For headphones, why do we need good, high, powered amp toYou don't. Many phones of this impedance are easily driven by a few milliwatts. Some 'phones require a bit of power and others don't. How much power they need is independent of their impedance.
move headphones with very high impedance (300-600 ohms).
Why are low impedance speakers harder to drive than high impedance speakers
I don't understand the electrical reason for this. I look at it from a mechanical point of view. If I have a spring that is of less resistance, and push it with my hand, it takes little effort, and I am not working hard to push it. When I have a stiffer spring (higher resistance) I have to work harder to push it. This is inversely proportional when we are looking at amplifier/speaker values.
So, when I look at a speaker with an 8 ohm rating, it is easier to drive than a speaker with a 4 ohm load. This does not make sense to me, although I know it to be true. I have yet been able to have it explained to me that makes it clear. Can someone explain this to me in a manner that does not require an EE degree?
Thanks
80 responses Add your response
Hi all, This is my first post in this forum :) According to Al's information, having the two extreme examples, I have the exact opposite question: For headphones, why do we need good, high, powered amp to move headphones with very high impedance (300-600 ohms). According to the ohm equasion, for the same power, as long as the amplifier "understand" that the output is not disconnected (as the "100,000 ohms" al's example), than the amp needs only a tiny current to "drive" the headphones. What am I missing here? Thank you all! |
Since the OP thinks in mechanical terms, this analogy may best describe how speaker loads affect an amp. Imagine the amp is a somewhat fragile flywheel that will fly apart at a certain RPM. If you apply a resistance (an 8 ohm speaker load in electrical terms) to the flywheel that is sufficient to prevent the flywheel from reaching critical speed it will not break. If you remove part of the resistance (think 4 ohm speaker) the flywheel will speed up. If you remove enough of the load, you eventually reach a point where the flywheel is spinning so fast it fails. Replace the concept of the flywheel speed with power output from the amp. The power an amp will produce is inversely proportional to the resistance to current flow. More resistance keeps the amp in check so to speak. |
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To me it seems like if you need twice the amount of electrons to flow to a 4 ohm speaker than to an 8 ohm speaker, the amplifiers would need to work harder and in return wouldn't this cause more distortion? Also, if it takes twice the amount of current to drive a 4 ohm vs an 8 ohm speaker, wouldn't this mean the 4 ohm speaker is less efficient. However, you would think the 8 ohm speaker would be less efficient because it has twice the amount of resistance to the current. I was told once a 4 ohm speaker requires twice the amount of current than an 8 ohm speaker. Because a 4 ohm amplifier delivers twice the amount of current, would this in turn supply twice the amount of information to the speaker to create more detail in the music? |
Am I close????You’re better than close; that’s exactly right :-) I’ll mention also that the following equations can be derived by substituting some of the terms in equation 2 in your post above into equation 1, and doing some algebraic rearrangements, and these equations may add some further clarity to what has been said: Power (watts) = (Volts squared) / Ohms Power (watts) = (Amperes squared) x Ohms It can be seen from these equations that for power to remain constant, as the number of ohms decreases voltage must decrease, while current must increase. Finally, to be precise I should mention that we’re simplifying all of this a bit by making the assumption that the load is purely resistive. Volts x Amps = Watts in the case of a resistive load, but things get somewhat more complex when the load has a significant inductive or capacitive component, in addition to its resistive component. Regards, -- Al |
Al, THANK YOU very much!
