[HN Gopher] Headphone and Amp Impedance (2011)
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Headphone and Amp Impedance (2011)
Author : Tomte
Score : 116 points
Date : 2024-02-17 18:49 UTC (4 hours ago)
(HTM) web link (nwavguy.blogspot.com)
(TXT) w3m dump (nwavguy.blogspot.com)
| lelanthran wrote:
| Interesting. Does the 1/8th rule apply to matching amplifiers
| with speakers as well, say for car audio?
| michaelrpeskin wrote:
| Digging deep into my memory here, but I always remember that
| when Zout==Zin that's what you want for maximum power transfer.
|
| My guess is that the 1/8 trick is for the typical
| characteristics of small-speaker headset. Big speakers may have
| different capacitance and inductance characteristics.
|
| I don't know though. That's the first time I heard that
| approximation, and it doesn't seem to align with anything that
| I learned in electronics.
| marcosdumay wrote:
| Why would one optimize for maximum power transfer at all?
| Worse, why would one un-optimize the power source by adding
| impedance so that it works at maximum capacity? It's much
| better to just leave the extra capacity there.
|
| That 1/8 rate is a property of our hearing, and so will apply
| to any system that creates sound. But it's a quite hard
| target to achieve in large bass speakers, so a lot of systems
| simply don't.
| ahartmetz wrote:
| The radio stuff is about "wave impedance", the speaker or
| headphone stuff is more like a power supply situation at (in
| RF terms) very low frequencies. You want your power supply to
| have low impedance so it doesn't sag and keeps tight control
| over its output voltage.
|
| There must be a mathematical description of these different
| situations but I can't be bothered to work it out. An
| important part of it is probably that an audio amplifier has
| feedback, an RF amplifier just emits a wave and the consumer
| must make itself compatible with it. Lower output impedance =
| more effective feedback control for an amplifier with
| feedback.
| screcth wrote:
| Analysis becomes much simpler when frequencies are low
| enough. Wavelength is large compared to the dimensions of
| the circuit so you can ignore the effects of wave
| propagation.
| willis936 wrote:
| That's true for RF, but not for DC. Audio is "DC" because
| transmission lines are well approximated as ideal wires at
| frequencies below 100s of kHz.
| formerly_proven wrote:
| That's for maximizing the power transfer, which is important
| in radio applications because you want to maximize SNR (and
| because this is the condition where no power is reflected).
|
| Power applications (like speakers or the power grid) are
| generally concerned with maximizing efficiency, which you get
| by lowering source impedance as much as possible.
| kazinator wrote:
| Unlike what other commenters are saying, this Zout == Zin is
| applicable to audio.
|
| For a fixed Zout much higher than zero, if we want to suck
| the most power out of the amplifier, we should use a matching
| Zin indeed. E.g. if an amplifier has 16 ohms output
| impedance, then we will draw the most power out of it with a
| 16 ohm load.
|
| Loads higher than 16 ohms will result in less current
| flowing.
|
| Loads lower than 16 ohms will cause power waste in the source
| impedance. For instance a zero ohm load will mean that all
| the power is dissipated in the 16 ohm source impedance: the
| zero ohm load draws current, but no power because it I^2R is
| zero.
|
| So from there, if we increase the impedance gradually, we
| obtain more and more power until we get to 16 ohms, and after
| that less and less power.
|
| In amplifiers we care about efficiency, not with operating at
| the theoretical point where absolute maximum power is drawn.
| With a near zero ohm output, we ensure that all the voltage
| is dropped by the load rather than wasted in the source.
|
| When we have a near zero ohm output impedance, Zin == Zout
| still applies! E.g. theoretically, the maximum power transfer
| from a 0.1 ohm amplifier would take place if we connect it to
| a 0.1 ohm load. And that reflects the trend in low-output-
| impedance audio amplifiers: the lower the speaker impedance
| you plug in, the power power you get: 16 ohms, 8 ohms, 4
| ohms, ... You just can't go anywhere near 0.1, due to the
| practical limitations in the amplifier: ability to deliver
| current without frying itself.
