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Setting the A/V Receiver Impedance Selector Switch

by June 21, 2015
Contributors: Paul Apollonio
Half Portion

Half Portion

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?

There is no universally adhered-to standard for how consumer loudspeaker manufacturers rate 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.

SPKA_impedance.JPG        SPKB_impedance.JPG

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.

 

About the author:
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Gene manages this organization, establishes relations with manufacturers and keeps Audioholics a well oiled machine. His goal is to educate about home theater and develop more standards in the industry to eliminate consumer confusion clouded by industry snake oil.

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Recent Forum Posts:

ntsarb posts on March 22, 2021 20:17
Thank you @PENG. Very interesting and useful information.
PENG posts on March 21, 2021 10:10
ntsarb, post: 1469677, member: 94157
* Why does the manufacturer need to reduce the rail voltage more than required to keep the power rating steady?

I believe they did it on as required basis. The issue is not so much “power”. In my opinion, It was unfortunate that in the beginning, manufacturers started rating their amps by “power” output instead of voltage and current. It is now too late to change.

Loudspeakers are sort of voltage devices, their sensitivities are often rated as X dB/1W/1m but should be more appropriately rated X dB/2.83V/1m. 2.83 V is picked because for the popular 8 ohm loads, it would be equivalent to X dB/1W/1m. The fact is, if you apply a voltage signal to the loudspeaker's terminals, it would make sound in proportion to the applied voltage, not to the “power input” as such. That is, the speaker may only consume only 0.1 watt in one moment but make a very loud sound, yet may consume 0.2 W in another moment and make a much quieter sound.

If an amp is rated 100 W into an 8 ohm resistor, then it can be rated 50 W into a 4 Ohm resistor safely, and that means the output voltage has to be reduced by half in order for the current to be the same. The current would be 4.8 A. (using your 38.47 V, 8 Ohm example).

Now if the manufacturer knows their amp can be rated higher than 4.8 A, then they wouldn't have to lower the rail voltage as much for the 4 Ohm setting, may be lowering it by 30% is enough, just an example. That's why the lower rail voltage would vary depending on the specific amp's rated voltage and current capability. So far so good? And you can see why I said amps should have been more appropriately rated for their voltage and current limits, than just “power” that by itself actually makes little sense? By the way, keep in mind, you really don't know how much “power” your speaker would actually consume, all you know is how much current it draws on moment by moment basis, as a good portion of the so called “power” would be consumed, or dissipated in the amp itself!!

Would dropping the rail voltage from 38.47V (in the following example) to 33.3V significantly increase power loss in the form of heat? If so, does this relate to the transformer's efficiency at different voltage?

No, dropping the rail voltage should result in less current so less loss, not more, all else being equal.
Copper loss = I^2 R for an resistor load. For an inductive load such as many loudspeakers, it gets more complicated as much of the loss would be dissipated in the amp, not just in the speaker. It would depend on the phase angle vs frequency characteristics of the speaker.

Here's a good article for you:
Phase Angle Vs. Transistor Dissipation (sound-au.com)

* In case more heat is produced by use of lower impedance speakers at same wattage, would it not make more sense keeping the heat production steady (i.e as if n 8Ohm speaker was driven) by adjusting the voltage accordingly? Why would the manufacturer drop the rail voltage more than that?

Yes, but you are assuming the manufacturers drop the voltage more than necessary, you don't really know that for sure, as you don't know their products current capability. Again, think current, and phase angle, not “power”.

* I have not fully understood the certification process, hence, my next question is: is the certification process biased towards use of higher impedance speakers, to the extent that it forces manufacturers to degrade the amplifier's actual capabilities (practically leading to the production of less heat with lower impedance speakers than the other way around)?

Not sure I fully understand what you are asking, but this is a complicated issue that I don't think there is a right or wrong answer even if you clarify your question.
ntsarb posts on March 20, 2021 21:08
Very interesting article. Thanks for sharing. A few things I don't understand, so here are my questions:

* Why does the manufacturer need to reduce the rail voltage more than required to keep the power rating steady? Would dropping the rail voltage from 38.47V (in the following example) to 33.3V significantly increase power loss in the form of heat? If so, does this relate to the transformer's efficiency at different voltage?

Example: if the system can supply 185W to an 8Ohm speaker, that means 38.47Volts output. At this voltage, a 6Ohms speaker would require (38.47^2 / 6 =) 246W, but if rail voltage was dropped to 33.3Volts (using the switch), the power rating would remain 185W.

* In case more heat is produced by use of lower impedance speakers at same wattage, would it not make more sense keeping the heat production steady (i.e as if n 8Ohm speaker was driven) by adjusting the voltage accordingly? Why would the manufacturer drop the rail voltage more than that?

* I have not fully understood the certification process, hence, my next question is: is the certification process biased towards use of higher impedance speakers, to the extent that it forces manufacturers to degrade the amplifier's actual capabilities (practically leading to the production of less heat with lower impedance speakers than the other way around)?

Many thanks.
PENG posts on December 31, 2019 16:21
hotrabbitsoup, post: 1359291, member: 90421
Thanks for the replies guys. Multiple secondary windings are common for the power supply transformers used in tube amps and is where I'm getting my inspiration from.


I stated that too, yes it is quite common, but for different purposes.
hotrabbitsoup posts on December 31, 2019 16:11
Thanks for the replies guys. Multiple secondary windings are common for the power supply transformers used in tube amps and is where I'm getting my inspiration from.

I think some cheaper amps, that actually have an impedence switch, implement the power restriction by using a tap on a single secondary winding that supplies a lower voltage to the power supply, but in a more expensive design I don't see why they couldn't use a second coil altogether. But, all that said I don't think it would be worth it, as you said, just make the transformer with the secondary coil that's going to work for all intended purposes in the first place.

But all manufacturers don't have acces to the same. Some shops have the size to invest in custom transformer production runs, some even wind themselves, while many others look to off the shelf parts.

I'll try and find some schematics of older 80s receivers with the switch and report back if I notice anything useful. I am EE too.
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