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Cab Modeling and IR Deep Dive - Helix vs Axe/others vs IR's (yet again)


roscoe5
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This haunts me...I literally lose sleep over this....so I've gone further down the rabbit hole on speaker modeling with Helix HX Cabs as my baseline, compared to Impulse Responses (IR) using solid state amps vs tube amps vs my own IR's made from my various tube power amp sections. I'm also considering my current preference of running Helix into a neutral tube amp, like Line 6 Spider Valve (Fryette PS2 in future), and Helix preamps into DT50.

 

The last bit of realism missing seems to be boiling down to tube amp-speaker interaction of different tube power amp types and speaker types, including Impedance Curves and dampening factors resulting from this interaction.  I think we all are realizing that a lot of magic happens at the cab/IR block :)

 

This seems to be addressable via amp matching tech like Kemper, BIAS, and Fractal.  It seems fairly consistent where you configure a similar model amp & cab then capture that final interaction with a real amp and cab via matching.

 

I get a very similar result (I think) when I capture an IR of my real cabs with the power section of a tube amp of similar type to the Helix amp that I want to pair it with.  You can test yourselves with this handful of different power amp IR's I created a while back.  

https://www.dropbox.com/sh/ch8ekey3nr4ro5l/AABr5NAVSAIm9ZzfzwG9eOWKa?dl=0

 

Redwirez provides different impedance curve IR's to mix with their standard library to get closer to this tube amp and speaker interaction.  

 

I think some of the 3rd party Helix patch guys are providing some tone match/imprint IR's with their patches.  

 

We Helix users and patch authors also seem to use EQ after the cabs to achieve a roughly similar result I think.

 

Fractal seems to have a very specific set of cabinet parameters to address this in AxeFx.  I'm wondering if additional similar Helix cabinet parameters could be a Helix wishlist or IdeaScale item and improve the overall realism. Maybe some of this tech is already in Helix and I am missing it.

 

http://www.fractalaudio.com/downloads/manuals/axe-fx-2/Axe-Fx-II-Owners-Manual.pdf

 

5.1.2 Amp Speaker Parameters

 

These parameters shape the virtual speaker impedance curve and the resulting resonances in the virtual power amp. Amp/Speaker interaction causes an increase in power amplifier response at certain frequencies, affecting the tone. Note that the power amp frequency response will not equal the speaker impedance if the NEGATIVE FDBK is greater than “0†(because negative feedback flattens the response curve).

 

LOW FREQ, LOW Q, LOW RES – Guitar loudspeakers have a low-frequency resonance, typically about 100 Hz. This shifts up slightly when the speaker is mounted in an enclosure. This resonance causes an increase in the power amplifier response due to the finite output impedance of the power amp. EFFECTS GUIDE Doc v15.0b 43

 

HI FREQ, HI RES – A loudspeaker voice-coil presents an inductive load to the power amp at high frequencies. This inductive load, in conjunction with the output transformer capacitance, creates a highfrequency resonance at the specified frequency.

 

XFRMR LF, XFRMR HF – These set the output transformer bandwidth.

 

SPEAKER DRIVE –This parameter simulates distortion caused by pushing a speaker too far. It interacts with the MASTER, which determines how hard the actual power amp is pushing.

 

XFRMR DRIVE – Controls how hard the virtual output transformer is driven. Higher values simulate a smaller, more easily saturated transformer.

 

 

All this said, I have no intention of changing over to another modeler and am staying with Helix.  I can get what I need by making my own IR's with matching tube amp type power sections.  I could certainly use a couple more cabs and mics to fill it out.  But I think this could be addressed by Line 6 and Helix and would love to see a parameter-based solution, or some kind of impedance curve matching between the Helix amp model and HX cab model that reads the amp type and speaker type...maybe similar to the way the guitar input impedance (Z-in) changes automatically on the amp type.

