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bmanic
KVRAF
 
7832 posts since 3 Feb, 2003, from Finland, Espoo

Postby bmanic; Thu Dec 07, 2017 5:31 am Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

jens wrote:
stratum wrote: I'd say it's irrelevant but you can nevertheless still fill a bug report to IKM and say their model is 20dB off the target. That's a reasonable complaint.


Thanks for revealing your agenda - that's settled then.


Indeed. It's rather amazing how narrow minded and tribal some people can be. Here is an original poster who created an absolutely awesome, superbly detailed post where most of the hyperbole is held in check..

.. then we get people like stratum here pissing all over the efforts with complete ignorance. Amazing really. Reminds us why there are struggles in this world when there are people like this who can't appreciate the effort the OP put into the first post, no matter what ones own opinion is.

@stratum: Dude.. seriously. :pray: :scared:

@alteregoxxx: Nice and informative detective work there. Please know that I'm sure there are tons of people in the silent majority that thoroughly appreciate your post. I found it extremely interesting and thorough. Wasn't planning on commenting without some tests of my own but stratum here really triggered me. :dog: :lol:
"I'm not trying to be anyone's savior. I'm just trying to think about the future and not be sad." - Elon Musk, TED talk April 2017
stratum
KVRian
 
1360 posts since 29 May, 2012

Postby stratum; Thu Dec 07, 2017 5:43 am Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice


Indeed. It's rather amazing how narrow minded and tribal some people can be. Here is an original poster who created an absolutely awesome, superbly detailed post where most of the hyperbole is held in check..

.. then we get people like stratum here pissing all over the efforts with complete ignorance. Amazing really. Reminds us why there are struggles in this world when there are people like this who can't appreciate the effort the OP put into the first post, no matter what ones own opinion is.

@stratum: Dude.. seriously. :pray: :scared:



OK guys, I have already said I had left the thread. Have fun and continue. Indeed that's a very interesting thread. I know what's wrong with it, but that doesn't mean I have the right the spoil the picnic. Acknowledged.
~stratum~
DaveClark
KVRist
 
166 posts since 8 May, 2007

Postby DaveClark; Thu Dec 07, 2017 10:36 am Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

Hi alteregoxxx,

Thanks for your efforts to compare sims to SPICE and posting your results.

Perhaps I missed them in your description, but it seems to me that there are some important details missing. For example, what is the load of your LTspice circuit? Did you attempt to model an electromechanical speaker with some purely electrical circuit, or did you just terminate it with a transformer into a resistor? Or something else?

Similarly, did you simply turn off the cabinet modelling for Amplitude 4 and other sims? Or are you running the waveform into some other sim or circuit model? The Amplitude 4 results look very suspicious for not having been properly terminated. Have you asked IK Multimedia about this? You may think that you should just be able to disconnect the cabinet and that the model should still behave appropriately, but "appropriately" could mean different things to different people.

In your discussion about memory (memoryless) and time variance (invariance) you may have confused some readers. As you probably know, any circuit that has a capacitor in it has memory. You probably also know that the 65 Twin Reverb has feedback from the transformer secondary back to the 12AT7 tubes before the pentodes and that this is also memory in an even more obvious form. What people call "sag" is also a form of memory. So when you posted that there is no "memory" (in quotes), you seem to be writing about something other than what most engineers, scientists, and other technical folks refer to as memory when discussing linearity, invertibility, memory, time invariance, etc. From what you wrote, some readers may not realize that time invariance does not imply memoryless, that these are two different attributes.

You wrote that for the first plot, the TH3 and the LTspice simulations were not notably different. In my experience in working with simulations of modifications of tonestacks, I disagree, somewhat. The frequency of the smallest magnitude is different between the two. The amount of dip below the peaks is different. The relative peaks of bass and treble are different - almost the same for the TH3 but about 3dB difference for LTspice. The lower frequency peak of the TH3 is significantly narrower than the corresponding LTspice peak. As you surely know, small differences on log plots can be large enough to be clearly heard, and I would expect these to results to sound different to me. Yes, the Amplitude 4 curve looks like it's way off, but that may be due to the termination as I mentioned above, or something like that. (It's too far off to be a trustworthy result.) The TH3 result may also be off; that is, the comparison may turn out to be worse if terminated in a similar fashion as the LTspice simulation.

