Exactly How the LINE2AMP Reamping Box Works (and Why) July 19, 2012 13:30 19 Comments
Why do we need a reamping box? People have been plugging line-level signals into guitar amps since long before the first reamping box hit the market, often with good results. This is true; in many cases, simply plugging the output of your interface into an amp and crossing your fingers is good enough. However, what we want is to be able to plug any line-level device into any instrument-level input and have it sound right every time. In order to do this, we need a device that addresses the following issues:
- matching volume levels;
- interfacing balanced and unbalanced gear;
- emulating line and instrument impedances;
- ground-related noise.
As so rarely happens in audio and electronics, there is a cheap, simple, and ideal solution to all of these problems: an audio transformer. Let’s look at the ways a humble audio matching transformer can make reamping a consistent and transparent process.
Attenuating the Signal
The nominal level for a professional line-level signals is +4dBu, or approximately 1.2 Volts rms. Guitar signals, on the other hand, vary between a few tens of millivolts rms to 1 Vrms or slightly more depending on the pickup. So, in order to to mimic the output level of an average guitar, we need to attenuate our line-level signal somewhat before sending it to an amp. The clear choice for attenuating voltage (“volume” in audio) in terms of cost and complexity is a couple of resistors set up as a voltage divider. However, since we’re already using a transformer to address the other two problems of reamping, we might as well use it to reduce the voltage as well. Modified and reused under the GNU Free Documentation License
To understand how we can use a transformer to attenuate the signal, let’s look at how transformers work. Transformers operate according to a funny feature of our universe called “electromagnetism” by which a changing electrical field induces a changing magnetic field and vice-versa. A basic transformer such as the one we’re using consists of two coils of conductive wire wrapped around a magnetically permeable core, usually iron. The alternating current running through the “primary” (input) coil creates a magnetic field which induces a corresponding voltage in the “secondary” (output) coil. From our point of view, two important things happen to between the input and output of a transformer. 1) The grounds are isolated (which will be important for solving another reamping problem), and 2) the voltage, current, and impedance are transformed in proportion to the ratio between the number of turns of wire in the two coils (the “turns ratio”). Since the issue at hand is volume, let’s discuss voltage for the moment and leave current and impedance for later. Edcor PC10k/600 transformer
A transformer reduces or increases the voltage of a signal in direct proportion to the turns ratio. If the turns ratio is 2:1, the voltage induced in the secondary will be half of that in the primary. The Edcor PC10k/600 transformer included in the LINE2AMP kit has a turns ratio of 4.1:1, so the output signal is 4.1 times smaller than the input signal. In audio terms, this equates to 12dB of volume attenuation, just enough to get us to an average guitar output level.
Achieving the Correct Impedances
Put simply, “impedance” is the degree to which a circuit opposes the flow of AC current. Impedance is similar to the more basic concept of resistance, except that it is frequency dependent. That is, a circuit may present more opposition to high-frequency than low-frequency signals and vice versa. A great way to de-mystify impedance is to think of it in a realm we're all familiar with: acoustics. Every physical medium presents a certain amount impedance to the propagation of sound. This impedance is not uniform at all frequencies. This is why, for example, we hear the bass drums of a marching band first and higher frequencies connote "presence" to our ears.
There are two impedance values involved when connecting one piece of audio gear to another: that of the output device, called "source" or "output" impedance, and that of the input device, called "load" or "input." These two values tell us (at least) two important things:
- how much of the output signal (voltage) will be transferred to the input device;
- how much power is required for the output device to drive the input device.
Generally speaking, we get optimum results in both of these areas when the output Z is at least 8-10 times lower than the input Z; this is called "bridging" the output impedance. There is a common misconception that the input and output impedances should be matched--forget it, this is not true for audio! So what are the "correct" impedances in a reamping situation? The modern standard for line-level input Z is around 10kΩ, while guitar output Zs vary from below 100Ω for active pickups to 50kΩ+ for passive. In the same way that our transformer induces a different voltage across the secondaries than the primaries, it also transforms impedances. But whereas voltage is transformed in direct proportion to the turns ratio, impedance is transformed in proportion to the square of the turns ratio. Thus, the impedance ratio for our 4.1:1 transformer is 16.81:1, or approximately 10k:600.
We achieve our 10kΩ+ line-level input Z by placing a resistor across the transformer secondaries. The impedance reflected in the primaries is then 16.81 times the value of this resistor (for the LINE2AMP kit I have chosen a 1k resistor). At this point, with only the transformer and 1k resistor, our output Z is equal to the line-level output Z divided by 16.81, i.e., arbitrarily low. We can raise this value to within the range of a typical guitar output Z by adding a resistor in series with the output signal. I chose 15kΩ for this job but any value between 0 (no resistor) and 50k will emulate a different type of pickup.
Balanced to Unbalanced and Ground Noise
Reamping involves interfacing balanced (actually “differential” is correct, but I’ll defer to the parlance of our times) and unbalanced gear. A balanced connection has three parts: signal “hot” (non-inverted, +, tip, XLR pin 2), signal “cold” (inverted, -, ring, XLR pin 3), and ground (shield, chassis, sleeve, XLR pin 1). Balanced gear takes for its signal the difference between the hot and cold signals and ignores anything common to both (including noise). The chassis pin exists to provide unwanted noise a quick path to ground; it is not part of the audio signal.
Modified and reused under the Creative Commons Attribution-Share Alike 3.0 license.Original rendering by Søren Peo Pedersen
An unbalanced signal on the other hand consists of simply signal (tip) and common (“ground,” 0 V, sleeve). In an unbalanced system, the common is the reference for the signal voltage, so any noise injected into the common will in turn affect the signal. In the case of reamping, we are attempting to connect a balanced output from our mixer/interface/whathaveyou to an unbalanced, instrument input. What happens when we do this without properly "unbalancing" the output signal? By connecting the sleeve pins of both devices together we are connecting the the noise return path of the balanced output to the audio common of the unbalanced input. Noise, meet audio. Transformers solve this problem by providing what's called galvanic isolation between the input and output stages. Recall that transformers transfer voltage from one winding to the next through electromagnetic induction, without providing a path for current to flow. Thus, they allow the balanced output stage to "communicate" the signal to the unbalanced input stage without connecting their grounds/commons. It is therefore impossible to a ground loop to exist between the two devices. That's it. An elegant solution to a simple problem. Please feel free to continue this discussion below with your questions, comments or corrections.