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I took a queue from Fractal's writeup about a 100Hz+ resonance in most guitar speaker cabs in their Axe manual, and that many impedance curves can result in a low end bump, then scoop and rise back up in the highs.

 

I made a simple Line 6 Helix preset with a Cali Rectifire amp model and stock HX 412 Cali Cab model.  I have placed a parametric eq between the amp and cab with a +4db 110Hz boost and a -3db 750Hz cut to attempt to simulate the Impedance Curve between tube amp and speaker cab.

 

The first half of the clip is eq off, second half on.   This feels MUCH more like my real Mesa Recto head and cab.  The resonance is much better.

 

I, and others, have been putting eq's after the cab, but it seems to improve the realism, feel and response of the whole amp/cab when placed between them.  The HX Cab seems to respond better and the overall sound is tighter.

 

Clip

https://soundcloud.com/michael-potts-995394155/cali-rect-impedance-curve

 

 

Preset

http://line6.com/customtone/tone/2735500/

 

In conclusion, the parametric eq between the amp and cab with some deliberate adjustment to simulate the effects of an impedance curve seems to add a touch more realism to the sound and response of the Helix HX Cabs.  This is most apparent in high gain IMO.

 

Of course this isnt a real impedance curve and doesnt capture all of the interaction between tube amp and cab.  But it does enhance the HX cab resonance and a little mid scoop that you might from the IC, bringing it closer to real for me.

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thanks Roscoe, .. i''m wondering , if you are using the parametric eq are you leaving it set at the default q setting? also are you doing any corrective eq pre or post to compensate ? i tried this idea on the litigator and i thought i may have to compensate for the low boost , thinking it might be a bit boomy but so far it sounds nice tho i am at bedroom volume levels so far.

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I modified the Q setting and gain to taste to try and match my real recto head and cab, which is a little boomy also.  I tried it in headphones and 8" studio monitors.  You could certainly tweak the gains.  I was really targeting the frequencies.

 

I think the power lies in having the eq between the amp and cab and tweaking it near those frequencies depending on your amp and cab models.

 

It may be overkill for clean to low gain sounds.  But really starts to show with mid to high gain sounds.

 

I may look at some of my tube amp/cab IR's in an analyzer app/addin and see if I can determine impedance curve effects of the different tube amp types and roughly translate it to some parametric eq settings.  But honestly, once I found the frequencies and had some ideas of what I wanted to accomplish, my ears seemed to work pretty well.

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....

 

I made a simple Line 6 Helix preset with a Cali Rectifire amp model and stock HX 412 Cali Cab model.  I have placed a parametric eq between the amp and cab with a +4db 110Hz boost and a -3db 750Hz cut to attempt to simulate the Impedance Curve between tube amp and speaker cab.

 

...

 

Very cool suggestion. I am experimenting with some IRs these days, and trying to compare with stock Helix mics/cabs. I notice some of the IRs I am playing with mention a boost at the 113Hz mark, along with some generalized statements about Lo/Hi cuts/boosts. I plan to test these things to see if I can get the stock mics/cabs to sound as good as some of these IRs. At the moment I am leaning towards IRs.

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Unfortunately the interaction between a speaker and tube power amp is very non-linear and would be hard to do more than approximate with an EQ between the amp and speaker/IR models. The interaction depends on the speaker, how hard it is driven (i.e., the elasticity of the cone edge and spider, cone flexing, changes in impedance due to different positions of the voice coil in the magnetic field), transformer saturation and most importantly negative feedback.

 

A simple IR produced from a sine sweep probably won't capture this too accurately either, since it will change depending on how loud the sweep is. 

 

This may all matter, but I'm not sure how much. Accurate modeling of existing physical devices that we love is certainly a good goal. But we shouldn't let that get in the way of innovation that helps produce new, different, and possibly better sounds. In the old days, amp designers would have loved to be free of some of the engineering compromises they had to make. We don't necessarily need to make those same compromises in the digital domain. There is certainly magic in the results they produced with those amps, speakers and cabinets. But maybe there's a new world out there waiting for us to explore.