Because your are an EE, I am curious as to why you decided to run white noise into the circuits. Most EEs would probably run sinusoidal sweeps because it's right there in all SPICE versions. Was it just easier to compare the amp sims with white noise? You could have, of course, created a sinusoidal sweep waveform and used that for the amp sims. Just curious.

Finally, you did not show the phase response. Did you look at that? Some people think that phase response is not important, but not most EEs. I've taken sounds and fed them through EQs I've created which modify only phase. The result can be a very different sound indeed.

Thanks again for your efforts. I look forward to reading more about your work.

Regards,
Dave Clark
alteregoxxx
KVRist
 
356 posts since 19 Sep, 2005

Postby alteregoxxx; Thu Dec 07, 2017 3:19 pm Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

DaveClark wrote:Hi alteregoxxx,

Thanks for your efforts to compare sims to SPICE and posting your results.

Perhaps I missed them in your description, but it seems to me that there are some important details missing. For example, what is the load of your LTspice circuit? Did you attempt to model an electromechanical speaker with some purely electrical circuit, or did you just terminate it with a transformer into a resistor? Or something else?


See my last answer, at the bottom of this reply.

DaveClark wrote:Similarly, did you simply turn off the cabinet modelling for Amplitude 4 and other sims? Or are you running the waveform into some other sim or circuit model?


Simply turned off the cab, no other sim or circuit model after the preamp/poweramp.

DaveClark wrote:The Amplitude 4 results look very suspicious for not having been properly terminated.


I firmly believe you can not have unproper termination in the vst amps sims world: if this could happen, you could not correctly use external impulse responses loaded in third parts plugins, as normally happens.
How do you think Amplitube or TH3 or whatever vst amp sim could ever interact "electrically" with a third part impulse loader?
I'll try to explain better; I can think of only 2 possible paths:

1)The developer of the amp sim did plan the interaction with the speaker load to happen when use their own native internal cab sims--> In this case, you should have really bad sound when turning off the native cab sim and using third party impulse loader vst and, obviously, electrical arcs, bad smells and, finally, fire because of the missing load on the transformer secondary.

2)The developer of the amp sim did plan that interaction to happen when use their internal cab sims but, moreover, the finicky developers coded their sims in a way that, when the user turn off the native cab sim, the code automatically "load" the power amp with a "general purpose" electrical load simulating a "generic" speaker as load-->Hence, no problem of unloaded power amp here so....

Moreover, as I wise person, before posting screenshots here, I also measured the frequency response of the Amplitube model with its own cab sim on, so, following your reasoning, with its power amp "loaded". And measured TH3 using this same Amplitube cab sim as "load" for TH3. Guess what? Amplitube was still 20dB off....

DaveClark wrote:Have you asked IK Multimedia about this?

I did not, sorry.

DaveClark wrote: You may think that you should just be able to disconnect the cabinet and that the model should still behave appropriately, but "appropriately" could mean different things to different people.


See my previous answer about load termination and amp sims.
DaveClark wrote:In your discussion about memory (memoryless) and time variance (invariance) you may have confused some readers. As you probably know, any circuit that has a capacitor in it has memory.

Capacitors store DC values. Did you read In that discussion that I mentioned "bias point shifting" ?


DaveClark wrote:You probably also know that the 65 Twin Reverb has feedback from the transformer secondary back to the 12AT7 tubes before the pentodes and that this is also memory in an even more obvious form.


Nope, that feedback is only...feedback. it's not memory because it happens instantaneously, the moment next to the one it happened it has no memory of what it did before. But we are defying in philosophy.

