With reamplification in mind, you first record a direct instrument signal from the guitar or bass during tracking, and later play that signal back through an instrument amp. Reamping can be a great solution if you have a limited amount of time for tracking and may not have the time to get the exact tone you want, or you aren’t sure what tone you really want. Even if you do get the tone you want at the time of tracking, you can still record a DI feed from the instrument as well, giving you additional flexibility later on if you change your mind.
Another scenario: A well-known guitar player has a reputation for constantly turning his amp up over the course of the evening, starting with it much too soft and ending up with it much too loud. For live recording, the crew records a DI feed off his instrument, then reamps the tracks later on in the studio.
For reamping you need a box that takes the balanced output from your console or recorder and turns it into a high-Z unbalanced output that matches the impedance of a typical instrument so that it can go into an amp. Since there is some interaction between the pickup and the amplifier input stage, it can also be worthwhile to have a device that models the source impedance of a typical pickup. And it’s important to be able to isolate the instrument amp ground from the rest of the studio grounding system.
One excellent way of doing this was patented by John Cuniberti and is sold commercially under the product name “Reamp.” This product requires a special transformer and while it does an excellent job, it only allows one sort of pickup to be modeled and it’s expensive.
How it works inside
With this project, I have taken a totally different approach. I am using a very inexpensive (in fact downright cheap) 600:600 ohm transformer because it is easy to manufacture a good low-ratio transformer cheaply. The output of the transformer goes into a 680 ohm load resistor which absorbs most of the signal going into it (and also makes sure the transformer is as neutral as possible because it’s properly loaded). A tiny amount of signal is tapped off of this and run through a volume control and then into some resistors and inductors that simulate pickup source impedances. Since the transformer is expected to be neutral and not to be part of the load simulation, we get around much of the high-cost componentry of earlier designs, at the minor expense of signal level. Even so, there is plenty of level available going into almost any amp.
The impedance switch allows you to select between a purely resistive (1 Megohm) source, or an inductive one (10Kohm plus 0.1 Henry inductance). These are somewhat simplified models, and it should be pointed out that the inductance of many pickups is much higher than that, but they both give good sounds that are roughly representative of some pickups.
The hard part of building this box is actually the metalworking. The box is a very solid die-cast aluminum one. The hole for the XLR jack is an 11/16" hole, and I have always done these by drilling out with a 3/4" hole saw (the ones sold for use with wood work nicely with aluminum if you keep them well-lubricated with cutting oil as you use them), then filed the hole out to size with chainsaw files.
A 1/4" hole for the switch and two 3/8" holes for the potentiometer and the phone jack will do nicely, but may also need to be filed out a little bit. The switch also requires a small hole to be drilled in the case to lock the front notched washer into place (see the top photo at right).
The switch is one that locks into place; you have to pull out on the toggle in order to move it. I find this a huge advantage for DI boxes and other gadgets used in stage environments and just decided to use the same switch here. If you substitute for another type of switch, be sure that it’s one with gold-plated contacts. Because the levels involved here are so small, switches with conventional contacts will tend to become unreliable after a couple of years.
The diagram and photo should be enough to help you assemble it. Be sure to cement the top of the tra nsformer upside-down to the case so that it doesn’t move around; a little epoxy or silicone goo will work nicely to keep it in place, and since you’re not actually getting any on the wiring you don’t need to worry about special adhesives.
All of the parts required can be obtained at Digi-Key, 1-800-DIGI-KEY
I think you’ll like the way the unit sounds—fellow contributor Darwin Grosse had one for testing, and really enjoyed it. The two different switch positions give very different tonal characteristics, both very useful on both guitar and bass.
Scott Dorsey (firstname.lastname@example.org) is an electrical engineer, and an audio engineer, residing somewhere in the quiet part of Virginia.