[AzN1337c0d3r]

The reason why "portable" headphones are designed with a low impedance is because whatever is powering your source is likely to be a battery. Your typical battery is most likely 1.5V (9V batteries are in fact, 6x 1.5 batteries connected in series). Most headphones require about a milliwatt of power to drive properly. Let's take a look at the math here:

Taking Ohm's law and reorganizing so current is on one side.
V = I * R -> I = V / R

Power:
P = I * V


Combining the previous 2:
P = V^2 / R

As you can see, the power is inversely related to the resistance (impedance).

Thus, as you lower the resistance, your power goes up (assuming your voltage remains the same)

There is a drawback to this though, the low-impedance headphones require much more current than high-impedance headphones to drive to the same volume levels (assuming the dB/mw are the same.) This is a big issue with capacitor-coupled outputs on most amplifiers. Capacitors can only hold so much charge, once that well runs dry, the signal starts distorting, which is a bad thing. You may ask, why don't they just make the capacitors bigger? There are 2 primary reasons. The first reason is one of economics, smaller capacitors cost less. The second one is that as you make a capacitor bigger, you also reduce the rate at which it discharges. If the music has sharp transients, this capacitor may not be able to discharge quick enough to avoid distortion, again, a BAD thing.

 

[compuryan]

oh gosh I love physics. Ohm's law FTW!

What I might add, to keep it simple, is that Impedence (Ohms) is RESISTANCE to CURRENT flow (amps), thus you will need more voltage to power the headphones. because:

Resistance = Voltage / Current

 

[ericj]

Quote:

Originally Posted by AzN1337c0d3r /img/forum/go_quote.gif There is a drawback to this though, the low-impedance headphones require much more current than high-impedance headphones to drive to the same volume levels (assuming the dB/mw are the same.) This is a big issue with capacitor-coupled outputs on most amplifiers. Capacitors can only hold so much charge, once that well runs dry, the signal starts distorting, which is a bad thing.

Um.

No.

The distortion you speak of exists for sure, but it exists regardless of whether the output is capacitor coupled or not.

The problem with the capacitor output is that the capacitor along with the impedance of the headphone driver forms a high-pass filter. You need an output filter cap of about 470uf if you expect to hear real bass with a 32-ohm headphone driver. You need much less of an output capacitor with higher impedance headphones.

With the low-Z cans and a too-small output capacitor (I've seen as little as 47uf), the bass is just severely attenuated.

The distortion you're describing happens when the power supply cannot service the pulse. This is the case whether or not the amp has a capacitor-coupled output.

It's very expensive to build capacitorless power supplies that are capable of servicing these pulses, so, the power supply has storage capacitors near the amplifier.

And yes, if the caps aren't big enough, and run out of juice before the pulse is finished, the waveform will fall flat before it's complete. And that sounds awful.

 

[rsaavedra]

Quote:

Originally Posted by Ichinichi /img/forum/go_quote.gif take the DT990s. other than different power requirements, how would they be perceptibly different if they were 60ohms instead of 600ohms? why have different impedances? are there implicit advantages and disadvantages aside from how easy they are to drive?

Now my take on this one.

For the amp there are differences with respect to having to drive different impedances. For low impedance loads the amp should in general be comfortable delivering current. For high impedance loads the amp should in general be comfortable in particular delivering high voltages.

Now this begs an explanation of the differences between current and voltage.

I'll use what I think is a very adequate analogy. (Learned about it from a CCNA course material.)

Think of water flowing through a water pipe. You can have water flowing through a given pipe very slowly, or really very fast, depending on the pressure pushing that water. Consider also two pipes, one 1/2" in diameter, the other one is 6-feet in diameter (like those huge underground metropolitan pipes.) Water could flow at exactly the same speed on those two pipes, but because of the differences in their diameters, the thin pipe could carry only very small amounts of water per unit of time, while the large pipe could carry possibly tons of water per second, even though the flow goes through it at the same speed as in the thin pipe.

Now keeping those pipe images in mind, think of electrons as the actual water. The amount of water flowing per unit of time would be analogous to the amount of electrical current flowing through a conductor. And the pressure making that water move with a specific "urgency" would be analogous to voltage.

