Fender Series 3000 Owners Manual - Page 45

• • • Impedance Watching Passive Devices 8-11 High-Frequency Flom/Dnver HF Input LF Amp 811 Passive Loudspeaker Crossover The Passive Crossover must be loaded with the correct impedance loudspeakers (impedance matching). 81/ Low-Frequency Woofer 0 Active Devices Fender 2244 Power Amplifier 0 Inputs 0 Ou puts Fender 2851 Loudspeaker System 411 Load seen by Power Amplifier Rohl Channel • 0 811 Loudspeaker System Do not overload the outputs of an active device like a power amplifier (impedance "watching"). Sit Loudspeaker System • 0 receive the same treatment as any other active device (next rule). 4) Active Devices An active device is one that uses batteries or AC power and has one or more tubes, transistors or ICs. Impedance watching for an active device means not overloading its output, that is, not connecting too low a "load" impedance to the output of the active device. A too-low impedance is an overload because the lower the impedance, the closer it is to a short circuit. It's usually very easy to follow this rule because almost every active device comes with a set of specifications that tells you the value, in ohms, of the lowest allowable load impedance. This is usually called the "rated" or "minimum" load impedance. Incidently, in almost every case, it's okay to connect a higher than rated load impedance to any active device. For the power amplifiers in your Fender 3000 Mixer, for example, the "minimum" load impedance is 4-ohms. That means you can connect any impedance down to 4-ohms to the Power Amp Out jacks. Since an 8-ohm loudspeaker is greater than 4-ohms, it is an acceptable load. So is a 16-ohm loudspeaker. Two, 8-ohm loudspeakers in parallel equal a 4-ohm load, so this is also acceptable. Four, 4-ohm loudspeakers in parallel equal a 1-ohm load, so this is definitely not acceptable. (See "Calculating Series and Parallel Impedances.") Connecting a too-low load impedance to a power amplifier will cause the power amplifier's protection circuits to operate (which increases distortion) and may, in extreme cases, cause damage to the power amplifier. For a line-level active device, like a limiter, the same rule applies. If the limiter has a rated minimum load impedance of 600-ohms, you can connect the output of the limiter to the input of any device whose input impedance is 600-ohms or higher. (The input impedance of most active devices is considerably higher than 600-ohms.) Some professional power amplifiers, on the other hand, have input impedances of 5000-ohms or lower. Connecting your 3000 Mixer, with its 10,000-ohm minimum load impedance to the professional power amplifier, with its 5000-ohm input impedance would reduce the output level and might also cause an increase in distortion. Impedance and Cable Length One more aspect of impedance watching involves the effect of cable length on the frequency response of high-impedance microphones. A toolong cable on a high-impedance microphone will cause a loss in highfrequency response, that is, the sound from the microphone will be dull and voices will lack intelligibility. This results from the interaction between the capacitance in the cable and the high impedance of the microphone which form a low-pass filter (a lowpass filter passes only low frequencies which means that it attenuates high frequencies). The lower impedance of 43