Parasound A 51 manual Technically Speaking, Audio Trigger Sensitivity Adjustment

TECHNICALLY SPEAKING 13

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 51 might be falsely triggered on by non-musical or noisy signals that can appear in the system, such as when you switch preamp inputs at high volume levels. If you set this control to 250 mV, the A 51 might not turn on during quiet musical passages. The detented position (click stop) at 12 o’clock corresponds to 100 mV. It’s a good starting point and will be suitable for most systems.

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 bal- anced 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 two-conduc- tor wires are less costly than the additional circuitry, higher priced XLR connectors and three-conductor wiring required for balanced connections.

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 separate 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 component receiving the signal they are equal, but 180 degrees out of phase with each other with respect to ground. To send and receive bal- anced 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 common 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 sideways through the terminal’s metal post. Before inserting 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.

Speaker Wires - Banana Plugs Instead of Bare Ends

For convenience, we’ve supplied ten gold-plated banana plugs with your A 51. To attach these plugs, first pull the flexible red and black insulators off the plugs and slide them on the + and - speaker wires. Next, use a wire stripper to remove insulation to expose about 3/4" (19mm) of bare wire. Insert the bare wire into the barrel of the banana plug and crimp the barrel down with a pliers or crimper to hold it in place. Finally, slide the colored insulator down the wire so it is over the barrel of the plug. The only exposed metal should be the tip of the plug.

Choosing Interconnect Cables and Speaker Wire

We are often asked to recommend specific brands of interconnect cables and speaker wire. 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 interconnects 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