I truly appreciate your full, detailed response that also went to the
heart of the issue. I think I am
beginning to understand. It seems like
the critical part (at least for me) is the following:
Ohm’s law – by itself - doesn’t seem to get at this. If I truly understand how the relationships work, there are several steps involved:
Therefore, when you reduce impedance, but keep power constant, the current increases but the voltage decreases. This is where the crucial piece of information applies to clarify that the reduction in voltage does not mitigate the increased energy required by an amp to increase the current. Am I close???? PS. Still not sure how the pipe analogy works here? |
I’ll add a few comments to Charles’ excellent answers. Swingfingers, first let’s change the word "sensitivity" in your question to "efficiency." Speaker sensitivity is usually defined on the basis of an input to the speaker of 2.83 volts, rather than 1 watt. 2.83 volts into 8 ohms corresponds to 1 watt, so the resultant SPL (sound pressure level, in db) is the same either way. But 2.83 volts into 4 ohms corresponds to 2 watts, so if the 87 db figure you referred to for the 4 ohm speaker is defined on the basis of a 2.83 volt input that speaker would produce only 84 db in response to 1 watt. So with the word "sensitivity" (which we’ll define as db SPL at 1 meter in response to a 2.83 volt input) changed to "efficiency" (which we’ll define as db SPL at 1 meter in response to a 1 watt input, although in some other contexts the term "efficiency" may also be used to refer to the ratio of acoustic power out to electrical power in), my answers to your three questions are: Q1)Yes. Q2)Yes, with the slight qualification that in the specific case of a class A amplifier the amp will dissipate (consume) less power internally (and therefore have a lower internal operating temperature) when it is supplying large amounts of power to the speaker than when it is supplying small amounts of power (or no power) to the speaker. And in that sense and to that extent (there are other factors that come into play, of course) a class A amp may be working less hard when supplying more power rather than less. Q3)Yes, a 90 db/1 watt/1 meter/4 ohm speaker will require a lower setting of the volume control to produce the same volume as a 90 db/1 watt/1 meter/8 ohm speaker. If the above are basically right, I don’t understand why an amp would need to work harder with a 4 ohm load than an 8 ohm load to put out the same spl in the same room. If the above are not correct, where did I go wrong?Keep in mind that the speakers referred to in Q2 are identical, while in Q3 they are not. In both situations referred to in Q3, the amp will deliver the same amount of power to produce a given SPL. For a resistive load power = voltage x current. The volume control setting controls the amp’s output voltage, while the impedance of the speaker determines how much current is drawn from the amp at a given output voltage. In the case of the 4 ohm speaker the lowered setting of the volume control that you correctly referred to will result in less voltage being supplied by the amp compared to the 8 ohm case, but the amp will be supplying more current at that lowered volume control setting than at the higher volume control setting of the 8 ohm case. Put simply, it is easy for an amp to supply voltage, as long as it is operated within the range of voltage it is capable of, but less easy for it to supply current. I’ll leave the hose analogy question to others, as I generally prefer to avoid using non-electrical analogies for electrical things. Hope that helps. Regards, -- Al |
Thanks for the reply. Could you please relate this to my 3 points above and the hose analogy as well? I see the formula, but don't know how that applies to needing less signal for an amp that can produce more watts because there is less resistance. How does that relate to a pump needing to work harder (or less hard in the amp world) to push the same amount of water through a smaller diameter hose? |
Hi, If you keep the Ohm’s law equation in mind it’s easy to explain. I = (current), V=voltage and R = resistance (speaker load impedance expressed as ohms). I=V÷R. So the smaller R becomes, the larger I becomes. If for example V=10 and R=10 then I will =1. If R is reduced to 1 then I now =10. Reducing the R (speaker impedance expressed in ohms) will increase the value of I (current demand). So a 2 ohms load (lower R) will demand more current (I) than a 8 ohm load (larger R). Charles |
Would it be possible to go back to the original question? This thread contains lots of great information,
but I am still struggling with the basics. I apologize for this “newbie” request, but I'm clearly misunderstanding something and would be grateful for some help! All other
things being equal (PLEASE SEE full caveats note at bottom), I thought the following were generally true:
If the above are basically right, I don’t understand why an
amp would need to work harder with a 4 ohm load than an 8 ohm load to put out the
