Setting the A/V Receiver Impedance Selector Switch
Half Portion
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.
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 time 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 many horsepower are in the engine 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.
See also:
Recent Forum Posts:
However, the points that I would like to make plain, and that I believe that Gene obfuscated, are as follows:
1. This may well cause distortion to increase, but ordinarily not unless you are playing so loudly that the amp is caused to run much hotter than it ordinarily runs.
2. The effect of the switch is not necessarily to lower the supply rails. In the case of my Onkyo receiver, it is manifest that the primary effect is to reduce the idle current, i.e., the operating mode of the amplifier is shifted further toward class B, where idle current is reduced and the amp runs cooler. This again may well cause an increase in distortion, but it is not necessarily the case that you will be able to hear the difference, whereas it is necessarily the case that you will use less electricity.
3. Even in the case where the lower impedance setting causes your amp to clip at a point in loudness where otherwise it would not clip, this is not likely to cause any damage to your speakers. This is a myth that started back in the early days of the battle waged by tube aficionados, who claimed that because clipping in transistor amplifiers produced stronger high-order harmonics as compared to tubes, that clipping in transistor amplifiers would be more likely to damage speakers.
On our beloved Internet, you can find several half-baked attempts to prove that clipping distortion will fry your tweeter. I have read several of them, and they all use erroneous, convoluted reasoning. The question that they end up answering is not the correct question. They do this because they are hell bent on finding an answer different from the one they get when they stick to the correct question.
The correct question, stated succinctly, is this: When sound is played at the same perceived volume level through an amplifier that is clipping and an amplifier that is not clipping, is the RMS power sent to the tweeter greater for the amplifier that is clipping than for the amplifier that is not clipping?
I have yet to see one of these so-called proofs stick to this correct question. In all of the analyses that I have seen, that purport to prove that clipping damages tweeters, it is virtually impossible to infer exactly what question they eventually did manage to answer.
Clipping does not increase the total RMS power that the amplifier delivers to the speaker. As such, it is manifest that the only way that it is possible for clipping to increase the power delivered to the tweeter is if the spectral power distribution shifts significantly from lower to higher frequency. For the most part, clipping removes some of the original high-frequency content and replaces it with harmonics of the lower-frequency content. Possibly, there is some shift in spectral content from low to high frequency. However, in the hypothetical case where the spectral power content is ordinarily distributed evenly between the woofer and the tweeter in a two way system, the entire spectral content below the crossover frequency would have to shift above the crossover frequency in order for the RMS power delivered to the tweeter to increase by 3 dB. Granted this is not entirely realistic because the spectral power division between the woofer and the tweeter is not ordinarily 50/50. But the point is that an enormous shift of spectral power would have to occur before the tweeter RMS power would increase by 3 dB, and way, way before that happens, you would hear the difference, not because you would hear the 3 dB increase in high-frequency content, but because when this occurs, the shift in overall tonal quality would be absolutely impossible to miss. It would be practically the same as disconnecting your woofer and turning up the volume by a tad.
This is what the audible effect would have to be in order for the power delivered to the tweeter to increase by a few dB. But even if you were so deaf that you could not hear this happening, this much of a shift in spectral power distribution does not occur. When an amplifier clips, the spectral power distribution takes on a character inherently similar to white noise. All you really need to ask yourself is whether you believe that if you were to play white noise loudly through your amplifier that is guaranteed to never clip, if this would burn out your tweeter. If you think the answer is yes, then you had better pay very close attention to the sort of music you play, because there is plenty of prerecorded commercial music where there are significant periods where the spectral power distribution is shifted more toward high frequency than is the case with white noise.
