Understanding impedence matching

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

Hi:
A couple things....

Elizibeth..how goes it? Your analogy to language is fine except it covers only part of the issue. If impedances are matched you get more power. (Power = current x voltage.)But a mismatch in impedances is not just for stability.

Power is not always the goal and sometimes you want to avoid it because you do not want to load down the prior circuit. For example, if you are in the front-end (pre) of a system with a small voltage that you want to pass to the amp you want a small to larger impedance in order to do it. One of the reasons a tube's grid is used to develope the signal is that it has an almost* infinite resistance. Perfect for developing a voltage signal. Anyway, what I wanted to emphasize is that in order to think about "matching impedance" you have to figure out what the goal is. transferring voltage or current.

Esteemed Dr. Greenman:
I am not a physicist but I know how amps work. You certainly are matching impedances from amp to speaker. This is at the heart of the OTL issue in fact. The primary purpose of a transfermer in a tube amp is to match the output tube plate circuit, which requires a large (some thousands of ohms) impedance as a load, to a speaker that is small impedance (2-8typically). The impedance is changed by the square of the turns ratio of the output transformer. This is the fist thing you must spec in a transformer. This is the primary reason conventional tube amps have transformers and why OTLs are, well, different. Now most folks don't need to know this except it makes it easier to understand that when they look at the 4 and 8 ohm taps on their amp they are looking at different taps to the transformer's secondary and using one tap or the other
changes the turns ration, which in turn, changes the nominal impedance match from amp to speaker.

A further note, most amps would do a lot better if the folks who built knew what speakers were going to be attached to them because they would know the load. Unfortunately we are all in love with separates.

Matching Z takes place through the entire system, is not limited to transmission lines as you seem to suggest, and IMHO, if folks understood it (and a little RLC) about 3/4 of the magic and synergy would disappear from the wires used in audio. It is fatal if overlooked with passive pres and such. There are many places you can go to get a little more on this simple point and, if anyone is interested, you might try online. Here is one short piece.

http://www.aikenamps.com/OutputTransformers.html


Sincerly
I remain
not a physicist, and

This is so cool. I'm all over google trying to make sense out of what you good people are saying. It's slow, it's slow. Clueless, you say the point about impedence is that I need to figure out what the goal is, transferring voltage or current. Can you elaborate on this? How do you tell what the goal should be?

I am not even going to touch all the other questions I have - I'll keep reading around and try to learn a little more about the basics.

Many thanks to you all. Happy listening. Chuck

Chuck --
investing a little time in understanding Ohm's law will take you a long way towards understanding these issues. It's a bit like learning a language: in the beginning it feels daunting and impossible and then, suddenly, you loosen up & speak the language... you've acquired the "knowledge" as it were.

OK, you may not be ready to design your own amps, etc -- but, as Clueless (and not a physicist:)) notes,

if folks understood ...(that)... (and a little RLC) about 3/4 of the magic and synergy would disappear from the wires used in audio

However, the magic of matching devices into a system would appear.

Try this link, too. It's a short introduction to electricity & components.
It can be fascinating!

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.