same spl in the same room. If the above are not correct, where did I go wrong? THE HOSE ANALOGY: I’ve heard the previously referenced analogy of water going through a hose many times, and each time it sounds backwards to me. It seems to me that if the flow of water – or amount of water moving through space over time – is to remain constant, a larger diameter pipe, or lower impedance, would make it easier for a pump (or amp) to push that water. If the flow remains constant, then as the hose diameter decreases, the pressure increases and the pump would need to work harder. Why is this not correct? Clearly, I must misunderstand some fundamental concepts! I’m not an engineer or science “type” so may need some baby steps. PLEASE NOTE: in order
to try to understand the basics, all of the above is based on simplistic
and theoretical situations, with all other things such as speaker
configuration and design, other components, room size, etc. being equal AND
with all components properly matched. I
understand that real world implementations may vary. |
Interesting thing is you need amp and speakers together to make sound. Either one alone says nothing. So you can judge each on paper or do measurements but absolutely cannot judge how one sounds exactly without the other. Only the two together and each pairing sounds different. Then as if that’s not bad enough there is this thing called room acoustics that means the same amp and speaker pairing will sound different depending not just on room but location in it. However natural selection is at work. Only the strong will survive no matter what. |
+1 on that for sure :) I thought the notion that speakers, being transducers, vary much more in sound that do amps was universally agreed upon, and should therefore be selected first. The notion that power amps vary in character as much or more than do speakers is one I disagree with.If you have no preference for tube or solid state then this is the way to go. If you have found that you prefer one over the other then you will need to get the amp first and then find a speaker to match. That is why in most cases, you start with the amp. It is tricky- to know that amp you have to hear it in a variety of circumstances to make an informed purchase. But if you do that, you are less likely to flush more $$$$$ down the loo trying to get the system to sound the way you want it. |
So, when I look at a speaker with an 8 ohm rating, it is easier to drive than a speaker with a 4 ohm load. This does not make sense to me, although I know it to be true. I have yet been able to have it explained to me that makes it clear. Can someone explain this to me in a manner that does not require an EE degree? You are in luck. I can explain this in terms that does not require any degrees. In fact due to the inverse law of reciprocal square roots you probably will not understand if you DO have a degree. The first critical thing is The Gnome. Gnomes are small and live inside your squeaker. Gnomes all belong to the Conservative Party - they RESIST change. If you want more resistance you must get more Gnomes. Second critical thing is Impudence. Impudence is the amount of resistance. However unlike resistance in a resister where there are always the same number of Gnomes, in a squeaker the amount of resistance (measured in Gnomes) varies with frequency. A squeaker may be described as having 8 Gnomes. In reality - and this includes my speakers - there is a variation in resistance. So at a lower frequency the impudence dips to as little as 4 Gnomes. The third critical component is Currant. Your amplifier feeds your squeakers with currants. So it goes like this: Your amplifier feeds 8 Gnomes at most levels of frequency. Each Gnome processes one currant at a time. But where the impudence dips to where there are only 4 Gnomes the speaker still needs to process 8 currants. But there are only 4 Gnomes, so what to do? A well designed amplifier, ie one that has a lot of currants, will pass twice as many currants to each Gnome. So the 4 Gnomes will process 2 currants per Gnome. The amplifier therefore has to put out currants at twice the rate. This is all known as Gnome's Lore. |
I was surprised when I learned that some choose their power amp first, then look for a speaker it can drive well. I thought the notion that speakers, being transducers, vary much more in sound that do amps was universally agreed upon, and should therefore be selected first. The notion that power amps vary in character as much or more than do speakers is one I disagree with. I feel the same way about phono cartridges (also transducers) vs. pickup arms and/or turntables, though to a lesser degree. |
It is not a matter of "harder to drive" but of a "proper amplifier" to match the lower impedance. Amplifiers have different "abilities" that need to be "matched" with any given speaker. Amplifier match is, subjective, and, YMMV, like everything else in audio. NO speaker has a pure 8 ohm resistance across the audio spectrum, by the way. Speakers are also electro-mechanical .....not mechanical. |
Excellent additional information from the always illuminating Ralph Karsten! Thanks as always. Modjeski designs his transformers and has them built to his specs (even winding them personally, for those willing to pay him to do so). Perhaps he does so in a way that takes light loading into consideration. I don’t employ it myself, needing all the power I can get for the rather insensitive 8 ohm loudspeakers I use the RM-200 with. |