DaveClark wrote:What people call "sag" is also a form of memory. So when you posted that there is no "memory" (in quotes), you seem to be writing about something other than what most engineers, scientists, and other technical folks refer to as memory when discussing linearity, invertibility, memory, time invariance, etc.


Again, "Sag" is a cool word to name guess what?....bias shifting.


DaveClark wrote:From what you wrote, some readers may not realize that time invariance does not imply memoryless, that these are two different attributes.

Again, read carefully the part about bias shifting: I splitted the concept in two parts. I said you can have a time invariant electronic circuit, but also that if you shift its bias point, the same signal that before gave you a result, after the bias shift will give you a different one. Now, you can call it memory if you like to call it that way. Truth is that it is simply bias shifting. Put the circuit in the same exact bias point it was before it was altered by an external signal, power supply fluctuation or whatever, give it a signal and it will alway respond with the same output. That's all.

DaveClark wrote:You wrote that for the first plot, the TH3 and the LTspice simulations were not notably different. In my experience in working with simulations of modifications of tonestacks, I disagree, somewhat. The frequency of the smallest magnitude is different between the two. The amount of dip below the peaks is different. The relative peaks of bass and treble are different - almost the same for the TH3 but about 3dB difference for LTspice. The lower frequency peak of the TH3 is significantly narrower than the corresponding LTspice peak.


Well, we're talking of a circuit that is more than 50 years old, subject to thousands of minor "mods". My LTSpice schematic is of aTwin Reverb '65 reissue. Where in the "original" Twin Reverb there were old 240Kohm pots now there are 250Kohm in the '65 reissue, some caps value are not standard anymore, so they too could be slightly different in the '65 Reissue...That's alone can justify some "little" differences. I can swear that measuring two identical built circuits never gave me same identical results! :)

DaveClark wrote:As you surely know, small differences on log plots can be large enough to be clearly heard, and I would expect these to results to sound different to me.

You know...It was not me estabilishing we can consider 3dB differences small enough :P
Jokes a part, For sure they are not identical. Nor I expected them to be, as I explained previously.
Comparing that minor differences to a frequency response that is 20dB off....well...Don't you think one is a "very minor" difference, the other is a gigantic one ? Perspective is the key of life!

DaveClark wrote:Yes, the Amplitude 4 curve looks like it's way off, but that may be due to the termination as I mentioned above, or something like that. (It's too far off to be a trustworthy result.) The TH3 result may also be off; that is, the comparison may turn out to be worse if terminated in a similar fashion as the LTspice simulation.

Uhmmm...I've no caught the logic....Th3 may also be off, but it agrees (Please, forgive my 3dB philosophy!) with LTSpice, Amplitube is massively off...Two results agree, more or less, one is compeltely off...I repeat, I do not believe termination has a relationship with all this.


DaveClark wrote:Because your are an EE, I am curious as to why you decided to run white noise into the circuits. Most EEs would probably run sinusoidal sweeps because it's right there in all SPICE versions. Was it just easier to compare the amp sims with white noise? You could have, of course, created a sinusoidal sweep waveform and used that for the amp sims. Just curious.


Simply to compare, right! (No slope to set on Spctrum analyzer, no reasoning on time integration to choose etc etc etc...)

DaveClark wrote:Finally, you did not show the phase response. Did you look at that? Some people think that phase response is not important, but not most EEs. I've taken sounds and fed them through EQs I've created which modify only phase. The result can be a very different sound indeed.

Please no! :help: Please, do not call for phase response to come in!! :-D
Audibility of phase response is a can of worm. I just don't want to start philosophals debates. Nevertheless, I'll give you my point of view: In some extreme cases, read large enough time shifts between spectrum components of a signal, it can be audible. But those cases cross the frontier with time delay audibility. All the other cases, I think phase delay is not audible. But that's just my opinion and I don't want to convince anyone that I'm right nor he/she is wrong.

DaveClark wrote:Thanks again for your efforts. I look forward to reading more about your work.


