Disable preamp in Amp Block?

nikki-k

Experienced
I want to use an outboard (guitar/bass) preamp with the Axe as a means to utilize the power amp modeling, the Cab Block, and the features both offer (simulated interactivity). Is this possible?

I could go direct with the preamp, or even utilize the Axe (strictly) as a post-preamp effects unit and then utilize a convolution plugin in my Dawquencer (IRs), but I would not benefit from the Axe's power amp and cab block modeling features.

Optimally, I suppose what I would love to have is the Axe to have Preamp Block, Power Amp Block, Cab Block, plus the existing Amp Block. I will create a coincident Wish List post, so if this post goes in that direction, no harm in deleting it Mods :) The intention of this post is not to "wish" for anything, but rather to find out if there exists a way to effectively "bypass" the preamp part of the Amp Block, and if not, what workarounds might exist, if any.
(apologies if this has been discussed (to death) previously; I did not find it with a cursory search)
 
Okay, found old posts. (sigh) Looks like "this will never happen" was the result. So, no "Wish" for it, as it "will never happen."

My original request stands then: Is anyone using an external guitar/bass preamp with the Axe and the Amp (& Cab) Block? If not the Axe for the power amp and cab modeling/IR/etc, then what?
 
Yes, I actually used (till recently) an engl e530 and a tech 21 psa as well as 2 other preamps with the axe ultra (those 2 are the ones I used the most tho). I ran them (with the amp sims in parallel) before the cab block, and found the results to be very satisfactory :)

I have not used them with the II, and I also didn't benefit from the poweramp part of the sim like you want, but I liked the sound. I can understand wanting them separate tho, but I remember reading that it was most likely a no-no somewhere. In the II you'd lose the benefit of the dynamic part of the modeling with the amp/cab interaction, but I doubt that it would be possible anyway, as the amps in the axe are just mathematical models, as opposed to the real world preamps, which are analog, and won't exhibit the push/pull phenomena that the axe would give with the mathematical models.
 
Nikki-k,
With my Ultra I sometimes used a Soldano X99 preamp in the efx look and put an amp block (the "tube pre" model) after the loop. That way I used the poweramp-sim in the very neutral "tube pre" model with the X99 - worked great. I´ve posted about it before, so do a search on my name and those posts and the threads regarding the use of external preamps with Axe Fx will be easily found.
Haven´t tried it with my Axe II though, but don´t see how it would be different.
Hope this helps.
 
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@Jon:
Cool! That is part of a wish that seems will never come to be, as I do love rack preamps and power amps, and the interchangeability that affords :D (Triaxis + 2:90, or maybe a.. gasp.. Marshall rack power amp, or a (sweet!) VHT, etc. So many combinations, more flexable than an amp, and different)

Nikki-k,
With my Ultra I sometimes used a Soldano X99 preamp in the efx look and put an amp block (the "tube pre" model) after the loop. That way I used the poweramp-sim in the very neutral "tube pre" model with the X99 - worked great. I´ve posted about it before, so do a search on my name and those posts and the threads regarding the use of external preamps with Axe Fx will be easily found.
Haven´t tried it with my Axe II though, but don´t see how it would be different.
Hope this helps.
Thanks! I will do that search :D

Couldn't you use the "tube pre" and change the tone stack around?
Feeding the front of an amp with a preamp is what I wanted to avoid, as the Axe is modeling.. an amp. However, the tube pre is a great idea, as it is not (theoretically) an actual "amp head." I thought of the various rack preamps being modeled (JMP-1, Triaxis), and though they are followed by an unknown "power amp," I could not find a way to do the inverse of that. Completely forgot about the tube pre... off to try :D
 
yes, the tube pre is a great idea, and i hindsight, seems totally obvious now! hahaha :) good call! ;)
 
I found an easy way to mix and match preamps and power amps from the different amp blocks, at least the way things stand in firmware 5.07. It requires the use of two amp blocks but works very well. It doesn't work well for a few extremely high gain models but it works for most of the models. It's based on the premise that while you can't completely disable a preamp, you can render it almost neutral with the correct settings. Here's the process I use to insure everything goes smoothly and I achieve the desired result:

For the preamp simply use an amp block and turn off the power amp by setting the Sag to zero. Adjust the preamp output Level to something close to unity - when you bypass the block the volume should be about the same or slightly louder than the when the amp block was active. Leave the block bypassed for now.

To place the power amp of your own choosing after the preamp, place a second amp block containing the power amp you wish to use after the preamp block and start with default values Then place the Tonestack POST preamp, make it active (very important), and set it flat (0dB on all controls).. Make sure the Master Volume is PRE-PI and set it relatively low for now to avoid power amp distortion.

