EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: Yamin on January 01, 2019, 07:10:35 pm
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Hi I recently got a Beyerdynamic DT100 headphone as a present . I noticed that it doesn't sound as "loud" or "clear" as some of the other headphones I have used. (But DT100 is the most comfortable). Anyway looking into it I learned that the version I have, has got impedance rating of 400 ohms, there is a DT100 version rated at 16 ohms too.
Could you please clarify the impedance specification, and about impedance matching. (Technically I know what Impedance means but I'm not sure how it works with in this context).
Why have different version with different impedance?
I use the headphone with different sound cards (audio interfaces) and mixers, one of the sound card I use it with is Focusrite Scarlet 2i2. The headphone output's impedance is rated at 10 ohms (again what does this mean).
Thanks so much for help.
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if the headphones have a power rating, then the impedance dictates what the measured voltage would be at the max power rating.
the reason being the amplfiier rails. If the amplifier has a +-5V supply but it can supply 50 amps, and the speakers require a higher voltage, then it wont work because they will be current limited, despite the availability of more current and power, there is not a high enough voltage to supply the speaker.
like you can throw a resistor of a certain size on a car battery and it will be fine (but if you throw a screw driver on it it will explode), but if you put the same resistor into a high voltage transformer of the same surge power capability as the battery, it will explode.
It's often easier to make a amplifier that runs on +-15V or whatever, then use a transformer to get the span correct to the speaker, then to make a amplifier that runs on +-1.5V or +-150V, with the same quality, or whatever the math comes out to.
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Headphones have inductance as well as DC resistance, the impedance is therefore the result of these two elements when fed with an AC signal.
For something like a personal music player the 400 ohm devices will be best as they will present the highest possible load impedance to amplifiers that are meant to drive headphones anyway. When listening to a HiFi amplifier the 16 ohm devices will be better as they are closest to the impedance of the speakers, although there may also be series resistors in the amp to reduce the level.
To match headphones to an existing source you have two options. The first is to use a pair of transformers where the turns ratio provides the matching. The second method is to use a series resistor which will match low impedance headphones to a medium impedance output BUT provide a lower sound level.
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I also forgot to add is there anyway of making an "impedance matching" device which will come in between the headphone output of a mixer and the headphone (bearing in mind that the headphone output is an amplified output). Ideally I don't want to have separate headphone amplifier which uses a line output.
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For something like a personal music player the 400 ohm devices will be best as they will present the highest possible load impedance to amplifiers that are meant to drive headphones anyway. When listening to a HiFi amplifier the 16 ohm devices will be better as they are closest to the impedance of the speakers, although there may also be series resistors in the amp to reduce the level.
To match headphones to an existing source you have two options. The first is to use a pair of transformers where the turns ratio provides the matching. The second method is to use a series resistor which will match low impedance headphones to a medium impedance output BUT provide a lower sound level.
Thanks, What do you mean by a personal music player? I checked the specs of the iphone 7 and its recommended that I use a headphone amplifier for headphone impedance rated greater than 100 \$\Omega\$.
With the series resistor matching method, what do you mean by a "low" and "medium"?
Thanks again :)
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Modern personal electronics (Phones and such) usually run from a supply of under 4V (Lithium ion cell), which means they cannot push much current into a high impedance load, like your 400R DT100s (Which were never the most sensitive of cans anyway).
With a 4V supply, we are probably looking at somewhere in the 1V region for RMS output, which into 400R gives about 2.5mW at clipping. The nominal performance is given as 94dB/mW which I assume is ref 20uPa, so 2.5mW makes the peak level about 98dB SPL or so.
Compare with say a 32R version, our 1V now gets us 31mW which is just over 10dB more power, but which will sound about twice as loud (Hearing is funny that way).
Now compare with a set of in ear units designed for use with personal electronics, these are usually both low impedance and designed to be FAR more sensitive then the DT series (which are really designed to be plugged into hifi or mixing desks), 110dB/mW or more is not untypical, so 30mW would easily hit 127dB which is well above the pain threshold and also into very fast permanent hearing damage.
Modern earphones designed for personal gear are usually of a relatively low impedance and are usually sensitive. Back in the day, it was common for audio gear to use +-15V rails (30V total) which could drive a reasonably large amount of power into a high impedance set of cans like your 400R set (In fact there was something to be said for higher impedance headphones as they caused much less heating in the amplifier and were less likely to deafen you when something went sideways).