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Yeah, I understand the nature of IR's and non-linear characteristics.   Knowing this and noting the difference between solid state amped IR's and tubed amped IR's, I went for the low hanging fruit of eq between the amp and Helix HX cab to approximate this effect.

 

I'm sure there are diminishing returns on the effort and cost of chasing every detail.  Maybe there is an opportunity to develop eq curves for specific amp and speaker type combinations for Helix.  

 

I believe that this is what Redwirez was trying to accomplish with their impedance curve IR's. @fremen here actually did a nice video comparison/demo of the Redwirez impedance curve IR's back in 2010.

 

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Unfortunately the interaction between a speaker and tube power amp is very non-linear and would be hard to do more than approximate with an EQ between the amp and speaker/IR models. The interaction depends on the speaker, how hard it is driven (i.e., the elasticity of the cone edge and spider, cone flexing, changes in impedance due to different positions of the voice coil in the magnetic field), transformer saturation and most importantly negative feedback.

 

A simple IR produced from a sine sweep probably won't capture this too accurately either, since it will change depending on how loud the sweep is. 

 

This may all matter, but I'm not sure how much. Accurate modeling of existing physical devices that we love is certainly a good goal. But we shouldn't let that get in the way of innovation that helps produce new, different, and possibly better sounds. In the old days, amp designers would have loved to be free of some of the engineering compromises they had to make. We don't necessarily need to make those same compromises in the digital domain. There is certainly magic in the results they produced with those amps, speakers and cabinets. But maybe there's a new world out there waiting for us to explore.

 

Along the same lines as your post I think perhaps even more important than 'accurate' modeling is just incorporating the degree of nuance and subtlety from the multitude of factors interacting in the old technology. That level of detail in a sound is ultimately what I hope we capture even if it is not used to emulate old amps and effects but instead constructed in a completely new way and composed of different elements and as you mentioned may yield completely new sounds.

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Im with Amsdenj on this.  I dont understand at all why people are so fixated on getting helix to sound exactly like something else.  There is nothing intrinsically good, better or great in the sounds that come from various old fashioned technologies...they  just happened to be the ones that became popular.  Modelling existing amps is useful in that it provides a familiar starting point. Beyond that an artist should be looking for something unique and original  - not copying.

 

Helix is capable of tremendous creativity.  Im pretty mainstream style wise....but even when I use my Mesa Combo I do NOT want it to sound like anybody else...of course since I bought the Helix I have not actually switched my Mesa on.  Probably never will again.

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Unfortunately the interaction between a speaker and tube power amp is very non-linear and would be hard to do more than approximate with an EQ between the amp and speaker/IR models. The interaction depends on the speaker, how hard it is driven (i.e., the elasticity of the cone edge and spider, cone flexing, changes in impedance due to different positions of the voice coil in the magnetic field), transformer saturation and most importantly negative feedback.

 

A simple IR produced from a sine sweep probably won't capture this too accurately either, since it will change depending on how loud the sweep is. 

 

This may all matter, but I'm not sure how much. Accurate modeling of existing physical devices that we love is certainly a good goal. But we shouldn't let that get in the way of innovation that helps produce new, different, and possibly better sounds. In the old days, amp designers would have loved to be free of some of the engineering compromises they had to make. We don't necessarily need to make those same compromises in the digital domain. There is certainly magic in the results they produced with those amps, speakers and cabinets. But maybe there's a new world out there waiting for us to explore.

 

Spot on IMO. Not that I didn't love the sound of my old Marshall Amp+Cab sound. But when I took the plunge into the digital domain and modellers I immediately realized that repro's of the "old world" tones was not my goal. New technology endless possibilities. I like the sound of both real amps+cabs and simulated amps/cabs. In my opinion there's no winner - just more options really.

