Question about amps


New member
Username: Bushd841

Post Number: 6
Registered: Mar-06
If an amp is rated for X number of watts at 8 ohms and the speakers are rated for 6 ohms, is it just a matter of plug and play?
Do any adjustments need to be made to the amp to match it to the 6 ohms impedance of the speakers?

Silver Member
Username: Davidpa

Portland, Oregon US

Post Number: 455
Registered: Nov-05
Keep the volume down, that is going to be the most important adjustment. The load the speakers are putting on the amp could harm both if you try to stretch the capabilities of the 8ohm amp. It will heat up, shut down, and cause distortion in your speakers due to the 6ohm load. BUT, if played at MODERATE levels, and not pushed to its limits, it will just "plug-n-play" like you've mentioned. An amp not rated to sustain a 6/4/2 ohm load is working much harder and depending on the user can cause big headaches, the distortion caused by the amp working overload will kill your speakers, and the load on the amp will cause it to overheat, and maybe even just decide to quit on you.

New member
Username: Bushd841

Post Number: 7
Registered: Mar-06
David, thanks very much for that information - you saved me a lot of grief.
I had been considering the Marantz PM7001 amp with the NHT Classic One speakers and their sub.
I'll have to rethink my speaker choice.
Thanks again!

Gold Member
Username: Nuck

Post Number: 7031
Registered: Dec-04
Dennis, the numbers are not the whole story.
Some speakers are quite flat in impedance, other may dip far below their stated bench value. This measurement changes all the time. A speaker, perhaps the NHT you mention, may be quite flat and not dip too far.
A receiver rated @8 ohms may be capable of going lower(for a bit) or longer, if you follow DP's knowlegable words.

For any particular pairing, things sometimeswork perfectly well, despite 8,6,4,2.

Some homework on the impedance swing of the speaker is a good start, regardless of the power source.
A good understanding of the power source,it's limitations and it's qualities should be undertaken as well.

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 10307
Registered: May-04

"If an amp is rated for X number of watts at 8 ohms ... "

If an amplifier is rated to deliver "X" number of watts into an eight Ohm load, it is literally an eight Ohm load. A large load resistor is placed on the amplifier's speaker output and this resistor's value is not the equivalent of a speaker. A real world speaker is a voltage/current driven motor which has both resistance and inductance, neither of which are constant with frequency or output level and the motor assembly of the speaker (the voice coil) produces what is called back electromotive force - meaning the voltage produced by the driver's motor (voice coil) has to go somewhere and it ends up back at the amplifier trying to drive the amplifier's outputs. How much the speaker fights back and how well the amplifier deals with this back EMF will be important in how well the two components mate.

Most speaker systems also utilize capacitors and inductors to shape the frequency response of the individual drivers. Multiple capacitors and inductors in line with the amplifier's outputs make the task of the amplifier even more difficult by altering the electrical phase angle of the speaker system. The combination of impedance and phase at any frequency will vary and will draw either more voltage or more current to drive the speaker. Most mass market receivers can produce a fair amount of voltage (for at least a short burst) but often do not do well when asked to supply large amounts of current. Therefore, it would be wise to disregard most numbers which suggest an amplifier is capable of driving "X" amount of watts into a real world "eight Ohm" speaker load. Certainly when you are dealing with mass market receivers the ability of an amplfiier to deliver its rated wattage into multiple speakers should be considered with, at best, a jaundiced eye.

" ... the speakers are rated for 6 ohms ... "

As you can see from this graph;
nominal impedance means little if anything when judging whether a speaker and amplifier will mate well. This nominal four Ohm speaker dips to 1.4 Ohms and spends little time above two Ohms. Due to the complex crossover the electrical phase of the system reaches a negative 62° combined with a 3.3 Ohm impedance at 112Hz. This is a frequency where plenty of energy can be produced in most music and therefore it represents a combination which would stress most amplifiers connected to such a dificult load even though an amplifier might be reasonably happy driving a four Ohm load resistor. As the text indicates, this is a rather brutal load for any amplifier despite the impedance being rather flat across the frequency bandwidth. If the impedance were higher, say above ten Ohms, and just as flat, the load would be far less difficult for any amplifier. You might not be considering a $16k pair of speakers, but, unfortunately, this sort of load behaviour is not limited to exotically priced speakers. Here's a bit on impedance and electrical phase, both of which will influence how well a speaker pairs with any amplifier.

You might step back a few pages in this linked article to read about sensitivity or what most specs refer to as "efficiency". A more sensitive speaker will require less power to produce the same amount of SPL's (volume) as a lower efficiency speaker. More power required to bring a lower sensitivity speaker up to a satisfactory volume will place the amplifier closer to its limits. Combine this with a more difficult impedance swing and electrical phase angle and you can have a serious problem with a speaker that is "rated for 6 ohms".

There are actually quite a few considerations to be taken into account before you "plug and play" any speaker and amplifier. If the two components have been designed to mate together by one manufacturer, there's a good chance the pairing will create no serious problems. However, if you are choosing speakers from column "A" and an amplifier from column "B", you should rely on a top notch audio shop to provide sufficiently good guidance to keep you from doing something you'll regret. If the salesperson cannot at least tell you the lowest impedance point of the speaker, I would shop somewhere else. You should be able to find some information on line for any speaker or amplifier you might be considering. If not, call the manufacturer(s) and ask for guidance before making a non-refundable error.


Gold Member
Username: Nuck

Post Number: 7048
Registered: Dec-04
What Vigne said.

New member
Username: Bushd841

Post Number: 8
Registered: Mar-06
Thanks everyone for all the terrific information. I'll read up on this before I proceed further.

Bronze Member
Username: Leonski

Post Number: 84
Registered: Jan-07
thanks for link to speaker articles. I am i/p of reading 'cover to cover'.
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