Regards, Dan.
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I wonder if a lot of modern "high impedance" headphones are in fact low impedance headphones with built in series resistors, in order to make the noise floor of lower cost opamps less apparent. True high impedance headphones need very little power to make a sound, to the extent they're used for crystal radios.
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Those were a slightly different thing, being a piezo crystal driver, very sensitive, very high Z but sounded poor due to the limited frequency response.
In ear earphones/earbuds are usually very sensitive at any impedance level, far more so then on ear headphones, which is a problem since the EU started mandating maximum levels for consumer gear based on the assumption that we were all using the nasty things.
Regards, Dan.
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There were magnetic high impedance headphones for crystal radios at one point, usually 2k impedance. I don't think they're common at all nowadays since the piezo type works even better for crystal radios.
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The impedance is 400 Ohms according to this spec sheet which I found by googling for Beyer Dynamic Dt 100.
https://north-america.beyerdynamic.com/amfile/file/download/file_id/4831/product_id/1478/
This is a much lower impedance than typical headphones for MP3 players and the like.
Recording studios often run on lower impedance. Apparently (as provided in the spec sheet) there is another version, the DT 102 which runs on 16 ohms which is closer to some loud speakers.
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Holy moly there are some fundamental misunderstandings here!
OK, so you guys remember that V*A=W, right? Good.
And you remember Ohm’s law? OK.
So, for a given headphone, its maximum wattage is some combination of a voltage and a current. The impedance of the headphones dictate whether it prefers a higher voltage and lower current (high impedance headphones), or lower voltage and higher current (low impedance headphones).
Portable devices generally have quite low voltage limits, but the batteries can provide significant current, so low impedance headphones do better here. Typical values are between 16-40Ω. This gives good volume even on portable devices.
Mains-powered amps generally have much higher voltage dual-rail power supplies, and thus can do much larger voltage swings, supporting high impedance headphones. Some divert a pair of speaker amps to the headphone output via relays, others simply use a separate op-amp headphone amp.
A speaker amp intended for speakers won’t have any trouble with headphone impedances far higher than the speaker impedance, simply because the maximum voltages needed for loud playback on speakers is far higher than needed for headphones. The current involved is negligible compared to loudspeakers.
So, with those fundamentals out of the way:
For something like a personal music player the 400 ohm devices will be best as they will present the highest possible load impedance to amplifiers that are meant to drive headphones anyway. When listening to a HiFi amplifier the 16 ohm devices will be better as they are closest to the impedance of the speakers, although there may also be series resistors in the amp to reduce the level.
That’s absolutely, 180˚ backwards. Portable devices like low impedance headphones. 400Ω aren’t going to be very loud at all on a portable device. 16-40Ω is the typical range.
The impedance is 400 Ohms according to this spec sheet which I found by googling for Beyer Dynamic Dt 100.
https://north-america.beyerdynamic.com/amfile/file/download/file_id/4831/product_id/1478/
This is a much lower impedance than typical headphones for MP3 players and the like.
Recording studios often run on lower impedance. Apparently (as provided in the spec sheet) there is another version, the DT 102 which runs on 16 ohms which is closer to some loud speakers.
No, that is far, far higher than typical headphones for portable devices.
It’s studios and audiophiles that tend to run high impedance headphones. (I have yet to come across a convincing explanation of why high impedance headphones are supposed to sound better than low impedance ones, but it’s a common belief.)
In ear earphones/earbuds are usually very sensitive at any impedance level, far more so then on ear headphones, which is a problem since the EU started mandating maximum levels for consumer gear based on the assumption that we were all using the nasty things.
Just a little niggle: headphones, earbuds, and in-ear monitors (aka in-ear earbuds) are three different kinds of earphones, not two. The volume characteristics of in-ear types are very different, since they significantly block outside noise by sealing off the ear canal, reducing the amount of volume needed compared to earbuds that do not seal off the ear canal. And I categorically disagree with you dismissing them all as “nasty things”. There are some very, very, very nice in-ear earbuds out there.
The EU limit is a bad design, IMHO, since it’s based on “maximum volume with the pack-in earphones”. So the second you plug in some other model of earphone, the maximum volume may be too high or too low. So dumb if you ask me... (Never mind that higher gain may be needed to boost quiet content...)
