14TECHNICALLY SPEAKING
Audio Trigger Sensitivity Adjustment
The Audio Sensitivity Control sets the threshold of the audio trigger signal. You can adjust this level from a maximum sensitivity of 50 mV (fully counterclockwise) to a minimum sensitivity of 250 mV (fully clockwise). If you set this control to 50 mV, the A 52 might be falsely triggered on by
Balanced and Unbalanced Lines
Recording and broadcast studios use balanced con- nections exclusively because of their inherent ability to reject noise and hum, thus assuring the best sound. Certain high quality preamplifiers and surround controllers built for residential use utilize balanced connections with XLR jacks for the same reasons.
All Parasound Halo series power amplifiers have balanced inputs with XLR jacks so you can take full advantage of their inherent noise reduction capability and superior sound quality.
Unbalanced connections with RCA jacks are found on all home audio equipment. RCA jacks and
In an unbalanced line, the positive audio signal appears at the center pin of the RCA jack and the negative signal on the outer shield wire, which also functions as the ground connection. Unbalanced interconnect cables are vulnerable to hum from an AC line, or other noise, such as RF (Radio Frequency), which can be reproduced through your loudspeakers. Since the unbalanced line’s ground also carries the audio signal, there is no way for the connected amplifier or preamplifier to distinguish between the audio signals you want and unwanted noise emanating from external sources.
Balanced lines are superior because they utilize sepa- rate conductors for audio and ground: two inner conductors carry the positive and negative audio signal, and a third outer wire connects the grounds and also shields the two signal conductors. When the positive and negative signals appear at the com- ponent receiving the signal they are equal, but 180 degrees out of phase with each other with respect to ground. To send and receive balanced signals requires special differential circuitry.
A differential input circuit amplifies only the difference between the positive and negative signals. For exam- ple, when a 1 Volt signal arrives at a balanced input stage, the differential input “sees” a positive 1 Volt minus a negative 1 Volt, or 2 Volts total. External hum and noise that somehow gets into a balanced line is common to both its positive and negative conductors with respect to ground. Therefore, it is canceled or rejected by the differential input circuit.
This phenomenon of rejecting noise signals common to both positive and negative conductors is called common mode rejection. Differential inputs are speci- fied according to how well they reject signals com- mon to both conductors. This is measured in dB and is called the common mode rejection ratio or CMRR.
Bare Speaker Wire Ends
If you plan to use connections with bare wire ends, use a wire stripper to remove just enough insulation to expose a 1⁄2” (13 mm) length of bare wire. You can insert the stripped wire into the hole that goes side- ways through the terminal’s metal post. Before insert- ing the wire, twist its bare strands to prevent any of the strands from making contact across the two speaker terminals. If you have a soldering iron, you can “tin” (apply a small amount of molten solder) to each stripped bare wire to prevent it from unraveling, fraying and oxidizing.
Choosing Interconnect Cables and Speaker Wires
We are often asked to recommend specific brands of interconnect cables and speaker wires. It’s true that with some amplifiers, sound quality will vary greatly according to interconnect cables and speaker wires. However, Parasound amplifiers use robust circuitry that sounds superb regardless of intercon- nects and speaker wires. Therefore, we feel that choosing a brand of cable for Parasound amplifiers is purely a matter of personal taste.
Ground Loops - Eliminating Hum and Buzz
Audible hum and buzzing noises in a system are usu- ally related to issues with the component grounds. Ground (sometimes called common) is a point of reference for voltages in virtually all audio and video components. Ground is supposed to remain at zero volts while the audio signal swings positive (voltage above ground) and negative (voltage below ground). If ground isn’t at zero, there can be an audible 60 Hz hum (or 50 Hz hum in regions with 50 Hz AC). The harmonics of these frequencies (120 Hz, 240 Hz, 480 Hz or 100 Hz, 200 Hz, 400 Hz) may add buzz in addi- tion to the hum.