^ 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.
Furthermore, there are many other considerations that speaker designers need to consider besides impedance. With the advent of amplification that can adjust power output to speaker impedance/sensitivity, speakers designers were now able to permit more attention on other speaker considerations such as wave form fidelity, dispersion characteristics, box resonances, etc.. |
bdp24 1-8-2017
For instance, the original Quad ESL's nominal impedance was 16 ohms, but
it’s impedance rose to 60 ohms at low frequencies, and fell to 1.8 ohms
at high frequencies---anything but an easy load! That impedance
characteristic is one reason the sound of the Quad ESL is so affected by
the amp driving it, and why almost no solid state amp is a good
match---it makes for overblown bass and missing highs.
BDP, a minor correction to your characteristically excellent inputs. The last phrase should be "it makes for missing bass and overblown highs." As you no doubt realize, and aside from some rare exceptions, in comparison with a tube amp a solid state amp will deliver more power into low impedances and less power into high impedances, for a given input signal level. Best regards, -- Al |
Hi Charles,
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.
Best to you Charles,
Dave |
Hi Bdp24,
For sake of this discussion I’m referring to speaker load impedance characteristics that remain "relatively flat. So my hypothetical comparison would be a 4 ohm vs a 16 ohm. A speaker such as the Quad that you cite with its extreme fluctuations renders nominal rating meaningless lf both the 4 and 16 ohm speakers are stable, the higher ohm speaker demands less current and work from the driving amplifier as predicted by Oohm’s law.
In reality there are speakers with both reasonably flat impedance characteristics and gentle phase angles. There are also those with widely fluctuating impedance and steep phase angles. It seems that the former would be preferable as it is far less demanding on a power amplifier assuming that either provides comparable sound quality.
Charles |
Also remember the complexity of the Crossover is directly proportional
To ohms,resistance to the drivers and demand affecting the Amplifiers
Constant changing ohms ,and Amplifier Load depending on demand.
A speaker can go from a 8ohm load to 2 ohms which Is very demanding
For a conventional driver Loudspeaker. Panel or stats ,can dip down to 1ohm .
This is where a Amplifier be rock solid .verify that the amplifier you are using
Can handle the extreme for a given Loudspeaker before purchasing it. |
While a 16 ohm speaker may seem to "present an enormously easy load for virtually any power amplifier", such is not necessarily the case, as that 16 ohm rating is merely nominal. For instance, the original Quad ESL's nominal impedance was 16 ohms, but it’s impedance rose to 60 ohms at low frequencies, and fell to 1.8 ohms at high frequencies---anything but an easy load! That impedance characteristic is one reason the sound of the Quad ESL is so affected by the amp driving it, and why almost no solid state amp is a good match---it makes for overblown bass and missing highs. Ralph Karsten of Atma-Sphere has already explained this a few times. Related to my post above, using the lowest impedance tap on a tube power amp not only usually results in the lowest distortion and best sound the amp is capable of, but the amp’s damping factor is also highest, leading to the flattest frequency response it is capable of, irrespective of speaker impedance characteristics. |
@mapman
Ahem, please see the example I posted, above. :)
The drop at resonance is no lower than Re. :)
I'm sure @Atmashpere will chime in, but depending on the amp, it's really the difference between the peaks and Re that makes a tube amp misbehave, as the high impedance will track this. If Re is high enough, it will be minor.
Best,
Erik
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Actually ports in ported speakers are perfect examples of why low impedance is harder to drive. Impedance at port frequencies is always low. Check any ported speaker impedance curve and see. After all the port itself has little or no physical resistance/impedance. Air blows right through largely "unimpeded". An amp driving the ported speakers well will result in a stronger air flow. One that cannot drive it well will result in little or no air flow meaning the port is not being utilized well to move air at the lower frequencies that it is intended to enable. |
Because of these properties of amplifier and speaker systems: - Amplifiers have a non-zero output impedance
- Voltage is proportional to impedance in a series circuit.
- Speakers usually present complex impedance, with a range of impedance magnitude and voltage / current phase angles
Let’s simplify this to a couple of statement: Amplifiers are current limited. This limit defines the maximum voltage at any given impedance. As the amplifier output impedance rises, or speaker impedance drops, the electrical frequency response at the speaker inputs goes from flat to tracking the speaker impedance. Want to understand why? Read on. As an introduction, see the first graph on my blog post on speaker impedance for a very easy to drive speaker: https://speakermakersjourney.blogspot.com/2016/12/crossover-basics-impedance.htmlWe’ll ignore phase angle, as it’s hardest to grasp. Let’s assume instead: - Tube amp output with about 1 Ohm impedance,
- An ESL such as a Martin Logan electrostatic, which has a panel impedance that varies from 4 Ohms at the bottom of it’s range to 0.3 Ohms at the top end.