Once you've done the above, set the Drive parameter in the second amp block to a very low value so that there is no distortion coming from the preamp. Don't set it too low. Just set it low enough so that there is no audible distortion. As in the case of the first block, use the overall output Level control to adjust the volume of the second amp block so that the bypassed sound is close to the volume of the sound when the block is active.

The preamp tubes are not being driven into distortion if you set the drive properly and the EQ in some amp models will be so close the the bypassed mode you'll hardly be able to tell the difference. If the EQ isn't flat (doesn't sound close the bypassed mode) an EQ block before the preamp block (with no overall gain) can usually be used to fine tune it to closely match the bypassed sound.

Now make both blocks active. There should be no distortion at this point from either block if you've set the levels per the instructions above. If you increase the drive in the first block you will get preamp distortion. If you increase the Master Volume in the second block you will get power amp distortion - maybe!

Here's the most important part. With the weak output of the preamp in the second amp block (to keep the preamp uncolored and undistorted) you may find you have insufficient gain to drive the power amp into distortion. This is where the XFormer Match control comes into play. If you turn the XFormer Match control in the power amp section up to about 5.0. you should be able to use the Master Volume control to get more distortion than you will ever want from the power amp since it behaves and sounds just like gain in the phase inverter. If you can't, it's likely that you set the drive on the preamp too low and are well under the threshold of preamp distortion. Go back and check the level and bring it up if you need to again without creating audible distortion in the preamp.

You can also now use the tonestack as well as the speaker resonance curve to push the power amp harder and get more distortion and different voicings without affecting the preamp in the first block.

Like I mentioned, for some of the very high gain amp blocks the preamp has so much coloration and gain that it's hard to get rid of. It's easy to use any preamp from any amp block (by disabling the power amp) along with the power amps of almost all of the low to mid gain amp block models, but it's more difficult to use the power amps of high gain models where the preamp associated with it has so much coloration that it's difficult to get rid of by turning down the gain and using corrective EQ.

Sorry this is so long, but it works only if you're very careful when you set the levels.
 
This has been a wish list item since early days.

It seems obvious that it would be a GREAT addition to the multitude of options we have with the axe, but Cliff has said that it would unleash too much behind the scenes stuff to the competition.

One can hope some day it will be an available option.
 
If separating the code into preamp and power amp sections reveals the secrets to the competition why not try something like this:

choose current amp models that represent a few popular power amp types. I.E. Marshall 800 for el34, mesa rec or mark for 6l6, vht deliverance for kt88/6550.

Then modify the code so from the input to the tone stack has a neutral unity gain effect no matter where any parameters are set. not disabling or bypassing them but making the end result that comes out of the processing equivalent to what goes in.

Name these models "Power Amp el34, 6l6" etc... and add them to the model choices.

I always wondered why an idea along these lines was never considered as far as I can tell.
 
I agree this would be an awesome option. In my last recording project before I got my Axe I used Amplitube and when it came time to final mix I was able to use 4 tracks per rhythm guitar, for my tracks it was 5150 stock & 5150 w/EL34 50 watt power section, and the other 2 were JCM800 stock & 800 w/6L6 power amp. This really helped create a depth to the sound.

I know that you can "sort of" do this with the tone stack switch but I have never been clear on if switching tone stacks just affects the tone or does it affect the drive & other preamp characteristics of the model as well.
 
I agree this would be an awesome option. In my last recording project before I got my Axe I used Amplitube and when it came time to final mix I was able to use 4 tracks per rhythm guitar, for my tracks it was 5150 stock & 5150 w/EL34 50 watt power section, and the other 2 were JCM800 stock & 800 w/6L6 power amp. This really helped create a depth to the sound.

I know that you can "sort of" do this with the tone stack switch but I have never been clear on if switching tone stacks just affects the tone or does it affect the drive & other preamp characteristics of the model as well.

If the tonestack location is PRE on the original preamp model then switching the type of tonestack affects both the tone, the drive, as well as other preamp characteristics of the model - the whole shebang!

If the location of the tonestack on the original preamp model is POST then switching the type will have almost no effect on the preamp drive, and it will affect the tone similar to an simple EQ placed between the preamp and power amp, and it MAY have an effect on "other preamp characteristics" since it's still "connected" to the last preamp tube (in theory).

Look at a schematic of a guitar amp and you'll see what I mean.

I think somebody should find a simple schematic of a guitar amplifier, label the sections, and explain what each part does. This would greatly help in understanding how the AXE controls relate to a real world amp. I'll do it when I find some time if nobody else will.