if your tube amp has 4, 8, and 16 ohm taps (typical in tube amps, though the RM-200 offers 2, 4, and 8 ohms), and you hook up an, say, 8 ohm speaker to the 4 ohm tap, the power available to the speaker will be less than it would be if connected to the 8 ohm tap. But, says Music References Roger Modjeski, a tube amp so employed will usually be producing not only less power, but also less distortion, and better sound. As Al mentioned, Roger calls this tactic "light loading". In addition to lower distortion, an additional benefit of using a lower impedance tap is that the amps output impedance will be lower---it will have a higher damping factor, and will interact less with the varying impedance characteristics of the speaker load, resulting in a more predictable frequency response.While generally true, a problem that can turn up when doing something like this is that the transformer can 'ring' if insufficiently loaded. In addition, with such a load, it will not be as flat across its bandwidth, as the transformer will tend to express less of its turns ratio and more of its inter-winding capacitance. The 'lighter' you load the transformer the more of a problem this becomes. So the result, while possibly reducing distortion in the output tubes, will be to **increase** distortion from the transformer (ringing) and degrade the frequency response. Of course, if the amp employs negative feedback some of this will get sorted by that, but a problem with negative feedback is that while reducing distortion overall (in particular lower ordered harmonics), it actually **introduces** higher ordered harmonics that otherwise may not have been present at all! Its best not to give feedback too many places to screw up. |
Excellent clarification from Al, with more specifics. No surprise there! Timbre77, you missed the important qualifying adjective of the amplifier scenario I described---"tube". As Al stated, while the power solid state amps create increases with dropping impedance, the opposite is generally true of tube amps, with the notable exception of the unique Music Reference RM-200, which actually behaves more like a ss amp in that regard. As Al detailed, the different impedance taps on a tube amp allow the amp to provide similar power to all the taps---that's one task of tube amps output transformers. However, if your tube amp has 4, 8, and 16 ohm taps (typical in tube amps, though the RM-200 offers 2, 4, and 8 ohms), and you hook up an, say, 8 ohm speaker to the 4 ohm tap, the power available to the speaker will be less than it would be if connected to the 8 ohm tap. But, says Music References Roger Modjeski, a tube amp so employed will usually be producing not only less power, but also less distortion, and better sound. As Al mentioned, Roger calls this tactic "light loading". In addition to lower distortion, an additional benefit of using a lower impedance tap is that the amps output impedance will be lower---it will have a higher damping factor, and will interact less with the varying impedance characteristics of the speaker load, resulting in a more predictable frequency response. |
I agree with Geoff’s post just above. Also, regarding: Timber77 1-15-2017While 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 |
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. 😄 |
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. |
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 concernedErik, 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. |
For best results, practically, in most cases, I think one has to first match speakers to the thing you have least control over changing (the room) and then match amp to speakers. The room is the biggest determining factor IMHO that determines the viable approach or approaches for getting the best sound results. I’m technology and product agnostic when it comes to good sound from there. Of course if one is willing or able to do extensive sound treatment and tweaking of the room that opens up even more ways to skin the cat. Each case is different. No one solution always works or sounds best in the end. Budget and funds are almost always a constraint. If one is more biased towards using specific technologies, whatever that may be, that’s fine as well. The only thing that really matters is people like and enjoy their endeavors. |
I agree with Unsound’s post just above. And while I consider myself to be a "speakers first" kind of person, as he is, I don’t see that as being inconsistent with Ralph’s statements, including: Often people have a preference about tubes and transistors- the speaker **must** be chosen to take that preference into account!!As I see it this is saying essentially that what kind of amplification one anticipates using, now or in the future, is one of the major factors to consider in choosing a speaker. That is not quite the same as saying "amplifiers first," rather than "speakers first." Or so it seems to me. It should also be noted, btw, that there are some speakers that will be equally suitable, or at least comparably suitable, for use with nearly all types of amplification. In those cases the resulting sonics will depend mainly on the intrinsic sonic characters of the speakers and the amp (as well as on speaker-room interactions, of course), with amplifier-speaker interactions contributing minimally if at all. Such speakers will typically have impedance curves that are relatively flat and do not have severely capacitive phase angles at any frequency, and combine relatively high efficiency with the ability to cleanly handle copious amounts of power. Best regards, -- Al |