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I wonder if a lot of modern "high impedance" headphones are in fact low impedance headphones with built in series resistors, in order to make the noise floor of lower cost opamps less apparent. True high impedance headphones need very little power to make a sound, to the extent they're used for crystal radios.
Not in any of the big brands, at least, like Beyerdynamic or Sennheiser. They spin different drivers for different impedances.
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(Never mind that higher gain may be needed to boost quiet content...)
And that any content loudness normalised to the EBU standard -23dBLuFS is going to need that gain in many circumstances.
This is actually a real problem with mobile 'phones even without the eu getting involved, Youtube for example seems to be loudness normalising for an integrated loudness of about -14dBLuFS, basically because the things do not have the ability to provide the gain needed to produce reasonable volume in the cans with a lower reference level and reasonably dynamic content.
A correctly done system should be able to get at least 20dB above whatever level you are actually listening at to accommodate dynamics, -23dBLuFS does that given a sufficiently powerful output stage, -14 not so much (But is still infinitely better then peak normalisation).
You do see youtube audio that has clearly been peak normalised to -14dBFS, that is just painful.....
But yea, low Z cans for portable devices, high Z (if you must, not sure there is much point) for proper hifi.
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I wonder if a lot of modern "high impedance" headphones are in fact low impedance headphones with built in series resistors, in order to make the noise floor of lower cost opamps less apparent.
Even lower cost operational amplifiers are not particularly noisy.
The problem is that bandwidth and slew rate of a compensated operational amplifier are inversely proportional to the input stage transconductance. This is why FET input parts are inherently faster; FETs have a lower transconductance. The common solution to this problem with a bipolar input stage is to use emitter degeneration resistors to lower the transconductance but these resistors add directly to the input stage noise. You can even sometimes figure out the input stage circuits of an undocumented bipolar operational amplifier by comparing the input bias current or input current noise and the voltage noise.
Only video operational amplifiers commonly do this and it was never common in integrated "audio" operational amplifiers. Where it *is* common however is integrated and discrete audio power amplifiers like the ones in the monitor speaker Dave reviewed not long ago.
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I use 600 ohm AKG240 and Beyerdynamic DT990 headphones as they can be directly driven by some op-amps on +/-15V rails, or a Pultec eq. It's the best fidelity I have. Designing a DAC using this approach worked very well, as you are in the soundfield and can immediately hear details, noise and other subtleties when changing a design.
For lower impedance headphones, I did build a few amplifiers but nothing sounding as clean as directly running off line driver op-amps.
Lehmann Black Cube clones were a waste of time, chinese fake transistors and no feedback around the output stage, and lots of voodoo and superstition around it. Almost cult level on some headphone forums.
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Hey tooki thanks, just wanted to ask what does the headphone output spec come into this context. For eg: For the Focusrite 2i2 its says
Maximum Output Level > +10dBu
Impedance 10 Ω
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I use 600 ohm AKG240 and Beyerdynamic DT990 headphones as they can be directly driven by some op-amps on +/-15V rails, or a Pultec eq. It's the best fidelity I have. Designing a DAC using this approach worked very well, as you are in the soundfield and can immediately hear details, noise and other subtleties when changing a design.
Hi floobydust, wow I would like to design and build one for my own. When you say directly driven where is the input taken from. The line output from the mixer/audio interface or the headphone outputs? Would really appreciate if you direct me to some design/build/explanation links or any resources I could dive into.
Thanks so much
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Holy moly there are some fundamental misunderstandings here!
OK, so you guys remember that V*A=W, right? Good.
And you remember Ohm’s law? OK.
Holy Moly - you never read any book on Impedance matching (nor even carefully reading others responses). Guess what? You made a fool of yourself and so arrogantly as well! Right? -No and there is more to electronics than Ohm's law in a DC circuit and that includes the subjects here. How really silly of you!
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Holy moly there are some fundamental misunderstandings here!
OK, so you guys remember that V*A=W, right? Good.
And you remember Ohm’s law? OK.
Holy Moly - you never read any book on Impedance matching (nor even carefully reading others responses). Guess what? You made a fool of yourself and so arrogantly as well! Right? -No and there is more to electronics than Ohm's law in a DC circuit and that includes the subjects here. How really silly of you!
