I always read and was taught by members here that the guideline was at least 10x but will work well together with anything greater, too. Identical match is definitely not correct.
Tim
Impedance match
My understanding for impedance matching a preamp or upstream source having a volume control (e.g., a DAC with volume control) to a downstream amplifier is that an appropriate “impedance match” is for the amplifier to have an input impedance at least 20-fold to 50-fold greater that the output impedance of the upstream preamp or source output impedance. One dealer told me that the appropriate “impedance match” between to such components is exactly that—an identical match of the same impedances.
So which is it?
So which is it?
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Tim's response is correct, of course. The following excellent article may provide clues to what might underly the dealer's misconception: https://www.soundonsound.com/techniques/understanding-impedance As the article indicates, the concept of numerically matching source and load impedances for transmission of analog audio signals originated in the early days of telephony. And it was necessary in that application, in pre-digital times, as a consequence of the fact that the distances involved (many miles) were a substantial fraction of the wavelengths of the signals. That concept was also carried forward into the early days of pro audio, in part because of the characteristics of the microphones that were used in those days. But for the most part that concept no longer applies even in pro applications. In modern home audio systems the only situations I can think of in which source and load impedances should be numerically matched are those in which RF frequencies are involved, such as in the transmission of AES/EBU and electrical S/PDIF digital audio signals. Matching is necessary when RF frequencies are involved in order to minimize or eliminate reflection effects that would degrade waveform quality. Regarding power amp outputs, unfortunately some manufacturers confusingly refer to the 4 ohm and 8 ohm taps of their amplifiers as having output impedances of 4 or 8 ohms, when what they really mean is that those outputs are designed to perform in an optimal manner when loaded by 4 ohms or 8 ohms. That can be seen in the measurements of many tube amps John Atkinson has provided in Stereophile's reviews. Depending on the particular amp the output impedance of 4 ohm and 8 ohm taps can be anywhere from a small fraction of an ohm to several ohms, or even more in a few designs, and has no particular relation to the load impedance the tap is designed for. Regards, -- Al |
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