I think choosing speakers first has it's advantages. More so than with other components speakers need to be paired more carefully with the room in which they will be placed. More so than with other components speakers are the most colored components, and vary in sound more so than other components. As such it behooves one to make sure those colorations are the least offensive. Working backwards from the listening position will less likely box oneself into technological obstacles. |
So by and large, @atmasphere re-expressed my explanations for output impedance with a lot of discussion about tubes and distortion. I may not agree with some of it, but the truth is, voltmeters don’t get pleasure from listening to music. Humans (and cats) do. So while I can say with precision that most solid state amps are more accurate, as far as measurements are concerned, I cannot tell you which you should buy instead of a boat, gun collection, or second home. :) Tube amps behave and sound different, and some sound quite wonderful. If that’s the sound you are entranced with, you should go that route, but your range and choice of speakers will be different. I agree that the idea of picking a speaker first is not the right way to go. You should find out which team you want to bat for first. Overall do you like very good solid state, or do you like tubes, and if either, what are their qualities you like? Also, what is your real listening style? Do you really sit in a throne for 3 hours at a time in front of two speakers alone? Is it usually background music? Do you sit with your partner, friends, etc? All these matter. :) Best, E |
If the former, 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.Ralph, you mentioned 'former' twice in the last part of your post. Bob |
When this is true, the 4 Ohm is usually 3 dB more sensitive. The low impedance causes extra current flow which provides for increased force against the same magnet.This seems to require clarification! Given two drivers of the same **efficiency** (1 watt, 1 meter), if one is 8 ohms and the other is 4, the 4 ohm unit will be 3 db more **sensitive** (sensitivity is measured at 2.83Volts at 1 meter; if this is 8 ohms that is one watt, at 4 ohms its two watts; two watts is double one watt and there is your 3 db ). That current will not flow as expected by the speaker designer if the amplifier output impedance is higher than about zero ohms. This is because as the output impedance is increased, more and more power will be dissipated in the output section of the amp rather than the load- dissipated in the form of heat. That’s a lot of amps! In particular, this is especially true of tube amps, whose output impedance can often be measured in ohms rather than fractions of an ohm. One might ask, ’what is the point of such an amplifier?’ and the answer has to do with how humans perceive sound. In a nutshell, we perceive volume, or sound pressure, by listening for the higher-ordered harmonics. This is because pure tones do not exist in nature, and apparently nature sorted out millions of years ago that listening for the higher-ordered harmonics is more expedient, as it would millions of years before anyone invented pure fundamental tones :) BTW this is very easy to prove with simple test equipment and is not a matter of debate. I’ve posted the way to prove this a number of times on this site. Since this is the case, a good number of designers (myself included) prefer to design amps that by intention do not make the higher ordered harmonics. To do this often requires a higher output impedance, because to do that means avoiding loop negative feedback (which is known to enhance higher orders and generate more of them at the same time; see Norman Crowhurst). Loop feedback lowers output impedance; without it the output impedance is therefore higher. It also happens that it is far easier to design such an amplifier by avoiding the use of semiconductors in the signal path. Semiconductors have non-linear capacitive elements inherent in their junctions (magnified by current through the junction) and these are known to create higher ordered harmonics in the distortion structure of the device (FETs and MOSFETs far less so than conventional bipolar devices; a particular device known as a varactor takes advantage of this aspect and is used as a variable capacitance to tune radio receivers). The bottom line is that if you are dealing with an amplifier designed to not make higher ordered harmonics (as opposed to just low THD in general), the usual voltage rules as defined in the quote above simply don’t work (and I explained why in my second paragraph). Put another way as a speaker designer you have to pay attention to driver efficiency rather than