You just made fool of yourself, not tooki. What impedance matching? This is not some high frequency transmission line where you care about signal reflection :wtf:. Half decent audio amplifier outputs the same voltage regardless of the load impedance/current flowing through it because basically has zero output resistance. Not to say speaker impedance depends on signal frequency. Typically rated impedance is just DC resistance. Also you wrote complete nonsense here:
This is a much lower impedance than typical headphones for MP3 players and the like.
Recording studios often run on lower impedance. Apparently (as provided in the spec sheet) there is another version, the DT 102 which runs on 16 ohms which is closer to some loud speakers.
Portable devices simply cannot output high enough voltage swing to drive high impedance headphones at reasonable volume. And you don't want to connect 16 ohm headphones to usual loudspeaker output of amplifier. Look on volume control funny and headphones will go poof in less than a second.
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I wonder if a lot of modern "high impedance" headphones are in fact low impedance headphones with built in series resistors, in order to make the noise floor of lower cost opamps less apparent. True high impedance headphones need very little power to make a sound, to the extent they're used for crystal radios.
That would make crappy and inefficient headphones. Adding resistor in series kills dampening factor.
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Could you please clarify the impedance specification, and about impedance matching. (Technically I know what Impedance means but I'm not sure how it works with in this context).
High impedance headphones need higher voltage for the same power, most headphone amps can't supply it.
Why have different version with different impedance?
High impedance headphones are like vinyl, an artifact from ancient time kept around by audiofools who want to be a special snowflake.
I use the headphone with different sound cards (audio interfaces) and mixers, one of the sound card I use it with is Focusrite Scarlet 2i2. The headphone output's impedance is rated at 10 ohms (again what does this mean).
That they put a series resistor on the opamp output, it's not really relevant for driving your headphones.
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Holy moly there are some fundamental misunderstandings here!
OK, so you guys remember that V*A=W, right? Good.
And you remember Ohm’s law? OK.
Holy Moly - you never read any book on Impedance matching (nor even carefully reading others responses). Guess what? You made a fool of yourself and so arrogantly as well! Right? -No and there is more to electronics than Ohm's law in a DC circuit and that includes the subjects here. How really silly of you!
You just made fool of yourself, not tooki. What impedance matching? This is not some high frequency transmission line where you care about signal reflection :wtf:. Half decent audio amplifier outputs the same voltage regardless of the load impedance/current flowing through it because basically has zero output resistance. Not to say speaker impedance depends on signal frequency. Typically rated impedance is just DC resistance. Also you wrote complete nonsense here:
This is a much lower impedance than typical headphones for MP3 players and the like.
Recording studios often run on lower impedance. Apparently (as provided in the spec sheet) there is another version, the DT 102 which runs on 16 ohms which is closer to some loud speakers.
Portable devices simply cannot output high enough voltage swing to drive high impedance headphones at reasonable volume. And you don't want to connect 16 ohm headphones to usual loudspeaker output of amplifier. Look on volume control funny and headphones will go poof in less than a second.
Yes headphones and speakers don't need to be impedance matched with the amplifier output and the impedance of the cable is of no consequence at audio frequencies.
Generally the amplifier's output impedance should be as low as possible, compared to the load, otherwise it will give a poor damping factor, resulting in peaks at resonant frequencies of the speaker. This is typically worse with speakers than headphones, as the resonance is more audible.
The output impedance of modern amplifiers is fairly low, often much less than an Ohm, so the maximum output voltages and currents are the prime limiting factors, as mentioed above.
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Headphone amps almost always have a few tens of ohms of buildout resistance, they are nearly never actually near zero impedance outputs (And here be audiophool infested waters).
In old hifi it was common for the headphone jack to be driven from the speaker outputs via a couple of suitable resistors....
As to the restive part of the load impedance, it is usually down to the winding resistance but for a single driver design that is of course equivalent to adding a series resistor (An electromagnet with resistance is electrically the same as a resistor plus a perfect electromagnet, at least at audio frequency)!
73 Dan.
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Headphone amps almost always have a few tens of ohms of buildout resistance, they are nearly never actually near zero impedance outputs (And here be audiophool infested waters).
Unless it's a tube amp or a flaky D-class amp, generally you'd need to add series resistance to get tens of ohms of output impedance because negative feedback tries to get it close to zero.
(https://www.eevblog.com/forum/chat/help-clarify-headphone-impedance/?action=dlattach;attach=613282;image)
In old hifi it was common for the headphone jack to be driven from the speaker outputs via a couple of suitable resistors....