At low frequency, say 300 Hz where the panel is 4 Ohms, the voltage at the panel is 4/(4 + 1) = 4/5ths = 80% of the amplifier output. At high frequencies, say 10kHz 0.3 Ohms the voltage stays at the amp, and the speaker gets 0.3 / (0.3+1) = 0.3 / 1.3 = 23% of the amplifier output, or about 25% of the amp output! But look what happens with an amplifier with very low impedance of 0.001 Ohms (aka high Damping Factor): At 4 Ohms: 4/(4 + 0.001) = 4/4.001 = 99.97% At 1/3 Ohms: 0.3 / ( 0.3 + 0.001) = 99.67% So in the range of speaker impedance from 4 to 0.3 Ohms, the amplifier output remains nearly rock-solid. |
Ports in ported designs are always a major factor it seems. |
As watts got cheaper with ss, designers could not put as much effort into creating high imp. speakers and could also experiment with low imp. designs. But my question is: %-wise, what contributes to impedance: the overall design (enclosure, drivers, venting, etc.) or the crossover? I've always suspected that the crossover had a big role, and fancy crossover topologies often make for low imp. speakers (I suspect). |
It’s all about the bass, about the bass,. Not the...
well it’s also about size and cost actually.
The reality is low power tube amp lovers have limited speaker choices at least if bass extension matters. Still good ones out there though if a low power tube amp is what one chooses to build around.
Making music especially bass is a lot of work. Somehow combo of amp and speakers must be up to the task else results suffer in comparison. A nice high pressure shower makes for a much better clean. The actual amount of water used might still vary widely though. |
Unsound,
That makes sense and with the advent of transistor power amplifiers I can see where the 4 ohm load speakers becomes more plausible to manufacture . One could make a case for either type of load characteristic. Higher impedance =easier load and less work for a given amplifier. Intuitively this seems to be ultimately more desirable.
There was a time when 8 to 16 ohm speakers were the norm and one could use modestly powered amps that were of high "quality ". 16 ohm speakers are quite rare these days. This would present an enormously easy load for virtually any power amplifier.
Charles |
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. Also as we know from experience the best sounding gear is not always the ones that measure lowest in distortion. Lower impedance tougher load speakers just means amps must work harder. Some are able do that much mbetter than others. Newer more efficient amp technologies like Class D perhaps do it best. Class D offers a much larger higher pressure "garden hose" for your speakers. For example I have 500 w/ch class d amps that can deliver almost 40 amps of current that are about the size of a shoebox. My harder load speakers shine and easily surpass my smaller easier load ones with these.
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Charles1dad, sometimes a speaker designer will add components to a cross-over to lower the impedance in order to make the impedance flatter/smoother. Sometimes lowering the impedance allows for more current delivery in order to facilitate stronger bass response. |
Lots of food for thought here. Thanks guys for all your responses to help me see this a little clearer. As charles1dad pointed out, in response to Ralphs explanation, it does seem to be (at least to me) in the best interest of design, to provide a speaker with a higher impedance value. So why would a designer go the other direction? Oh, boy, where will this be leading the thread |
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. |
gs5556,
That is a very good analogy!
I’ve often wondered why given the reality of Ohm’s Law, why do many speaker designers revert to 4 ohms rather than 8 ohms (or higher) speaker impedance. As Al noted in his reply, Ralph (Atmasphere) has pointed out that the higher the impedance load ,the lower the amplifier distortion produced (which is a desirable result you'd think).
It would seem that a concerted effort to design easier to drive high impedance speakers would yield obvious benefits. The most logical explanation for the plethora of lower impedance speakers is that the higher impedance speaker is probably more difficult to implement to get things right and presents more design challenges.
Charles |
For the same reason you lose water pressure in the shower when the toilet is flushed. More water is drawn, lowering the pressure in the tank. When the speaker has a high impedance, it’s like a faucet that is slightly open. The pressure (voltage) in the tank (amplifier) is maintained as there is little flow (amperage) through the piping (speaker cables). When the impedance drops, the faucet opens which draws more water (amps) causing the pressure (voltage) to drop. If the amplifier does not have enough ’pressure’ it cannot supply the current for the speaker driver to move.
That is why amplifiers that maintain voltage throughout the impedance swings of speakers are heavy and expensive. They have to act as a voltage source regardless of the current demands of the speaker. This requires a bigger transformer, bigger power supply capacitors and high current output devices to put it all togther. |
How about this simple example. A regulator on an alternator of a car stops the alternator from putting out more power than it is designed to do. If not regulated the alternator will push as much power as you demand of it until it melts.
A speaker load that has less resistance or push against it or back pressure, will allow an amp to keep putting out more than it can handle. (over heat or melt outputs.) If an amp can do 100 watts pushing against 8 ohms it will want to do 200 into 4 and 400 into 2. If the heat sinks or power supply wasn’t built to handle that it melts down.