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I totally agree with not being bound by old amp tech and embracing new tech. There are some good approaches, physics and lessons learned in the past generations of amp tech though that can help us. Personally, it does me good to revisit the old stuff from time to time.

 

This week's journey led me to the parametric eq and surgical settings between amp and cab to improve the overall feel and response. I just went back through all my patches and replaced all IR'S with Helix HX cabs fine-tuned them with upstream eq.

 

I know it sounds like an exageration, but Helix feels like a new machine to me now. The Helix cabs have great consistency, contol ability and predictability. I think they just needed some tuning beyond of their block settings. I might actually be ready for FRFR.

 

P.S. I did end up pulling down the 110Hz bump to +3db from +4db when I ran the patches at stage volume. May be the Fletcher Munson effect.

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Great dialog on the topic from the CabIR.eu thread here...

 

http://line6.com/support/topic/18056-impulse-responses-by-cabireu/page-2

 

 

 

Yes, this makes 100% sense! The convolution / impulse response tecnique is a "linear" concept. It represent frequency response and time based events (reverb tail). So, no "non-linear" events can be achieved with this concept, like Gain, dynamics, harmonics, etc. That said with "some" i meant the frequency changes, the impediance (frequency) curve from the cabinet - low damping real (tube) power amp will generate. The impediance curve depends on the cabinet, the negative feedback from the power amp and other parameters...
Long story short: Your findings match the tecnical behaviours:)

 

 

 

Good! Thanks - I'll not spend any more time wondering why the Drive setting is so non-responsive with the preamp+T combo! As you described, the full+I combo is the way to go with Helix.

 
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Great thread! This thread discuss exactly the edge of the missing link, modeling can`t achieve perhaps not 100%, because the interaction between real cabinet and real tube power amp could be for sure modeled, but based on the assumption, how the impediance curve of a given guitar cabinet would look like. In the digital world, where the amp and cab block is separated, the Amp may can model the influence of impediance curve, but don`t know the exact impediance curve and low/high resonance of a given cab IR. Every cab will produce a dedicated response to the actual amp. `Cause the IR don`t have this information, nor can it transfer this information to the amp block. Nethertheless, we should not overrate this issue, because in fact, we can achieve great and authentic tones out of our "virtual" amp rigs, right?

To be honest, i ain`t own a HELIX unfortunately (Axe-Fx here ...sorry), but this issue is the same to all modeling concepts, because of tecnical barriers, as i tried to explain. IMO!

So, let`s have a look to this plot, i made years ago ... (FYI: The Amp and Cabinet was never used for cabIR.eu IR shootings!)

(picture attached below)

 

Here we see:
BLUE: AMP DI (low damping factor). The current feedback of the given Amp it "see" of an actual, dedicated Speaker Cabinet. This frequency response depends on certain parameters, some of them: reactive load of the speaker cabinet, damping factor of the power amp, negative feedback, also resonance/depth & presence control settings of an tube power Amp, ...
This influence in the frequency, we baked in to some degree in our www.cabIR.eu "T_ube Amp" Voicing, dedicated to the specific guitar cabinet.

GREEN: AMP DI (high damping factor). As we see, the curve gets almost linear above the low-resonance peak, we see at 117Hz from the blue line. IOW: This Amp is NOT very linear below this area.

 

RED: This is, what i called "I_dealized NULL Amp". The non-linearity of the green line was mathematical removed. The result is the "ideal" linear power amp with a theoretical ∞ high damping factor. These "I_dealized NULL Amp" IRs don`t contain ANY power Amp influences, just the frequency response of the cab/speaker + mic + mic position. The rest should be the job of the Power Amp simulation of the modeler. Although some people like using there modelers with real tube power Amps to passive FRFR (what the T_ube Amp" IR versions would get closer... Or others just like these more scooped IRs...

So, when the approach is: As close, as the real thing, it does make sense to distract any influences of a power amp, when shooting IRs, because this should be the job of the virtual power amp simulation, which should also provide the nonlinear events like harmonics, gain, distortion, etc.

