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)
Mic level: About 2mV (0.002 volt) to about 1V depending on how loud the source is, and on how sensitive the mic is.
Instrument level: Typically 0.1V to 1V for passive guitar pickups; up to 1.75V for active pickups.
Semi-pro or consumer line level: -10 dBV (0.316 volt).
Pro line level: +4 dBu (1.23 volts)

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:
dBu = 20 log (V/0.775)

So, expressed in dBu, here are the levels above:

Mic level: About -52 dBu to +2.2 dBu.
Instrument level: -17.7 dBu to +7 dBu.
Semi-pro or consumer line level: -7.8 dBu.
Pro line level: +4 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.

IMPEDANCE
Impedance is resistance to AC electrical current, such as an audio signal. Impedance is measured in ohms. For best sound quality and highest voltage transfer, the input impedance of the connector you are plugging into should be at least 7 to 10 times higher than the output impedance of the source (such as an electric guitar or microphone).

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
In short, input impedance should be at least 7 times higher than the source's output impedance. The output voltage of a signal source should fall within the input-voltage range of an equipment input.

With these facts in mind, let's look at the Presonus Firebox specs:

Line input impedance: 10K ohms (too low for an electric guitar)
Instrument input impedance: 1M ohms (1,000,000 ohms -- more than high enough for an electric guitar. No direct box is needed).
Mic preamp input impedance: 1.3 K ohm (a good impedance for most low-impedance mics).

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).
Line In Sensitivity Range: -40dBu to +10dBu nominal,+25dBu maximum (handles semi-pro and pro line levels)
Mic input impedance: 4 K ohms (high enough for any low-impedance mic or direct-box output)
Line input impedance: 20 K ohms (too low for an electric guitar. A direct box is needed).

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.

SUMMARY
When you use the Alesis Multi-mix 8 mixer, use a direct box between your electric guitar and a mixer mic input.
when you use the Korg Pandora and PreSonus Firebox, plug the guitar into the Pandora, and plug the Pandora output into the Firebox instrument input. Those connections will give the best sound.

That's a lot to swallow. Hope this helps.

brubart