Re: Questions on impedance matching

Ted Rook

Hi Goran,

if I understand correctly the system is bandwidth limited two ways, first because the forward
path through the amplifier is bandwidth limited, and also because the amplifier feedback
network is usually bandwidth limited. The wanted signal sent by the amplifier into the line is
therefore bandwidth limited. It follows that reflected energy is already bandwidth limited
before it passes through the feedback network. I would like to understand your question
better, perhaps I have not understood? Please can you explain your thoughts?


On 13 Feb 2018 at 19:04, Göran Krusell wrote:

Hi Ted,

is the last part of your discussion correct? I am not all that convinced
since your are not taken
the limited amplifier bandwidth into account.


2018-02-13 18:37 GMT+01:00 Ted Rook <rooknrol@...>:

David, I believe the following to be accurate but I am no physicist :-)

The reflection arrives back at the source with a delay that is
proportional to twice the distance
between source and load, the reflection is travelling at approximately
half the speed of light.

For the sake of argument assume the signal is a 20kHz sinewave, the period
is 50usec, the
half-period is 25usec. Here is a question, given that the reflection
travels at half the speed of
light how long would the line have to be for the reflection to arrive back
at the source with a
delay of one half period, 25usec?

This question can be rephrased: The line is 5 feet long, the reflection
travels at half the speed
of light, the signal is 20kHz sinewave, the reflection arrives having
travelled 10ft with a small
delay, what fraction of one period (50usec) is this delay?

The amplifiers in question are negative feedback amplifiers. The physical
length of the
feedback path around which the feedback signal must travel from the
amplifier output in
order to reach the input network and do its work is a small distance,
inches. The feedback
signal travels this path at approximately half the speed of light. The
reflection signal arrives
back at the source after a delay analysed in the paragraph above. It is
attenuated and the
residual is summed at the output and combines with the feedback signal
that travels back to
the input network of the amplifier. Because the distance around the
negative feedback loop is
small the feedback signal arrives back at the input very quickly, and long
before the reflection
arrives from the load which is 5ft away. The situation might be summarized
this way: the
negative feedback loop is very fast compared to the reflections coming
back from the load.
Errors induced by reflections are cancelled out immediately by the
feedback loop.


On 13 Feb 2018 at 8:55, David Berlind wrote:


Thanks so much for your various responses.The link to Dr. Leach's work is
Probably enough bedtime reading for a year or more (at the pace I consume
such texts). I
may have to build that Fisher KX-200 for fun. The voltage-db and power-db
conversions were
very helpful. In your opinion, is there a typical target decibel
improvement? Regarding
reflections, I'm glad I have company in @jafinch78 on the confusion front
(I'm not alone!).
You said "In this condition reflected energy is attenuated by the source
impedance behaving
as a low impedance sink." I thought I saw or read somewhere that the
refections interfere
with the output signal in transit. For example if the reflection is 180
degrees out of phase, it
practically cancels the signal. Do I have that incorrect? I'm trying to
understand why distance
or frequency renders this effect moot in audio. Seems like it's just
physics to me (yes, we're
at the limits of my understanding!).

thank you.

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