Yamaha P-2200 Owner's Manual - Page 45

range, the clipping may sound like an irritating sizzle that only happens on certain sounds. Similarly, clipping in the low frequencies can cause bass notes to sound fuzzy or muddy, or it can cause mid-range frequencies to be harsh. Yet because the clipping only takes place on certain sounds, it may not be immediately apparent that clipping is the source of the problem. With graphic equalization, the choice of a cut-only device (rather than a boost and cut device) may help solve the problem because, since boost is not available, clipping problems are reduced. With cut and boost graphics, parametrics, or other types of equalizers, the system operator must be aware of the potential clipping problem, and attempt to avoid it. SPEAKER PROTECTION The maximum sustained output power of the P-2200 in- to an 8-ohm load is at least 230 watts. Few, if any, single speaker systems are capable of absorbing that much power on a continuous basis. Most speaker systems, however, are capable of absorbing short duration peaks of considerably higher power than their rated continuous power capacity. The ability to produce these peaks without distortion is a major advantage of a large power amplifier like the P-2200. The speaker, however, must be protected against the abuses of excessive average power, sudden large peaks, DC current, and frequencies outside its range. The following are methods of achieving some degree of protection against these abuses. Fuses Yamaha does not recommend the use of any type of fuse as speaker protection. Fuses are slow-acting devices of inconsistent quality, and do not offer adequate protection for speaker systems. They are mentioned here only because they are used in some systems. Standard fuses may be capable of protecting a speaker against excessive average power, but they are too slow to successfully protect a speaker against sudden peaks. Fast-blow, instrumentation fuses, with improved time response, may blow on normal program peaks and needlessly disrupt the program. Slo-blo fuses, on the other hand, may not blow quickly enough to prevent loudspeaker damage due to voice coil overheating. If fuses are used, whenever possible fuse each loudspeaker separately so that a single fuse failure will not stop the show. A fuse will protect a loudspeaker against one common fault of a DC coupled amplifier: DC at the output. The slightest DC offset from a direct coupled preamplifier will be amplified and appear at the power amplifier's output as a larger voltage with the power amplifier's large current capacity behind it. Even though there is no immediate audible affect (the extra power draw may cause some amplifiers to hum slightly), the loudspeaker is forced to absorb the DC power output of the amplifier. Since it cannot convert this DC power into acoustic power, the speaker overheats. Small amounts of DC voltage can shorten the life of a loudspeaker, and any large amount of DC will cause sudden, catastrophic failure. Fortunately, the input of the P-2200 is not DC coupled so any DC voltages from preamplifiers, etc. are not amplified, and cannot reach the speaker. The only time DC voltage could appear at the P-2200's output would be in the event of a severe electronic failure inside the amplifier, a very unlikely event. Capacitors Inserting a non-polarized capacitor in series with a high frequency driver can protect it against excessive low frequency energy. The capacitor acts as a 6dB/ octave high pass filter. Especially on a biamplified system, this kind of protection is desirable. For a biamplified system (or triamplified system), choose a protection capacitor by the following formula: Value (in microfarads) = (Where " p " =3.14, "f" is the crossover frequency divided by two, and Z is the nominal impedance of the driver.) The same formula can be used to choose a capacitor to insert in series with a low quality 70-volt speaker transformer to avoid excessive current flow at low frequencies (see Page SIX 13). Measure the impedance of the transformer primary at the lowest frequency of interest (which will probably be somewhere around 100Hz) with a speaker load connected to the secondary. Choose the protection capacitor by the above formula with Z = the measured impedance of the transformer, and f = the lowest frequency of interest divided by two. The voltage rating of the capacitor chosen must be greater than the maximum expected total peak to peak voltage that will ever appear at the driver's terminals. For the P-2200, this is equal to the sum of its positive and negative supply voltages, which is 160 volts. The most common types of capacitors used for driver protection are non-polarized electrolytics. Because of the inductance associated with an electrolytic capacitor, it may be paralleled with a mylar capacitor of about 1 / 1 0 the value in microfarads to reduce high frequency losses. Limiters A limiter is not normally considered a loudspeaker protection device, but it may be one of the best and most practical. A "squared up" or "clipped" waveform causes a loudspeaker cone or driver diaphragm to move to one position and stay there, then move back to the extreme opposite position, and stay there, etc. Because there is still power flowing through the voice coil, but there is no voice coil movement, the power is converted to heat. If a limiter is placed before the power amplifier in a system, it can be adjusted to prevent peaks from reaching a level that would cause the power amplifier to clip, which may avoid burned out loudspeakers. Transformers The 70-volt transformers used in "constant voltage" commercial sound systems lend a certain amount of protection to a loudspeaker. They will not pass DC current, and most of them will not even pass subsonic Fig. 71 - Typical use of Auto-Transformer for Speaker Impedance Matching which also helps protect the Speaker from damage caused by DC at the Amplifier's Output.