You're welcome! I'll post some more screenshots, maybe in a day or two, adding in the party Peavey Revalver 4. I'll take the time to reply also to your first question with some examples.
Image
DaveClark
KVRist
 
166 posts since 8 May, 2007

Postby DaveClark; Fri Dec 08, 2017 11:05 am Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

Hi alteregoxxx,

Thanks for your detailed responses to points I wrote about.

1) You said that improper termination would cause problems with third-party cabinet IRs.

I agree that this is a likely result, but my question actually has two parts: What did you use for termination of the amp sims? What did you use for termination of the LTspice simulation? If these are different, then your results are not fully valid. It is very important to ensure that you have terminated the amp sims in the same manner as the LTspice simulation, or somethng very close to this. Unless I missed it, you didn't say what you did for LTspice. I have used a transformer model, no transformer model, resistive loads, etc. They all sound very different, much like what you are showing.

Much more likely, you have simply gotten the bass knob set to completely different values between Amplitube and LTspice without realizing it. From your plots, it appears to me that increasing the bass knob might put the red line right up into the orange one.

Something such as this is very wrong, probably something simple. That is why I don't really think that 20 dB off versus 3 dB off is a valid comparison of error. Not completely impossible, but very unlikely. Do you really believe that guitarists cannot hear that the bass is 20 dB too low? Even Amplitube DSP programmers should be able to hear that!

If I have time, I'll look at the Amplitube model you are working with and see if I can determine anything. I also have ngspice (modded to allow binary signals) and LTspice.

2) You said that in the VST world, you cannot have the termination incorrectly set.

Well you certainly can have this! I didn't bother to set the terminations to anything particular in my own guitar amp sims (LG2Amp, etc.) when the cabs are turned off. If you turn the cabs off, interfacing them to any external cabinet modelling program is your problem, not mine. I allow cabs to be turned off simply to remove the cab effect so that they can be used for acoustic guitar, piano, voice, etc. I did it to remove a filter, not with the intention to allow other filters to be used.

If users heard differences between amp sims with the same cab, they would attribute those differences to the amp sims' modelling, not to different terminations of the circuit, even though the latter may be correct. That is similar to what you did.

3) Unless I misunderstood you: You still insisted that there is no memory. You also said that feedback is instantaneous.

I am very surprised by this because these claims are simply not correct, assuming that you are using the technical definition “memoryless.” This is not philosophy at all; I am speaking about fact. A simple RC lowpass filter is not memoryless; look at the basic equation for voltage of a capacitor which involves an integral over time. In the digital world, most DSP implementations of circuits contain n-1, n-2, etc. and some contain y[n-1], y[n-2], etc. These are also not memoryless. It is the existence of memory that allows filters to work. Bias-shifting is not instantaneous. It can't be.

Capacitors cannot instantaneously change voltage, and inductors cannot instantaneously change current, hence reactive circuits have memory. Guitar amps have reactive circuits (and reactive parasitics) so have memory.

4) You say that phase discussions are usually philosophical or usually something like a philosophical discussion.

These discussions are often about two different things, so people talk past each other, as we say in the English language. It is true that absolute phase of an infinite single-frequency signal, as far as we know, cannot be determined by listening. (I suspect that one could almost prove this from logic alone.) However, when a temporary signal has more than one frequency, shifting the relative phases can audibly change the sound. Music is composed of many temporary sounds so can be audibly changed by shifting the phases of the constituent frequencies relative to one another.

I hope I understood what you wrote correctly and did not misinterpret what you wrote.

Thanks once again for your discussion. I'll let you know if I get some time to look at this myself, but I was planning to rewrite my plugins for CPU efficiency and clean them up a bit. (Lots of trial code is in all of them.)

Regards,
Dave Clark
alteregoxxx
KVRist
 
356 posts since 19 Sep, 2005

Postby alteregoxxx; Fri Dec 08, 2017 3:27 pm Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

DaveClark wrote:Hi alteregoxxx,

Thanks for your detailed responses to points I wrote about.

