[Falcon]

I have noticed some manufacturers of amplifiers like NAD for example often have the same power ratings for 4 and 8 ohm speakers. Others often give a much higher rating when using lower impedance speakers.
What are the pros and cons of amplifier design when the power ratings are the same with different impedance? Or, why would designers use one design strategy over another?
Thanks!

[Speedskater]

Don't put to much stock in any manufactures specs.
But if an amp's power almost doubles at lower impedances, it means that the higher price is because of that feature. High power into low impedance is very expensive.
* * * * * * * * * *
There was an old blog about how one major manufacture would write specs differently for different price lines even thou the internal circuits were the same.

[Falcon]

Your comments seem correct given I have the Luxman M900U that can double output into the lower impedance--and it was an expensive amplifier. While that is a cool spec., I don't know that paying the high cost got me greater listening pleasure. I also have an NAD 275BCEE (a relatively cheap amplifier) that has the same output, 150 watt/channel at 4 or 8 ohms and I really can't hear any difference between the two. Perhaps my habit of listening at relatively low volume is part of the reason I experience that. Perhaps the Luxman would shine at ear splitting levels!

[Freo-1]

Your comments seem correct given I have the Luxman M900U that can double output into the lower impedance--and it was an expensive amplifier. While that is a cool spec., I don't know that paying the high cost got me greater listening pleasure. I also have an NAD 275BCEE (a relatively cheap amplifier) that has the same output, 150 watt/channel at 4 or 8 ohms and I really can't hear any difference between the two. Perhaps my habit of listening at relatively low volume is part of the reason I experience that. Perhaps the Luxman would shine at ear splitting levels!

There's some truth in that.  Doesn't even have to be ear splitting levels,  just levels that approach live non amplified music. 

[I.Greyhound Fan]

There's some truth in that.  Doesn't even have to be ear splitting levels,  just levels that approach live non amplified music.

Of course it does matter with speakers that are 4 ohm or lower and low sensitivity.  That Luxman probably would not break a sweat but the NAD might with such speakers.

And lets not forget about high current amps vs low current.  That's another discussion.

[Freo-1]

Of course it does matter with speakers that are 4 ohm or lower and low sensitivity.  That Luxman probably would not break a sweat but the NAD might with such speakers.

Agreed. I was thinking more of distortion levels when the volume is cranked up.  Watts are watts, which gets back to the speaker load vs. the amp.

[FullRangeMan]

Watts are V x A=W
So 50V x 2A=100W
So 20V x 5A=100W

So W are a relative value.
What are important are big transformers and big capacitance, stay alert on low weight amps, big transformers are heavy and expensives unless the amp are Class D.

[I.Greyhound Fan]

Your comments seem correct given I have the Luxman M900U that can double output into the lower impedance--and it was an expensive amplifier. While that is a cool spec., I don't know that paying the high cost got me greater listening pleasure. I also have an NAD 275BCEE (a relatively cheap amplifier) that has the same output, 150 watt/channel at 4 or 8 ohms and I really can't hear any difference between the two. Perhaps my habit of listening at relatively low volume is part of the reason I experience that. Perhaps the Luxman would shine at ear splitting levels!

There is something wrong if you cannot hear a difference between the 2 amps.  That Luxman can output 12 wpc in Class A, 300 wpc at 4 ohms and an instantaneous burst of 1200 watts at 1 ohm.   Even at low volumes it should be more dynamic.  It should be more transparent,  resolving and detailed.  Of course a lot depends upon the preamp, source, speakers, cables and music.

[Letitroll98]

My son is putting his audio system together with thrift shop buys so he'll often send me pics of gear, is this any good.  My advice is pick it up, if it's heavy, it's good.  How it relates to your question is higher power in lower impedance means a stiffer power supply, bigger transformer and supply caps, i.e. more weight. The second reason is amplifier class.  Class A bias means the transistor is always on through positive and negative swings of the signal, so with a good power supply it will double it's wattage as you halve the load.  Class AB will split the signal between two transistors, but will still deliver more wattage into 4ohms than 8, just not double.  Class D amps tend to deliver the same wattage regardless of load, although some Class D topologies do up the wattage as load decreases.  I'm not an EE so there may be errors in my descriptions, but that's the general idea.

[Freo-1]

Class AB amps can double the wattage from 8 to 4 ohms if designed to do so.   Models from Pass Labs are cited as an example.

[RDavidson]

Your comments seem correct given I have the Luxman M900U that can double output into the lower impedance--and it was an expensive amplifier. While that is a cool spec., I don't know that paying the high cost got me greater listening pleasure. I also have an NAD 275BCEE (a relatively cheap amplifier) that has the same output, 150 watt/channel at 4 or 8 ohms and I really can't hear any difference between the two. Perhaps my habit of listening at relatively low volume is part of the reason I experience that. Perhaps the Luxman would shine at ear splitting levels!