sensitivity. This is why back in the old days, many speakers had mid and tweeter level controls. They were not there to adjust the speaker to the room, they were there to adjust the speaker to an amplifier of unknown voltage response (high output impedance). The approach is trickier, but has the advantage of less overall audible distortion (which the ear converts to tonality, often favoring that tonality over actual frequency response errors!). In most cases this design approach is to avoid ’brightness’ and ’harshness’; two audiophile terms used to describe the presence of trace amounts of higher ordered harmonic distortion. The way I see it, if a system **always** has brightness (which will be found to not be toned down by a treble control because it does not arise from a frequency response error) then the best it will sound will be like a nice stereo rather than real music. 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 former, 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. |
Not sure anyone cares about this, but I thought I would chime in with more information now. In general, many good drivers are available in 4 and 8 Ohm versions. When this is true, the 4 Ohm is usually 3 dB more sensitive. The low impedance causes extra current flow which provides for increased force against the same magnet. The choice here usually has to do with whether designers will put them in parallel and matching other drivers in the system with less wasted heat. Some speakers really can't help it. Like ESL's. Essentially they are moving capacitors. The Apogees of old were/are essentially just wires suspended n a magnetic field. Getting up to 1 Ohm impedance was as high as they could get! Any decrease in the current (i.e. increased impedance) would cut the force down proportionately. In some cases I have seen crossover design deliberately lower impedance to below 4 Ohms to make speakers seem more "discerning" or "demanding." This added nothing to the sound quality of them. They just added to the snobby appeal. Best, E (yes, I'm a snob) |
atmosphere: "One other point not previously mentioned is the effect of speaker cables! At lower impedances they tend to be far more critical, where shorter distances and much larger gauges are required to prevent loss of definition and impact. This can be a pretty big deal as the series resistance of the speaker cable can have a pretty dramatic effect on the effective output impedance of the amplifier! By contrast the speaker cables are far less critical at 16 ohms- which is why a lot of us kids got by with hardware store zip cord in the old days." At the risk of taking this thread into the contemptuous nether regions of many others (please no!), this may explain a lot regarding contrary experiences of many on this forum regarding speaker cables' impact on sound. Good point Ralph. Dave |
you took my statement out of context. Here is what I actually said:Yes- it was not to contradict you, but to use the phrase as a talking point, as I see that approach recommended a lot. Then later people sell off the gear in search of that holy grail... I think if you start with the grail first you're less likely to sell and waste the cash. Atmashere, the OP said nothing of ESL's. I did make mention of : "Most typical speakers...."Correct- in fact by 'most typical' is about 95% of the market. In high end, its still about 85% and so is 'mostly accurate' :) For those outside of that percentage (Charlesdad's speakers are box speakers but they are an example) that voltage thing just does not work. This is entirely due to the designer wanting the speaker to work with a particular kind of amp. In Charlesdad's case, the speakers were originally envisioned on a set of our M-60s and later that manufacturer started making SETs, which work much the same way. SETs in general are much happier on higher impedance loads despite often having 4 ohm taps and they tend to make constant power rather than constant voltage owing to zero loop feedback. In a tube amp, the 4 ohm tap is not an efficient way to use the output transformer, which often means not only less power (lost due to heat) but also less bandwidth, sometimes up to an octave lost on the bottom end. So its often really worthwhile to avoid low impedance speakers with tube amps even if you have the taps on the output transformer! |
Mapman, I don't get the sense of "pundits" taking sides. The OP raised an interesting question regarding the popularity of 4 ohm speakers. This generated a very good discussion. We agree that one fine aspect of High End audio is the many choices/paths that lead to a good sounding home audio system. Your system and mine are at opposite ends of a component/system spectrum, yet we're both very happy respectively. This is a good outcome. I appreciate the multitude of perspectives provided here. Charles |