1) You said that improper termination would cause problems with third-party cabinet IRs.

I agree that this is a likely result, but my question actually has two parts: What did you use for termination of the amp sims? What did you use for termination of the LTspice simulation? If these are different, then your results are not fully valid. It is very important to ensure that you have terminated the amp sims in the same manner as the LTspice simulation, or somethng very close to this. Unless I missed it, you didn't say what you did for LTspice. I have used a transformer model, no transformer model, resistive loads, etc. They all sound very different, much like what you are showing.


For LTSpice : Transformer model loaded with 4 Ohm resistor. And this is the only way I could do it, because I didn't want to see the speaker freq response coming in....

Amp sims have been measured simply putting their cab sims off. Please, could you explain me how many ways you know for " properly terminating an amp sim" ? You have no way of forcing an amp sim coded by someone else to "electrically interact" with the world after the blackbox the amp sim it's closed in...The only thing you can do is to turn its cab sim on or off...

And, let me contraddict you about the "They all sound very different": I can assure you that the frequency response of the preamp alone, in LTSpice, is 99% identical to the frequency response you can measure at the push pull transformer output.
Look for yourself, only 2dB off at 40Hz...:

Image




DaveClark wrote:Much more likely, you have simply gotten the bass knob set to completely different values between Amplitube and LTspice without realizing it. From your plots, it appears to me that increasing the bass knob might put the red line right up into the orange one.

Something such as this is very wrong, probably something simple. That is why I don't really think that 20 dB off versus 3 dB off is a valid comparison of error. Not completely impossible, but very unlikely. Do you really believe that guitarists cannot hear that the bass is 20 dB too low? Even Amplitube DSP programmers should be able to hear that!


Nope. Look at the screenshots...you can see the knob for TH3 and Amplitube...Same positions...Same for LTSpice...

"Do you really believe that guitarists cannot hear that the bass is 20 dB too low?"

Me for sure can. In fact all this started beacause I tought (and still think) Amplitube Twin Reverb 65 model sounds thinny and not like a real Twin. At least it didn't sound at all like the solid state model I realized and was testing on a breadboard with oscilloscope, spectrum analyzer, ears and a guitar...

Don't know...Maybe Amplitube DSP programmer are bass shy!

DaveClark wrote:If I have time, I'll look at the Amplitube model you are working with and see if I can determine anything. I also have ngspice (modded to allow binary signals) and LTspice.


Please, do it.

DaveClark wrote:2) You said that in the VST world, you cannot have the termination incorrectly set.

Well you certainly can have this! I didn't bother to set the terminations to anything particular in my own guitar amp sims (LG2Amp, etc.) when the cabs are turned off. If you turn the cabs off, interfacing them to any external cabinet modelling program is your problem, not mine. I allow cabs to be turned off simply to remove the cab effect so that they can be used for acoustic guitar, piano, voice, etc. I did it to remove a filter, not with the intention to allow other filters to be used.

If users heard differences between amp sims with the same cab, they would attribute those differences to the amp sims' modelling, not to different terminations of the circuit, even though the latter may be correct. That is similar to what you did.


So, in your models, if the user turn off the cab sim, you leave your modeled transformer unloaded? And the output of your modeled cirtcuits doen't oscillate?? Uhmm...now THAT is really strange! :D

DaveClark wrote:3) Unless I misunderstood you: You still insisted that there is no memory. You also said that feedback is instantaneous.

I am very surprised by this because these claims are simply not correct, assuming that you are using the technical definition “memoryless.” This is not philosophy at all; I am speaking about fact. A simple RC lowpass filter is not memoryless; look at the basic equation for voltage of a capacitor which involves an integral over time. In the digital world, most DSP implementations of circuits contain n-1, n-2, etc. and some contain y[n-1], y[n-2], etc. These are also not memoryless. It is the existence of memory that allows filters to work. Bias-shifting is not instantaneous. It can't be.

