WHAT GOOD IS SPEAKER AND CABLE IMPEDANCE ANYWAY?

We’ve all heard it said, some speakers need high current amps and some don’t, why? The normal explanation is that a low impedance speaker has a “high” current draw from the amp. Is this true, really? Yes, and no. Impedance of the speaker does not really tell the true story.

A speaker is a motor. A funny looking motor that goes in and out instead of around and around. The most fundamental thing to know about a motor, is that it won’t move at all when the signal is reactive power (capacitance or inductance). The reactive power is stored and discharged in the motor, but does NOT move the motor. The motor is moved when the power factor (amount of the signal that is real, or true power verses Reactive power) is greater than zero.

 Active (Real or True) Power - watts (W), power drawn by the electrical resistance of a system doing work.
 Reactive Power - volt-amperes reactive (VAR). Reactive Power is power stored in and discharged by inductive motors, transformers and solenoids. This makes heat, but no work (unless you want to burn-up something!).
 Apparent Power - volt-amperes (VA) and is the vector sum of the active and the reactive power.

A speaker’s impedance is the vector sum of real power and reactive power. In a power delivery system, the utility company will charge you a premium if tour power factor isn’t “real” enough, as this hurts efficiency. You can hang a capacitor or inductor across the system to improve the power factor to a higher number. Capacitance and inductance or opposites one another, so larger of the two is essentially what you have left when you sum them. Capacitance is drawn on the Y or vertical axis, inductance on the negative / down Y axis and resistance is in the right horizontal axis. There is no left “imaginary” resistance axis as zero resistance is simply zero, it can’t be less than that and is always the absolute value of the “vector” magnitude.

So the big issue with speakers and amplifiers, is that a speaker is a TERRIBLE motor! A speaker is less than 5% efficient at best. Again, this is simply terrible. And THAT is why speaker impedance is so misleading to amplifier load response. A graph of impedance with respect to frequency is a white lie. The amplifier wants a resistive load, so the response curve of the REAL (resistive) power load, not the Apparent Load (impedance) with respect to frequency is much more meaningful. There are some graphs that show “phase” reactivity, but you really want to see EACH factor (real / active and reactive / imaginary) separately with respect to frequency. Me, I want to see the real value, as THAT is what’s allowing your amplifier to do work across the load. Most of the impedance vector magnitude of a speaker is imaginary, and of little use. Small changes in the real power are very significant is such a lousy load.

A 4-ohm impedance value at a set frequency can be more resistive than an 8-ohm impedance load, for instance. And, when you take into account that a speaker is 5% efficient, the real resistive load is darn near a short across the input of ANY speaker! So to say any amp is high current is an understatement like no other. ALL audio amps are high current devices! How this even works is a mystery. Would you hook-up a $15,000 electronic amplifier to a 4-ohm resistive load and expect good things to happen? Common sense says no, but we do it. Don’t forget all that imaginary capacitive and inductive energy will randomly superimpose itself on one another and become real, and try to do “work” at inopportune time and places.

With such terrible efficiency, speaker cables can have a big effect on sound. A seemingly small amount of reactive influence can change the overall efficiency 1-% or so, making the load look more real. This is a BIG change, 25% or so! You don’t “hear” the speaker cords so much as you change the real component of the signal to something that isn’t trying to destroy your electronics. Electronics like the load on, not off! A dynamic driver speaker is a big inductive component (coil of wire moving in a magnetic field) so speaker leads are best LOW inductance to not make a bad situation even worse. A few speaker designs are capacitive, but that is an exception.