If one did a study of speaker size versus nominal impedance for speakers with good bass extension, I suspect an correlation would be seen between speaker size and impedance. Also a correlation between size and efficiency for certain. Bottom line is way more people these days prefer speakers with smaller form factors that still do extended bass well to fit into their lifestyle and that puts more demand on the amps to do the work needed. Granted that there is also a motivation for makers to make speakers smaller as well to keep costs down and profits up. Its much harder technically to get a lot of good sound out of a smaller package than a larger one. It requires higher quality drivers capable of more output with low distortion be designed. A lot of speaker innovation these days revolves around exactly that. Smaller speakers with better sound is the holy grail to a great extent. Some do it much better than others. Granted though it does COST more to make a larger speaker of certain high quality than a smaller one. more of anything always costs more. Add in that amps are more efficient than ever and can do more work with smaller packages and less electricity and the story is complete. Lower power tube amps SETs in particular but OTLs and others as well have to rely more on the speakers to pick up the slack and do the work more efficiently. That’s the speakers problem then, not the amps. But all buyers need is for the amp speaker combo together to be up to the task however that is achieved. So not really a fair thing to judge the world from the perspective of low powered tube amps. Its a different paradigm as Atmasphere accurately refers to it that one can choose from if it fits their needs better, but obviously only a small % of even high end audio enthusiasts choose to go that way for good reason just as some do. Those are the facts. Its nice to have a choice of two different ways to do about things and still get excellent results. One side will never be made to see things the others way. Each is focused on doing things a different way for good reasons. Each approach will have its pros and cons compared to the other but pundits will naturally attempt to stage the comparison in terms favorable to their preferred approach. |
Whoa, Ralph, you took my statement out of context. Here is what I actually said: "Usually it comes down to approach: picking the speaker you love and then finding the right amp or vice versa, or in a fortuitous alignment of skill and circumstance, finding both without significant compromise." Vice versa means "with the main items in the preceding statement the other way around", thus indicating your approach as one of two alternatives. To say, as you did, that the first approach (speakers then amp) does not also work is counter indicated by many fine sounding audio systems selected based on that approach. Best to you Ralph, Dave |
Well, if high efficiency speakers are more difficult to build by a factor of 10! that pretty much confirms my suspicion as to why their numbers are much less compared to the many 4 ohm speakers available. Why would a speaker designer "bother" with the increased difficulty particularly knowing the popularity/high usage of solid state amplifiers. This makes sense as most SS amplifier owners with gravitate to 4 ohm speakers for the most part. Charles |
Bruce Thigpen, designer and builder of the Eminent Technology LFT-8b magnetic-planar loudspeaker, states in the product’s literature that he could have made the speaker any impedance he wanted, and chose a nominal 8 ohms. The LFT-8b is a hybrid (employing a dynamic woofer below the 180Hz x/o frequency), the m-p drivers themselves being closer to a 12 ohm load. If you bi-amp the speaker, the m-p’s are a very easy load for a tube amp, being not only about 12 ohms, but also primarily resistive, not capacitive or reactive. |
So why would a designer go the other direction?Lower impedances have appeared as a means to get more power out of solid state designs. But this is very different from getting lower distortion! So a lot depends on what your goals are! I think the trend towards speakers with lower impedance corresponds to the trend for speakers to be smaller yet more full range to fit into people’s lifestyle.The size of the speaker has nothing to do with its impedance. The two are unrelated design aspects. Impedance at port frequencies is always low. Check any ported speaker impedance curve and see.This statement is mostly false. Ports are usually placed in the peak of a cabinet resonance (which will be seen by a peak in the impedance) as a means of reducing the peak and spreading it out. If properly placed, two lessor peaks with a dip in between will be seen, but overall usually represents a higher than nominal impedance. Usually it comes down to approach: picking the speaker you love and then finding the right ampI don't buy that this works! Often people have a preference about tubes and transistors- the speaker **must** be chosen to take that preference into account!! Otherwise you may never get satisfaction and a lot of money down the loo. ^ Lest anyone get the idea that the increase/decrease of power output by ss amps into decreased/increased impedances would suggest that frequency linearity would be compromised, the opposite is actually the case, they actually provide better frequency linearity. Most typical speakers will decrease/increase their sensitivity in direct proportion to the increased/decreased impedance changes.In the specific case to which this quote refers, a Quad ESL was the speaker and most definitely does ***not*** fit this