Capacitors cannot instantaneously change voltage, and inductors cannot instantaneously change current, hence reactive circuits have memory. Guitar amps have reactive circuits (and reactive parasitics) so have memory.


Don't really want this go on for ever: it's not the argument of the topic.
Nevertheless, I guess language barrier doesn't allow me to better communicate my "vision of things". So far so good.

Low pass filters, hi pass filters (and so on...) have memory. Because they have capacitors and or inductors in them. So what? What audible effect do you think a capacitor memory has on the sound that pass trough the latter?
I'll try (last time, I swear! :) ) again with an example.Let's try:

Take a "soft" signal, think of it like the guitarist caressing the guitar strings, call it A, and pass it trough a low pass filter; record the output coming out of this low pass filter, depurated from its DC content, hence leaving only the signal.
Do you think you'll obtain a different output passing again this same "caress" signal in the filter, let's say 20 seconds later during which you "maltreated" the poor low pass filter with an hard rock riff? I don't think so. What will be different? Only DC levels on the capacitor of the low pass. Good. DC levels in guitar amp affect bias point and THAT has an effect on "compression", distortion and whatever adjective guitarists all around the world use to define the sound of an amp when pushed, because it influences the way active elements, the only one that are non linear (I know I know...capacitors are not ideal etc etc etc...but please...don't start on this too...).
Bias shift take time to move? Agree! Never said the contrary! I was only trying to give a "different point of view" on the "memory" argument. But I give up, I promise. :lol:


DaveClark wrote:4) You say that phase discussions are usually philosophical or usually something like a philosophical discussion.

However, when a temporary signal has more than one frequency, shifting the relative phases can audibly change the sound. Music is composed of many temporary sounds so can be audibly changed by shifting the phases of the constituent frequencies relative to one another.


Whitin the limits already exposed in my previous answer, I repeat that I do not agree.
I'm not able to hear 360°, 530°, 110° phase shift of one frequency component of a complex signal.
Maybe it's a limit of mine. :)
When phase imply longest enough TIME DELAY, I think it can become audible.

This is what I discovered from my own tests. I do not pretend others agree with my own perception.


DaveClark wrote:
Thanks once again for your discussion. I'll let you know if I get some time to look at this myself, but I was planning to rewrite my plugins for CPU efficiency and clean them up a bit. (Lots of trial code is in all of them.)

Regards,
Dave Clark


Hope you'll find some time to post here your own results.
Image
electro
KVRAF
 
4212 posts since 5 May, 2002

Postby electro; Fri Dec 08, 2017 5:11 pm Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

TH1 and TH2 have the same algorithm for Twin Blackface and TH1 came out in 2009. Does anybody know if they updated it for TH3?

Amplitube Fender 2 is very recent and much better modeled than Fender 1 which also dates back to about 2009.
Intel Core2 Quad CPU + 4 GIG RAM
alteregoxxx
KVRist
 
356 posts since 19 Sep, 2005

Postby alteregoxxx; Sat Dec 09, 2017 6:18 am Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

Ok, here again. Let's recap using some images (In the meantime I've added Guitar Rig5, PeaveyRevalver4 and also LG2Amp, from ExeConsulting developer, to the "battle"). The following are comparison on the TwinReverb 65 amp model (exception is LGAmp2, that is not, from my knoledge, specifically modeled on any amp; nevertheless it has in the various tone stack filters ad the user disposal, also one that is "Fender like", so I decided to see how it behaves).

Fender Twin Reverb '65 comparison:

Image
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Image

JCM800 comparison:

Image
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alteregoxxx
KVRist
 
356 posts since 19 Sep, 2005

Postby alteregoxxx; Mon Dec 11, 2017 6:30 am Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

Just some quick consideration on the reasons about this small analysis I performed.

