Why are low impedance speakers harder to drive than high impedance speakers

 Ralph, you mentioned 'former' twice in the last part of your post.

Ach!

That should read:

Again, this all comes down to intention. Is your intention to get the system to sound as good as it can or is it more important to simply play loudly? If the latter, than some of the lower impedance speakers and higher power transistor amps will be of interest; if the former, then you will be very careful to be matching the speaker to the amplifier (and not the other way ’round) and most likely avoiding lower impedances in general.

-as Al corrected.

So while I can say with precision that most solid state amps are more accurate, as far as measurements are concerned

Erik, I don't think this statement is correct, and here's why. If you look at the specs, the lower distortion and apparent constant voltage characteristics of most solid state amps looks great! The problem is, that bit of paper ignores how our ears perceive sound.

This takes a bit to grasp! To give you some idea, most of us know that the ear employs a logarithmic approach to sound pressure. This is why we use the VU scale of decibels.

So take this concept, but apply it to harmonics. The ear seems to use something that looks very much like a logarithmic approach when it comes to how sensitive it is to harmonics- being less sensitive to lower orders and far more sensitive to higher orders on what looks much like an inverse logarithmic function.

The fact that the ear is more sensitive to higher orders has been known for decades and should not be a matter of debate! This is very easy to prove with simple test equipment.

Add to that the fact that the ear is tuned to be most sensitive to bird song frequencies (Fletcher-Munson). This fact arises out of evolution and is millions of years old- birds are the first warning of a predator in the area!

So the fact is that if the ear does not care about the lower harmonics so much, then logically we should be designing to eliminate the higher orders, especially since the tools that the amplifier designer has in the tool box all have certain limitations. For example, as I stated earlier (and as been stated by Norman Crowhurst, a universally recognized sage), loop negative feedback is known to add additional harmonics and IM distortions (the harmonics can go as high as the 81st and the intermodulations occurring at the feedback node in the amp). In this way an amp with feedback will usually sound brighter than an amp without, even though on the bench they both measure flat.

So what is more 'accurate'?  Low distortion on paper is meaningless unless we also know what it is that makes it 'low'. Its one thing if we can see the lower orders in the harmonic distortion spectrum. But if we are to take how the ear perceives sound into account, the higher orders should really be a lot lot lower than they are currently with all 'low THD' amps. And by that I mean **at least**  2 orders of magnitude! 

Just seeing 'low THD' doesn't cut it.

****This is ignoring how the ear works!!!**** (fist bangs tabletop)

The fact is that as far as the ear is concerned, the distortion of most amps with seemingly really low THD is that the distortion is higher. Its easy to hear too- which is why tubes still exist in the marketplace 60 years after being declared obsolete. Its why the tubes/transistor thing has been going on longer than the internet!

(if the tubes weren't doing something right, they would have been gone long ago. How many flathead V8s are still in production? If you got 'none' then you probably also know its because they are obsolete. There is a huge difference between being declared obsolete and actually **being** obsolete!)

In essence, the bench specs are an excellent example of the Emperor's New Clothes. This is because you have to ignore the obvious coloration of brightness/harshness/brittle in order to really say that its more accurate. The bench spec thing still has its roots in the 1960s and has not changed much since then (its mostly based on an idea of low distortion and flat frequency response while totally ignoring what the ear perceives; its actually tuned to the eye rather than the ear).

Put another way- we like to think our amps are low distortion because that is how they look on paper. That appearance is false- we're not measuring the right thing. Try to wrap your head around the fact of the ear's crazy sensitivity to higher ordered harmonics and use **that** as a baseline instead. If you can make that translation, you will see that most amps are fairly high distortion and not accurate at all.

The steering wheel is smaller, requires more 'brut' force.......

bdp24
1,906 posts
01-07-2017 7:17am
In a related matter, Roger Modjeski of Music Reference recommends hooking up your speakers to a tube amp on the lowest impedance tap that provides the power you need. So if an amp puts out 45 watts at 8 ohms and 30 at 4, and 30 watts is enough for your needs (with the combination of speaker sensitivity, room size, listening level, etc.) with an 8 ohm speaker, use the 4 ohm tap for lowest power amp distortion and best sound.


THE last part of the  sentence is usually true, however the logic is totally off.
No audiophile amplifier will go down in output in that fashion , if its 45 watts at 8 ohms then it should be 90 watts at 4 ohms....If the wattage is dropping then the amplifier should not be trying to drive the lower impedance load in the first place.

 
timber77

bdp24
In a related matter, Roger Modjeski of Music Reference recommends hooking up your speakers to a tube amp on the lowest impedance tap that provides the power you need. So if an amp puts out 45 watts at 8 ohms and 30 at 4, and 30 watts is enough for your needs (with the combination of speaker sensitivity, room size, listening level, etc.) with an 8 ohm speaker, use the 4 ohm tap for lowest power amp distortion and best sound.

THE last part of the sentence is usually true, however the logic is totally off.
No audiophile amplifier will go down in output in that fashion , if its 45 watts at 8 ohms then it should be 90 watts at 4 ohms....If the wattage is dropping then the amplifier should not be trying to drive the lower impedance load in the first place.

the impedance of the speaker generally dictates the best sounding amplifier tap. Sometimes speakers have widely varying impedance curves that means experimentation is probably necessary to see which tap sounds best. Depends on the impedance curve of the speaker, though. You can't get something for nothing. There's no free lunch. 😄

I agree with Geoff’s post just above. Also, regarding:

Timber77 1-15-2017
No audiophile amplifier will go down in output in that fashion , if its 45 watts at 8 ohms then it should be 90 watts at 4 ohms....If the wattage is dropping then the amplifier should not be trying to drive the lower impedance load in the first place.

While the maximum power capability of high quality solid state amps will of course often be twice as much into 4 ohms as into 8 ohms, tube amps do not behave in that manner. A tube amp which has an output transformer and provides 4 and 8 ohm taps will generally be designed to have a maximum power capability that is the same or similar when a 4 ohm load is connected to the 4 ohm tap as when an 8 ohm load is connected to the 8 ohm tap. And an output transformerless tube amp will typically have a greater maximum power capability into an 8 ohm load than into a 4 ohm load (and often an even higher capability into 16 ohms).

In the situation bdp24 referred to, where an 8 ohm load is connected to a 4 ohm tap, maximum power capability will usually be reduced in comparison to the amp’s capability when an 8 ohm load is connected to the 8 ohm tap or when a 4 ohm load is connected to the 4 ohm tap. The degree of that reduction will depend on the specific design, as will the desirability of the "light loading" (i.e., 8 ohm load connected to 4 ohm tap) that Mr. Modjeski recommends.

Regards,
-- Al