Re: Questions on impedance matching


Ted Rook
 

I wish to revise something I said in the third paragraph about audio 600 ohm working below. I
should have also stated that the more significant change, more significant than the value of
the output impedance, was the choice to design equipment having only high-value bridging
input impedance and no low impedance input for line matching. Low impedance inputs
gradually disappeared in professional gear and so far as I know were never used in hifi gear.
Today it is probably impossible to find a low impedance input in audio and hifi, except for high
quality microphone preamps, where impedances in the 1000 ohm region are common. It is
unecessary to even know they ever existed, but the topic of bridging came up and this has
some history.

Ted

On 15 Feb 2018 at 14:51, Ted Rook wrote:

David,

To understand the meaning of "bridging" in audio we have to enter the dark cobwebbed world
of broadcasting in the 1940s and 1950s...........

One way to think about the word "bridging" is when connecting two units at line level, for
example between a tuner and an amplifier. If the tuner is a valve era unit having a 600 ohm
output impedance then there is a choice of connection methods, terminated method and
bridging method. Terminated method: Chose an amplifier having a 600 ohm input
impedance. This combination is identified as "terminated" which means the 600 ohm source
impedance is terminated by the 600 ohm amplifier input impedance and everything is hunky
dory. The primary thing this achieves is the correct signal transfer voltage level, attaching the
600 ohm amplifier causes the tuner output to be its nominal value. Note that when we
connect another amplifier to the tuner at the same time the load impedance is two 600 ohm
loads in parallel, 300 ohms, this causes the tuner output voltage to fall, the system is no
longer calibrated. When the second amplifier to be connected has an input impedance much
greater than 600 ohms, say 10kohms, then the combined load impedance of the two
amplifiers is 600 ohms in parallel with 10k, which is slightly but not much less than 600 ohms.
This method of attaching a high impedance load is known as "bridging", it allows multiple
device to be connected with minimal change to the load impedance and hence minimal
change to the signal level, things remain calibrated. The actual values of the imput
impedances of the amplifiers don't matter so long as they are more than 10x the output
impedance of the source. Bridging does not require knowledge or control of the impedance.

Eventually people realised that the 600 ohm output impedance was more trouble than it was
worth and the old concept of terminating audio lines to achieve nominal signal level was
displaced by the use of much lower values of output impedance combined with high values of
input impedance. Now there is no longer a dependancy of calibration of signal level on the
correct load impedance being present. In this new system the load impedance is always high,
the line is never terminated, signal levels don't change when multiple devices are connected,
altogether a better situation, one of those win-win combinations.

This is why I made the statement we never match impedances in audio, meaning input
impedances are never chosen to be the same value as output impedances. The system
chosen uses low output impedances and high input impedances.

One final thing. In audio there is a tendency to use technical language loosely. Impedance
matching in electronics has a precise purpose and meaning. But the in audio, especially hifi,
word "matching" may mean other things besides impedances. For example choosing the
appropriate power amplifier in terms of wattage to suit the speakers, for example high
efficiency speakers do not require high power amplifiers whereas low efficiency speakers do.
This is "matching" but it is not impedance matching. Another example of "matching" in audio
is the choice at the output terminals of a tube amp of an impedance tap to suit the speaker
impedance, for example 8 ohms in both cases. This may be thought of as "matching" the
amplifier to the speakers however note that the output impedance of the tube amplifier is not
8 ohms (it is probably around 0.5 ohms) and so this is not an example of impedance
matching.

Ted



On 15 Feb 2018 at 6:52, David Berlind wrote:

Actually, I actually want to thank everyone for broadening the discussion beyond my original
question. My understanding of impedance matching is so much better as a result and I'm not
sure that I would understand it as well were it not for all the tangentially connected issues,
use cases, and thorough explanations. For example, understanding the RF scenario greatly
improved my understanding of the close-quarters audio scenario. All of the tangents were
really worthwhile to me. I am still re-reading all of them to look for details I may have missed.

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