First of all, it hasn't to be considered an exhaustive and complete analysis. My intention was only to perform a quick check, to verify if at least the frequency response of the simulated amp model, that is the simplest part to model, was right. From my point of view, if the amp sim is not able to mimic not even the frequency response of the original, there are few chances that it will reproduce the more complex aspects of the original amp accurately, for example the dynamic behaviour of the latter.

This doesn't mean that such an amp model can't sound good on its own, instead, you could like it even better than the original! Nevertheless, its modeling can not be defined accurate. It can only be considered a a good sounding amp on its own, but, despite marketing claims, not an accurate model of this or that amp, sorry....

I've to add that I've also checked for accuarcy in modeling the dynamic behaviour of these amp sims, but I've not posted any screenshot to avoid overcomplicating an already complicated topic...
It has not been a super-accurate check in testing THD in LTSPice versus all the amp sims, because such a test, would require accurate matching on the signal level feeding the amps, and this is a complicated matter when you are comparing a "real" simulation in LTSpice, where you can use "real" signal voltage, versus an amp sim that lives in the VST world, where the levels are confined in a -1 to +1 range...you know...
Nevertheless I exploited a characteristic of the JCM800 to conduct a "reasonably significative" analysis: the JCM800 circuit exhibit a "shift" its frequency response when subjected to low input signals levels versus high signals levels. In other words, I've simulated the JCM800 circuit in LTSPice, stimulating it with noise at 5-6 different voltage levels, from really low to really high. Did the same with amp sims, and looked for "similarity" in the frequency response "shift".


With this considerations in mind, I'll give my point of view on the amp modeler I've tested, both in frequency response accuracy terms (more objective evaluation) and dynamic behaviour terms (more rough, semi-scientific evaluation):

AMPLITUBE:

I'm a bit puzzled by the Amplitube modeling of the Twin Reverb 65, especially beacuse that is an "Officially approved by Fender" specimen. Frequency response seems to be really off.

Their generic "British tube lead 1", not officially approved by Marshall, seems instead, paradoxically a lot more accurate in modeling the behaviour of a JCM800. Dynamic behaviour seems accurate too. Good!

To the people that were suggesting that the Twin Reverb 65 is part of their "old" Fender1 collection, and so that it would have been better to investigate the "new" Fender2 collection, I reply saying that, unfortunately, in this second collection, they did not model a Twin Reverb65; They focalized more on the "tweed" models. Probably they think their first collection already did a good enough job in emulating Fender amps they modeled at that time. Who knows....

OVERLOUD TH3:

They seems to model accurately all the amps I've tested, included the nuances that leads to a frequency response alteration on certain amp models, for example, when having the volume pot at really low or really high values. Dynamic behaviour it's good too. Really good Job!

DIGITECH RP360:

Despite not being as meticulous as Overloud TH3 people in modeling "frequency response volume pot nuances", they did a discrete to very good job (depending on the specific amp model) in modeling the amps I've tested, both, in frequency response and dynamic behaviour. Considering the price of this unit, this is a really really really good sounding solution, except for their cab simulation "system", that completely sucks.
For example, their Twin Reverb 65 2x12 cab, has a frequency response that have some peaks and deeps, like real guitar cabs must have, but problem is that it extends "almost flat" up to 20Khz: no guitar speaker behaves this way, that's sure.
I've discovered, however, that it is this way, because, when you connect headphones to the unit (and behave this way also when you set the unit in what they call "mixer mode", as opposed to the "amp mode") they "superimpose" a strange low pass filter to the the cab sim selected by the user, and their conjuncted work act to "resemble", finally, the frequency response of a guitar speaker...well...a bad one, I mean...
In other words, try the Digitech RP360 with external guitar speakers impulses and this unit simply SHINES! Try it using its internal guitar cab simulation system and you'll fall in a spiky mosquito fuzz world.
An alternative solution, is to use the equalizer included in its internal effects modules and cut the hell around 4Khz (8-12dB), boost 4-5 dB at 125Hz, cut a couple of dB at 8Khz. It works more or less, from good to really good, with all the cab simulations included in the unit.