The reason someone else in this discussion said it is not necessarily true is because other factors include efficiency of the speaker, but this is only a factor if your taking into account that your trying to fill a room with a certain SPL level, and woofer size because you need more amperage to move a larger motor. |
First, I wouldn’t say that 4 ohm speakers are **necessarily** harder to drive than 8 ohm speakers, as there are many other variables involved. Including the efficiencies of the speakers; how the magnitudes of their impedances (the number of ohms) vary over the frequency range; the phase angles of their impedances at various frequencies (which describe the degree to which the impedance becomes partially inductive or partially capacitive at various frequencies, rather than purely resistive); etc.
But yes, typically a speaker having a low nominal impedance such as 4 ohms will be more difficult to drive than one having a higher nominal impedance such as 8 ohms. Adding to what has already been said, perhaps a good way to envision that is to consider a pair of extreme examples.
On the one hand let’s say that all the amp is driving is the input impedance of another amplifier, as would be the case if the amp were only connected to the speaker-level input of a powered sub. It might then be seeing a load of perhaps 100,000 ohms, which would result in the sub responding to the voltage being put out by the amp in question at any instant of time, but per Ohm’s Law (thanks for bringing that into the discussion, Ghosthouse) drawing essentially negligible current from that amp. In that situation the amp in question would hardly know that it is connected to anything at all, and the power it would be putting out would be essentially zero. (Power into a resistive load equals voltage x current). (In saying this, btw, I'm putting aside the fact that tube amps having output transformers should not be operated unloaded while processing a signal, that being a separate issue).
At the other extreme let’s apply a load of essentially zero ohms to the amp, by shorting its + and - output terminals directly together with a heavy gauge jumper. I think most will recognize that the amp would be incapable of putting any kind of reasonable signal into that near zero ohm load, because per Ohm’s Law creating a non-zero voltage across a zero ohm resistance requires infinite current. And as the amp attempts to do that the result is likely to be either that it goes into a self-protective shutdown, or a blown fuse, or damage.
Obviously a 4 ohm load comes closer to being a direct short than an 8 ohm load, and an 8 ohm load comes closer to being a negligible load than a 4 ohm load, so there you go!
One additional point: As Ralph/Atmasphere has stated here many times, for various reasons both solid state and tube amplifiers will exhibit measurably better distortion characteristics when driving 8 ohms than when driving 4 ohms.
Regards,
-- Al
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Hi crazyeddy,
I can't explain the math to you, but the inverse proportionality that you spoke of has to do with the fact that impedance is the AC version of resistance. While it is also measured in ohms, it is not the same as DC resistance.
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And, remember most of the amps power is going to the lower frequencies. So, if the impedance dips at low frequecies, your amp will have a hard time keeping up with the demands of the speaker.
Like the water in the hose instead of being restricted by a nozzle, is now having to provide the same pressure in a larger diameter hose.
HTH
Bob |
"Current production is a more useful indicator of amp "grunt" than rated wattage (or so I think)."
Nice job ghosthouse.
Best to you,
Dave |
Maybe unsound will see this and chime in;
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I'll embarrass myself by saying I have always thought of it as a hose with water going through it. If you have a narrow opening at the end (say, using a nozzle), which equates with high impedance in my analogy, you can get a powerful jet of water from the hose with your normal water pressure or even less, but if you take the nozzle off (low impedance) you won't get the same powerful jet of water unless you significantly increase the water pressure (in the analogy, a much more powerful amp). I'm sure Al will give the best technical explanation. |
I ain’t Al but one of the few things I sort of understand about electronics is Ohm’s law.
From Wikipedia
Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:
I (current; amps) = Voltage divided by Resistance (ohms)
At the risk of embarrassing myself, as resistance decreases the current required for a given voltage goes up.
As an example:
1 volt/8 ohms = 0.125 amps;
1 volt/6 ohms = 0.167 amps;
1 volt/4 ohms = 0.25 amps
1 volt/2 ohms = 0.5 amps
So, for a constant voltage, current requirements double going from 8 ohms to 4 ohms and quadruple going to 2 ohms. Current production is a more useful indicator of amp "grunt" than rated wattage (or so I think).
The other thing to remember is that for a given speaker design, resistance might not be constant across the frequency range. Low resistance dips occurring at different points can result in differences in how things sound between two amps with similar wattage ratings but differing in current output capability.
That’s what I think I know. |
A low impedance speaker requires greater current from the amplifier to result in the same voltage at the output.
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It's about the amp. Amps like to put out at 8 ohms. The mechanical analogy isn't a good one. Instead, think about your past gf's. |
I've heard it described like this: remember that the amp is not just pushing against a spring in one direction, but rather pushing and pulling back and forth. Less impedance, e.g. a lighter spring in your analogy, means more slop, more wiggle room, that the amp must now cope with in order to control the woofer.
Almarg, please weigh in and set us all straight. ;-) |
