The down and dirty details on the gear and the science of sending recorded guitar tracks back out to an amp
By Michael Schulze
Over the years this magazine has featured many articles on guitar cabinet miking, and there are a lot of articles on the topic in this online library. We’ve suggested comparing different mics, combining mics at the front, back, and even inside the cabinet, trying mics at different distances, mics pointing away from the cabinet, mics submerged in a mug of beer, mics swung vigorously in a circle above your head for a Leslie type effect, and on and on.
The problem with trying all those options is that your guitarist is itching to shred, and might not want to play the same old Yngwie Malmsteen licks over and over for 25 minutes while you decide whether fresh or flat beer has more “sparkle”. On a slightly more serious note, I have on occasion realized too late after the session that the guitarist got so excited during the magic take that they played harder and harder, and the track has ended up way more distorted than I realized.
Reamping can help you in these situations! In reamping, you record your usual guitar audio track, along with the direct output from the guitar before it gets to the amp. After the session, when the guitarist is at home in their jammies, you can run the recorded direct guitar signal back to the amp, or to a different amp, or to a bank of 27 amps. You can play your pre-recorded guitar track over and over again, trying different amps, miking techniques, and brands of beer to your heart’s content. At 4:30 AM, once you have your golden tone (aggressively hopped), you simply record the result to a new track or tracks, and you’re done!
Or, if you’re not very confident of your engineering skills, you can record direct guitar at home and then bring the tracks to a pro studio with great amps, great mics, and great engineers.
You can also reamp bass, of course, and why stop there? You can reamp any track: vocals, horns, acoustic guitar, whatever. How about bagpipes through a 100 Watt Marshall head with full stack?GWAR does that. For now, though, we’ll concentrate on electric guitar.
By the way, the word “reamp” comes from “reamplification” and was the brand name of the first device specifically designed to do this, which we’ll discuss below. Nowadays, Radial Engineering owns the Reamp name with the blessing of inventor John Cuniberti (who still has a cool history/applications site at reamp.com), and sells products based on his original designs. The name’s trademarked, but it has now been associated in general use with the reamplification process.
You might think all you have to do is plug your guitar into the 1/4″ input of your USB interface, record it, and then connect the 1/4″ output of the interface to your guitar amp, but it’s not that simple. The line-level output of your interface is way louder than the output of your guitar, so you are going to have to turn the output level way down to avoid overloading the input of the amp into extreme distortion. Not only that, but there is another problem, which is that your USB interface and your guitar amp don’t love each other in the same way that your guitar and amp do…
There is a magical love relationship interaction between an electric guitar and a guitar amplifier. To “impede” is to get in the way of something, to hinder the progression of something, and impedance (written Z in electrical equations) is a measure of how much a signal is hindered in getting through a circuit. For example, a length of wire has an effectively zero impedance, current flows down it easily… but a broken wire has an infinite impedance! Electric guitar pickups have a relatively high output impedance, which means that it is hard to draw very much signal out of a guitar pickup.
Think of a huge tank of water with a regular garden hose coming out of it. It will take a long time for that garden hose to fill a swimming pool, because the water can only come out so fast. In this example we would say that the swimming pool has a very low input impedance, meaning there is very little impeding the flow of water into the huge pool. The high impedance output of the small garden hose will not be a very efficient way to fill the pool.
But what if, instead of filling a pool, we want to squirt water at our next door neighbor’s kid over the fence? We can put on one of those spray nozzles that has a very small hole at the end. That hole is much smaller even than the small diameter garden hose, so when we screw it on, the water in the hose gets impeded, the pressure builds up, and we are able to squirt water all the way across the yard at little Billy next door. Look at him run! He doesn’t even know where the water is coming from! The small, high output impedance garden hose does a fine job of providing all the necessary water pressure to the even higher impedance spray nozzle.
So it is with an electric guitar output. Impedance is measured in Ohms. A typical electric guitar has an output impedance of about 33,000 Ohms, which is pretty high. To give you a comparison, the impedance of the main outputs on your USB audio interface is only about 50 Ohms, and the output impedance of a microphone is usually about 150 Ohms.
On a guitar amplifier like the Fender Deluxe Reverb there are two inputs, labeled 1 and 2, on each of the 2 channels. The #1 inputs have a very high input impedance of 1,000,000 (1 million) Ohms, or about 30 times the output impedance of your guitar. Plugging into this input gives you the loudest, brightest sound because it is like that spray nozzle. The #2 inputs have a lower input impedance of 136,000 Ohms, only about 4 times the output impedance of the guitar. Plugging into this input makes it harder for the pickup to keep the pressure up, and the result will be a slightly quieter and noticeably warmer sound, which can be very, very lovely.