And it makes sound crappy because of low dampening factor which means speaker lives on it's own instead of being tightly controlled by amplifier. Adding series resistor is just a cheap way how to add headphone output without caring about performance.
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I see lots of mobile 'phones and similar in that list, compare with say hifi or mixing desks or such (Which are the proper use of the DT100 and friends) and you will find that well north of 10R is normal.
They use explicit buildout resistance (Most of this stuff runs on mains power and has internal rails that can give 10V++ at the amplifier output which is well excessive for cans).
The only place you tend to see DT100s is radio studios, they don't sound that good, but are damn near indestructible, and spare parts are available.
Regards, Dan.
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I see lots of mobile 'phones and similar in that list, compare with say hifi or mixing desks or such (Which are the proper use of the DT100 and friends) and you will find that well north of 10R is normal.
10R or even 50R output impedance is completely fine for 400R headphones but not for 16R headphones.
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This is what Sennheiser says dumbed-down for commoners.
https://sennheiseruk.happyfox.com/kb/article/260-headphone-and-amplifier-impedance/ (https://sennheiseruk.happyfox.com/kb/article/260-headphone-and-amplifier-impedance/)
If the headphone impedance is lower than the output impedance of the device they are plugged into, this lowers the damping factor, which may result in a significant change from the published frequency response and also cause distortion in the headphones.
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Is there anything which can go in between the headphone outputs of a mixer and the DT100, eg: transformer or small headphone amp. I couldn't find any plug and play solution around the web.
It would be great to build one myself.
How would a op-amp HP amplifier design vary if using the line outputs of the mixer or if I'm using the headphone output of the mixer as the input for the op-amps?
Thanks
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Holy moly there are some fundamental misunderstandings here!
OK, so you guys remember that V*A=W, right? Good.
And you remember Ohm’s law? OK.
Holy Moly - you never read any book on Impedance matching (nor even carefully reading others responses). Guess what? You made a fool of yourself and so arrogantly as well! Right? -No and there is more to electronics than Ohm's law in a DC circuit and that includes the subjects here. How really silly of you!
Holy Moly!
Despite the name "power amplifier," in audio the device which drives the speakers (including headphones) is really a voltage amplifier which is capable of sourcing and sinking the current required by the generally low-impedance loads.
(Yes, impedance matching is done for RF power amplifiers ...)
There is no impedance matching in modern audio. Everything is bridged: output impedances as low as possible (made easy by op-amps) and moderate input impedances (10 k-ish for line level audio). Instrument amplifier inputs are high impedance (1 M is common) so as to not load down passive pickups (usually between 5 k and 10 k).
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Holy moly there are some fundamental misunderstandings here!
OK, so you guys remember that V*A=W, right? Good.
And you remember Ohm’s law? OK.
Holy Moly - you never read any book on Impedance matching (nor even carefully reading others responses). Guess what? You made a fool of yourself and so arrogantly as well! Right? -No and there is more to electronics than Ohm's law in a DC circuit and that includes the subjects here. How really silly of you!
:-DD :-DD :-DD
I’ll be the first to tell you that I don’t really understand transmission lines — but I also know that headphones (and audio in general) doesn't fall into that category. Luckily, others here that do understand both audio and transmission lines have already confirmed what I said and explained why.
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Is there anything which can go in between the headphone outputs of a mixer and the DT100, eg: transformer or small headphone amp. I couldn't find any plug and play solution around the web.
It would be great to build one myself.
How would a op-amp HP amplifier design vary if using the line outputs of the mixer or if I'm using the headphone output of the mixer as the input for the op-amps?
Thanks
I suppose that in theory a transformer would work, but whether appropriate models are even made any more, I don’t know.
A simple op-amp headphone amplifier (running off proper dual rail power of appreciable voltage, like ±9V at minimum) would work fine. You can buy them as finished amps, as Chinese boards on eBay, or build your own in an afternoon.
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I'm surprised a mixer is having difficulty driving headphones, given that sort of thing is usually a professional or prosumer bit of hardware.
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Is there anything which can go in between the headphone outputs of a mixer and the DT100, eg: transformer or small headphone amp. I couldn't find any plug and play solution around the web.
It would be great to build one myself.