The most popular combination (or one of the two most popular and my favorite):

TONESTACK > PREAMP > PHASE INVERTER > POWER AMP > OUTPUT TRANSFORMER > SPEAKER

Rewritten:

TONESTACK (a type of EQ)

> PREAMP (makes distortion or not and boosts signal)

> PHASE INVERTER (something the power amp needs so the tubes work in what's called "push-pull mode, a "cooperative" mode that allows class AB or class B operation which is much more efficient than class A which needs no phase inverter)

> POWER AMP (self explanatory, can be class A or more popular and powerful class B or AB)

> OUTPUT TRANSFORMER (reduces the output voltage of the tubes so the speaker gets the lower voltage it needs, and boosts the output current to the speaker)

> SPEAKER (self explanatory)


Guitar amps are relatively simple devices. There aren't that many designs, which is one reason a lot of the models sound very similar.
 
The problem with these ideas is that there is no flat tonestack, even if set to "0" and Active there is still colorization. The "Tube Pre" uses the Vintage TS. You can fudge setting and get lots of usable sounds, but none as good as a real-deal power amp sim IMO.

My solutions are the Two Notes Torpedo products which have really good power amp sims (both single-ended and Class AB El34, EL84, 6L6, and KT88, in both Triode and Pentode modes, with Presence and Depth feedback controls), as well as state of the art Cab Sims which allow real-time mic positioning and Load Boxes.

Two Notes Audio Engineering | Advanced speaker miking simulation
 
The problem with these ideas is that there is no flat tonestack, even if set to "0" and Active there is still colorization. The "Tube Pre" uses the Vintage TS. You can fudge setting and get lots of usable sounds, but none as good as a real-deal power amp sim IMO.

I agree with that, but you don't need a flat tonestack, you can also use an EQ before the preamp, and using your ear, an EQ, and the tonestack, you can get damn close to flat if you're willing to take the time in most cases - not all. The active tonestack is extremely close to flat, but the preamp circuit adds colorization that must be removed, again in most cases.

I think it's almost as good as a real-deal power amp sim but a PITA to do. It wouldn't be possible without the XFormer Match parameter.

Thanks for the links.
 
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@DJD100:
Ahhh! I believe Michael Wagener swears by the VB-101 (hardware), IIRC. Checking out the plug-in version.

I have no real problems when tracking; the issue is having extremely limited ability to track. I have to be absolutely sure no neighbors will be offended, and that no reps for the landlords are out and about (duplex, so not too much of an issue). Come warmer weather, we have two in-window AC units, so those months are out completely for "live tracking." Maybe ver6 will be the firmware to change the game. My primary reasons for wanting to use the Axe for power amp + cab are the power amp modeling, and the interactivity between it and the cab that is modeled as well. If this is not possible, then I suppose I could just relegate the Axe to effects loop duties, and secondary amp models, and go full on software for the power amp + cab. Tracking with an amp at low volume is not an issue at all... it is the mics and needed volume, proper recording environment, etc that I need to... replace :D
 
@Search, Jon, Rocket, Funeral, etc...
Thanks for all of the suggestions :D

Going to try the suggestions and see if anything "clicks." As I said, going to wait for ver6 and see if it changes things. I have various preamps I like a lot, and it would have been nice to have a way to use the Axe. I was not that pleased with the "preamp DI + plugin IR" route previously. Since an IR made utilizing a power amp or power section of a head and a cab will not *properly* replicate the two via convolution (solely), it is the power amp (and amp->cab interactivity) that I was hoping to have.
 
...the interactivity between it and the cab that is modeled as well.

The method I suggested will admittedly have no interactivity between the preamp and power amp blocks.

In many guitar amps I've seen schematics for the coupling between the preamp and the power amp's phase inverter is a single capacitor. This being the case, I don't see how there could really be any significant interactivity between the preamp and power amp. I've always considered them to be separate entities with no interaction between them.

HOWEVER, there may well be circuits that couple in a way that does provide for some sort of interaction - if they exist I'm simply not aware of them.

I run my A/Cs until it's cold, then turn them off to record. It's a pain and it gets hot again fast but it works. The electric company likes it! :D
 
"Need" is a relative term...

If you're happy with the Axe as a power amp sim great, though in my case the Torpedo stuff is much more realistic (and I did use eq to improve things when attempting to use the Axe as a power amp sim with my tube pre's, but the accuracy just wasn't there).

There is much more to it than just whether or not the non-bypassable tone-stack is semi-flat, additional params such as tube type, operating class, single-ended or push-pull, phase-inverter, triode or pentode, etc.