Guitars with high-output humbucker pickups sometimes work better in the #2 input when you are going for clean sounds, and better in the #1 input when extreme distortion is your dark god. Guitarists and engineers frequently try all inputs on an amplifier to find the one that has the type of sound they are looking for, and it is all based on the ratio of the output vs. input impedance.
So, going back to your USB interface, connecting its 50 Ohm output impedance to your amp will give you a nice, bright sound, but not that magic, soft warmth that results from the 1 to 4 ratio between your 33,000 Ohm pickup and the 136,000 Ohm input. The USB interface might even sound a bit too bright and cold connected to the 1,000,000 Ohm input. So we need a way to connect the guitar into the proper input impedance, record it dry, and then get the proper output impedance as we connect it to an amp. This way it will sound just as it does when you plug directly into your amp.
There are two ways to do this, and we will start with the old-school way from back in the analog tape days, before it was even called “reamping”, and then look at reamping itself. But whichever way we use to get our recorded clean guitar signal out of our DAW and back to the amp, we first have to get it in… and to do that we need a DI (Direct Injection) or “direct box”.
A DI has a 1/4″ input and a 1/4″ parallel output. The guitar is plugged into the 1/4″ input. You connect the direct output to your amp and mic it as you normally would. Between the DI input and the parallel output is nothing but wire, so the amp receives the exact same signal it normally would. Since we are going to have tons of flexibility later, for the tracking session you can just point a good old Shure SM57 or Audix i5 at the amp, a few inches away and somewhere between the center and edge of the loudspeaker. That will sound fine for now, and you could even record it along with the clean signal; the main reason to have it now is so the guitarist hears something inspiring when he or she plays.
On the other side of the DI you will find a 3-pin XLR output. The signal on this output is a low impedance (150 Ohm) microphone level signal. You plug this signal into a mic input on a preamp, mixer, or interface, and record the direct guitar signal to an audio track in your DAW. (See Figure 1.)
At this point you might decide that all this reamping business is too complicated. Why not use any of the fantastic amplifier simulation plug-ins on the market to cop your golden tone and be done with it? You can certainly do that, with excellent results. But you can still do better! The DI you plugged your guitar into has an input impedance of its own, but how high? You probably want to make sure that your guitar “sees” the kind of input impedance you would choose on a real amp so that your amp simulator plug-in gets the same slightly loaded-down signal a real amp would. So let’s talk a bit more about DIs.
DIs were not invented for reamping. Rather they were invented so that bass and keyboards could be sent up the same snake as all the mic signals and recorded direct, as these instruments sound pretty good recorded direct… unlike a guitar, which really needs the warm coloration of an amp to sound good.
These early DIs had nothing inside them but a single transformer, a piece of iron in a square or round donut shape with two separate coils of wire wrapped around it. (See Figure 2.) The wire is coated with a thin layer of plastic so that electrical current cannot flow between the wire and the iron, or between adjacent turns of a coil if they are wound close enough to touch each other.
If we connect an audio signal to one coil, it will generate a magnetic field that will travel through the iron doughnut and cause an identical signal to come out of the other coil. You could connect a light bulb to the coil on the right and it would glow. In this kind of circuit we would call the coil on the input the primary coil and the coil on the output the secondary coil. If the secondary coil has half the amount of wire as the primary, then the signal coming out of the secondary will be half as loud as the signal going into the primary. This is how this type of DI converts your guitar signal into a lower-voltage microphone level signal.
Because this type of DI needs no source of power to operate, it is called a passive DI. Passive DIs generally have an input impedance close to that of a guitar amp’s lower impedance input. The Radial Engineering ProDI is an excellent and affordable passive DI with an input impedance of 140,000 Ohms, pretty close to the 136,000 Ohm impedance of the #2 input of that Fender amp. Using this kind of DI will warm up the sound of the direct signal you are sending to your amp sim plug-in.
What if you want to simulate the brighter #1 amp input? In this case you might try an active DI. This type of DI requires a battery, a wall wart power supply, or the phantom power from your microphone preamp or mixer to operate. Instead of a transformer, it uses transistors and other active electronics. The advantage of an active DI is that it can be designed to have any input impedance desired. Some active DIs have a very high input impedance, like the Countryman Type 85’s 10 million Ohms. This DI will give you a brighter, crisper sound, which can be really great for bass guitar when you want a very tight, edgy sound. (Impedances this high also do wonders for the tone of acoustic guitars’ built-in piezo pickups, which often sound “quacky” into lower impedances.)
