We followed the progression from synthesized to convolution reverb… but that didn’t make synthesized reverb obsolete. Compared to the visual world, synthesized reverb is more like a painting, and convolution is more like a photograph; they both have their place.
With amp sims, the convolution-based guitar cabs — which are based on loading impulse responses (IRs) — can sound excellent. But there are some aspects of IR-based cabs that bother me, like that sort of phasey sound and the occasional “fizzy” frequencies. So, how about going in the opposite direction — from convolution to synthesized cabs? After all, a cabinet simply represents a particular frequency response. Before digital took over the world, I did cabinet emulations with analog EQ. Equalizers weren’t as sophisticated as they are now, but the emulations still did the job. Can today’s precision EQs synthesize cabinet responses that sound good enough to substitute for CPU-hungry cabs with IRs?
Spoiler alert: synthesized cabs, based on filters that emulate analog technology, can sound really good, and they’re more editable than IRs. They don’t necessarily sound better or worse but offer a different, musically useful tone. Combining synthesized, “analog” cabs with bi-amp or multiband processing creates an alternate universe of satisfying guitar sounds. Synthesized cabs also require less CPU power, so with hardware, you can create presets that may be a challenge with IR cabinets. For example, with a Helix Native multiband preset that uses two amps and four IR cabs, using EQ-based cabs cuts the CPU power by about 30%.
Okay, you don’t believe me — I understand. So, let’s listen to an audio example with the Line 6 Helix that shows how using only the Parametric EQ and Low and High Cut EQ blocks can create an “analog” cab that sounds quite similar to an impulse response-based cab. And while this article is based on the Helix, it applies to other amp sims, as well — hardware or software.
Audio Example 1
The first four measures are the stock Helix Essex A15 amp and 1×12″ Blue Bell cab. The second four measures are the Essex A15 amp and an EQ-based cab. Aside from the cab, the other parameters are identical.
First Things First
The Helix Parametric EQ has three stages along with high- and low-cut responses. Physical cabs roll off the highs and lows as a given, so the EQ-based cab emulations described here consist of two Helix EQ stages: a Parametric EQ stage followed by a Low and High Cut EQ. The latter has cutoff frequencies of 100Hz and 6kHz, respectively (in some cases, a high-cut frequency of 7.2kHz gives a little more brightness). The parametric’s high- and low-cut stages can provide further trimming.
Guitar amps typically have a high-frequency and a low-frequency boost along with a midrange cut. However, this is a very broad generalization; real cabs have a plethora of frequency-response peaks and dips, which IRs can emulate. Figure 1 shows the screenshot of the EQ settings used to emulate the 1×12″ Blue Bell cabinet used in Audio Example 1.
How to Program the EQ
One of my goals with analog cabs is to emulate some of the stock Helix IR cabs, which I can then use in presets when I need to save CPU. Another one is to come close to a stock cab but then modify it so that it’s better suited to a particular project.
My development setup has two channels, each with an effects chain consisting of (in this order) a white-noise tone generator, the Helix Native plug-in, and a spectrum analyzer. One channel has the Helix set to the IR cab to be emulated, the other Helix uses a Parametric EQ followed by the High Cut and Low Cut EQ stage. I adjust the Parametric EQ to create a response curve, as displayed by the noise spectrum, to be as close as possible to the curve generated by the IR cab (fig. 2).
Matching white-noise response curves will get you closer to the sound you want, but even the slightest variations in filter frequency, Q, and gain/cut settings can make a huge difference in the overall sound. This is where you can trust only your ears, not frequency response graphs and white noise. So, after getting close, it’s time to insert an amp and start playing guitar.
Audio Example 2 plays the Voltage Queen amp and 1×12″ US Deluxe speaker, first with the IR cab, then with the amp and the EQ-based cab.
Audio Example 2
Figure 3 shows the EQ settings for the 1×12″ US Deluxe speaker (remember that this is followed by the Low and High Cut EQ I used for every EQ-based cab). In some contexts, I prefer the analog cab sound.
Improving the Sound of Cabs
One advantage of this technique becomes clear when you have a speaker-cab IR that you like, but it’s not quite ideal for your needs. For example, I liked the overall sound of the 1×12″ Celest 12H cab, but there was a higher-frequency component I didn’t particularly like — I wanted a more midrangey tone to fit better in a particular mix. EQ to the rescue: I again used the white-noise test to approximate the 1×12″ Celest 12H cab, but then I tweaked the cab EQ to give the desired tone. Audio Example 3 plays the Divided Duo amp going through the stock Celest 12H cab, followed the EQ-based version.
Audio Example 3
Here’s the screenshot of the Celest 12H cab emulation (fig. 4).
Create New Cabinet Sounds
Being able to replace, or improve upon, existing IR-based cabinets with “analog” cabs that draw little CPU has its advantages. But, let’s be realistic — the Helix Parametric EQ doesn’t have enough stages to match the digital precision of IR-based cabs. Fortunately, sometimes coming close creates a different — but equally useful — cabinet sound (fig. 5).
If you check out this technique, you may be surprised at how even the slightest changes in EQ settings have a major effect on overdriven or distorted guitar sounds. Although emulating an existing cabinet is a great place to start with your EQ settings — you know it’s going to be fairly close to reality — being able to create entirely different cabs can be pretty useful.
Are IRs Not Cool Anymore?
Of course not! IRs do what they do better than anything else. But the best attribute of amp sims in general is that they offer options the physical world can’t offer; and when it comes to Helix, there are two viable ways to generate cabinet responses. IRs aren’t always the right tool for the right job — sometimes analog cabs will give the sound you want.
Then again, sometimes combining the two is even better. One technique is to choose an IR cab for one channel of a stereo sound and an analog cab for the other channel. The sound is close enough to still have the same character, but there can be enough differences to result in a wonderful stereo image. I’ve also started to mix in analog cabs with multiband presets — some bands use IRs, while others use analog cabs. This can often free up enough CPU to allow adding more effects or to use stereo effects instead of mono.
I still feel we’re only starting to tap the full potential of amp sims in general — and Helix in particular. I hope you find your experiments with analog cabs useful!
