Understanding impedence matching

A couple of practical points specific to your components: since your pre has an output impedence of 800 ohms, using the 1:10 rule your power amp should have an input impedence of at least 8k, which is easy to find. (My Alephs, for example, have an unusually very low input impedence of 10k ohms.) Secondly, and perhaps more importantly, use of a tube amp with a high output impedence (let's say above 1 ohm) WILL couple with your speakers' variable impedence differently as a function of frequency, resulting in the somewhat unpredictable trampolining of its freq response that can make tube amps so much fun (!?). However, a 6ohm speaker load min is pretty high, so MOST tube amps will show minimal timbral distortion. Again, think of the varying load response the amp sees as it's 1 ohm impedence meets a 6-20 ohm load across 30-20kHz window. (Yes, study ohm's law.) Then imagine what happens with a more common lower impedence load, often varying down to impedence minima of 2-3 ohms!
There can then easily be a 10x change in "load" across the frequency band, severely modulating the response. Usually the upper bass and mids get plumped up, the highs rolled off. Only works well a small percentage of the time since it's such a wild crapshoot. So even though your speaker is relatively amp friendly I'd chase an amp with a low output impedence to maximize your odds of a reasonably flat frequency response without having to tweak the hell out of your room, or other consequent bandaids. Good luck.

If the circuitry is stable and well designed, the source output impedance should match that of the load input impedance. If the load contains a high level of reactance, especially at sharp phase angles, the source output impedance should be measurably lower than that of the load input impedance. If our audio gear was designed properly, we wouldn't be using a 1 / 10 ratio or anything close to that.

The only company that i've ever seen that offers a product line that takes advantage of such design features would be Sierra Audio ( i "think" it was them ). Jud Barber of Joule Electra has also made comments similar to this, but not to the point of promoting matching input / output impedances. Sean
>

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
>