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1:1 - Let`s talk A/B testing actual Amps to their Axe-Fx II Counterparts

Der JD

Inspired
I've a/b'd my Peavey 6505 head and Rectifier cab to the Axe model. Ran the Axe through the Peavey power amp. The Axe seemed slightly flatter, more one-dimensional. Difficult to say if this was due to tone or feel.

I tried it with power amp sim in the Axe off and on. Greatly preferred it off.
 

Casper

Inspired
This is not a new argument.

This reminds me of the same issue 20+ and more years ago. Only then, the comparison was rack to power vs head/combo. There was always something lost with a tube rack preamp to tube power amp combo vs a single head, or even better a combo amp. There was never an explanation to be had, but having separate units always lost some feel. Heads always had more whomp.

Maybe Cliff can explore this. It may not be obvious with a straight circuit diagram modeling ideal components. Maybe the answer to the magic is not easy to define - maybe some sort of combo of shared and imperfect supplies, mechanical chassis or maybe proximity effects.
 
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Sixstring

Axe-Master
This is not a new argument.

This reminds me of the same issue 20+ and more years ago. Only then, the comparison was rack to power vs head/combo. There was always something lost with a tube rack preamp to tube power amp combo vs a single head, or even better a combo amp. There was never an explanation to be had, but having separate units always lost some feel. Heads always had more whomp.

Maybe Cliff can explore this. It may not be obvious with a straight circuit diagram modeling ideal components. Maybe the answer to the magic is not easy to define - maybe some sort of combo of shared and imperfect supplies, mechanical chassis or maybe proximity effects.
FWIW, Cliff posted this which sounds like it address the issue.


All models are affected. The harder the virtual power amp is driven the more noticeable the improved preamp modeling will be.

Before I was treating the models as separate preamps and power amps as though you were using a rack system with a preamp unit and a power amp. I did some tests and noticed that there is quite a bit of interaction between the power amp and preamp in an integrated amp (combo or head). So now the modeling feeds the power supply voltage from the power amp algorithm back into the preamp algorithm. As the B+ sags (and the screens droop and bounce) the preamp is affected as it will in a real amp.

What happens is that as you hold a note or chord the B+ sags. The preamp voltage sags as well but at a slower rate due to all the extra capacitance and resistance between the screen voltage and the preamp tubes. Eventually the preamp voltages sag enough to compress 10-20% depending on the amp. This is a couple dB or so of compression. It's a slow compression though so your pick attack is unaffected.

I don't think I hear much, if any, difference but the feel is definitely different. It feels a lot easier to play like I don't need to press on the strings as hard.

I imagine the low end will be affected as the compression will cause less drive into the power amp and therefore less saturation at the resonance of the speaker.
 

Philip34

Experienced
I will try this. I have a Mesa MarkIII with 'simul-class' power amp -> open back cab 1X12 Black Shadow. I can run this either 15Watt Class A on EL35s, or 60Watt Class AB on 6L6s, so i can try both ways. Which i will do just as soon as i can get my Axe FX II.
Is that a green stripe Mk3? We will be waiting for that comparison. I miss my Mk3.


By the way, Cliff, I'm really curious: what is the difference between a Mark IIC+ and a Mark III green stripe?
 

BillyZeppa

Power User
Is that a green stripe Mk3? We will be waiting for that comparison. I miss my Mk3.


By the way, Cliff, I'm really curious: what is the difference between a Mark IIC+ and a Mark III green stripe?
Nope it's a purple stripe - a bit smoother than the other stripes. :pride:
 

PacoCasanovas

Fractal Fanatic
When i use the Koch Studiotones FX Send (only Preamp) OR Line Out (inbuilt Power Soak = ON; Line Out is in reference to the schematics behind the PowerAmp) and go from their into the Matrix GT1000 into the Guitar Cabinet, something gets to be lost. The tone / notes sticks more at the speaker, note seperation get a bit lost, the "magic" disappear.
The Matrix is a flat responce amplifier with a high damping factor. There will be no influences from the speaker (resonance hump or inductive coil impedance rise) affecting your tone. That is what you're missing when comparing the Matrix against a tube amplifier. You can't have those effects with a pre-setted powersoak - if you don't use a real speaker in parallel you won't get similar results. Most powersoaks were just linear acting resistors and use some capacitors and a dummy coil to simulate the speaker influences - some came close, but most can't.....

This test wont work IMO.....

Since the AxeFx does all the "magic" stuff between output tubes, transformer and load - the Matrix is a true flat responce, high damping power ampifier for FRFR applications. The goal for the Matrix products are transfer functions of MOSFETs in their output stages and improved inputs for guitar application, which is a "feel-thing" and act like as we expect to hear from any good guitar power amp! But it does not colorize the sound....

Regarding the Speaker Page in the Advanced Section: Not all amps react the same, some amps have minor- others have major interaction between load and their output stage. It depends on the amount of the load, on the transformer, on the NFB-line on the cabinet (!!!) were the speakers were mounted - if it's open- or closed back, it also depends on the wiring of a cabinet (mostly if various types of speakers were used).......

I really, really like to make this experience without using a Tube-Poweramp. Is this possible???
It is..... ;)
 
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PacoCasanovas

Fractal Fanatic
Yes.

