Measuring and matching audio output impedance? - Page 1
Forum section: electronics / beginners

author: Peabody
author_rank: Super Contributor
timestamp: July 09, 2023, 05:09:43 pm
content: I do pretty well with digital stuff. Analog - not so much. So I have a new Roland digital piano, which in deference to my neighbors I will be playing mostly on headphones. After reading reviews I bought a pair of 7Hz Salnotes Zero earbuds, and tried them out. They were awful. No bass at all, and generally very tinny, thin and shrill. I have cheap earbuds that came with my Sandisk Clip MP3 player that sound much better. So I'm wondering why the Zeros sound so bad - are they just bad, or is it just a bad match for my Roland? Or is the Roland the problem?I've always heard that for best performance the output impedance of the amp should match the impedance of the speakers. But I thought that just affected overall gain level, not EQ. Anyway, I created a 440Hz sine wave .wav file with Audacity, and played it back on the piano, with my scope connected to the earphone output. And I found the resistance across the output that reduced the waveform by 50%. My understanding is that the output impedance is equal to that resistance. That resistance turned out to be 100 ohms.I can't find any information on the impedance of the Zero earbuds. Can I measure that the same way? If I can figure out a way to do the hookup, it seems that should work too.I'm a bit at a loss to know what to do about headphones or earbuds. How important is it to have an impedance match to have good equalization (i.e. - good bass response in this case)? I don't see any impedance spec for a lot of headphones, so maybe it's not really that important.Anyway, did I measure the output impedance the right way? Does 100 ohms sound kinda high? It did to me, but my reference is 8-ohm speakers, which may not be relevant.I think I also need to record regular piano output using the headphone jack as a line out, and play that back through my stereo system, just to be sure the Zero isn't accurately reproducing what the piano is outputting. Hard to believe that would be the case, but I need to be sure.

author: langwadt
author_rank: Super Contributor
timestamp: July 09, 2023, 05:18:59 pm
content: impedance for most headphones are ~32 Ohm100 Ohm impedance for a headphone output sound very high, I'd expect it to be much lower like a few Ohm, sure it is not a line out?

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2023, 05:42:56 pm
content: Some high end headphones have high impedance, and ideally they will require a headphones amplifier capable to drive headphones with more than the standard 32 ohms.Most often the specified ohms value in the datasheet is in fact the DC resistance. Impedance is a complex number (amplitude+phase) and varies with frequency, so it's not a single number like "32\$\Omega\$", should be a chart with phase and amplitude over the entire audio spectrum.Most often the quality difference perceived could be because the casual phones you use to have before were emphasizing the bass and treble, a marketing trick so the ear will hear the music sound "better".Could be a very bad audio amplifier as well. Try other jack output sources you might have around, PC/laptop soundcard, mobile phones, FM radio, etc. I doubt any consumer amplifier would go under 20Hz, let alone the advertised 7Hz of your headphones. My guess is most probably it is not the amplifier's fault, just that different headsets sound differently.Hard to say the exact cause if you don't have other known good reference to compare against. In minutes, the ear will accommodate with the new sound anyway, so it doesn't really matter.

author: langwadt
author_rank: Super Contributor
timestamp: July 09, 2023, 06:02:08 pm
content: Quote from: RoGeorge on July 09, 2023, 05:42:56 pmHigh impedance headphone should just be quiet with an amplifier that's only has voltage for low impedanceQuote from: RoGeorge on July 09, 2023, 05:42:56 pmI don't think the "7Hz" is anything but a name