Your signal leads (pre to power amp, CD player to pre-amp ETC) are all darn near ZERO current. Just the opposite if speaker leads. Here, the lead is two conductive plates in parallel. THAT is a capacitor. So these leads want low capacitance per unit length to appear “invisible” to the signal. Remember that the input load is as high as you can design it, and the output load resistance is as low as you can design it, within practical limits. This is done on purpose as the signal is supposed to drop across the input (very high impedance load) and not the output (very low impedance load). The load voltage drop is current in the circuit time the resistance of each component along the circuit. If you had an infinitely high input load, the entire signal would be across that load, for instance. Physics says we can’t do that since we need some current to do work, so we just make the input load ten time or more the output load value. The cable between the output and input is just a distortion generator (filters the signal like a low-pass filter) that gets worse the longer it is. It steals signal, and adds a passive low-pass characteristic to the signal. A three dollar 12” lead can sound better than a three hundred dollar 3 foot lead.

Now hear this, there is NOT cable impedance values with audio cables! I repeat, RF impedance isn’t useful at audio. All common dielectric material is not frequency stable below 1 MHz, and the termination methodology is NOT matched loads. Example; 75-ohm impedance cable matched to a 75-ohm input load. We eliminate RF cable impedance concerns by purposefully setting the input resistance very high, making the cable as “invisible” as we can. The cable still is resistance (steals voltage that never makes it to the amplifier or pre amplifier) and capacitance (distortion it adds as a passive low-pass filter), but it isn’t meaningfully acting like a complex vector impedance anymore. We get hung-up on RF cable design and apply it to audio. Don’t. Impedance at audio is NOT stable with respect to frequency so the values are impractical to spec.

The reason people think that RF impedance and audio are related, is the published capacitance value of RF cable at low frequencies. A simple to use RF cable impedance formula is a constant, (101670), divided by the capacitance (pf/foot) times the velocity of propagation of the dielectric.

101670 / 20.5 Pf/Ft * 66% = 75-ohms. This is a common solid dielectric RG-59 cable.
101670 / 17.3 Pf/Ft * 78% = 75-ohms. This is a common foamed dielectric RG-59 cable.

It’s nice to know that the capacitance is measured at 1KHz! Hey, this is AUDIO influenced per unit cable length! The dielectric isn’t too important at audio as long as it is getting you the right capacitance (changed by the combination of the thickness of the insulation and the amount of “air” foamed into the insulation. The higher the RF impedance of a cable the lower the capacitance, all things being the same. But, it is NOT the RF impedance that is doing work at audio. Did you ever-wonder why parallel polyethylene insulated wire separated by an inch on a dielectric substrate sound so good? They have super low capacitance that’s why. Look inside your device at the RCA female plug, it’s essentially this arrangement. A single wire well removed from the ground plane results in very low capacitance. You want the same thing in a cable as practical as can be managed.

There is no skin affect at audio as much as people want to believe it. Why? This effect is a pain in the ass at RF, why do we want it at audio? The signal is diffusion coupled all the way through the copper wire (or any wire) at audio. There are no equations or measured values that prove other wise…just talk.

Copper, gold, silver, aluminum and tin are all OK to use once you understand why they are used. We want to avoid high resistance contacts, especially on low current high impedance signal leads. Copper has low contact resistance and resists oxidation at room temperatures with respect to time but is terrible as temps and time go up. It can turn BLACK with oxides if heated enough. Gold is nice in that it is oxidation resistant at all temps and times periods. But, gold has higher resistance than copper. Silver is weird in that it is super prone to oxidation at ANY temperature, but silver oxide is “soft” and easily defeated with pressure. So you can have low resistance but need to use self-cleaning sealed contact connectors to keep it. Aluminum gets a bad rap, as it also loves to be oxidized at room temperature. Aluminum oxide is one tough coating that is not easily defeated. Aluminum contacts need to be sealed from oxygen to last. Tin is poor man’s gold and it has a much wider oxidation time and temp window than copper. The overall resistance of the wire is the parallel value of the two coating over length. So, the thicker copper wire core is substantially what the lead acts like. The pretty coatings just insure the contact resistance is low so the signal makes it to the load, and isn’t dropped across the RCA plug. Why get the signals there if the oxidized coating can’t get it across the RCA plug? Speaker leads aren’t as sensitive to contact resistance, as they are high current but, that doesn’t mean they aren’t at all. To an extreme, many an aluminum wire contacts have caught on fire from oxidized contacts.