Week 3: Electrical Theory; History of Recording

Electricity

 * High Voltage - Power (plugging into the wall for actual power)
 * Low Voltage - Connections between components

Circuit
A prescribed pathway for electrons to flow

There are three states for a circuit to be in...
 * 1) Closed - Circuit is on
 * 2) Open - Circuit is off
 * 3) Short - When there is something other than the prescribed pathway interrupting the flow

Voltage
Potential energy, energy at rest it is measured in “Volts”, this is the force that moves or pushes the electrons through the circuit.

Resistance
Something that causes work to be done by a circuit, resistance is measured in "ohms" (Ω)

Current
The amount of electrical energy that can safely move through a circuit, this is measured in amps.

Wattage
A measurement of how much work can be performed by a electrical circuit
 * W = V x A (watts equals volts times amps)
 * W = 120 x 20 (2400 watts can safely pass through)

Ohms Law
Often V = I (current) x R (resistance)

LOW VOLTAGE
This does not require a license

Impedance
Measured by ohms, the letter "z" is often used to differentiate impedance from resistance.

Impedance is frequency dependent resistance

All gear has a impedance ratings. Low-impedance microphones are professional grade. They tend to use a standard XLR. Don't run a high-impedance microphone long distances.

A trick to remembering impedance situations is this:
 * "Low into high won't fly."


 * "High into low won't go."

Generally speaking, you want to go from your lowest impedance and scale up.

Balanced Vs. Unbalanced
You always need to know if you've got balanced or unbalanced cables, inputs, and outputs.

If I have at minimum I'll have two wires, the hot and the ground. For example the Quarter-inch TS cable has the hot wire through the tip, and the ground attached to the sleeve. This is an unbalanced cable, and subject to RFI (Radio Frequency Interference)

Balanced - Will have three wires, the hot wire, the cold cable, and ground. For example the quarter-inch TRS has the hot wire through the tip, the cold wire through the ring, and the ground through the sleeve.

Using a balanced signal will boost your signal by about 3 dB, this is accomplished by sending your signal through the hot cable, and sending your signal inverted through the cold cable. Once it gets to the balancing amplifier the signal is switched back around to achieve the boost. An additional effect of this is that RFI picked up along the cable is flipped out of phase and is cancelled out.

Direct boxes (DI)

Can take high-impedance signal and turn it into low-impudence signal

Can take a instrument level box into a mic level output. (+10dBv to -4dBm)

Can take it unbalanced to balanced.

A passive DI conditions the signal through a transformer.

An active DI requires external power,either from a battery or phantom power (48v).

Amplify - Increase the signal

Attenuate - Decreases the signal.