Connecting 4-ohm Speakers to an 8-ohm Receiver

Q: How do I connect a set of four ohm speakers safely to an eight ohm system?

A: There are many variables to consider when connecting 4 ohm speakers to your system. Please consider the following:

Define your system

Are you using a Receiver or separate power amp to drive your speakers?

Receivers: Most midfi Receivers may have problems adequately driving a 4 ohm load. However, many of the better Receivers today have a large enough power supply, heat sink area, and current capability in the amp sections to handle 4 ohm loads. You are usually safe running these speakers on the Flagship Receiver models from: {Yamaha, Denon, Onkyo, Harman Kardon, NAD, Nakamichi).

NOTE: Some Receivers have an impedance selector switch. In most cases we recommend the 8-ohm or more setting.  The manufacturer puts them there for UL/CSA approvals as well as easing consumer concerns about driving low impedance loads. These switches step down voltage feed to the power sections which will limit dynamics and overall fidelity. Keep the switch set for 8 ohms regardless of the impedance of your speakers and ensure proper ventilation of the Receiver.

For a detailed explanation on how to set the impedance switch on your receiver, read:  Setting the AV Receiver Impedance Switch

Update: 09/06/05

Upon further investigation on this topic we have discovered that in most cases the switch is doing nothing more than reducing the rail voltage. Please check out our measurements on page 2 of the Yamaha R-XV4600 review for more information on this.

As you can see the measurement differences between the "low" setting (less than 8 ohms) and the "High" setting (8 ohms or more). This is the reason I usually recommend keeping this switch in its default "High" setting and using common sense when mating a receiver with inefficient 4 ohm speakers in large rooms.

All the "Low" setting of the switch is doing is stepping the rail voltage down so when UL tests the amp at a specified distortion level, the amp will achieve that distortion level sooner since it runs out of headroom more quickly than it would in the "High" (8 ohm or more) setting. This in turn generates less heat since the amp isn't driven as hard. You really aren't buying any protection for driving low impedance loads as you actually risk clipping the amplifier more since it can run out of headroom more easily. The switch is there more for certification purposes.

The reason you don't see this switch on separate amps is twofold:

• They typically have more heat sink area, and bigger power supplies and can better manage the heat
• They aren't UL certified and don't have to meet the requirement.

Separate Power amps

Most separate power amps have more than a sufficient power supply and power amp sections for driving 4 ohm loads with ease. In fact many of them will perform twice as good with a 4 ohm load as opposed to 8 ohms if their design is robust enough to pump out the extra current. Again, make sure you have proper ventilation so that you are not frying eggs on top of the amp, risking thermal meltdown.

What Other Specifications Are Available for Your Speakers?

Although a particular loudspeaker may be rated for 4 ohms (nom), it may actually provide a more stable load for an amp to drive than another speaker rated at 8 ohms. This has to do with the inductive reactance nature of loudspeaker systems. A speakers impedance varies as a function of frequency. An improperly designed loudspeaker may have nasty impedance dips at certain frequencies. This may potentially drive an amp into oscillation if it becomes too problematic. A well designed 4 ohm speaker system will usually present a more ideal load to an amp than a poorly designed 8 ohm speaker.

What About Sensitivity?

The higher the sensitivity of a speaker, the louder it will play at a given power level than a less efficient design. Higher efficiency does not necessarily mean it's a better speaker. There are many trade off's in high efficient loudspeaker designs. However, this topic is beyond the scope of this article.

How Can I Get the Most out of my A/V Receivers Amp Section

• Use bass management and set all speakers to small.  This allows the power hungry bass to be fed to your powered subwoofer and will take the load of the internal receiver amps thus preserving more dynamic range.
• Use external amplification (when needed) for at least the front 3 channels
• Select reasonably high efficient (>89dB @ 1 watt/meter)  8 ohm loudspeakers
• ALWAYS keep the impedance switch set to the 8 ohm (high) setting, regardless of your loudspeaker impedance