Impedance match

For audio impedance matching really comes into play for transformer coupled stages like tube amplifier outputs or phono inputs.

Other than that 10X the source impedance is the general rule although some manufacturers may recommend higher.

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

10x is usually the recommended amount, but usually the vendor should post this in the spec.

It is instructive to look at some of the reviews at Stereophile for tube preamps. The main issue is the volume control, and response at the ends of the spectrum. This will give you a very good idea of how tube electronics will perform when feeding other electronics.

The issue for tube amps feeding speakers is more complicated because since speaker impedances tend to be complicated, with multiple peaks and valleys.

Thanks, posters!
+1, Al, for the article link.

 

noble100

    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.

With source to pre, pre to amp.
Once you have a "Output to Input" impedance ratio of say 1:10 or more, your fine.
 
We had a large participant demo at our audio society meeting, about 35-40 "golden ear'ed" audiophiles were present.
I designed a switchable on the fly input impedance changer on an excellent amp that was in a very good system.
 
This impedance changer change the "O/I" impedance ratio in 20 increments from 1:100 down to 1:5. It was at 1:5 that only two "super golden ear'ed" audiophiles "thought" they could hear a difference for the worse, but they also both said they probably couldn't pick it in a blind A/B.
All said they heard a difference at 1:3.
AC levels were checked to the mV for all ratios to be the same.

Cheers George