These days, rather than miking my guitar amps to track clean tones in my home studio, I record direct into my computer and get my sounds using digital “fingerprints” of these amps I’ve generated using impulse response (IR) technology. Frankly, the tones I get using amp IRs with direct-to-interface recording are 99-percent identical to the sound of a miked clean amp ... without the hum and hiss. The process is like using an amp simulator plug-in, but rather than the edgy, artificial tones they often yield, IRs sound startlingly real. As an added benefit, you skip the endless preset scrolling that often comes with using amp sims. It has made me an IR convert.
Through the miracle of IR technology, you too can easily make tone clones of your clean guitar or bass amps, and quickly load the digital replicas into your favorite digital audio workstation (DAW) for late-night sessions or just plain hassle-free recording. If this sounds intriguing, let me take you through the step-by-step process of making your own clean amp IRs. I’ll use popular software from Apple and Ableton to illustrate the procedure. We’ll start with the basics.
Typical impulse responses are made by playing and recording a sine sweep (a succession of sine waves spiraling up in frequency) through a system (i.e., speaker plus room plus mic), and then “deconvolving” the recorded signal to derive an impulsive sound, or IR, that reflects the sonic characteristics of the system. You then load the IR into a “convolver,” which can be a plug-in or hardware box. The sound fed through the convolver reflects the frequency response, phase, and ambience captured in the IR.
As a Premier Guitar reader, you may be familiar with the technique of using IRs of speaker cabinets to accurately reproduce the sound of an amp plugged into a speaker. This typically involves running the speaker output from an amp head into a load box, and then feeding the line out of the load box into your audio interface. The interface sends the signal into your chosen DAW software running a cabinet IR plug-in that’s loaded with a 4x12, 2x10, 1x12, or whatever cabinet style you have available. This is an excellent way to record your amp’s tones—including overdriven sounds—without having to set up microphones and make a racket that’s inappropriate for most domestic home studios.
Using techniques that are similar (but a bit easier) than making cabinet IRs, you can make IRs of the whole amp experience—tone control settings, cabinet, speaker, and, if you like, spring reverb. If you use distant mic techniques, you’ll have the sonic imprint of your recording room, too. What you cannot capture with amp IRs, however, are qualities that are non-linear, such as overdrive, distortion, or compression.
Before we dive into creating IRs, let me convince you that modeling clean amps with IRs is worth the effort and yields sounds that are essentially indistinguishable from an actual miked amplifier. You’ll be able to compare a phrase played through a 20-watt, 1x12 Fender Deluxe Reverb II tube combo (Fig. 1) and miked with a Shure SM57 to an identical phrase recorded direct into the DAW and then processed with an IR made using the same Deluxe Reverb, settings, and mic. (I’ll describe this IR-crafting process in a moment, but for now, let’s focus on the A/B comparison.)
To ensure there was no variation in performance from example to example, I captured the initial phrase using a Boss RC-2 looper. The looper pedal then ran into either the miked Deluxe Reverb II or directly into my Apple Logic Pro X DAW and Impulse Response Utility software. My audio interface was a RME Babyface Pro. I kept the phrase short to facilitate our comparison test.
Fig. 1. — The 20-watt tube combo featured in both the miked amp and IR clips: a fantastic 1985 Fender Deluxe Reverb II. The volume was on 3 (loud, but not loud enough to noticeably distort), and the 12" speaker was miked with a Shure SM57 positioned at the center of the cone.
For even more comparison fun, I added a third example. Once again, I ran the looper directly into the DAW, but this time the signal fed an amp simulator plug-in, rather than the IR plug-in.
To avoid confirmation bias, initially I’ll identify the audio clips only by number. Later, I’ll reveal which of the three clips is the actual miked amp, which is the direct recording convolved with an IR, and which is the direct recording processed with an amp sim. These examples have no EQ, compression, or other processing.
Ready? Check out Clip 1, Clip 2, and Clip 3 online.
Now let’s crank up the Deluxe Reverb II’s glorious spring reverb and try another A/B test. We’ll skip the amp sim, which means one of these clips is the direct recording run into an IR and the other is the miked amp. Listen to Clip 4 and Clip 5 and see if you can distinguish one from the other.
Now for the big reveal: In Clip 1 and Clip 4, you hear the miked amp; Clip 2 and Clip 5 are the direct guitar feeding Logic’s Space Designer convolution plug-in hosting the Deluxe Reverb II’s IR. Clip 3 is the direct guitar running into the amp sim—in this case, Logic’s Amp Designer.
Fig. 2. — The Amp Designer settings used in Clip 3. How does this compare to the Deluxe Reverb II’s IR?
Close, but no cigar.
A couple of things stand out in these clips: The actual amp recordings (Clip 1 and Clip 4) have a constant low-level hum and hiss, particularly when the reverb is engaged. I was tempted to remove it with Izotope’s Denoiser plug-in but thought it best to leave the recordings unprocessed.
Anyway, what is remarkable to me is not only how the guitar tone is darn near identical between the miked amp and IR clips, but that the spring reverb in Clip 5 is perfectly captured with the IR, just without the extra hiss. And while I got close in Clip 3, the amp sim version, I couldn’t replicate the Deluxe Reverb II tone in Amp Designer, so I gave up tweaking. Fig. 2 is a screenshot of Clip 3’s amp sim settings.