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Re: Class D Amplifiers--Is It Just Me or What?
You left out
The linked article covers (one of) the reasons FOR high input impedance as well:
You left out
or everyone agrees to disagree.
The linked article covers (one of) the reasons FOR high input impedance as well:
The solution to this problem is to dispense with the idea of matched impedances completely, and use what is called voltage matching instead. The idea here is to engineer the equipment to have the lowest possible output impedance and a relatively high input impedance — the difference between them must be at least a factor of ten, and is often much more. Modern equipment typically employs output impedances of around 150(omega) or below, with input impedances of at least 10k(omega) or above. With the minuscule output impedance and relatively high input impedance, (the cable impedance can be disregarded completely in comparison) the full output voltage should be developed across the input impedance.
Relatively high-impedance inputs such as these are called bridging inputs, and they have the advantage that several devices can be connected in parallel without decreasing the impedance to any significant degree — the voltage developed across each input remains high and the source does not need to supply a high current. (A low impedance is often referred to as 'loading' the output or circuit, because of the high current it demands.) Let's have another look at our earlier example, where a console output is feeding two tape machines. Say each machine now has an input impedance of 30k(omega); connecting two in parallel will only reduce the combined input impedance to 15k(omega), which is still substantially higher than the 150(omega) output impedance of the console. Hence, the input voltage will be virtually unaffected — I calculate a loss of 0.04dB, in fact! Even connecting a third device to the output, the impedance would only fall to 10k(omega) — the level would fall by a further 0.05dB, which I don't think anyone would hear! Because bridging inputs make studio work so much easier, the idea of voltage matching is now employed almost universally in line-level audio equipment, irrespective of the actual reference signal levels used.
Relatively high-impedance inputs such as these are called bridging inputs, and they have the advantage that several devices can be connected in parallel without decreasing the impedance to any significant degree — the voltage developed across each input remains high and the source does not need to supply a high current. (A low impedance is often referred to as 'loading' the output or circuit, because of the high current it demands.) Let's have another look at our earlier example, where a console output is feeding two tape machines. Say each machine now has an input impedance of 30k(omega); connecting two in parallel will only reduce the combined input impedance to 15k(omega), which is still substantially higher than the 150(omega) output impedance of the console. Hence, the input voltage will be virtually unaffected — I calculate a loss of 0.04dB, in fact! Even connecting a third device to the output, the impedance would only fall to 10k(omega) — the level would fall by a further 0.05dB, which I don't think anyone would hear! Because bridging inputs make studio work so much easier, the idea of voltage matching is now employed almost universally in line-level audio equipment, irrespective of the actual reference signal levels used.
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