Speaker Impedance and amplifier design
There’s been a lot of techno speak on the various forums to readers about the advantages of using loudspeaker impedances of specific values, some even discussing the merits of 4 Ohms as only being used in very high end systems. The question here is based on another question: If a great sounding system works well driving an 8 Ohm why would you then think a 4 Ohm would make it sound better?
Certainly car audio is often found with 4 Ohm speakers, sometimes in parallel to get 2 Ohms. Does it sound better than 4 Ohms?
Is a system designed to drive an 8 Ohm speaker then inferior to that which can drive a 4 Ohm speaker?
An amplifier designed to drive a 4 Ohm speaker can drive an 8 Ohm or higher value impedance speaker if the output stage is semiconductor.
An amplifier designed to drive an 8 Ohm speaker may not necessarily be designed to drive a 4 Ohm speaker if it is transistorised.
If we halve the impedance we double the potential power output of the amplifier which means more current, more heating which requires a larger power supply.
Tube amplifiers designed to drive an 8 Ohm speaker will drive a 4 Ohm speaker with increased distortion. If you increase the impedance of the load you are risking damage to the output transformer.
- Tube amplifiers don’t like a “no load” situation.
- Transistorised amplifiers don’t like a short circuit situation.
- No matter what, a well designed crossover network in the speaker system will add a new dimension to your listening pleasure.
- No matter how you design the crossover it will generate distortion and power wastage.
This applies to both 4 Ohm and 8 Ohm speaker systems.
- Now we have an amplifier which is designed to drive a 2 Ohm load which many car and pro audio amplifiers can do. Would you prefer to use one 2 Ohm speaker or 4 x 8 speakers in parallel. Would you have a sound stage rather filled with multiple speakers which has a nominal impedance of 2 Ohms or just a stereo amplifier driving two 2 Ohm speakers? The permutations are endless. The manufacturers don’t care – the amplifier manufacturers do!
- In the real world and I am throwing away some of the bigger conspiracy theories surrounding damping factor if you throw in Ohm’s and Kirchoff’s Laws you’ll learn how to balance the power output of equally sized loudspeakers with various impedances.
- A fast rule of thumb is that the lower the overall impedance of the loudspeaker system the higher the output spec demand on an amplifier.
- An amplifier can be made more cheaply by using high current low voltage transistors in the output stage as opposed to high voltage transistors. This was evident in the good old days. High voltage high current Mosfets are no longer that expensive.
- Usually car audio, because of the lower supply rails would be designed for 4 Ohm speakers. This was also evident in the good old days. With switching supplies boosting rail supply to more than 80V or split rail +40V – 0 – -40V this is no longer the case.
- Most amplifiers designed to power 8 Ohm speakers may only have one push pull set of output transistors (NPN/PNP) in the output stage. Driving 4 Ohms the amplifier may be designed to carry many more in parallel. This pushes up pricing but not necessarily the output quality.
- There is absolutely no bearing on the assumption that a 4 Ohm sound system sounds better than an 8 Ohm system. I have heard many 8 Ohm systems which sound significantly better than 4 Ohm systems.
- In an enclosure and with a passive crossover network the input impedance is all over the place once driven from an audio source. It’s out of the scope of this article to cover this suffice to say that it can play havoc on an amplifier designed solely for an 8 Ohm load, one of the big reasons leading to the incorrect rationale that 4 Ohms is better than 8 Ohms. See next paragraph.
- An amplifier which can drive very low impedance loads as well as those that are either unstable or capacitive are inherently more expensive than those that cannot.
- In the 80s when I dabbled with club systems I never found 12 or 15″ 4 Ohm loose loudspeakers – they were all 8 Ohms. All car loudspeakers were 4 Ohms. This may have been different overseas.
- IMO 8 Ohm speakers are easier to configure in an enclosure to have a nominal impedance of 4 Ohms, your best match for club systems (bridging amplification). With four in parallel you get 2 Ohms, even better for the right amplifier.
- Loudspeaker crossovers come in all shapes and flavours and frankly if you want to talk about the most bang for your buck, active crossovers and multi-amping is the best solution. (where an amplifier is dedicated to a specific frequency response). Hence two or three stereo amplifiers may be used to drive bass, mid and high. The efficiency is much higher and the overall result cleaner. Pro equipment and car power amplifiers are often found with active input filters, usually to drive bass bins.
In conclusion, whether it’s 4 or 8 Ohms configure your system for best match without burning out the amplifier. If you are using a 4 Ohm loudspeaker on an amplifier designed to drive an 8 or 4 Ohm speaker system make sure that the power out rating coincides with your speaker impedance. The manufacturer may have spec’ed the output rating with the higher value impedance.
[Ed’s note: Depending on the sensitivity of your hearing often playing a system at very low powers a is lot more revealing than pushing pedal to the metal].