Understanding impedence matching

Ccryder,

The interface between the TT/cartridge and the preamp does
have to be impedance matched. However, cartridge makers and
preamp makes have standardized this interface to be a 47k ohm
transmission line - so you don't have to do anything special.

Likewise, all your interconnects are 75 ohm transmission lines
which is also standardized - and therefore impedance matched.

However, for the power amp to speaker interface there is an
intentional mismatch - so as Sean states - the speakers
can't feedback to the amp and modulate it.

For this interface, you check the specs of the power amp
or receiver - that will tell you the minimum load impedance
that the particular amp will handle properly.

Best wishes.

Dr. Gregory Greenman
Physicist

Ccryder,

First - when you are matching speakers and amps - you are not
doing "impedance matching". Impedance matching is when you
are dealing with transmission lines - like the coax or
twin-lead antenna cable going to your TV. You want to match
impedances so you don't get internal reflections. Imagine
light going through air, then hitting a glass window or
aquarium. There is a mismatch in impedance and you will get
partial reflection at the surface. That's why you can see
yourself in a window - like a partially silvered mirror.
If you match impedances - you don't get the reflection.

What you are doing in matching speakers and amp is not
impedance matching. The lower the impedance of the speakers
the more current has to flow through them for a given
voltage. For example, a 4 ohm speaker [ on average ] needs
twice the current as an 8 ohm speaker given the same drive
voltage. The amp may not have the current reserves to
supply the current required. Thats why an 8 ohm speaker
is less taxing on the amp than a 4 ohm speaker. You follow
the amp manufacturer's recommendation for the load impedance.

If you have too low an impedance, the current reserves of
the amp will be taxed. At low volumes, it'll work - but as
you crank up the volume, and hence voltage, the amp has to
deliver more and more current to the load. If it runs out
of current, the amp will "clip" - it will chop the top off
the music waveform. That type of distortion is death to
speakers - it will fry the tweeters.

Dr. Gregory Greenman
Physicist

Clueless,

I'm sorry - you are misusing the term "impedance matching".

When you have tranmission lines - and you do "impedance
matching" - you make the impedance of one element exactly
the SAME as the upstream components.

For example, if you have a transmission line with twin-lead -
the type that's used for antenna connections to TVs - that's
a 75 ohm transmission line. When you want to terminate the
transmission line - you use a 75 ohm resistor. That's
because a 75 ohm resistor looks to a twin-lead transmission
line like an infinite length of twin-lead transmission line.

THAT is impedance matching - making the impedances the SAME
hence the term "matching".

I would use the term "coupling" not "matching" in the example
you give with the tube amp. Even with the transformer -
the output impedance of the tube amp is not 2-8 ohms.

In fact, the ratio of the output impedance of the amp to
the load impedance of the speaker is called the "damping
factor" and it is not unity [ 1.0 ] in a good stereo setup.

For the amp to exert good control on the speaker - the
damping factor is usually a few hundred. That is the
output impedance of the amp is around a few thousand ohms.

In an audio system - the interconnects ARE transmission
lines - typically 47k ohm transmission lines.

I believe we have a semantics problem here. You evidently
well understand how amps and speakers work. However, you're
evidently unfamiliar with the terms "transmission lines"
and "impedance matching". These are well defined terms in
science and electrical engineering.

Courtesy of the Institute for Telecommunication Sciences:

Transmission line definition:

http://www.its.bldrdoc.gov/fs-1037/dir-038/_5565.htm

Impedance matching definition:

http://www.its.bldrdoc.gov/fs-1037/dir-018/_2679.htm

Dr.Gregory Greenman
Physicist

Sean,

You are correct about the amp output impedance.

I know I have an amp with a damping factor of 400 into an
8 ohm load. I got my ratio upside down.

The amp's output impedance is 20 milliohms.

Let me correct some more of my previous post:

Twin lead antenna cable is a 300 ohm transmission line.

Coax cable is the 75 ohm transmission line.

The connection between an audio cartridge and the phono
preamp is the 47k ohm transmission line.

Dr. Gregory Greenman
Physicist

Sean,

Well said in your explanation.

I was being "loose" in my use of the word "control". In a
way, a speaker that can modulate the amp is not under "control".

You are quite correct that in the case of the interface
between the speaker and amp - "impedance matching" is the
last thing you want to do - since as you state - it would
allow the speaker to transfer what limited energy it has
back to the amp.

It is desirable for the interface NOT to be "impedance
matched". That way - only the amp - which has more power
than the speaker - can dictate what is going on.

As you state - there is a purposeful impedance mismatch
at the amp / speaker interface so the speaker can't talk
back to the amp - but of course - the powerful amp can
make the speaker do its bidding.

Dr. Gregory Greenman
Physicist