|
| But, so yes, Zin == Zout is relevant, but in the majority of
| the audio amplifiers built, which have very low output
| impedance, that theoretical point occurs at low value of
| Zout/Zin which usually out of reach of the absolute current
| delivery capability of the amplifier.
| instagib wrote:
| Smith charts
|
| https://en.m.wikipedia.org/wiki/Smith_chart
|
| You can put an amp 50ohm output into a 75 ohm cable or
| connectors but there will be negatives. The further away from
| output impedance the more negatives. Loss, distortion, and
| worse roll off, etc. Cables are fun too. for much higher
| frequencies, the less of the actual copper is used. The signal
| uses the surface of the wiring. For lower frequencies it uses
| the entire strand/solid copper wire.
|
| RF mostly uses 50ohm but not always. Cable video (tv) is
| usually 75ohm.
| kazinator wrote:
| The 75 ohms is meaningful when there is no reflection (like
| when we imagine sending a signal into an infinite piece of
| the cable).
|
| If you are using a short piece of the cable to convey a
| voltage signal between two pieces of equipment (like the
| output of one amplifier into the input of another), it has no
| meaning. The voltage on the other end rises almost instantly
| and is reflected back.
| willis936 wrote:
| This whole blog is a treasure for new EEs. The great mystery is
| that the author disappeared while working on a DAC revision but
| has kept their site running for over a decade.
| Hamuko wrote:
| Do you need to put any effort into a Blogspot site to keep it
| online?
| anthomtb wrote:
| You need to pay hosting bills. Or keep a server running
| somewhere in your home.
| Hamuko wrote:
| Aren't Blogspot blogs free and hosted in Google's DCs?
| proto-n wrote:
| Blogspot is google's blogging platform
| squarefoot wrote:
| I'm very skeptic of that 1/8th rule. Usually output impedance
| should be as low as possible, so that the resistance added by the
| cable becomes negligible in comparison, and its intrinsic
| capacitance won't turn it into a low pass filter. Unless I'm
| missing something, at least in audio, amplifiers with lower
| output impedance are always better than those with higher output
| impedance.
| notfed wrote:
| > most headphones work best when the output impedance is less
| than 1/8th the headphone impedance
|
| This doesn't seem to disagree with what you're saying?
| squarefoot wrote:
| 1/8 max seems still too high to me. For example it would be
| satisfied by a 12.5 Ohm Zout amp with a 100 Ohm headphone,
| but that wouldn't be ideal in case of long cables. I'd aim to
| much lower impedance anyway, especially today that it's easy
| to obtain with common parts.
| Kirby64 wrote:
| Honestly I don't know what reasonable headphone output today has
| these high output impedances. They're all class D amplifiers with
| quite low impedance directly to the power supply. This 1/8th rule
| is kind of pointless... you just specify output voltage drive at
| various loads and measure frequency response. Most do this very
| well. No need to faff about with damping of responses.
| atoav wrote:
| Jup, but when the post was written good class D headphone amps
| weren't as readily available as it is today, if I recall
| correctly.
| pclmulqdq wrote:
| Class A and AB amps often have significant output impedance
| due to how they are constructed, and used to be the standard.
| They still are the standard for HiFi audio. The other types
| of amps are generally pretty low impedance.
|
| Edit: Amp types as pointed out by another commenter.
| smolder wrote:
| I wouldn't call Class A "standard", unless you're including
| class AB in that? Pure class A is a bit esoteric since it's
| wildly inefficient compared to D or even AB.
| kazinator wrote:
| Class AB amplifiers (basically class B but with an overlap
| region when switching between the output devices) have very
| low output impedance when they feature negative feedback.
| Which is the standard for hi-fi audio.
| kazinator wrote:
| I also never heard of this 50-ohm-1996 thing. Any headphone
| output worth its salt in the last 70 years is going to have
| near zero ohm output impedance.