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2023, 07:02:36 pm
content: Yes, though quieter also means low ear sensitivity for bass and treble, so it might sound "muffled" only because the volume is lower. That is why some audio amplifiers have "loudness" correction, where bass and treble are artificially emphasized the more the volume potentiometer is lower, so to compensate for the average ear perception.For a fair comparison the perceived volume should be adjusted the same for each headphones before comparing. For example, play at first only a 432Hz tone and note the volume needed to produce the same perceived audio. Then test with music, each headset at the volume you determined as being the same at 432Hz.(Side note, instruments tuned in 432Hz instead of 440Hz sound warmer and more pleasant to me. Search for passages played with 442 vs 440Hz tuned instruments on YouTube, to hear the difference, if any).In theory for higher impedance headphones should only differ the power, not the spectrum.To answer the questions about impedance matching, that is important only when one wants to achieve a maximum power transfer, for randament efficiency, or against reflections. For example, it matters a lot to match the impedance of a radio antenna (for maximum power transfer), and it matters a lot for impedance matching in a LAN cable (to minimize the reflections in the cable and thus the possible data errors caused by reflected signals). For audio, neither of these phenomena are relevant.Impedance matching does not matter in the audio output amplifier. Speakers are usually driven in voltage, and they are expected to have mostly resistive behavior (e.g. 32+i0for casual headphones), while the audio amplifier's output impedance is very small, close to an ideal voltage source with zero ohms impedance.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2023, 07:04:36 pm
content: Audio power amplifiers (including low-power amps for headphones) rarely "match" the output impedance to the load impedance.Instead, they are designed to drive a range of load impedances, with negative feedback reducing the output (source) impedance to a much lower value.In the case of headphones, you need to ensure that their impedance is not too low for safe operation of the amplifier.(WWII-era"dynamic" headphones for short-wave receivers were typically 2000\$\Omega\$, and the vacuum-tube amplifiers used with them would not be happy driving your 100\$\Omega\$ devices.)The "maximum power theorem" is often mis-applied to power amplifiers, which have finite power supplies.That result (load impedance = source impedance) would give you maximumpower gain, which is appropriate for matching an antenna to an amplifier input (for example).It does not give you the maximum useful power from a power amplifier, which is limited by power supply voltage and current capability.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2023, 07:07:40 pm
content: Quote from: RoGeorge on July 09, 2023, 07:02:36 pmYears ago, 435 Hz was common for A above middle-C. An article I read decades ago inScientific American, written by an elderly Russian whose perfect pitch had earned him a full-ride at the Imperial Conservatory in St Petersburg before the Bolsheviks, complained that recent trends to tuning up to 445 Hz made everything sound sharp to him.

author: bdunham7
author_rank: Super Contributor
timestamp: July 09, 2023, 07:08:11 pm
content: Quote from: Peabody on July 09, 2023, 05:09:43 pmNo, for quality audio the effective output impedance of the amplifier should be much, much lower than the minimum impedance of the driver, whether it be headphone or speaker. Decent audio amplifiers for 8R speakers will typically have an output impedance of less than 0.1R, of course YMMV and there are exceptions. Headphone amps should have output impedances as well, but 1-2 ohms is probably good enough for most headphones. Headphones commonly range from 16R to 600R, so again YMMV. 600R headphones require a higher drive level, but that doesn't have much to do with output impedance. The reason you need low output impedance is that the actual impedance of any driver varies wildly with signal frequency. Unless your amplifier output is very 'stiff' (doesn't vary with load) then the impedance changes of the driver will alter the overalll frequency response--and more.100R sounds like a regular line output, regardless of how it is labeled. If that's all it has, you need to get an appropriate headphone amplifier.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2023, 07:10:30 pm
content: I meant 432Hz, sorry for writing 442Hz. I'm not a musician and I've mixed the numbers.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2023, 07:14:46 pm
content: I'm not sure if the trend towards higher tuning pitch has continued.I often attend concerts of music from 5 or 6 centuries ago, with instruments that are reproductions of the historical units, and the musicians pay a lot of attention to tuning pitch and different temperaments (fitting 12 notes into an octave with different keys)."Equal temperament" (now ubiquitous for pianos with only 12 keys per octave) is an engineering approximation that sounds the same in all keys (equally bad, if you will) to allow Bach and successors to modulate (change key) into all the possible variations.From one source: "Some idea of the variance in pitches can be gained by examining old tuning forks, organ pipes and other sources. For example, an English pitchpipe from 1720 plays the A above middle C at 380 Hz, while the organs played by Johann Sebastian Bach in Hamburg, Leipzig and Weimar were pitched at A=480 Hz, a difference of around four semitones. In other words, the A produced by the 1720 pitchpipe would have been at the same frequency as the F on one of Bach’s organs." Also, the same source states: "My US Civil War era horns are high pitch (A=454) as are most of my pre 1900 instruments." He continues with a survey of old church organs: 380 to 480 Hz.See:https://capionlarsen.com/history-pitch/

author: magic
author_rank: Super Contributor
timestamp: July 09, 2023, 07:57:53 pm
content: Quote from: Peabody on July 09, 2023, 05:09:43 pmCorrect. You could try the same at 44Hz to see if output impedance increases due to AC coupling.As bdunham7 says, speakers are designed to be driven by low impedance and modern headphones too, although in the past there were some attempts to standardize 120Ω output impedance for headphone jacks. It is unclear if HP manufacturers ever gave a damn about it, maybe in the era of 600Ω. Modern battery powered gear must be low impedance to provide any useful voltage swing into heavy loads and hence things like IEMs must work right from such outputs to be market-viable.Again, bdunham7 explains that changing output impedance changes frequency response if impedance of the headphone varies with frequency. There are HPs where almost no such variation exists (these don't care much about impedance) and others where it varies a lot.