Not only do your listening levels matter, but more importantly the thing that matters is the load your speakers present to the amp. If you listen at low levels and are using low sensitivity speakers, you're not likely to get great sound regardless of amp you use. Some speakers simply don't sound good at low levels. KEF LS50's are the perfect example of this. They are pretty low sensitivity and not an easy load. One should really get speakers that are suitable to one's preferred listening levels, room size, and listening distance...which can take a lot of trial and error.

[I.Greyhound Fan]

Not only do your listening levels matter, but more importantly the thing that matters is the load your speakers present to the amp. If you listen at low levels and are using low sensitivity speakers, you're not likely to get great sound regardless of amp you use. Some speakers simply don't sound good at low levels. KEF LS50's are the perfect example of this. They are pretty low sensitivity and not an easy load. One should really get speakers that are suitable to one's preferred listening levels, room size, and listening distance...which can take a lot of trial and error.

My Magnepan 1.6's are an good example of speakers that sound better with some volume.

The preamp matters as well.  I had a Pass X1 preamp to go with my X250 amp.  It sounded great  at low volumes.  I sold the X1 and bought a BAT VK51se preamp which sounds way better than the X1 but at low volumes the X1 had it beat.

Many of the AVA amps and preamps sound very good at low volumes.

[Falcon]

Based on some of the responses, perhaps the reason I don't hear a difference between amps is the speakers are Harbeth 40.2 which allegedly are easy to drive. They are rated at 8 ohms and maybe dip to 6.

[Freo-1]

Based on some of the responses, perhaps the reason I don't hear a difference between amps is the speakers are Harbeth 40.2 which allegedly are easy to drive. They are rated at 8 ohms and maybe dip to 6.

If you listening at lower volume levels,  then you may not pick up any significant difference between amps.  Pretty sure if you crank it up, you will notice deltas. 

[RDavidson]

That's possible. I've never owned Harbeths so I don't know how well they respond to different amps. But with the various Omega speakers I own or have I've owned, I hear very distinct differences in all amps I use with them. I have 5 different amps I rotate with them because it is fun to hear these differences in presentation. Some amps have better imaging. Some are more airy. Some have deeper bass and control. It is very obvious. I typically listen around 70dbs. I know this isn't a true-to-life "realistic" level, but it is comfortable. Something to note too is that I have a pretty small listening room that doubles as my office. My ears are about 7 feet from the drivers. As I listen near-field, room effects don't come into play too much. This makes me wonder if your room is dominating what you're hearing versus hearing what your speakers are doing.

[Freo-1]

The Harbeth 40.2 is a fairly benign load to drive.  Reviews state that they sound great with just a 35 watt amp.  So based on that, doesn't surprise me that you don't find a y real delta in sound at low volume.  The amps are not getting taxed to work,  so it makes sense not to here much if any difference.   


Turn up the volume, and you should hear differences.  Don't think it's any worry.

[FullRangeMan]

Hi Falcon,
6-8 ohms is a benign load but you have another obstacle to obtain definition that allows you to hear differences between these amps or differences among cables etc, your speakers are very difficult to drive very low sensitivity only 86dB along the minimum amp power are 100W, not to mention the complex 3 way xover, these boxes are not a sweet pear.

Technical Specification:
-   Monitor  40.2
Sensitivity:      86dB/W @1m
Suggested amplifier:   100–200W
Power rating, programme:   650W
Impedance, nominal:      6-8 ohm

[Falcon]

I appreciate all the thoughtful answers. A few posters said the Harbeth are easy to drive which could affect sound while another said they were hard to drive. If memory serves,  Harbeth advertised the 40.2 as easy to drive I'm guessing because the impedance doesn't drop very low. But a lot of folks pick up on the efficiency of 86db. to make the claim they are hard to drive. I rarely turn my amplifiers up very high since there always seems to be plenty of volume so I'm wondering what the opinions are out there on the 40.2 being easy or hard to drive. Perhaps it's in-between?
Thanks!

[FullRangeMan]

If your amp are 500W they are easy to drive, if amp only 100W or less they are hard to drive.

[Freo-1]

I appreciate all the thoughtful answers. A few posters said the Harbeth are easy to drive which could affect sound while another said they were hard to drive. If memory serves,  Harbeth advertised the 40.2 as easy to drive I'm guessing because the impedance doesn't drop very low. But a lot of folks pick up on the efficiency of 86db. to make the claim they are hard to drive. I rarely turn my amplifiers up very high since there always seems to be plenty of volume so I'm wondering what the opinions are out there on the 40.2 being easy or hard to drive. Perhaps it's in-between?
Thanks!

Check out this review:


https://www.theabsolutesound.com/articles/harbeth-monitor-402-loudspeaker




The reviewer states that while the Harbeth can handle high wattage, it sounds just fine with a 35 watt amp.  So, I would say it was fairly easy to drive. It would benefit from a high powered amp, but not needed to enjoy, especially at lower volume levels.