So, what happens to the frequency response, when shooting with tube amp coloration (T_ube Amp) or exclude this influences (I_dealized NULL amp)? I did a little "slideshow", you can click thru and "see" how the frequency response change to a dedicated cabinet / power amp (so, not related to the example picture above...):

https://www.cabir.eu/en/content/12-pro-ir-series-libraries-II

 

(Related to the second "Virtual `scoop` Tube Amp color knob" slideshow. The first one shows the "virtual" P_Ush Amp version, containg only influences of the actual Tube Amp in the low res area.

Anyhow: The most important impact seems to have the low-resonance frequency. The curve & the center frequency depends on the cab. Closed classic 4x12" cabs are mostly around the 115Hz area. But 1x12" open back combos could also have this los resonance in the 50+ Hz area! For that reason, all IR-libraries at cabIR.eu naming the low resnance frequency close the gap (as described above) a bit more... But as i said: Don`t overrate this whole issue!

I don`t know the paramters, the HELIX support. As the OP described, the Axe-Fx has such parameters you can manually adapt to an used IR. But the problem stays: Normally, you don`t know the exact impediance curve. So, most of the guys don`t touch this "speaker tab".

The idea from the OP, emulate this resonance curve using a PEQ maybe useful, maybe not. At the end, EQing is justified, when it helps to achieve the sound, you`re looking for. Period ;)
Adapt from the BLUE line in the picture, i would recommend a bell filter boost with higher Q-factor for the low resonance ... and a shelving  with lower Q-factor for the high resonance....

 

post-2339826-0-45852500-1482179123_thumb.jpg

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Awesome, thanks for the info @CabIReu!  

 

As of now, Helix has no Cab/IR block controls similar to AxeFX and I dont think that the amp models are really addressing the interaction between amp & cab either.  I think the Helix cabs are REALLY flat, maybe by design.  IMO, IR's generally seem to have frequency enhancements that the HX Cabs do not.

 

PEQ between the Helix amp and cab seem very effective in simulating this.  The Kock 212 plot is very useful to me personally.  I think I was pretty close with my PEQ settings above for the HX Mesa 412 cab.

 

I'd love to see similar plots for different Marshall cab speaker types (Greenbacks, Seventy 80's, V30's, etc.) with different Marshall amps.  I know you guys do a lot of these.

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Perfect! Thanks again!  I really appreciate the expert input.

 

I'll definitely look into some of your IR's.  I like the way you think :)

 

P.S. You should consider adding this info to your tips and tricks page.  PEQ seems just as effective giving a tube feel to solid state amp-based IR's, at least in Helix anyway.

 

https://www.cabir.eu/en/content/category/3-academy-ir-tips-tricks?id_cms_category=3

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Unfortunately the interaction between a speaker and tube power amp is very non-linear and would be hard to do more than approximate with an EQ between the amp and speaker/IR models. The interaction depends on the speaker, how hard it is driven (i.e., the elasticity of the cone edge and spider, cone flexing, changes in impedance due to different positions of the voice coil in the magnetic field), transformer saturation and most importantly negative feedback.

 

A simple IR produced from a sine sweep probably won't capture this too accurately either, since it will change depending on how loud the sweep is. 

 

This may all matter, but I'm not sure how much. Accurate modeling of existing physical devices that we love is certainly a good goal. But we shouldn't let that get in the way of innovation that helps produce new, different, and possibly better sounds. In the old days, amp designers would have loved to be free of some of the engineering compromises they had to make. We don't necessarily need to make those same compromises in the digital domain. There is certainly magic in the results they produced with those amps, speakers and cabinets. But maybe there's a new world out there waiting for us to explore.

 

+1, but:

"A simple IR produced from a sine sweep probably won't capture this too accurately either, since it will change depending on how loud the sweep is."