You can have very tiny amounts of water moving slowly, for instance, a rain drop. Similarly, you can have small amounts of current with small voltages. For instance, think of a digital watch battery, which offers very small currents at very small voltages.

You can also have small amounts of water moving extremely fast. For instance, think of the water flowing through the grooves in the tire of a car going over a puddle at 100 mph.

Similarly you can also have tiny amounts of electrical current flowing at very high "pressure". An example of this would be a mild electrostatic discharge, which usually involves relatively small amounts of current, but with somewhat high voltages. (At least according to the CCNA course material, not completely sure of this myself.)

On the other hand, you can have a really huge amount of water flowing at a very slow speed. For instance, you can probably outrun the flow of water in a slow river. Yet, because of the size of the river, the amount of water flowing there could be very likely and in total in the order of hundreds or thousands of tons per second.

You can also have large amounts of current flowing at low "pressure" (voltage). One example of this would be a car battery. 12 volts is a relatively small voltage (even a few tiny watch batteries can give you 12 volts). But the amount of current generated by a car battery is in the order of several amps, and that's several orders of magnitud higher than the amount of current watch batteries can deliver.

You could also have very large amounts of water moving at very high speed though. For instance, if you dropped a 10 ton metal container full of water from a plane at 20.000 feet high. Or think of a 10 mile long meteor containing just ice, entering the Earth's atmosphere at 50.000 mph.

One analogous electrical example, lots of current and voltage happening at the same time, would be a storm's lightning bolt, which exhibits very high amounts of current as well as very high voltages. And that's of course why lightning bolts can be so destructive, just the same way a fast and large enough meteor can be so tremendously destructive.


In general, you can have water systems that can comfortably handle large amounts of water at slow speeds, while some other water systems might be able to handle smaller amounts of water, but at higher speeds. Some are better suited for one case, and some for the other. (Some for both though.)

Similarly, some circuits and amplifiers are better prepared to handle high amounts of current (required for low impedance loads), even though they might have limitations in producing large voltage swings quickly enough (required for high impedance loads.) Or rather, some amps might be able to produce relatively large voltage swings very quickly, but ultimately, moving large amounts of current per unit of time might be less "comfortable" for them.

So all in all, some amps might handle high impedance loads more comfortably, while some others might be better at driving low impedance loads. - Some good amps might be comfortable with both types of load though.

 

[AzN1337c0d3r]

Quote:

Originally Posted by ericj /img/forum/go_quote.gif Um. No. The distortion you speak of exists for sure, but it exists regardless of whether the output is capacitor coupled or not. The problem with the capacitor output is that the capacitor along with the impedance of the headphone driver forms a high-pass filter. You need an output filter cap of about 470uf if you expect to hear real bass with a 32-ohm headphone driver. You need much less of an output capacitor with higher impedance headphones. With the low-Z cans and a too-small output capacitor (I've seen as little as 47uf), the bass is just severely attenuated. The distortion you're describing happens when the power supply cannot service the pulse. This is the case whether or not the amp has a capacitor-coupled output. It's very expensive to build capacitorless power supplies that are capable of servicing these pulses, so, the power supply has storage capacitors near the amplifier. And yes, if the caps aren't big enough, and run out of juice before the pulse is finished, the waveform will fall flat before it's complete. And that sounds awful.

At the risk of derailing this thread:

I am curious as to why amp designers don't put ridiculously large capacitors (something is the millifarad range) into their designs then, if there are no drawbacks besides a few extra $ to buy said caps. I always thought this was due to the RC characteristic.

 

[ericj]

Quote:

Originally Posted by AzN1337c0d3r /img/forum/go_quote.gif At the risk of derailing this thread: I am curious as to why amp designers don't put ridiculously large capacitors (something is the millifarad range) into their designs then, if there are no drawbacks besides a few extra $ to buy said caps. I always thought this was due to the RC characteristic.

In the power supply or the output?

In the power supply, if you go overboard on storage capacitors you run the risk of blowing up your batteries and/or diodes.