author: Peabody
author_rank: Super Contributor
timestamp: July 09, 2023, 08:14:22 pm
content: The piano is the Roland RP107. It doesn't have a line out, at least officially, but has two headphone outputs, one 1/4 inch and one 1/8 inch. The manual is very clear that these are both headphone jacks. See attached excerpt from the manual. I tried both, one with an adapter, and they sound the same.However, I did record some piano playing to my computer using a headphone jack as line out, and it sounds perfectly fine.But I think I've answered the question about the Zero earbuds. Using an adapter, I plugged into the Phones output of my receiver, and played some Chopin from CDs. I had the Zero in my right ear, and the MP3 player ear bud in the left ear, and I alternately plugged in one jack, then the other. The MP3 earbud sounded fine, but the Zero earbud sounded just like it did on the piano - very strong treble, non-existent bass, brassy, tinny, shrill. So the Zero is just no good.That still leaves the question of finding earbuds that will work with the piano so I can have my MP3 player back. I guess they are officially called IEMs now - In-Ear Monitors. I have big ears, and wear glasses, so I'm not sure I would find full-size headphones that would be comfortable for extended periods. Anyway, if my measurement of the output impedance is correct at 100 ohms, then I need to look for IEMs with higher than average impedance, and may also need to get a headphone amp.Thanks very much for the comments and explanations. I was clearly wrong about matching the two impedances being the right target.

author: tszaboo
author_rank: Super Contributor
timestamp: July 09, 2023, 08:32:47 pm
content: As others said it, you don't match it, you drive it with close to zero Ohm. Large output impedance is bad. Not only will it make the headphone quiet, it will make distortion. A headphone is not a perfect resistance, so you introduce a frequency and headphone dependent EQ. Look up the Nvavguy's blog, good read, he explains how to design good amplifiers. Though his part choices are sometime strange.

author: David_AVD
author_rank: Super Contributor
timestamp: July 09, 2023, 08:51:27 pm
content: The headphone output on devices that also normally drive speakers is often a feed from the speaker output, via a series resistor per channel. This is effectively a high impedance output.These resistors are most commonly in the range of a few hundred Ohms which means the headphone transducer becomes the lower part of a resistive divider.For low impedance headphones, the reduction is fairly large. High impedance headphones (600 Ohms for example) don't see as much reduction and still get enough level.A lot of modern hi-fi amplifiers have a dedicated headphone driver circuit with a low output impedance.In the end though, it seems like those earphones are just plain bad. Maybe try some better headphones such as AKG, but don't skimp. Good headphones cost real money.

author: bdunham7
author_rank: Super Contributor
timestamp: July 09, 2023, 09:00:06 pm
content: Quote from: Peabody on July 09, 2023, 08:14:22 pmTwo reasonably priced products you might look at are:AKG K240 Studio headphones:https://www.amazon.com/AKG-K240STUDIO-Semi-Open-Professional-Headphones/dp/B0001ARCFA/ref=asc_df_B0001ARCFAAny variety of NwAvGuy's O2 headphone amplifier--these are more widely available used nowadays, look for Massdrop or JDS versions or if you are ambitious, build your own.https://jdslabs.com/product/objective2-printed-circuit-board/

author: Peabody
author_rank: Super Contributor
timestamp: July 09, 2023, 09:31:16 pm
content: Quote from: bdunham7 on July 09, 2023, 09:00:06 pmThe problem I have with over-the-ear phones is that my ears from top to bottom are 70mm. The few headphones that provide this measurement for their products seem to be 60mm. And I wear glasses, which complicates things. So that's why I was looking at IEMs. But do you (or anybody) have AKG K240s that they could measure?

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2023, 09:34:13 pm
content: If there is a small capacitor in series with the output, scaled for a higher load impedance, then a lower load impedance will see a loss in low-frequency (bass) signal.

author: John B
author_rank: Frequent Contributor
timestamp: July 09, 2023, 09:51:14 pm
content: ~100R output impedance is fairly typical for driving headphones. Generally you would prefer a low output impedance if driving speakers, but headphones are a special case. The reason is headphones come in a wide range of impedances from 32 to 600 ohms, and you have to account that anything in that range could be plugged in.If the output impedance is low, and the output voltage is sufficient for a 600 ohm headphone, it will probably blow a set of 32 ohm ones - and maybe your eardrums too. It doesn't take much power at all to create very dangerous sound levels when the speakers are in your ear!The output resistance sort-of evens out of the response for varying headphones.As for recommendations, I would go for actual studio headphones, not earbuds. Beyerdynamic makes some good ones which can be had a reasonable price. I have a bunch of DT770s, though there are many good options at a reasonable price. There's also a wide selection of unreasonable prices too when it comes to headphones.

