Re-amping

Let’s say you’re a guitar player and want to record your track of a song. Typically you would enter the studio with your guitar, amp and cabinet and spend record what’s needed. But let’s say you decide to record your guitar without any processing by pedals or amps, glue the good takes together and then process that track with pedals and amps. That process is called re-amping.

Now let’s say you want that guitar track processed by a plugin, not hardware. The process of recording would be almost the same, but the process of getting the plugin to sound like an amp is quite a challenge. Today if we want to emulate the whole rig, it is best if it’s done in two parts: NAM and IR. IR is much older, and it stands for Impulse Response. NAM stands for Neural Amp Modeler and it’s just a few years old. IR is used to capture the sonic characteristic of reverb and guitar cabinets, the most. It can do other things, but for now let’s just say that IR in our case is used to emulate guitar cabinets. NAM in our case is used to capture and emulate preamp. It can also capture full rig but you can’t use any effects that are time based, like reverb or delay. The reason why I like NAM for preamp and IR for cabinet is because that way I can use IR cabinets captured over past two decades. As NAM is relatively new, it still doesn’t have such a huge ecosystem.

Preamp and NAM

One of the great things that came out of AI hype is Neural Amp Modeler, or NAM for short. To capture NAM, you have to separate preamp from the rest of the gear. As stated before, you can capture NAM of your whole rig, but today I want to capture just the preamp. In my case it is Engl E570. It’s pretty rare so I’d like to be able to replace it with a NAM loader pedal (hardware pedal that loads NAM and emulates my preamp). If you’d like to profile preamp that is part of amp head or combo, you’ll need to use FX loop. To be precise, you’ll use Send on the FX loop as that is the preamp’s output. What is needed for that beside preamp is audio interface, computer and re-amp box. My choice of operating system is FreeBSD, but once you have JACK configured, the rest is the same as Linux. On the mentioned computer I will use JACK and Ardour to capture the sound of preamp. Computer is connected to audio interface via USB, and in my case it is Presonus AudioBox 1818VSL. It has line level outputs, which is almost exclusively the case on audio interfaces, so I can’t just plug it’s output to the preamp’s input. The impedance and signal level of line output do not match those of the guitar input on the preamp. To convert line to guitar level, I am using Radial Engineering EXTC-Stereo. It has inputs and outputs on one side, which are connected to the audio interface, and send and receive on the other side, which are connected to the preamp. To be precise, audio interface output is connected to EXTC, send of EXTC is connected to preamp input and preamp output is connected to audio interface (line in). If I call computer + audio interface just PC to make it short, this would be the “diagram” of connections

PC -> EXTC -> preamp -> PC

What we want is to send a signal from the PC, process it with the preamp and send it back to PC for analysis. To do that, create Ardour session at 48kHz and create two tracks with names “sweep” and “capture”. Both tracks need to be mono. Connect sweep’s JACK output to the output on the interface which is connected to reamp box. Connect capture’s JACK input to the input on the interface which is connected to the preamp. That way you can record the output of preamp in capture track and play some signal on the sweep track. Sweep is the term left from IR, and I will explain why it’s called that in the follow up post. For now all you need to know is that it’s not “normal” sound, it is a signal created so that AI can learn from it. To be precise, AI will learn from sweep and captured signal and it will produce information how to digitally transform the sound so it is the same as if we just did the reamp. Input on sweep track needs to be disconnected and output on capture track, too. That way we are minimizing causes of eventual problems. To get the sweep signal, go to tone3000 and download “sweep signal” from the page (downloaded file will be called T3K-sweep-v3.wav). Now all you need to do is arm your capture track and record the output of the preamp while the sweep signal is fed to the preamp’s input. Once that’s done, align start and end of sweep and captured clip. Also, set session start and end to the clip’s start and end. Normalize the captured clip. Now we need to export sweep and capture to separate, mono files at 48kHz and 24bits. To do that, go to

Session -> Export -> Stem Export

If you don’t already have a format for 48kHz (or session rate) at 24bits, create one. We need wav files that are in no way further processed. That means disable trimming and normalization. In the Time Span tab select session range and only that one. In the Channels tab, select sweep and capture track and disable Apply track/bus processing. Now export the track to wav files.

Go to tone3000’s capture and upload dry (sweep) and wet (capture) wav files. Follow the form and wait for AI to process all epochs (100 by default). Once that’s done, you’ll have NAM file published on Tone3000 platform. Congrats!

Tips

For better performance I suggest compiling jack from ports with SOSSO library enabled. It can dramatically reduce DSP usage in JACK/Ardour.

To test your NAM you can use neuralrack-lv2 to load it together with some IR. Nice thing is that tone3000 already has some IRs, so download some and test it in neuralrack-lv2.

While there is also a way to only upload the capture, it never worked for me. The reason is that I couldn’t make Ardour produce the exact same number of samples in the capture file, that the sweep file has. With stem export, tracks are exported to files with the exact same length. Briefly talking to Tone3000 support, they told me that input signal should be uploaded only if custom signal is used to train the AI. I would love to not waste resources by uploading both files, but I always get an error while producing the NAM if I upload only the capture.