How to record direct? Please explain impedances and levels.
I want to record my electric guitar direct. Should I plug the guitar into a line input, instrument input, or direct box? I don't understand all the impedance and level specs on my equipment. I have a direct box, a Korg Pandora effects processor, a PreSonus Firebox audio interface, and an Alesis Multi-Mix 8 mixer.
Reply: Input and output voltage specs, and input and output impedance specs, can be very confusing. I'll try to explain.
TYPICAL EQUIPMENT LEVELS (input and output voltages)
Note that "instrument level" can overlap mic level and line level.
Some manufacturers use dBu instead of volts. Here's an equation to convert volts to dBu:
So, expressed in dBu, here are the levels above:
Mic level: About -52 dBu to +2.2 dBu.
A direct box is used to connect an electric instrument to a balanced XLR mic input, not a line input. That's because most direct boxes reduce the level of the input signal when they convert the signal from high-Z unbalanced to low-Z balanced.
For example, if a mic's impedance is 200 ohms, the input impedance of a mic input (that you plug the mic into) should be 7 to 10 times higher, or 1400 to 2000 ohms. If you look at the input impedance spec for mixer mic inputs, it is typically around 1500 ohms. Similarly, the impedance of an electric-guitar pickup is typically 20K to 40K ohms (20,000 to 40,000) ohms. So the ideal input impedance of a guitar amp (or direct box input, or instrument input) is at least 7 to 10 times higher, or at least 280K to 400K ohms.
In general, the input impedance of a direct box (or an instrument jack) is higher than the input impedance of a line input.
UNDERSTANDING YOUR EQUIPMENT SPECS
With these facts in mind, let's look at the Presonus Firebox specs:
Line input impedance: 10K ohms (too low for an electric guitar)
Now let's look at another example, the Alesis Multimix-8 FireWire mixer specs:
Mic In Sensitivity Range: -60dBu to –10dBu nominal, +5dBu maximum (should handle most mic-level signals unless the source is really loud or the mic is really sensitive).
Now let's consider the Korg Pandora Toneworks PX4 specs:
The Pandora Toneworks PX4 is an instrument-level multi-effects processor. It's instrument-level in and out. According to the Pandora manual, you "connect the output jack to your guitar amp or headphones, etc." Since you would normally plug an instrument into a guitar amp, that tells you that the output of the Pandora is instrument level.
Plug your guitar into the Pandora guitar input (1/4" phone jack). Plug the Pandora output (stereo 1/4" phone jack) into the Firebox instrument input (1/4" phone jack). Plug the Firebox FireWire port into your computer's Firewire port, and record in your computer. In this case, you don't need to use the Alesis Multi-8 mixer or the direct box.
That's a lot to swallow. Hope this helps.