There is no current feedback in a typical guitar tube amp. It is voltage feedback. Your Koch has no feedback.
I guess he refers to the solid state amps with current feedback to simulate the interaction between load and power amp.... ;)

Current feedback on the output stages is only done by solid state amps who want to simulates the interaction between load and a tube amp, something what usually wont work because of the high damping factor of the solid state amplifier. Damping factor is the ratio between Output-Impedance and Load-Impedance. Tube amps have a much lower damping factor than solid state amps....
 

guitardoc

Experienced
What I realised is that apart from the strange artificial hiss the AC30TB model produces (which I mentioned in the other thread) the sound from

Guitar->AxeFx->Matrix->AC30TB combo speakers

seems to die significantly earlier than from

Guitar->AC30TB combo.

I don't know why that is though. May it be possible that the signal coming from AxeFx or Matrix is not as 'hot' or long lasting as from the actual amp?

Btw, is there a difference in controlling the volume from the AxeFx output or the matrix input? Usually I use bridged mode and open up the matrix full throttle and adjust per AxeFx output taper. Tried it the other way round and didn't notice any which doesn't necessarily mean that there is none. Any experiences?
 

PacoCasanovas

Fractal Fanatic
I don't know why that is though. May it be possible that the signal coming from AxeFx or Matrix is not as 'hot' or long lasting as from the actual amp?
Since the Matrix amp does not interact with your Vox Speakers, you hav to simulate the correct interaction inside the AxeFx to get similar results. The Matrix only amplifies that signal.....
Remember - the default settings all refers to a miced amp... ;)
 

guitardoc

Experienced
Hmm, that may pose a little problem. I go directly to FOH via output 1 and as an old school Rock'n'Roller I need the din on stage to feel what I'm playing, so to matrix and guitar cab via output 2 (per FXL block before the cab block which comes last in the grid).

The FOH sound is just awesome, no question. What buggers me is the sound on stage coming from the guitar cab (as mentioned). Since I obviously can't make changes to the FOH line since that would deteriorate the sound that goes to the FOH - what are possible options that I can utilise to enhance the stage sound which only affect output 2 and lead to the desired result? I'm out of my depth here, since I think simple EQing will probably not suffice, will it?
 

Morphosis

Fractal Fanatic
(...)the default settings all refers to a miced amp... ;)
If i understand Cliff right, the modeling with default values has nothing to do with miced or unmiced. The modeling consider the load of a "matching" (proper) speaker cabinet of the actual model:

So when modeling or measure you connect a convenient Guitar Cabinet as load to the amp, this load (guitar cabinet) could also be the draft for the speaker tab impedance curve (or resonance?!) settings and so all the "current feedback" interactions between PowerAmp and Cab will be considered when modeling and measurements?
So, the comparison guitardoc has made describes exactly my A/B comparison question in my OP:
Guitar->AxeFx->Matrix->AC30TB combo speakers

seems to die significantly earlier than from

Guitar->AC30TB combo.
So he compared the [AC30TB Sim]+[Matrix Amplifier] vs. [actual AC30TB] ... both into the identical cab.

In my understanding this should result in a nearly "spot on" comparison. If this is not happen i can imagine different reasons:

- Maybe AC30`s are so different from each other like Plexis. Perhaps Cliff`s model based on a different type as the one from guitardoc?
Cliff: "I intentionally limited the midrange notch since, IMO, it's a design flaw in that tone stack. But you can recover that behavior by setting the Mid control fully CCW" Source "Furthermore, modern AC30's don't have this flaw. They use a Fender-style tone stack with a fixed mid resistor" Source
source: AMP (block): list - Axe-Fx II Wiki

- The speaker used when modeling was another type as guitardocs used - This would require small adjustments on the speaker tab page, IMHO. That said, i don`t think those will be made the "night and day" differences, at least not those, guitardoc mentioned.

@guitardoc: Nice comparison in general exactly what i am asking for! When you did that: Did you take care about comparable parameter settings on the Axe-Sim and the AC30? Especially in reference to Hi / Low input; Hi-Cut, etc...? Did you try it also instead in Bridge mode in stereo mode with only one side of the amplifier? Don`t know, if bridge mode will influence the amplifying quality to audible differences (aside of loudness)

I don't know why that is though. May it be possible that the signal coming from AxeFx or Matrix is not as 'hot' or long lasting as from the actual amp?
Cable quality? Try it with the shortest you`ll find, don`t know...
 

kmanick

Fractal Fanatic
you can branch down off of the main line of blocsk and place a few things in before the Fx Loop block to experiment with. I generally use a GEQ and a PEQ and try to balance off Frequencies that differ from the Cab IR(s) I have going to FRFR. I have also used a Multiband compressor a few times (that was interesting). I find that without adding at least the GEQ , my sound going directly to my poweramp cab seems to suffer a bit, with some slight low mid boost and high end cut it brings the sound much more 'alive". this is going to differ form cab to cab but with my Jsx->2X12 recto these adjustments help considerably.
one of my typical patches (now they are a little different because I am going W/D/W but this is the idea)


Hmm, that may pose a little problem. I go directly to FOH via output 1 and as an old school Rock'n'Roller I need the din on stage to feel what I'm playing, so to matrix and guitar cab via output 2 (per FXL block before the cab block which comes last in the grid).

The FOH sound is just awesome, no question. What buggers me is the sound on stage coming from the guitar cab (as mentioned). Since I obviously can't make changes to the FOH line since that would deteriorate the sound that goes to the FOH - what are possible options that I can utilise to enhance the stage sound which only affect output 2 and lead to the desired result? I'm out of my depth here, since I think simple EQing will probably not suffice, will it?
 
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