A transformer or headphone amplifier would be suitable. Suitable high fidelity transformers are very expensive but headphone amplifiers can be found for a reasonable price.
How would a op-amp HP amplifier design vary if using the line outputs of the mixer or if I'm using the headphone output of the mixer as the input for the op-amps?
The only variation would be the gain or input attenuation to accommodate different signal levels.
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Holy moly there are some fundamental misunderstandings here!
OK, so you guys remember that V*A=W, right? Good.
And you remember Ohm’s law? OK.
Holy Moly - you never read any book on Impedance matching (nor even carefully reading others responses). Guess what? You made a fool of yourself and so arrogantly as well! Right? -No and there is more to electronics than Ohm's law in a DC circuit and that includes the subjects here. How really silly of you!
:-DD :-DD :-DD
I’ll be the first to tell you that I don’t really understand transmission lines — but I also know that headphones (and audio in general) doesn't fall into that category. Luckily, others here that do understand both audio and transmission lines have already confirmed what I said and explained why.
I never imagined that you know jack about transmission lines. However, I still am the first to tell YOU that you know nothing about impedance matching which applies to more than transmission lines. You know nothing about power transfer - and you still think you have something to contribute instead of something to learn. Read some books on electronics - read past the page on Ohms law.
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Bruh... I know my limits. Learn yours, since you’re the one spouting abject nonsense.
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However, I still am the first to tell YOU that you know nothing about impedance matching which applies to more than transmission lines. You know nothing about power transfer - and you still think you have something to contribute instead of something to learn. Read some books on electronics - read past the page on Ohms law.
The most efficient power transfer is when source has much lower internal resistance than load. Impedance matching you talk about but don't explain is some sort of nonsense.
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Holy moly there are some fundamental misunderstandings here!
OK, so you guys remember that V*A=W, right? Good.
And you remember Ohm’s law? OK.
Holy Moly - you never read any book on Impedance matching (nor even carefully reading others responses). Guess what? You made a fool of yourself and so arrogantly as well! Right? -No and there is more to electronics than Ohm's law in a DC circuit and that includes the subjects here. How really silly of you!
:-DD :-DD :-DD
I’ll be the first to tell you that I don’t really understand transmission lines — but I also know that headphones (and audio in general) doesn't fall into that category. Luckily, others here that do understand both audio and transmission lines have already confirmed what I said and explained why.
I never imagined that you know jack about transmission lines. However, I still am the first to tell YOU that you know nothing about impedance matching which applies to more than transmission lines. You know nothing about power transfer - and you still think you have something to contribute instead of something to learn. Read some books on electronics - read past the page on Ohms law.
I did transmission lines in college so have a fairly good understanding of them, but I've not used most of it, so I admit I'm rusty.
Yes, impedance matching does apply to areas, other than transmission lines (more later) but audio is not one of them, unless you're talking about old telephone lines, which is not what this thread is about.
How long are your headphone speaker cables? The wavelength of 20kHz is 15km through a vacuum, but it will be a bit shorter in a cable, so call it 10km, assuming a velocity factor of 2/3, so unless your cables are hundreds of metres long, impedance matching is a non-issue.
The only other reason we might want impedance matching is for maximum power transfer, but this in't applicable to modern amplifiers. To achieve maximum power transfer, the load impedance, should be matched to the source impedance, but assuming the source impedance is resistive, the maximum efficiency will be 50%, as half of the power is dissipated in the source. To get a decent efficiency, the source impedance needs to be much less than the load, ideally zero. Old valve amplifiers had a very high output impedance, so a transformer was required to enable it to drive a low impedance speaker efficiently, but nowadays it's completely unnecessary. Modern amplifiers have a very low output impedance, so impedance matching transformers are obsolete.
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the maximum efficiency will be 50%, as half of the power is dissipated in the source.
Actually it can be higher. With class D you can go over 90%, AB (most of analog amplifiers) can do over 60% efficiency.
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the maximum efficiency will be 50%, as half of the power is dissipated in the source.
Actually it can be higher. With class D you can go over 90%, AB (most of analog amplifiers) can do over 60% efficiency.
I know, re-read my post and if you still don't understand it, Google maximum power transfer.
EDIT:
Hint: if the source impedance is matched to the load, you have a potential divider.
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I know, re-read my post and if you still don't understand it, Google maximum power transfer.
Ok, I missed power transfer thing.