rule! The Quad has impedance curve arising from capacitance and not resonance of a driver in a box (for starters, there is no box). Planars in general don't fit that rule either. I feel like several points need clarification and at least one needs to be made. First the clarification: If sound quality is your goal, your amplifier investment dollar will be best served by a speaker of 8 ohms or more, all other variables being equal (and unfortunately they almost never are). By this I mean that no matter what amp you have, its not in your best interest to make it work hard if you want the best sound out of it that is possible! It does not matter what sort of amp- tubes, transistors or class D. The problem is the same- as you decrease impedance the distortion in the amp goes up. Unfortunately the kind of distortion that is going up is the kind you really don't want- because its the kind that is pretty audible! You don't hear it as breakup or crackle though, you hear it as tonality. Usually this means that the sound will be harsher and less detailed. This is because the distortion components are usually recognized by the ear as harshness and it take a vanishingly small amount of distortion to do this! This is because the human ear uses higher ordered harmonics to gauge how loud sounds are, and as a result is evolved to be far more sensitive to them. This is in fact why distortion can be so low as to be hard to measure, yet it can still manifest as brightness and hardness. So to reduce distortion and make the amp sound smoother, use a higher impedance load. Steve McCormick, a well known solid state amplifier manufacturer, sent a note to Paul Speltz, who is known for the 'anticables' but also for an autoformer known as the ZERO. The ZERO allows you to drive a 4 ohm speaker while the amp is loaded at 16 ohms. What Steve said in his letter was that even though his amps had no worries doubling power into 4 ohm loads (and BTW are very well built and designed IME) that the fact of the matter is they sound better driving 4 ohms via the ZEROs. This is simply due to reduced distortion. One other point not previously mentioned is the effect of speaker cables! At lower impedances they tend to be far more critical, where shorter distances and much larger gauges are required to prevent loss of definition and impact. This can be a pretty big deal as the series resistance of the speaker cable can have a pretty dramatic effect on the effective output impedance of the amplifier! By contrast the speaker cables are far less critical at 16 ohms- which is why a lot of us kids got by with hardware store zip cord in the old days. The reason 4 ohms is such a big deal has more to do with transistors than anything else. Back when tubes were the only game in town, speakers were usually either 8 or 16 ohms. They were often a lot more efficient too, as acoustic suspension hadn't been invented (Henry Kloss, who was the inventor of that was a co-founder of Acoustic Research, who in turn made the AR-1, the world's first acoustic suspension speaker about 1958). Obviously several things at once were happening at the end of the 1950s. Acoustic suspension (far less efficient) was getting started, and so were transistors. At first the two were not significant. But as silicon transistors (and consequently higher power) became more available (which really started in the late 60s or early 70s) the need to get a bit more output out of the amp started to take off. BTW, high efficiency loudspeakers are a **lot** harder to build, by nearly a factor of 10 over acoustic suspension, so you can see that speaker manufacturers sensed a profit motive (as did the amp manufacturers by going solid state- a similar motivation is occurring now with class D). However by getting a possible 3 db more output out of an amplifier suddenly became a big deal; acoustic suspension allowed a smaller size but paid the price in efficiency. Anything that might make them appear easier to drive was helpful. But in the world of high end audio, where tiny little things can make a difference, the need for 4 ohms is almost non-existent (at least not if sound quality is the goal). There really isn't a speaker technology (like planars for example) that actually **need** to be 4 ohms as opposed to a higher impedance. In most cases, this simply happens because the speaker manufacturer does not realize the smoothness and transparency benefits that are possible with the same amp if the impedance is increased! |
Most typical speakers will decrease/increase their sensitivity in direct proportion to the increased/decreased impedance changes.Electrostatics being a notable exception, though, including the ESL-57 which bdp24 was referring to. Some other exceptions are referred to in the Paradigms In Amplifier Design paper Ralph has often referred to. In the case of speakers that have been designed to sound their best when driven by tube amplification, such as the ESL-57 (which was designed before solid state amplification existed), frequency response at the output of the speaker may very well be most flat when the frequency response of the signal provided to the input of the speaker is not flat. In voltage terms, that is. Best regards, -- Al |