In either case, they're just so friggin huge.

The Sennheiser DSP Pro has 100uf output caps, and probably because that was what they could get in a 9mm tall surface mount cap when they built it.

There's at least one guy in the diy forum who likes to use photo flash caps with extreme ratings in the output coupling of tube amps, says it reduces phase distortion. I don't know if it really does or not.

 

[65535]

The lower the impedance (ohms) the more current it takes to drive the headphone at any given voltage, now with a lower impedance the headphone is easier to drive but won't be as load. A higher impedance headphone will require higher voltage but less current. Thats what gain switches do on amps increase voltage. As you increase the voltage you increase the speaker travel, thats what causes them to get louder, but with high resistance you need more voltage to allow the current to move the driver. In general low impedance phones are good for mobility with or even without amps, high impedance are better suited for home.

 

[manhattanproj]

so in the grand scheme of things:

low gain switch on the amp - low impedance phones
high gain switch - high impedance phones

correct?

 

[niktheblak]

You also have to remember that it's current that moves the voice coil and not voltage. Current, or moving charge, creates the magnetic field that exerts force on the voice coil causing it to move.

The higher the resistance the less current there will be in the circuit at a given voltage level. So if you push the resistance up, you'll need to pump up the voltage in order to reach the same current level. Ergo, high-impedance headphones require a lot of voltage but will not necessarily suck more current than a low-impedance one. Naturally driver efficiency also affects the current/voltage requirements.

 

[Awk.Pine]

This website does a good job of explaining how output impedance and input impedance interact to distort frequency response. Basically, you want an output impedance that is very small relative to the input impedance; or, more precisely, small relative to the impedance variation of the driver over audible frequencies.

Which makes me wonder: how much does impedance vary in a given headphone driver? what about balanced armatures? electrostats?

Anyhow, I'm glad to see a discussion that points out how "power" in headamps can mean different things.

 

[Kees]

Quote:

Originally Posted by rsaavedra /img/forum/go_quote.gif Now my take on this one. For the amp there are differences with respect to having to drive different impedances. For low impedance loads the amp should in general be comfortable delivering current. For high impedance loads the amp should in general be comfortable in particular delivering high voltages. Now this begs an explanation of the differences between current and voltage. I'll use what I think is a very adequate analogy. (Learned about it from a CCNA course material.) Think of water flowing through a water pipe. You can have water flowing through a given pipe very slowly, or really very fast, depending on the pressure pushing that water. Consider also two pipes, one 1/2" in diameter, the other one is 6-feet in diameter (like those huge underground metropolitan pipes.) Water could flow at exactly the same speed on those two pipes, but because of the differences in their diameters, the thin pipe could carry only very small amounts of water per unit of time, while the large pipe could carry possibly tons of water per second, even though the flow goes through it at the same speed as in the thin pipe. Now keeping those pipe images in mind, think of electrons as the actual water. The amount of water flowing per unit of time would be analogous to the amount of electrical current flowing through a conductor. And the pressure making that water move with a specific "urgency" would be analogous to voltage. You can have very tiny amounts of water moving slowly, for instance, a rain drop. Similarly, you can have small amounts of current with small voltages. For instance, think of a digital watch battery, which offers very small currents at very small voltages. You can also have small amounts of water moving extremely fast. For instance, think of the water flowing through the grooves in the tire of a car going over a puddle at 100 mph. Similarly you can also have tiny amounts of electrical current flowing at very high "pressure". An example of this would be a mild electrostatic discharge, which usually involves relatively small amounts of current, but with somewhat high voltages. (At least according to the CCNA course material, not completely sure of this myself.) On the other hand, you can have a really huge amount of water flowing at a very slow speed. For instance, you can probably outrun the flow of water in a slow river. Yet, because of the size of the river, the amount of water flowing there could be very likely and in total in the order of hundreds or thousands of tons per second. You can also have large amounts of current flowing at low "pressure" (voltage). One example of this would be a car battery. 12 volts is a relatively small voltage (even a few tiny watch batteries can give you 12 volts). But the amount of current generated by a car battery is in the order of several amps, and that's several orders of magnitud higher than the amount of current watch batteries can deliver. You could also have very large amounts of water moving at very high speed though. For instance, if you dropped a 10 ton metal container full of water from a plane at 20.000 feet high. Or think of a 10 mile long meteor containing just ice, entering the Earth's atmosphere at 50.000 mph. One analogous electrical example, lots of current and voltage happening at the same time, would be a storm's lightning bolt, which exhibits very high amounts of current as well as very high voltages. And that's of course why lightning bolts can be so destructive, just the same way a fast and large enough meteor can be so tremendously destructive. In general, you can have water systems that can comfortably handle large amounts of water at slow speeds, while some other water systems might be able to handle smaller amounts of water, but at higher speeds. Some are better suited for one case, and some for the other. (Some for both though.) Similarly, some circuits and amplifiers are better prepared to handle high amounts of current (required for low impedance loads), even though they might have limitations in producing large voltage swings quickly enough (required for high impedance loads.) Or rather, some amps might be able to produce relatively large voltage swings very quickly, but ultimately, moving large amounts of current per unit of time might be less "comfortable" for them. So all in all, some amps might handle high impedance loads more comfortably, while some others might be better at driving low impedance loads. - Some good amps might be comfortable with both types of load though.