I agree with that, but you don't need a flat tonestack, you can also use an EQ before the preamp, and using your ear, an EQ, and the tonestack, you can get damn close to flat if you're willing to take the time in most cases - not all. The active tonestack is extremely close to flat, but the preamp circuit adds colorization that must be removed, again in most cases.

I think it's almost as good as a real-deal power amp sim but a PITA to do. It wouldn't be possible without the XFormer Match parameter.

Thanks for the links.
 
There is much more to it than just whether or not the non-bypassable tone-stack is semi-flat, additional params such as tube type, operating class, single-ended or push-pull, phase-inverter, triode or pentode, etc.

If the preamp can be made very close to flat, and the signal level is kept very low (as my instructions call for) it will respond in a very linear fashion and should add no significant coloration to the sound. The circuit configuration becomes more and more irrelevant as the signal level is reduced. Even very non linear devices when operated on a small part of their transfer curve characteristic become very linear.

The small amplitude of the signal is what allows the preamp to become "null". If you read my very long explanation of how to set it up it results in the preamp never having a signal large enough in amplitude to cause a significant non-linearity. That's a fact.

Again, it doesn't work with some of the extremely high gain preamps, that I acknowledge. I also believe you when you say a real hardware power amp gives better results. I've experimented with a Peavey tube power amp and the results are definitely unlike the power amp sims in the AXE. I suspect (and I could be totally wrong) that the output transformer has a lot to do with the difference. OTs are extremely complicted and difficult to model. The physical construction of an OT is very simple, but the electrical characteristics are unbelievable complicated. Almost all of the parameters are distributed rather than discrete, which makes it a nightmare to duplicate in software. It's also inherently non-linear and exhibits hysteresis - an odd form of non-linearity that makes it even more difficult to model. That's why I tend to think the OT may have something to do with it (as well as the inoperable XFormer Drive parameter and the XFormer Match parameter that doesn't act like a real OT) and I TOTALLY believe you about your experience with the hardware. Whatever works - more power to you! :D

IIRC, the power amp modeling is supposed to undergo significant updates in 6.0. Time will tell whether they make a big difference.
 
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Well, that's the problem now isn't it (linear response)? If one is pushing one's tube PI/power amp into it's compression/distortion sweet-spot then one needs non-linear response, that's the whole point of the exercise!

If one is desiring linear response with the output tube/OT freq response curve then the Axe will do a sub-standard job of it. The preamp gain and tone coloring are always in the mix to some degree, and can never be fully eliminated.

All that said, if you get the tones you like jumping through the Axe's hoops then great, enjoy it, but if you really want accurate non-linear tube power amp sounds for use with external preamps then look elsewhere (like Two Notes, which has virtual power amps designed specifically for use with external preamps).

I tried everything as I really wanted the Axe to do this (I'm a 25 year audio engineering pro, had my Ultra for years etc), including begging FAS to make the change, but it wasn't to be. If you're interested and have a low-latency computer DAW, download the Two Note's Demo VST Plug and try their EL34 power amp sim to hear the difference (NOTE: They modeled their NFB Loop uniquely, so start with the Presence Control on 100%). Two Notes also has a 15 day demo license on their full versions though you'll need an iLock2 to use it (VST, RTAS).

If the preamp can be made very close to flat, and the signal level is kept very low (as my instructions call for) it will respond in a very linear fashion and should add no significant coloration to the sound. The circuit configuration becomes more and more irrelevant as the signal level is reduced. Even very non linear devices when operated on a small part of their transfer curve characteristic become very linear.

The small amplitude of the signal is what allows the preamp to become "null". If you read my very long explanation of how to set it up it results in the preamp never having a signal large enough in amplitude to cause a significant non-linearity. That's a fact.

Again, it doesn't work with some of the extremely high gain preamps, that I acknowledge. I also believe you when you say a real hardware power amp gives better results. I've experimented with a Peavey tube power amp and the results are definitely unlike the power amp sims in the AXE. I suspect (and I could be totally wrong) that the output transformer has a lot to do with the difference. OTs are extremely complicted and difficult to model. The physical construction of an OT is very simple, but the electrical characteristics are unbelievable complicated. Almost all of the parameters are distributed rather than discrete, which makes it a nightmare to duplicate in software. It's also inherently non-linear and exhibits hysteresis - an odd form of non-linearity that makes it even more difficult to model. That's why I tend to think the OT may have something to do with it (as well as the inoperable XFormer Drive parameter and the XFormer Match parameter that doesn't act like a real OT) and I TOTALLY believe you about your experience with the hardware. Whatever works - more power to you! :D

IIRC, the power amp modeling is supposed to undergo significant updates in 6.0. Time will tell whether they make a big difference.
 
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