Understanding impedence matching

Thanks everyone. I have begun my inquiry into Ohm's law and its implications. I've gotten the E=IR concept down, but as to the implications, well I gotta keep studying. But since there is some conviction from you that understanding this does, in fact, help demystify the process, I am eager to learn.

Again, my thanks. The more I explore the web, the more amazed I am by its power to connect strangers.

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

"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."

This one is wrong on both counts.

"For the amp to exert good control on the speaker - the
damping factor is usually a few hundred."

Damping factor doesn't determine the amount of "control" that the amp has over the driver. This is one of the most widely misunderstood and ill-repeated concepts in the industry.

Power transfer and loading characteristics determine the amount of control the amplifier has over the driver. The more power that the amp can deliver into the driver with the least amount of reactance within the passband being used, the more "control" or "influence" the amp has over the driver.

Damping factor is simply a measure of how susceptable the amplifier is to having a specific impedance speaker modulate its' output. The bigger the impedance mismatch, the less power that the speaker can load back into the amp. This is a good thing since speakers are not purely resistive and will always demonstrate various levels of reactance. Impedance matching increases power transfer and communications in both directions. Since we don't want the speaker "talking back" to the amp, we purposely design in an impedance mismatch. We simply want the amp telling the speaker what it should do, not the other way around.

The closer the output impedance of the amp to the input impedance of the speaker, the easier it is for the speaker's reactance to modulate or interfere with the amplifier's normal operation. This is true of both SS and tube designs, but more-so with tubes due to their typically higher output impedance. Hence the wide frequency response deviations found in SET or OTL amps when connected to various speakers / levels of reactance & impedances. This demonstrates the previous rules i stated above i.e. when reactance is involved, the source output impedance should be much lower than the load input impedance. Using this approach, the higher levels of reactance are somewhat minimized due to the reduced ability to modulate the output of the source.

"That is the output impedance of the amp is around a few thousand ohms."

I'll give you the benefit of doubt here and assume this was a mistake. The output of an amp should be infinitesimally low, not around a few thousand ohms. If such were the case, the amplifier would be loaded down and current limited before you played the first note. You probably meant to say that "the output impedance of the amp is around a few thousandth's of an ohm. Sean
>

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