 

There maybe differences related to capture volume, but as long the speaker is driven within his specificiations, and also the chassis works as intended, no "bigger" particular vibrations, etc. than i doubt, that the differences would be really audible. Maybe for the gras root listeners.

 

For the rest: Well, non-linear issues, convolution can`t represent in common, true. Those artefacts the modeling algorithm must add...

 

Attached again a plot. Same cab, same mic, same mic position. It`s a shoot from a friend, so i can`t say more about, but he mentioned the only difference between the both was a volume increasement of the measurement signal (sine sweep) of about ca. 10db, he state. Also, i can`t exclude measurment errors while capturing. I did only the plots for the friend...

post-2339826-0-62668600-1482182822_thumb.png

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One heck of a Fractal wiki collection on the subject...

 

http://wiki.fractalaudio.com/axefx2/index.php?title=AMP_block_parameters#LOW.2FHIGH_RESONANCE_FREQUENCY.2C_LOW.2FHIGH_RESONANCE_Q.2C_LOW.2FHI_RESONANCE_.28SPEAKER_IMPEDANCE.29

 

One way to find the SRF is to put a Filter block after the amp block. Set the type to Peaking, Q to 5 or so and Gain to 10 dB. Start with a Freq. of 50 Hz. Play some chugga-chugga and slowly adjust the Freq. until you hear and feel the cabinet resonate. Make a note of the frequency. Remove the filter block and set the amp block SRF to match. 4x12s typically have an SRF of between 80 and 120. Open back cabs are typically a bit lower

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  • 3 weeks later...

So I just tried something pretty cool that may give us the best of both Helix HX Cab and IR worlds.

 

Redwirez created Impedance Curve IR's to be used in series with standard solid-stamp-amp created IR's to give them more of a tube feel and response.  They made ICIR's for different speaker types; Celestion Blue, Celestion G12-75, Celestion G12-M25 Greenback, and Celestion V30.  

 

Placing these between a Helix amp block and HX single or dual cab block really give another dimension of sound, feel and control to the HX Cabs and can liven things up more like full IR's.  They seem to well after the cab too with a slightly different effect that you may prefer.

 

Since Helix has a mix control on the IR block, I found that I can just use the Redwirez 100% IC IR's and adjust the Helix IR block and mix control and level as needed. 

 

https://www.redwirez.com/mixIR2UsersGuide.pdf

 

Impedance Curves (Z-Curves)
The Redwirez collection includes impedance curves of various speakers. These can be used to
reproduce the effect that a tube amp can have on the frequency response of a speaker. Compared to
solid-state power amps, tube amps have a higher output impedance. As a result the connected
speaker's frequency response will change, causing its frequency plot to look more like its impedance
curve than when driven by a solid-state power amp. You may appreciate the more scooped tone, or
the added thump and sparkle. Some people perceive the low-end bump around the speaker’s
resonance point as “warmthâ€. You should run these in a serial block, before or after the cab, the order
is not important.
The IR labeled "100perc" is the impedance curve as it was measured. This is a bit extreme if your goal
is to simulate a real world tube-speaker effect. Files with 90perc, 80perc, etc. represent "toned down"
versions of the impedance curve for less of an effect, where “90perc†is 90% wet, “80perc†is 80% wet,
etc. 50-60% wet is equivalent to the real world effect we saw with many of the tube amps we tested.
 
I wonder if Line 6 could integrate something like this into their HX Cab blocks.
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This thread made me think of something. When recording, we try hard to eliminate the impact of the room. This is because when things get played back in a room, the room effect is doubled and this sounds unnatural. Similarly when we mic a guitar amp into FOH, we only get one microphone. However if you run Helix into a FRFR and mic that - you get two. That's going to magnify the impact of the microphone on the overall tone.

