Yamaha P-2200 Owner's Manual - Page 18

THE DISTINCTION BETWEEN PROFESSIONAL AND HI-FI EQUIPMENT In most applications, a variety of auxiliary equipment will be connected to the P-2200, including: mixers, tape machines, compressors, graphic equalizers, echo, time delay, and reverb units, and just about any other audio electronics imaginable. Regardless of the function of auxiliary equipment, it will undoubtedly fall into one of two general categories, professional type or hi-fi type. The following criteria place most "semi-pro" equipment in the hi-fi classification. The distinction between professional and hi-fi equipment is important primarily because it affects the way it will be used with the P-2200. Brand name, size, panel colors, durability and subtleties in function are not the significant differences. What matters is that professional equipment and hi-fi equipment usually operate at different input and output levels, and require different source and load impedances to function properly. The P-2200 is designed to function well with other professional equipment, although it has high enough input impedance and sensitivity to yield excellent results with hi-fi type equipment if a few precautions are observed. (These precautions are outlined in the Installation section of the manual.) The following paragraphs explain how the specific requirements differ for professional and hi-fi (or semi-pro) equipment. IMPEDANCE The inputs of a piece of professional audio equipment are usually designed to be driven from a low impedance source, nominally 150 to 600 ohms, and its outputs will drive low impedance (600 ohm or higher) loads. (Power amplifier outputs are not considered in this discussion.) Professional input and output circuits may be unbalanced, but they are often transformer isolated (balanced or floating), and use dual conductor shielded cables, with 3-pin XLR type connectors or Tip/Ring/ Sleeve phone plugs. The P-2200's inputs are unbalanced due to cost and adaptability factors. To internally balance the inputs of the P-2200 would require two matched input transformers with heavy shielding (to avoid hum pickup from the P-2200's power transformer). Induced hum in low level circuits, especially in low level transformers, can be a problem with any power amplifier, or other high current device (such as a DC power supply). High quality external transformers with less shielding can achieve the same results with a substantial cost savings. In addition, the user can choose the optimum impedance ratio for a given situation, increasing the P-2200's adaptability. Either the "matching transformer box" or "step up transformer box" described on Pages SIX 3, and SIX 4 are suitable, so long as they are kept several inches away from the P-2200. Hi-fi (and semi-pro) equipment generally is designed to be driven from a 5,000-ohm (or lower impedance) source, and its output will drive 10,000-ohm (or higher impedance) loads. Hi-fi input and output circuits are usually unbalanced, and use single conductor shielded cables with 2-conductor connectors, either standard phone plugs or phono plugs (also called RCA or pin plugs). Occasionally, the inputs of a piece of hi-fi or semi-pro equipment are professional XLR connectors which have been converted to a 2-wire, unbalanced circuit by internally connecting either pin 2 or pin 3 to pin 1. The nature of unbalanced, balanced, and floating circuitry is discussed further in the Appendix of this manual. For the purpose of this discussion, the most significant point is that an unbalanced circuit is somewhat more susceptible to hum and noise, especially if there is any irregularity in the grounding system. NOTE: THERE IS NO CORRELATION BETWEEN "BALANCED" OR "FLOATING" AND CIRCUIT IMPEDANCE. Low impedance and high impedance are relative terms. A 150- to 250-ohm microphone is considered low impedance, whereas a 10,000-ohm mic is considered high impedance. A 600-ohm line is considered low impedance, whereas 10,000-ohm, 50,000-ohm or 250,000-ohm lines are all considered high impedance. Sometimes, mics and lines with an impedance of 600 ohms to about 2000 ohms are considered "medium" impedance. NOTE: THE IMPEDANCE OF A CIRCUIT SAYS NOTHING ABOUT ITS LEVEL. While the exact transition between low and high impedance is not clearly defined, the distinction is still important, primarily because the output impedance of a source determines the length of cable that can be connected between it and a load before a serious loss of high frequencies occurs. The losses occur because all cables, and especially shielded cables, have some capacitance between their conductors. Some guitar coil cords may measure as high as 1000 picofarads total capacitance! A source impedance (such as a high impedance mixer output) and the capacitance of a cable form a type of low-pass filter a filter that attenuates high frequencies. This filtering effect, can be reduced by using low capacitance cable, by shortening the length of the cable, by using a low impedance source or by some combination of these methods. Fig. 32 - The Source's Output Impedance and the Cable Capacitance act as an "RC Lowpass" Filter which Attenuates High Frequencies.