Studio Diary #17: Getting A Better Recorded Guitar Tone By Using Impulse Responses In The Right Place

The whole point of last autumn’s home studio revamp was to get things to the point where I could start recording music again. However, as I mentioned in my 2019 review of the home studio gear, I wasn’t sure I’d got my signal chain order sorted out.

It was a good call.

What’s The Problem?

I have real guitar cabs, but I don’t mic them up. Partly because I’m not very good at it, and mostly because I don’t have the space to leave the mics out between recording sessions.

So I’ve been using impulse responses (IRs for short) for many years now.

An impulse response is an acoustic model of a speaker cabinet + the mic used + the preamp used + any outboard effects (like compression and EQ) used at the time. They’re a perfectly acceptable substitute for recording a mic’d up cab.

To use an impulse response, you need something called an impulse response loader (IR loader for short). There’s a lot of IR loaders to choose from. As far as I’m aware, all the major ones sound the same.

I’ve been using a DAW plugin as my IR loader.

It’s never sounded right – it’s never sounded like the kind of tones that folks on YouTube seem to effortlessly get – and until now I’ve never understood what I was doing wrong.

The problem is that my DAW plugin is in the wrong place in the signal chain.

Why Signal Chain Order Matters

If you think of the signal chain in a pro audio studio, it roughly goes like this:

• guitar into amp & cab (perhaps with pedals involved too)
• mic in front of cab into some sort of preamp
• the preamp is normally part of a channel strip on a studio console, which colours the sound on the way in
• the channel strip normally includes EQ and compression
• additional outboard equipment can be patched in too

and it’s the output of that signal chain that then hits the recording software.

In a home studio, we still have preamps, they just live on our audio interface. Some interfaces (like the UAD Apollo that I use) can emulate what a console channel strip does to the incoming sound.

Look at where the speaker cab & microphone is (the part that we replace with an IR). It’s right at the front of the signal chain. Now look at where an IR in my DAW is, by comparison.

It’s in totally the wrong place.

So how do we fix it?

Option 1: Channel Strip Plugins In The DAW

The first way we can fix this is to move as much of the signal chain as possible into our DAW. If we do that, we can control the order that everything happens in, and make sure that the IR loader runs before any channel strip plugins and other effects.

It’s not perfect. The preamps on the audio interface will still be colouring the sound on the way into the DAW, but it’s good enough.

Folks like Waves and Slate Digital (and loads of other people!) sell plugins that you can run in your DAW to emulate classic recording console channel strips. I’ve never used them myself, but they’re held in high regard by people who do this for a living.

My rig’s a bit different. Instead of running plugins in my DAW, I’m offloading those plugins to my audio interface, a UAD Apollo. It’s like having a physical console channel strip between my guitar amp and my DAW. It means that I don’t have to buy a better computer, and (arguably) the plugins that run on the Apollo sound just like the original hardware they’re emulating.

And it’s why an IR loader plugin in my DAW is in totally the wrong place.

Option 2: Buy An IR Loader Device

I’ve fixed this by buying a physical device to go between my guitar rig and the preamp on my UAD Apollo. It’s effectively a little computer that runs IRs on the input signal, and sends it off to the preamp of my audio interface.

There’s quite a few devices to choose from.

I’ve gone with the Two Notes Torpedo CAB M. It’s about £250 GBP at the time of writing, which is far cheaper than alternatives from Two Notes themselves, Boss and Universal Audio. I’ve paired it with a Two Notes Torpedo Captor (which I already had), because the CAB M doesn’t include a load box of its own.

You must use a load box to avoid blowing the output transformer in your guitar amp!

What Are The Trade-offs?

The main trade-off is that the Two Notes Torpedo CAB M can only run two impulse responses at a time. That’s the equivalent of only being able to have two microphones on a speaker cab at the same time.

Sometimes it’s nice to blend three IRs together – for example, to incorporate a room mic or a mic at the back of a cab too. Equally though, constraints can sometimes be better for creativity. In practice, it hasn’t been a negative for me.

So far, I haven’t found any other trade-offs.

And I’m finally happy with my guitar tone. So much so, in fact, that I haven’t plugged an amp into my actual speaker cabs since I added the CAB M to my rig.