Impendance - Resistance & the story of "Z"

A very simplified definition, though not technically correct, would be:

Resistance: opposition to current that is constant no matter what the frequency (60 hz, 2kHz, 1MHz, etc.)

Impedance: opposition to current that varies as frequency varies.

A resistor gives a constant resistance at all frequencies.
A capacitor gives more "resistance" as freqency decreases.
An inductor gives more "resistance" as frequency increases.

The audio signal has a varying frequency, from around 10Hz to well over 20kHz. So it becomes an issue with cable designs to account for this in order to minimize the effect of cable resistance. For speaker cables, a lower overall impedance improves the amplifers' damping factor which minimizes the distortion by spurious signal oscillation from speaker-to-amp-to-speaker. As far as interconnects go, Sean covers it very well.

Gs: Your comments about resistance being frequency independent and impedance acting the opposite i.e. changing as frequency is altered was a very good point and easy to understand in the manner presented. I had worked that into my original post, but when trimming things down, it got lost in the shuffle. Kudo's to you for highlighting this basic but very important aspect of understanding electronics.

This is why testing amplifiers into dummy loads ( resistive loads that remain constant with frequency ) isn't a very accurate representation of how amplifiers actually "load up" into various speakers. That's because speakers are complex impedances where reactance varies with frequency. In some cases, reactance and therefore the impedance can even change as the drive levels are altered. At 1 watt at 100 Hz or 10 KHz, the amp might see what appears to be an 8 ohm load. Increase the drive level to 10 watts at that same frequency and the amp may see what appears to be a very different impedance.

Trying to achieve a smooth or "flat" impedance curve* is something that many manufacturers attempt to do by adding parts to the crossover network, but in the real world, those parts only add more clutter between the amp and speaker. In most of the speakers that i've heard where the designers placed great importance on this aspect of design and performance, the end result was that it tended to "suck the life" out of the music. There is something to be said for designs using minimal or no crossover at all. Sean
>

* a "flat" curve sounds kind of contradictory, doesn't it??? : )

the story of Z sound great and is very interesting and if this is true that the smaller the Z number the better then i hope that Jopherfi or someone can tell what co. this is so i can look into getting some of these cables to listen to. thank you .

I want to thank all who took the time to respond and educate the non-EE’s (me) on this complex topic. I now have a better understanding of things. The reason I asked in the first place was because the manufacturer of the pre-amp I just purchased suggested that a long interconnect to the amp would be OK if I used and IC that kept the resistance low. I will now go back and ask if he meant resistance or impedance. (Does anyone know what he may have meant? He does not publish an output impedance spec for the pre-amp).

Sean, in particular, thanks for explaining the concept of an impedance transformer as related to interconnects. I see now how the electrical parameters of the components connected along with IC’s have a big impact on the “complex impedances” of the system. Understanding that the impact of a cables “Z” (nominal impedance) depends on its application (amp-speaker or source-pre-amp) wouldn’t one now question the cable manufacture that say’s that a lower “Z” for both IC's and SC's is always better? Perhaps I missed something.

Regards,

Joperfi: As i mentioned, most amplifiers have a very low output impedance and most speakers have a relatively low ( several ohms ) input impedance. Using a cable that is somewhere between these two figures will typically provide the best results. If one is trying to alter the sonics of a poorly matched system, introducing some type of colouration via series resistance ( makes the cable sound leaner due to less current flow / reduced bass output ) and / or high inductance ( makes the cable sound warmer and smoother due to high frequency roll-off ) can do that. Using a cable that offers both high series resistance and is inductive would act somewhat like a band-pass filter i.e. reduced bass and soft highs with "reasonable" mids. This might be okay for a system or speaker that was very boomy and bright. Having said that, it would be better t fix the system than to have to band-aid it though.

As far as interconnects go, the source output impedance may be anywhere from a very low impedance ( 20 - 50 ohms ) up to several hundred ohms. The input impedance of most preamps or amps is quite high i.e. several thousand ohms up to 100 - 200 thousand ohms. You have to remember though that we are talking about complex impedances here, not just straight resistance. This complex impedance can consist of XXX ohms of resistane combined with XXX values of capacitance and / or inductance. As such, it is possible to find a cable that "cancels" or minimizes the reactive part of the complex impedance found on the input of the device, allowing the source component to see what effectively looks like "pure" resistance. Since reactance is equivalent to "active resistance" or the load "fighting back" against the source, minimizing this encourages power transfer and can improve linearity via reduced ringing / improved phase & transient response. Obviously, this can get phenomenally tricky and for most folks, becomes a matter of trial and error. As such, due to the differences in voltage to current ratios and the impedances that we are dealing with, the electrical characteristics for a speaker cable and an interconnect are different. As a side note, something that works well as a low impedance speaker cable will typically work pretty well as a power cord too. That is, so long as it can pass enough voltage / current without potential safety hazards coming into play. Sean
>