Setting the A/V Receiver Impedance Selector Switch

Originally published: Aug 2, 2010

What if you went to your favorite local restaurant, ordered a meal and the waiter served you 1/2 the portion but charged you full price?  Would you feel cheated?  Believe it or not,a good deal of home theater enthusiasts are spending good money on the latest and greatest A/V receiver from their favorite manufacturer and serving the same half-portion to their speakers.  I can't tell you how many times I've run into threads on our forums or on AVS Forum where users either were utterly confused about how to set the impedance selector switch on their new A/V receiver or they decided to set it to the low setting because their speakers were rated below 8-ohms and the user manual said to do this, or Joe the Plumber set his this way and we all want to be like Joe.  Some even set the switch to the low setting while still running 8-ohm speakers, thinking it will be better.

This article explores why the impedance switch exists and its intended purpose.  Because of liability and the litigious society we live in, I can't tell you to ALWAYS set the impedance switch to the high setting for 4 ohm loads, but I can show you the facts on what this switch does along with supportive data for you to make your own educated decision.

If you call the manufacturer, they will tell you to ALWAYS set the switch to the low setting when using 4-ohm rated speakers mostly due to liability. UL/CSA labs may instruct you similarly.   It’s a damned conspiracy!  Well, not really.  I know it sounds crazy to go against the manufacturer's recommendation, but hear me out before you shoot the messenger.

 How Should You Set the Impedance Selector Switch on Your AV Receiver? MUST WATCH!

The Basics

Let's back up a bit and define a few basic terms to gain a better understanding of the topic at hand.

• Voltage – Is electromotive force. This is defined as a potential electrical pressure difference between two points in a circuit  measured in volts (V).
• Current - flow of electrons in a circuit measured in amperes (A).
  
• AC (Alternating Current) – The electrons move to and fro in the circuit in alternating direction.
• DC (Direct current) – The electrons move in a single direction in the circuit.
• Resistance – The measure which is the inverse of electrical conductance to direct current. This also can be considered as the ratio of electromotive force (Volts) divided by the flow of current (Amperes).
• Impedance – is a complex measurement of opposition to current flow in an  (AC) circuit.  With AC, or alternating current (alternating at any frequency higher than Zero Hertz, which is Direct Current) impedance can be represented as the complex combination of Resistance (DCR) and Reactance (“Resistance” to AC current flow).  AC ”Resistance”, technically called Impedance is a frequency dependent, complex measurement including both a magnitude and phase component.  This complex quantity is often represented as the letter “Z”.  
• Power -  is equal to the product of Current and Voltage times a power factor, resulting from the phase difference (if any) between the flow of the current, and the presence of electromotive force (Volts).  This product is measured in watts (W).  (In DC circuits, or even AC circuits where the load is purely resistive, the phase is zero, and the power factor is one, so the equation is simple Watts = Volts * Amperes)

What About Loudspeaker Impedance?

Loudspeaker Impedance is often stated as a single rating in ohms.  This is done for the sake of simplicity, as few ,if any, real loudspeakers present a constant load to their amplifiers.  Typically, the magnitude of the loudspeaker impedance can range from a few ohms to many hundreds of ohms. Loudspeakers are electro-mechanical transducers that operate with AC signal input.  They will also operate at DC, but only long enough for the VC to go one direction and jump out of the magnetic gap.  As a result, specifying a loudspeaker by its DC impedance or voice coil resistance is a little bit like trying to guess how much horsepower the engine produces based on the number of doors on a car.  At and near the resonant frequency of the loudspeaker, its impedance often rises to more than 100 ohms.  The nominal impedance is basically a conservative notion of how low the speakers impedance will go over the range of frequencies it is operating over, so that musical spectrum in that range will not cause the amplifier to be overloaded if the amount of current drawn by the loudspeaker is too high. As we can see from the impedance magnitude curves (bold blue) and phase (light blue) for the measurements below, the absolute value of the speaker's Impedance varies enormously, and it is the area on the curve where the magnitude is lowest that poses the greatest current demands on the amplifier. This is especially true when this low flat region corresponds to that range of frequencies where much musical information lies.  It is the impedance in this low region that was typically used to define the loudspeakers “nominal” impedance. Based on our definitions above, and measurements below, it's easy to see that a loudspeakers impedance is NOT constant but instead a function of frequency which can also vary drastically from the minimum or “nominal” impedance of the loudspeaker.

      

Impedance/Phase of two competing speakers (Left Pic: SPK A; Right Pic: SPK B)

Both of these speakers are rated at 8-ohms by their respective manufacturers.   Yet when you look more closely at the curves, they look drastically different not only from each other, but from the straight horizontal line that would represent a purely resistive impedance.  You can see Speaker A (left pic) never dips below 8-ohms at any frequency.  In this case the manufacturer rated the speaker very conservatively.  Speaker B exhibits several dips into the 6-ohm region measuring lower than 5-ohms below 20Hz. This particular loudspeaker lacks a high-pass section for its midrange speaker, so at low frequencies those midrange speakers are in parallel with the woofer, creating a high current demand on the amplifier, which can cause it to shut down.  This happened to me personally when this speaker was driven with extremely low frequency content at high output levels using a very beefy Marantz Integrated amplifier rated at 200wpc. Despite the fact that there is little musical content near or below 20Hz, the amplifier still sees that speaker as a dangerous load when driving it.  If this system is using a turntable, and if there is a slight warp to the record, the combination of phono cartridge and RIAA equalization curve may be producing a demand for output at 15Hz from the amplifier/loudspeaker combination that could be larger in magnitude than the entire audible musical spectrum! The RIAA curve made for LP's and phono cartridges uses far higher gain at the lowest frequencies than the highest.  Those of us not old enough to remember when our music was sold on LP records may have never witnessed this.  Suffice it to say, those who favor LP's over digital media must be proud owners of high order subsonic filters as part of their electronic arsenal.  The effect of even a modest amount of low frequency energy in the subsonic range can cause the loudspeaker, especially vented designs, to move wildly causing gross distortions under extremely high excursions they were never designed for.

There is no universally adhered-to standard for how consumer loudspeaker manufacturers rate loudspeaker impedance!  The EIA published a standard which has for many years been the defacto standard for determining nominal loudspeaker driver impedance.  That standard stated the impedance would be measured at 400Hz, and the voice coil resistance should not be below 6.4-ohms for an 8-ohms speaker, or twice that for a 16-ohm speaker.  That standard has become less and less common in the business as the race for sales created a pressure for manufacturers to use ever lower DC Resistance's (DCR's) on their voice coils to increase the apparent efficiency by drawing more power (lower impedance loads draw more power than higher impedance ones when attached to amplifiers) than the competition.  For equally efficient systems, the 4 ohm speaker should be 3db higher than the 8 ohm speaker having identical efficiency! 

Realizing the fact that impedance is a complex and greatly variable quantity, don't get hung up on an absolute number for impedance.  It's important to look at the loudspeaker's impedance curve and efficiency to understand how it will play with the amplifier it is coupled with.  Impedance dips at low to middle frequencies where much of the power is present in music can be far more stressful on linear class A/B amplifiers than dips in impedance magnitude at high frequencies, where demands for power are relatively small.  The opposite is true for Class D amplifiers, some of which choke when presented with low impedance dips at high frequencies because of potential interactions with their output filter.