Check out the specifications of any DI you plan to buy, as some active DIs, like the dbx DB12, have a lower input impedance (140,000 Ohms in this case), and may sound more like a passive DI. And this is especially true of the “Hi-Z” guitar inputs found on many DAW interfaces nowadays… there is no official definition of “Hi-Z”, so you’ll want to make sure your interface has a 1 million Ohm or higher impedance for the best possible tone.
There are also special little boxes, like Radial’s Tonebone Dragster, designed specifically for direct guitar recording. These devices present a tasty impedance load to the guitar and provide a 1/4″output you can plug directly into the 1/4″ instrument input on your audio interface, which probably has a very high input impedance. The Avid Eleven Rack audio interface has a 1/4″ input that has software-controlled adjustable impedance.
Reamping with a DI
Now we are going to proceed with our original plan of firing up the tubes in our CrunchMaster 100 amp and tweaking through the night. The old-school way to do this involves the same DI that we recorded with. Our forefathers, the intrepid recording engineers of the 1970s, discovered that they could take a pre-recorded direct guitar signal off the reel-to-reel tape machine and run it backwards through a passive DI, patching the tape output into the DI output with a female-to-female XLR adapter, then patching the 1/4″ input of the DI to the amp.
The passive DI works in reverse, and you get a high impedance signal back out of the DI that will interact nicely with the amp input. You can try the same thing with your DAW and audio interface, if you have the proper cables. You may have to turn down the output level from the DAW to avoid overloading the amp input. Note that this trick will not work with an active DI, as transistors do not like to be run in reverse!
Reamping with a Reamp
Or… you can buy a little box that is specifically designed to do this and eliminates all the hassle and guesswork. The Radial ProRMP Reamp is a passive box with a female XLR input, a ground lift switch to eliminate hum, and a high impedance 1/4″output to feed your amp. Simply connect the ProRMP between your audio interface and your amp and you’re good to go. It also has an output level control, so you can sit right in front of your amp and tweak the level to get exactly the level of crunch or clean that you desire. (See Figure 3.)
Radial also makes fancier devices, like the Radial X-Amp, an active reamper that lets you send signal to 2 amps at once… or at the extreme end of this trend, the Radial JD7, which lets you reamp into up to 7 amps at once! You can use it just to plug in and play into multiple amps at once without a DAW in between, and it (like some other Radial products) has an adjustable Drag control so you can dial in the exact input loading you desire. (The Dragster mentioned above is a small box that only provides this function.) And there are other reamplification boxes worth a mention, each with its own special features, such as the Little Labs Redeye 3D and Pepper, which offer so many possible interconnections and adaptation functions it’s a shame to just call them “reamp boxes”.
So how do we do this?
When recording, I prefer a very high impedance active DI to record my track. I don’t want to color the sound on the way in, because I use a nice reamping box with a Drag control on the way out. Also, a passive DI will load down and color the signal coming out of the direct output on the DI, which changes the sound the guitarist is sending into the pedal board and amp while recording. Guitar stomp boxes have different impedances, and a high impedance DI will avoid changing the interaction that is already happening there. (See Figure 4.)
You can put the DI before or after the guitarist’s effects pedalboard. Putting it before the pedalboard gives you the unaltered guitar signal, so you can do whatever you want later. Putting it after the pedalboard lets you record everything the guitarist did effectswise, so you can keep the entire sonic performance and just run it through cool amps. Why not put a DI before and after the pedalboard and record both? Because you are not OCD like me, that’s why!
Once you understand how impedance affects tone and how reamping lets you replay a tonally-sweet performance over and over again, the principles can be applied to all manner of playback evil. With a single output reamp box, you can record your direct guitar track through your favorite tube amp for a nice crunch, then run the same track back through again with more distortion to blend in a bit more mayhem, or combine a totally clean sound with a crunch sound for a bit more clarity. You can hear this done on Elton John’s track “Philadelphia Freedom,” where clean and distorted tracks are combined from the same single performance.
You can run your direct track through your pedalboard and use two hands and two feet to manipulate your pedals as if you had a dedicated crew of pedal gnomes running your rig. You can take your laptop, interface, and an amp to strange places that have AC outlets, like some creepy industrial factory basement. Set the amp at one end of the room and a mic or two way over at the other end to capture some scary Goth ambience. Anything is possible, once you make reamping a regular part of your guitar recording process.
All diagrams courtesy Radial Engineering. Photos by Michael Schulze.