Analog noise

Analog noise phenomena include the following:

• Hiss - There are two main sources of hiss. The first is caused by electromagnetic interference, line losses, and inefficiencies within the electronic components themselves caused by heat, material hysteresis (the lag in the material as it changes state or moves electrons from one place to another), and simple random noise. This creates part of the noise floor inherent in all analog signals. The second source is actual physical noise caused by the read device rubbing against the recording medium. (In records, this is known as surface noise, and some audio fanatics actually use a system that keeps the record wet while it is playing to address this.)
• Hum - This is electromagnetic noise caused by feedback. This feedback could stem from a ground loop, or it could be because you have your turntable too close to your speakers.
• Wow and Flutter - This is due to speed variations in recording and playback. Wow is a slow variation, and flutter is a fast variation. It is primarily found in cheap tape decks that have poor motor control.
• Rumble - This is low-frequency noise, usually caused by resonance coming through the stylus from the record surface. Proper isolation is the key here, using foam rubber feet and/or a very massive platform under the turntable. A record clamp also helps by coupling the lighter record to the body of the player.
• Pops and clicks - these aren't just caused by dust on the record surface, they are also caused by electromagnetic discharges due to static buildup that can occur on both phono styli and tape heads.
• Clipping - this is a phenomenon shared by digital when it comes to amplification, but applies to analog in the recording process. If the recording level is set too high, the medium becomes saturated by the signal. This is a problem found mostly in home tape recording, and can be prevented by proper setup.
• Tracking error - This is not a noise per se, but it causes a loss of signal, and possibly distortion. In a tape, if the heads are not positioned so that the recording and playback gaps are exactly perpendicular to the tape, the magnetic tracks that contain the information are either poorly laid down, or poorly read. In a turntable, if the cartridge is poorly aligned, or if the anti-skate (the tracking force counterweight) is set inaccurately, the stylus will press more heavily on one side of the groove than the other, causing stereo balance problems, increased surface noise, and irregular wear.
• Tracking pressure - In a tape deck, this is head-to-tape contact, and in turntables it pertains to tracking force. If the tape doesn't make good contact with the heads, there is a loss of signal, and if there is too much pressure, it can cause the tape to stretch, which leads to wow and flutter, and even breakage. In a turntable, if the tracking force is set too high, the stylus will dig in and damage itself and the record, and if it is set too low, it will not track properly, and skip (which will also damage the record and stylus.)
• Wiper noise This is noise caused by dirt in the controls. Volume knobs, equalizer slides, balance controls, and other resistive sound modulation devices can get dirty from corrosion, and make noise when operated. A squirt of contact cleaner fixes this.

The reason audiophiles spend so much time "tweaking" their systems is to remove as much of these things as possible by addressing the causes behind them and removing and reducing them from the system. These methods include proper grounding, vibration isolation (packing the turntable with plasticine, using spikes and lead-filled speaker stands, etc), clean surfaces, quality interconnects, and meticulous set-up to ensure proper alignment and recording and playback levels.