What a wonderful time it is to be alive and a guitarist. At no other time in history could the average six-string enthusiast have a collection of top shelf guitar amps without having to sell a kidney to afford them. I’m talking about amp modeling/simulation, specifically software amps. Yeah, I know there are purist out there , “amp sims suck, nothing sounds like a real tube amp cranked to infinity”. I agree, a cranked tube amp does sound quite glorious, but it’s also quite effective at angering the neighbors and attracting the police. There’s a time and place for everything, unfortunately for many guitarist we rarely have it for a Marshall dialed up to ten.
My first experience with software amp simulation was with a demo version of Native Instruments Guitar Rig. I sat in front of my computer for days, staring at the inspiring GUIs of vintage looking amps. My finger tapped my mouse button tweaking the amps knobs like I was the Eddie Van Halen of the computer world. But much to my chagrin, no matter what amp model I tried and no matter how much tweaking I did, my tone always sounded flat, thin, and totally uninspiring. My first inclination was to declare software amp sims sound like crap, but after watching a video of Dweezil Zappa getting some great tones out of the very same Guitar Rig software, it made me want to dig deeper into what influences the tone of amp simulation software. I will now share with you, with the best of my limited technical ability, what I discovered to be the missing piece to my software amp puzzle.
The root of my problem was an impedance mismatch. In simplest terms, impedance (symbol Z) is the amount of opposition a circuit has to an AC current and is measured in ohms (symbol Ω). In the guitar world the AC current is the signal coming from the guitar’s output jack and the opposing circuit is the guitar amp. It should also be understood that the guitar has an output impedance and therefore we can summarize that the amp’s input impedance sees the guitar’s output impedance. The whole concept of impedance got rather techie and riddled with algebraic equations causing me to conjure up undesirable memories of high school; fortunately I didn’t have to dig too deep into the subject to get what I needed to get out of it. Basically all I needed to understand was the relationship between the output impedance of my guitar and the input impedance of what I was plugging it into, in my case a Strat into a MOTU 8pre audio interface. Here are some of the key points I learned that might be helpful to you:
- Ideally, the input impedance of what I plug my guitar into should be at least ten times the output impedance of my guitar. This insures maximum voltage transfer from the source (guitar) to the load (amp, mixer, audio interface, etc). Maximum voltage transfer = maximum frequency response.
- A passive pickup equipped guitar’s output is high impedance (Hi-Z) and is typically around 10,000 ohms (10k ohms) but can range from 6k ohms to 15k ohms.
- Most mixing consoles and audio interfaces have balanced XLR inputs that are low impedance ( Lo-Z) and are typically around 2K ohms and balanced 1/4″ inputs called TRS inputs which are Hi-Z with a typical impedance of 10k-20k ohms.
- Some mixers and audio interfaces have dedicated Hi-Z instrument inputs with an impedance ranging from 500k ohms to 1,000,000 ohms (1M ohms) which is equivalent to the input on the majority of guitar amps.
- A computers sound card’s line input is usually Lo-Z.
So, if the input impedance should be at least ten times the output impedance and a typical passive pickup equipped guitar has an output impedance of 10k ohms, this means what ever we are plugging the guitar into should be at least 10 x 10k ohms, or 100k ohms. Most guitar amps have an input of 500k ohms to 1M ohms which is why I never cared about this stuff before, seems like the guitar and amp industries got on the same page giving us guitar players one less thing to worry. It’s when we start plugging our guitars into devices other than guitar amps when we can start having impedance mismatches and as a result, ugly tone.
A good rule of thumb to go by is “don’t connect a Hi-Z output to a Lo-Z input”. So if you’re thinking about running down to your local Radio Shack to get a fist full of adapters so you can plug straight into your audio interface’s XLR input or your computer’s sound card 1/8″ line input, don’t do it! Of coarse, good tone is subjective. If thin, lifeless, fizzy tone is your thing, then by all means go for it, you won’t harm anything.
Many audio interfaces have balanced, Hi-Z 1/4″ TRS inputs. Although they are Hi-Z and will accept an unbalanced 1/4″ cable, i.e. guitar cable, they typically do not have a high enough impedance, roughly 10K-20K ohms (when in doubt read the manual). You may ask yourself as I did “if my guitar’s output impedance is 10k ohms and a TRS input is 10k ohms, doesn’t that match perfectly?” The simple answer is no, there will be a degree of signal loss. The higher the input impedance is when compared to the output impedance the better the voltage transfer is and the better the frequency response will be. There is theory and formulas that prove this, but that goes way beyond the scope of this article. Just remember the 1:10 output to input impedance ratio and you’ll be good to go.
Now that I was armed with the basics of impedance and it’s effect on my guitar signal, it was time to research ways to make my guitar and input devices play nice with each other.
Let’s start by identifying some solutions that many guitarist already have in their possession. Many stomp box effects have a high input impedance and a low output impedance making them a good choice to buffer your guitar signal before it hits your choice of input device. As long as the stomp box isn’t true bypass you don’t need to have the effect turned on, just put it between the guitar and audio interface. I made a list of some popular stomp boxes with their input impedance:
- Boss DD-3 Digital Delay – 1M ohm
- Boss TU-2 Chromatic Tuner – 1M ohm
- Boss CS-3 Compression/Sustainer – 1M ohm
- Ibanez TS-9 Tube Screamer – 500K ohms
- ProCo Rat Distortion – 1M ohm
You most likely won’t hear a difference between a 500K ohm and a 1M ohm input impedance. If you do, the 1M will be slightly brighter. If you think your guitar sounds too bright or just want to tame down your single coil pickups you can look for something in the 300K range or lower. The lower the impedance, the darker the sound. Just stay conscientious of the 1:10 ratio, if you go below that things can start to get funky. If you have a pedal not listed check the manufacturer’s website, it should list its input impedance under the pedals specs.
Another option is to use a hardware amp modeler such as a Line 6 Pod HD or a Behringer V-amp Pro which both have a 1M ohm input impedance. Just make sure you bypass the units modeling and effects if you want just your guitar signal to pass through. I actually have a V-Amp Pro and using it in this manner made a significant improvement.
The option that I chose as my default technique was to use my Peterson Strobo Stomp tuner which also acts as an active direct injection box (DI). It has a 1M ohm input impedance and a balanced XLR output that I plug into my MOTU 8-Pre balanced XLR input. Balanced cables reject noise and don’t degrade your signal over long lengths like unbalanced cables can. I now have a three foot guitar cable going to my Stobo Stomp and a XLR cable carrying the signal the rest of the way to my audio interface. This has made a remarkable improvement in my signal-to-noise ratio. Fortunately, you don’t need to shell out a chunk of cash for a Strobo Stomp, just about any active DI will do the trick. Check out the Behringer DI100, not a bad unit for around $34.
From what I have read, passive DI boxes aren’t really recommended for this application. They tend to have a much lower input impedance than an active DI and require a high quality transformer in order to produce good frequency response making them rather pricey. Some people swear by them though for their warm tonal characteristics.
Lastly, if you are just getting started and don’t have an audio interface yet or have a stash of cash under mattress, just buy yourself an audio interface with a dedicated instrument input. Just make sure you read the specs to ensure it has the input impedance your looking for.
Hopefully some of this information will help you achieve success with your software amp. Of course, there are other factors that influence how they sound and I am by no means an expert on software amps. If you have some other tips on this subject and can add to my post, please do so in the comments section. I would be grateful as I am sure others would be too.