Here is more detailed information from the AUDIOVISION trade magazine (Issue 4/03) on the subject of impedance settings of the AV Receiver:
Actually, it should be quite simple: Connect the speaker cable to the amplifier and speakers with the correct polarity and you're done. Unfortunately, many manufacturers of home cinema receivers put stumbling blocks out for the users in the form of specifications on the rear of the device and/or in the operating instructions. These usually look like the following (translations): Minimum connection impedance six ohm! Or, speaker impedance of six to eight ohm. And what is the problem? There are hardly any six ohms and only a few eight-ohm speakers available in Germany. Most of the speakers sold in Germany have a nominal impedance of four ohms. So theoretically, you shouldn't connect them to most of the receivers and amplifiers.
To explain why it is possible after all, we would like to briefly explain the electrical engineering principles: The impedance of a speaker is nothing other than its electrical resistance which it puts up against the flow of current.
• Low resistance = high current
• High resistance = low current
Important to know for operation with an amplifier is that the impedance is not the same for all frequencies but can, for example, change between 50 and 200 hertz by 50 per cent. So it isn't entirely correct to speak of impedance for a speaker per se, rather one must always consider the behaviour across the entire range between 20 and 20,000 hertz. To still achieve a reasonable connection value for amplifiers (which, by the way, says nothing about the output impedance of the amplifier itself but rather only which impedances may be connected to it), the creators of the DIN Standard have applied the following stratagem: Only the minimum value of the impedance in the audible range is decisive for the nominal impedance value. This is where the most current flows and the amplifier is subjected to the highest load. Speakers with eight-ohm nominal impedance may only exhibit a minimum of 6.4 ohms, and those with four-ohm nominal impedance 3.2 ohms. Six ohms are not defined in the DIN (which is applied internationally), but one could use the above values to interpolate that they may exhibit a minimum of 4.8 ohm.
From the definition, it is apparent that the nominal impedance is only a rough benchmark because it says nothing about how the entire course of the impedance looks. No matter whether it only reaches less than 6.4 ohms at one point and is otherwise beyond that, or if the impedance moves close to 3.2 ohms across the entire range, but does not drop below that. The last speaker is surely much more stressful for the amplifier but has just a nominal impedance of four ohms than the other higher ohm one.
Conventional speakers usually exhibit one to two minima in the bass range, and there is one frequently in the treble range. These minimum values are usually not especially broadband. Aside from the minima, the impedance is usually far higher than the nominal impedance. The amplifiers are really stressed in the bass range because firstly, this is where the most energy is for music and especially the home cinema sound, and secondly, they have to let the most current flow through the impedance minima here.
And lastly, the main reason why many manufacturers don't want four-ohm speakers to be connected: In extreme cases - for deep frequencies of consistently 3.2 ohm - the amplifiers in multichannel operation become very hot. A lot of current also means a lot of power loss and thus a lot of waste heat. However, much has to come together before that happens - namely high performance for a longer period of time and setting up the device without sufficient ventilation.
But this extreme case is seldom met in practice. As mentioned above, most of the speakers are not as critical by far, and it is extremely rare to listen to an exorbitant volume for a longer period of time. That is why it is not a problem from a technical point of view to connect speakers with four-ohm nominal impedance to all modern home cinema amplifiers and receivers.
However, there is one small catch: The question of warranty. Should a defect occur to the device - which doesn't even have to do anything with the speakers - and the manufacturer notices that four-ohm speakers were connected despite the specifications to the contrary, he could refuse repairs due to improper use. As discussions with manufacturers and distributors have shown, this is highly unlikely because they are also aware of the problems.
And for purely legal reasons, Lautsprecher Teufel may not give you a warranty either that your amplifier or receiver will tolerate four-ohm speakers. We can only pass along one experience: For several years, the listening tests of the trade magazines are done exclusively with four-ohm speakers, and not a single defect has occurred as yet even though the devices are driven to their limits. Also, the performance of each device is usually measured at four and eight ohms irrespective of the specifications of the manufacturers, and the load here is also considerable. It has been a long time since measurement laboratories have seen failures due to low load impedance.
There are still some special cases, which should be briefly mentioned: Amplifiers/receivers with impedance switches give cautious natures the possibility to stay on the safe side under all possible conditions, even with the warranty. The four ohm setting of the corresponding switch reduces the supply voltage of the power amplifiers so much that excessive heat cannot occur even under unfavourable circumstances. However, the maximum power is then reduced somewhat.
During the listening tests of the trade magazine, these devices are always switched to the "eight-ohm" setting to tease out the maximum performance reserves. A defect here has yet to occur. When choosing devices, take a close look because there are devices that only permit speakers with a minimum of six ohms despite the impedance selector switch, like the Pioneer VSX-D2011, for example. This is remarkable in another respect: It possesses a THXSelect license. This license is only issued if certain measured values comply with that are determined at four ohms for at least the front and centre speakers.
If the THX laboratory had proceeded according to the manufacturer's specifications, the VSX-D2011 would never have received a licence, as it would not have been permitted to perform the necessary measurements. By the way: this is also the case for other THX Select devices. But you needn't worry here: The THX logo not only offers additional security, but also a good lever should a manufacturer once refuse a warranty.
The AUDIOVISION TIP
Overheating is the main problem that can occur through the combination of four-ohm speakers with power amplifiers that are not released for this.
Therefore, in case of doubt:
• Always set up the AV Receiver or amplifier in well-ventilated places and never in a closed housing or close to the heater.
• Occasionally check the temperature of the device.
• Reroute the bass elements via set-up at the subwoofer, if possible.