X2
This is exactly how I allways picture it.
Like water flowing through wide or narrow pipes, either at great speed or at slow speed.
The width of the pipes determines the resistance (wide pipe: low resistance, narrow pipe: high resistance).
The ammount of water passing is the current.
The speed of the water represents the amount of volts.
The water pressure represents the power.

 

[rsaavedra]

I'd like to resurrect this thread just to add a very helpful link I found in another forum:

ELECTRICITY MISCONCEPTIONS SPREAD BY K-6 TEXTBOOKS

 

[nullstring]

Quote:

Originally Posted by rsaavedra /img/forum/go_quote.gif I'd like to resurrect this thread just to add a very helpful link I found in another forum: ELECTRICITY MISCONCEPTIONS SPREAD BY K-6 TEXTBOOKS

Interesting read. It didn't really correct any ideas I had.. other than semantics of language, and the "sea of electrons" idea

This, however, was rather interesting-
"Electric charges are easily visible to human eyes, even though their motion is not. "Electricity" is not invisible! Never has been. When you look at a metal wire, you can see the charges of electricity which would flow during electric currents. They are silvery/metallic in color. They give metals their mirrorlike shine. Some metals have other colors as well, brass and copper for instance. Yet in all cases, the "metallic"-looking stuff is the metal's electrons. A dense crowd of electrons looks silvery; "electric fluid" is a silver liquid. And if metals weren't full of movable electrons, they wouldn't look metallic. "

 

[rsaavedra]

Quote:

Originally Posted by nullstring /img/forum/go_quote.gif Interesting read. It didn't really correct any ideas I had.. other than semantics of language, and the "sea of electrons" idea This, however, was rather interesting- "Electric charges are easily visible to human eyes, even though their motion is not. "Electricity" is not invisible! Never has been. When you look at a metal wire, you can see the charges of electricity which would flow during electric currents. They are silvery/metallic in color. They give metals their mirrorlike shine. Some metals have other colors as well, brass and copper for instance. Yet in all cases, the "metallic"-looking stuff is the metal's electrons. A dense crowd of electrons looks silvery; "electric fluid" is a silver liquid. And if metals weren't full of movable electrons, they wouldn't look metallic. "

Yes I also found that part quite interesting.

Also the comment about a coulomb in copper beign about the size of a grain of sand. One Ampere being one coulomb per second, then an Ampere is "one saltgrain-sized blob, moving each second, squeezing itself into whatever sized wire. The tiny saltgrains are going by bip, bip, bip, once per second ... In 30-gauge wire the saltgrains would be almost undistorted, and so the charges would move at about 0.4 mm/sec during a 1-amp current. " Really highly explanatory and highly visual descriptions and clarifications.

 

[steven_1026]

A little bit off topic but, it seems to me that most high end headphones are very hard to drive. Why is that?