|
| Some headphone amplifiers use cheap integrated circuits that
| can only drive outputs no lower than around 150 ohms, like the
| JRC4556 (a device that NavGuy has written quite a bit about,
| incidentally).
|
| I suspect that's where that idea might come from? If you have a
| JRC4556 that doesn't go lower than 150 ohms, and the headphones
| are 32 ohms, you can pad in some series resistance to make up
| the difference.
| kragen wrote:
| this is great, i've been reading a lot about headphone impedance
| this week, but from the perspective of 'i want to use 3.5mm jacks
| for super simple data communications and power for small devices,
| so how do i keep from burning out headphones if someone
| accidentally plugs them in'
|
| (also it turns out that these jacks briefly short-circuit as you
| plug and unplug them)
|
| because 'super simple' excludes power negotiation schemes like
| usb-c, whatever power i deliver to small devices is also
| available if someone plugs in a resistor, and earspeakers are
| pretty similar to resistors at dc, and normally driven at only
| about a milliwatt
|
| the vast majority of current headphones (and headphone-driving
| amplifiers) out there today conform to the android specs (though
| probably approximately 1% of the headphones of interest to
| nwavguy)
|
| jack:
| https://source.android.com/docs/core/interaction/accessories...
|
| plug:
| https://source.android.com/docs/core/interaction/accessories...
|
| relevant appnotes on driving them include david guo's
| https://www.analog.com/media/en/technical-documentation/appl...
| and the anonymous
| https://www.digikey.com/Site/Global/Layouts/DownloadPdf.ashx...
| frabert wrote:
| This is unwanted advice, I know, but here is what I would do:
| I'd make it so that both the transmitter has a very high output
| impedance (so that plugging in an headphone causes its output
| to drop to near zero) and the receiver has very high input
| impedance (so that it doesn't bog the tx down).
|
| It will cause some noise issues, because a very high impedance
| front end will pick up stray radio transmissions, but I think
| it's doable given a small enough bandwidth
| kragen wrote:
| just in case this wasn't clear, plugging headphones or other
| audio equipment into these 3.5mm jacks is not the intention
| of this design; rather, i want to minimize the chance of
| destroying expensive equipment if it happens by accident
|
| noise is pretty irrelevant on the power rail. but if i put a
| high constant impedance on the power rail--say, a 100O
| resistor in front of 5 volts--none of the devices powered
| from it can draw much power, 25mA being the limit in that
| case
|
| (for the data line, sure, resistance is not at all futile,
| resistance is a perfectly good solution, along with some
| clamping diodes)
|
| what i have in mind is two things:
|
| 1. maybe use a trrs connector and use the second sleeve for
| the power rail? that way the only thing that ever gets
| plugged into it by accident is a microphone, which i think is
| unlikely to burn out at 5 volts (even if normally it's not
| subjected to more than 2.2 volts)
|
| 2. also, limit the current with a linear current source. this
| design simulates as having about 10O impedance over the
| 0-100mA range and a few kilohms once you exceed the current
| limit: http://tinyurl.com/23qvuylw
| frabert wrote:
| You could do what USB does and limit the current until the
| device signals it wants full power!
| geon wrote:
| > i want to use 3.5mm jacks for super simple data
| communications and power
|
| That sounds like an all around bad idea.
| icegreentea2 wrote:
| It's not the best idea, but it's not the worst idea either.
| The original Stripe reader used the 3.5mm jack for example.
| It's even been used in some power applications
| (https://www.amazon.ca/UY-CHAN-Original-Soldering-
| Replacement...)
| krallja wrote:
| Square*
| kragen wrote:
| it's like a ratburger: it's not good, but hey, it sure is
| cheap
| ahdsr wrote:
| ZSA uses this for their keyboards. The moonlander I'm typing
| this on uses a headphone jack for all of the above [1]
|
| [1] https://www.zsa.io/moonlander/
| em3rgent0rdr wrote:
| Apple's iPod Shuffle 2G did charging and USB syncing over its
| 3.5mm TRRS socket.
