While newer Line 6 tech (e.g., the Stadium series) may avoid this entirely, current Helix users should not be left with a compromised experience, particularly since the fix likely falls within current DSP capabilities.
  • Potential impact on tone: Fixing the issue could alter the tone of affected models slightly, but that’s only because the current tone may already be subtly incorrect due to the unintended DC presence.
Summary:

I’m raising this not as a nitpick, but because it potentially affects the fundamental accuracy of the amp modelling on Helix. I believe Line 6 should investigate this DC offset issue and implement fixes where practical, both for sound quality and user confidence in the product.:

The amps I've noticed with noticeable DC (not just noise from ripple but where DC remains when ripple is at 0):

- 2203 (and all Marshall's besides the JTM45, Park P75 and 2204)
- German Xtra Red + Blue (XTC)
- Cali Texas (Mesa Lone Star)
- Cartographer
- The older Panama 5150
- Cali IV
- Grammatico
- Fullerton
- Super Vib
- Mail Order Twin
 
BenIfin said:
You should tag this so D.I can see it and pass it on to the team for Augora if it hasn't already been addressed.
Click to expand...

I tagged him in another post, and I've posted bits here and there recently. Nothing's been acknowledged yet but Im pretty sure someone would be aware now. I'm assuming it's probably not an area DI works in day to day, and probably for someone else in the team to tackle. Hopefully it gets acknowledged and looked into.

Even if its the by-product of something benign and isn't interfering, it doesn't particularly fill me with confidence that its there and I think its just better its fixed properly. I can't see how it wouldn't have SOME kind of effect, and it could be very easy to inadvertently run something with DC into a process where it would cause problems.
 
great... the TGF logo will now be removed from further Line 6 YouTube presentations....


jk
They will soon be handpicking new crew from TGF
Freaks And Geeks Nerd GIF
 
MirrorProfiles said:
I’m raising this not as a nitpick, but because it potentially affects the fundamental accuracy of the amp modelling on Helix. I believe Line 6 should investigate this DC offset issue and implement fixes where practical, both for sound quality and user confidence in the product.:
Click to expand...
Excellent work!
I’m curious if what you've discovered might also relate to unexpected harmonics—something I’ve often thought of as intermodulation distortion or something along those lines (though my understanding is still limited).

When testing amps or overdrive models, I usually use a single-frequency test tone to analyze the harmonics they produce—looking at both odd and even harmonics and their levels relative to the fundamental frequency.

If I’m not mistaken, some of the "weird" amps you listed (by the way: please check also the Matchstick models if you can!) also seem to generate unusual harmonic frequencies—not just harmonics above the test tone, but also unexpected ones below it and then their multiples.
It really made me wonder: could this be a fundamental characteristic of the real analog circuits themselves?
 
Lele said:
Excellent work!
I’m curious if what you've discovered might also relate to unexpected harmonics—something I’ve often thought of as intermodulation distortion or something along those lines (though my understanding is still limited).

When testing amps or overdrive models, I usually use a single-frequency test tone to analyze the harmonics they produce—looking at both odd and even harmonics and their levels relative to the fundamental frequency.

If I’m not mistaken, some of the "weird" amps you listed (by the way: please check also the Matchstick models if you can!) also seem to generate unusual harmonic frequencies—not just harmonics above the test tone, but also unexpected ones below it and then their multiples.
It really made me wonder: could this be a fundamental characteristic of the real analog circuits themselves?
Click to expand...
No idea. It was some particularly odd sounding distortion and artefacts that led me to see if something was wrong, and I still think the Ecstasy doesn’t sound right. Testing the preamp on its own seemed absolutely fine so I imagine there’s something else going on.

Impossible to say what the cause of it is, but the likelihood is it’s a multitude of complex things going on and changing one tiny thing will be unlikely to change some other behaviours. I don’t expect this bug to be related to other things but it’s hard to know. I’d imagine modelling the power supply, voltages, bias, etc behaviours is very complex and has to be simplified/approximated in order to work the way it does. Which probably has some trade offs.

Pretty sure I checked every amp (albeit quickly) and I didn’t notice the DC thing on the Matchless.
 
MirrorProfiles said:
No idea. It was some particularly odd sounding distortion and artefacts that led me to see if something was wrong, and I still think the Ecstasy doesn’t sound right. Testing the preamp on its own seemed absolutely fine so I imagine there’s something else going on.

Impossible to say what the cause of it is, but the likelihood is it’s a multitude of complex things going on and changing one tiny thing will be unlikely to change some other behaviours. I don’t expect this bug to be related to other things but it’s hard to know. I’d imagine modelling the power supply, voltages, bias, etc behaviours is very complex and has to be simplified/approximated in order to work the way it does. Which probably has some trade offs.

Pretty sure I checked every amp (albeit quickly) and I didn’t notice the DC thing on the Matchless.
Click to expand...
maybe it does relate to some artifacts or the disembodied distortion JT refers to
Be interesting to see if they Test Agoura XTC red vs HX if the problem was resolved
 
Lele said:
Excellent work!
I’m curious if what you've discovered might also relate to unexpected harmonics—something I’ve often thought of as intermodulation distortion or something along those lines (though my understanding is still limited).

When testing amps or overdrive models, I usually use a single-frequency test tone to analyze the harmonics they produce—looking at both odd and even harmonics and their levels relative to the fundamental frequency.

If I’m not mistaken, some of the "weird" amps you listed (by the way: please check also the Matchstick models if you can!) also seem to generate unusual harmonic frequencies—not just harmonics above the test tone, but also unexpected ones below it and then their multiples.
It really made me wonder: could this be a fundamental characteristic of the real analog circuits themselves?
Click to expand...
You might just be picking up intended behavior there, like ghost notes etc.
 
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