 

Maybe the same is true of cabinet models and IRs. No matter how accurately the model simulates any given cabinet, mic and room, then end result is going to be played through yet another speaker in a room, or at least headphones. These speakers will have their own impact on the tone, and its likely much greater than some of the details of the cabinet models. 

 

Perhaps there's an argument to be made for not modeling everything in the cabinet models because the result is going to be played through some FRFR which is another cabinet, thereby doubling the impact of what was modeled and producing a less natural tone.

 

Certainly factors like damping factor, negative feedback, and variable impedance will be different for a guitar cabinet driven by and overdriven tube amp and a FRFR driven by a high-power solid state amp. But the impact will still be doubled.

 

Note once the power amp clips, there's no more negative feedback (because there's no more gain) and damping factor drops significantly. This happens all of a sudden and would be something that would also have to be modeled.

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Got to agree with Amsdenj:

 

"No matter how accurately the model simulates any given cabinet, mic and room, then end result is going to be played through yet another speaker in a room, or at least headphones. These speakers will have their own impact on the tone, and its likely much greater than some of the details of the cabinet models. "

 

I can get the perfect model/IR .....whatever from you and maybe load it into my Helix and I copy all your equaliser and delays and all effects settings then I still have to go through MY speakers. Granted I have FRFR but even they are not perfect. And maybe your beautiful tone came via your tube amp. And mine is now digital solidstate in my Line 6 L3t's

 

And then I play via my Fender Elite noiseless SSS with different strings than your Ibanez or Fender HSS ........and then it all sounds different anyway.

 

So chasing tone which you heard by all kinds of scientific means is almost impossible and if you get it then it will be only the once in your room at your sound level via your equipment and you cannot really transfer it etc etc.

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I made a simple Line 6 Helix preset with a Cali Rectifire amp model and stock HX 412 Cali Cab model.  I have placed a parametric eq between the amp and cab with a +4db 110Hz boost and a -3db 750Hz cut to attempt to simulate the Impedance Curve between tube amp and speaker cab.

 

 

Ive read where if you cut 4-5db from 500 Hz this also helps with IRs. Im on the fence here because on some it "really" helps, and on others it "kinda" helps. I guess it depends on the IR and how it was made.  

 

So chasing tone which you heard by all kinds of scientific means is almost impossible and if you get it then it will be only the once in your room at your sound level via your equipment and you cannot really transfer it etc etc.

 

 

So boost around 110Hz and cut around 500 Hz, and start with the amp set at 50% on bass, mids and treble. Adjust to taste, and YMMV depending on the equipment.

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I was reading through the support doc for the Ownhammer California Duo Mesa IR pack I just purchased, and found this info.  I haven't tried it in Helix yet as the OH IR's sound good as is, but thought it may be useful.

 

http://www.ownhammer.com/support/docs/duos/California-Duo.pdf

 

THE POWER AMP

This library’s captures were driven by a mostly neutral tube power amplifier. While the overall frequency response is largely even like that of a solid state reference amplifier, the common tube amp deviation traits are present that both liven and thicken up the sound ever so slightly. As such they are ideal as-is with accurate modeling platforms and tube amps sent to dummy load + line out devices. For platforms that need the little extra scoop of modestly configured guitar tube amp driven files, this is quickly and easily accomplished by implementing the following simple post processing adjustment: Page 4

 

SOUNDING LIKE GUITAR TUBE AMP DRIVEN IR’S

As aforementioned, the IR’s in this library were driven with tonally neutral tube based power amplification. There is a very quick, simple step that can be applied to replicate the sound of IR’s that were instead driven by a guitar tube power amp with the Presence and Depth set to 0, which results in a mid scoop. To simulate this sound, following the cabinet IR loader add an EQ with a parametric bell curve set to -3 dB at 400 Hz. Adjust the Q/bandwidth to roughly where the edges of the curve start to make the initial cut around 100 Hz on the low side and 2 kHz on the high side. If necessary, adjust the Q/bandwidth to taste from here to best suit your sound source and tonal preference.

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