PEAVEY REVALVER 4:

Similarly to Overloud TH3, they have really accurate modeling going on here. The possibility of tweaking the amp sims circuits in a "SPICE like accurate way" it's simply mind-blowing!!
They also seems to have paid particular attention to the power amp-speaker interaction: It's more than a suspect that this is the reason for that "particular" peak in the really low region, around 50 Hz, for the TwinReverb65 emulation, and the "light" boost in the high freq region in comparsiono to the LTSpice simulation.
Revalver 4 has in fact a setting for choosing the power amp-speaker interaction, where the user can choose between 4 possible "kind of interaction". Moreover there is also the possibility to tweak the "fspeaker interaction" parameter, read the amount of interaction between the speaker and the power amp.
In tube power amps,in fact, being the latter naturally functioning as quasi-current drivers, their interation with guitar speakers (that have a peak impendance in the region between 40-100Hz, usually, and a rising impedance starting from 300-400 Hz) cause a "boost" at "low" freq and "high" freq.
Considering that for my simulation in LTSpice I've used as load for the output transformer a 4 Ohm resistor, hence not a guitar speaker, I should have set this "fspeaker interaction" parameter to the value 0: I've discovered this after I had already posted the screenshots, sorry.

Nevertheless, you can see that the power amp-speaker interaction, can't cause such big differences in frequency response as the ones visible for the Amplitube case. This, in conjunction with my previous LTSpice graph, should also turn away the suspects that the user @Dave Clark had about this point.
Hence, a part from that peak caused by my fault, Revalver 4 results seems to be spot on with LTSpice results. Outstanding.

GUITAR RIG 5:

It seems to be sufficiently accurate in terms of frequency response. However, on the TwinReverb 65 analysys, it seems to deviate from the LTSpice results when analyzing the Twin Reverb 65 with the switch "bright" active (this analysis is not shown in my screenshots for none of the amp sims, nevertheless I've tested also that :-) ).
JCM 800 seems sufficiently well emulated.


LG2AMP (from exe consulting, read the user @Dave Clark):

This is not intented to emulate any specific amp, at least from my knowledge, so there is no sense in evaluating its accuracy in emulating a TwinReverb65.
It has to be intented, as from my understanding, as a general purpose clean amp.
Nevertheless, if it's supposed to emulate a generic amp, in the meaning of a generic TUBE guitar amp, judging from its frequency response in the high frequency, I'm humbly dubious on how (and if) the effects of grid stoppers resistors/non-zero impedance source stage feeding another following stage/miller capacitance have been modeled: in all the guitar amps, emulating such aspects of the real tubes, should lead to a subtle (or less subtle, in function of the circuit topology) hi-freq attenuation. Here I can see no such effect.
Still, this is a really good sounding amp sim! Bravo!
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Ichad.c
KVRian
 
1007 posts since 8 Feb, 2012, from South - Africa

Postby Ichad.c; Mon Dec 11, 2017 10:43 am Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

Thank you for the detailed analysis, was a very interesting read.
oxxyyd
KVRist
 
30 posts since 11 Oct, 2012, from Stockholm

Postby oxxyyd; Tue Dec 12, 2017 5:14 am Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

Most interesting read. Have you done any comparison of distortion (harmonic contents etc) at some levels, say half and full gain?
alteregoxxx
KVRist
 
356 posts since 19 Sep, 2005

Postby alteregoxxx; Tue Dec 12, 2017 4:14 pm Re: How good amp sim emulate real circuits? Amplitube '65 Twin reverb vs TH3 '65 Darkface vs LTSpice

oxxyyd wrote:Most interesting read. Have you done any comparison of distortion (harmonic contents etc) at some levels, say half and full gain?


No, as I already said, this would require taking some time and thinking on matching levels of test signals in LTSPice vs VST world. It's doable for sure, but requires a little bit of thinking and test on my side. As soon I'll have some spare time maybe I'll give it a try :)
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