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?

    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.
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:

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.

-- 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.

    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      

     Interesting experiment.  You used slightly different phrasing, but thanks for validating the general impedance matching guideline of the amp input impedance being at least 10x the output impedance of the preamp or source component (if the source is connected directly to the amp) or, in your phrasing,  an output /input ratio of 1:10.


Yes when the output impedance to input impedance (O/I)  ratio got down to 1:5 then a couple listeners said they could hear the dynamics may have been getting a little effected, but they said it was very small and don't think they could of blind A/B'd it.
But most of the 40 odd listeners said they could just detect 1:3 ratio. So a safe bet would be say 1:6 or higher.

Cheers George 
Thanks for the excellent input, George.

An additional point everyone should be aware of is that how objectionable a given low ratio is likely to be depends not only on the ratio itself but also on how much variation that ratio has over the frequency range.

For example, if the ratio is say 3:1 at the worst case frequency (i.e., at the frequency at which the ratio is lowest), but the impedances that are involved don’t vary much over the frequency range, and hence the ratio doesn’t vary much over the frequency range, the consequences would be a slight and inconsequential reduction in gain; an increased sensitivity to cable effects (especially if the low ratio is due mainly to a high output impedance of the component providing the signal); and perhaps a small degradation in the distortion performance of the component providing the signal.

However if the low ratio involves the kind of output impedance characteristic that occurs in the case of many tube-based components, where the output impedance may be a few hundred ohms in most of the spectrum but may rise in the deep bass region to a few thousand ohms at 20 Hz, the consequences of that same 3:1 ratio (at 20 Hz in this case) will include significant deep bass rolloff as well as frequency-dependent phase shifts in the bass region. Which are likely to be much more noticeable and objectionable than the effects described in the preceding paragraph.

Or putting it all another way, 3:1 may be fine in some cases, while 8:1 may be unacceptable in some cases, depending on how the impedances and consequently those ratios vary over the frequency range.

-- Al
None of this makes it easy for us non pro-audio educated to make a correct purchase of a pre or amp/s
Tweak1, if one picks a sufficiently high impedance ratio, say 20-fold to 50-fold (or more), then one might cover all of the frequencies where the low impedance ratio is avoided. 
What I mean is (and I do not consider myself a typical component purchaser is: this topic has never been explained or mentioned in any component manuals

I have probably flipped through 10 preamps/amps in the last 15 years

I can't help but wonder how much it had to  d with impedance mismatches 
I think the concerns around "impedance mismatch" are somewhat exaggerated for most "typical component purchasers". By far, most of these purchasers either buy an integrated amplifier/receiver or solid state separates where the (separate) preamp almost always has an extremely low output impedance. Tube electronics, particularly separate tube preamps could pose some matching concern but those are usually purchased by more demanding, and sometimes more knowledgable  customers.

Matching concerns for all passive preamps is even more critical, if one doesn’t get at least a 1:6 ratio or higher from source to input and from output to amp then the wrong conclusions will be drawn about passives and the way they sound.

If done right they will give more transparency and better dynamics than preamps can hope to give. And save you a great deal of money as well by not buying $$$$K active preamps.

Better still is going direct if your source has a volume control, but there are things to look out for here too. As well as hearing your source without preamp colourations, you may not like it's sound, as it's ruthlessly revealing and dynamic.

Cheers George