|
| And for people concerned about accidentally shorting a power
| line when inserting the plug, hypothetically could use a
| socket with built in switches so power is only sent once the
| plug is fully inserted into the socket.
| semi-extrinsic wrote:
| Why not just go with RJ11 (6P4C) instead? The PCB mount
| connectors are cheap, cables are easily available even from
| most local electronics stores, up to 10m lengths.
| shadowpho wrote:
| What's wrong with type c connectors? They are dirt cheap,
| psu/cable is already done for you, thoroughly tested, and easy
| to solder(6pin version)
| amelius wrote:
| What I want is an electrical equivalent circuit, that models not
| only the ohmic losses but also the reactance and the output
| power. For different kinds of headphones and speakers. And then a
| way to tune the parameters in that model to an actual device.
| em3rgent0rdr wrote:
| SPICE model ideally.
| screcth wrote:
| Why don't sources try to measure the frequency response of the
| driver + cable + headphone circuit and apply a digital
| prefiltering step to compensate these distortions?
| kazinator wrote:
| Because it has to be done individually for each headphone. It
| would require specialized equipment, like a super accurate
| reference microphone capable of accurately listening to a
| headphone speaker, in the same way as your ear.
|
| The best thing is for the speaker manufacturer to assume zero
| ohm output impedance from every amplifier, and for them to do
| their work to get their speaker to have a close to flat
| response. (Or whatever: some markets prefer a huge bass.) Then
| it has that designed response in any such amplifier.
| Kirby64 wrote:
| You don't generally want to compensate, as impedance that isn't
| flat doesn't necessarily mean that the output SPF isn't flat.
| kazinator wrote:
| This business of damping or lack thereof, is really just a
| frequency response change.
|
| The concern with the resonance of the speaker is overblown.
|
| When you boost a frequency range electronically with an
| equalizer, that is also resonance (electronic rather than
| mechanical).
|
| Instead of your speaker ringing for a couple of cycles, the
| circuit is doing it.
| boomskats wrote:
| For context, the NwAvGuy O2 amp[0] design is pretty famous now.
| The dude even lays it down right there[1] in the comments of the
| linked article. And then soon after that iirc he just disappears?
|
| [0]: https://nwavguy.blogspot.com/2011/07/o2-headphone-amp.html
| [1]: https://nwavguy.blogspot.com/2011/02/headphone-amp-
| impedance...
| dusted wrote:
| I find myself guilty of deep-diving into headfi-territory
| audiophilia once in a while, this time it got to me as well and I
| started thinking how neat it'd be to finally do it and get some
| high-end replacement for my 250 ohm dt990 pro cans, and I've
| heard both the hifman arya and sennheiser hd800s and know they
| are quite a lot better.. but THEN comes the amp question... and
| the rabbithole deepens.. and then the DAC and in the end I give
| up, every time.. this time too! I've also heard the o2 amp, and
| it's quite nice.. In the end, a year ago or so, I went a bit
| overboard and decided that since my subjective experience of even
| cheaper studio monitors have been on-par with even the most
| expensive headphones, I should go all-in on that account
| instead..
|
| I somehow managed to convince myself that a pair of Genelec 8350s
| would solve all my problems, so I replaced my Presonus Eris E5s
| with them, and replaced the scarlett 2i4 with a Yellow Tec Puc2
| lite, and got the calibration box and mic and the silly volume
| knob (so I could pour pure PCM at 0dB straight into the monitors
| DACs).. Of course, audiophilia is a sickness, and it never ends,
| so I've been looking at a rather pricey stereo-subwoofer setup to
| go along.. but fortunately, my room can't support that at the
| moment..
|
| I guess what I hate about gear, is what others love, that there's
| no one right, no one truth, and that subjective experience has to
| be taken into account.. I don't want subjective, I want the
| objective truth, accurate reproduction.. but what is even that..
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