author: bdunham7
author_rank: Super Contributor
timestamp: July 09, 2023, 09:53:06 pm
content: Quote from: Peabody on July 09, 2023, 09:31:16 pmI also wear glasses and my ears are 80mm from the bottom of the earlobe to the top (which I never knew before today--and I don't think of them as all that big, mobody calls me Dumbo or anything like that... ) The openings in the K240s are only ~60mm, but the way the cushion is attached there is space between the cushion and the driver/screen and the inside point at which the cushion attaches gives you enough room, probably 90-100mm, for your ears to slip inside. These are the only headphones I can wear for hours, which is why I recommended them. They are open-back, so they don't need to fit tightly at all and they don't. They dont squish my glasses into my face either. Of course, YMMV.

author: David_AVD
author_rank: Super Contributor
timestamp: July 09, 2023, 10:55:36 pm
content: I've used a variety of closed and open back "over the ear" headphones over the last 40 years and in my experience the open back have always sounded better. Not as good for isolating your listening from people close to you of course. I have use the AKG K240 and liked them. Be aware that some AKG models are available in two different versions - low and high impedance.

author: bdunham7
author_rank: Super Contributor
timestamp: July 09, 2023, 11:58:14 pm
content: Quote from: David_AVD on July 09, 2023, 10:55:36 pmIIRC, the old model was 600R and the newer one (what I have) is 55R, so if buying used you need to watch for that--the 600R will definitely need a powerful amplifier. I think the newer version is regarded as a bit cheaped-out over the old, but I like them. People who are used to the exaggerated booming bass of Beats or other mass-market models may think they are bass-shy, but there's a reason they are called 'Studio'.

author: David_AVD
author_rank: Super Contributor
timestamp: July 10, 2023, 02:26:15 am
content: I just checked and the ones I use at work for testing the headphone output on amplifiers are the K270S. They are 55 Ohm, semi-open back.

author: Ray CI
author_rank: Contributor
timestamp: July 10, 2023, 03:11:14 am
content: Here are the specs to your earbuds:https://www.headphonesty.com/2022/09/review-7hz-salnotes-zero/In general, if you measure with an Ohm meter from Shield to either Tip or Ring, that will give you an approximate "per-channel" impedance. If you measure from Tip to Ring that will give you the combined impedance of both drivers. Since your buds have a detachable cord, just measure at the socket.BTW: The same way most processed food contains too much salt, transmitted audio contains too much bass. Most people now expect music to have a lot of bass. Genuine studio recording headphones (and studio equipment) have an intentionally "uncolored" response and do not over-emphasize bass the same way most consumer-level equipment does. This is a carry-over from days of vinyl because, very low frequencies do not render well on vinyl -especially on the inner tracks closest to the spindle hole.Anyhow, headphone and earbud sound quality is highly subjective. Try a pair of cheap logitech "computer headphones". They usually rig those to have a very rich bass response.RegardsRay

author: Ray CI
author_rank: Contributor
timestamp: July 10, 2023, 03:21:42 am
content: Other trivia:Headphones impedances vary considerably and commonly stated resistances sometimes represent a sum of both channels or, just a single channel. Also, the resistance is actually a reactance and varies considerably across the range from 20 to 20kHz. A pair of "64" Ohm headphones will likely have an actual DC resistance of 56 Ohms per-speaker (always called a driver in headphone language) with a combined resistance of 112 Ohms. Due to "Marketing Jargon", some brands will say these are 64 Ohm and other brands will advertise them as 128 Ohm headphones. Oddly enough, the RIAA and other Standards groups specify that the stated resistance of a speaker/driver should be the lowest value when sweeping from 20-20kHz. This is rarely the actual case.Typical "pro-audio" headphones are usually (about) 150 or 300 Ohms per channel and are typically advertised as being 300 or 600 Ohm headphones. When headphones first became popular in the hi-fi world, the amplifiers would simply attenuate the normal speaker outputs with various resistor values. ... You guessed it, with 57, 220 and 330 Ohm resistors. The higher resistance values also makes it easier to produce a final stage LPF without needing very large value caps.High end dedicated headphone amps will have selectable output impedances to match the typical pro-audio impedance levels. For Class A and AB headphone amps, a damping factor (ratio between amp output impedance and headphone load impedance) between 3 and 10 is considered desirable.I'm building a pure class A amp right now to work with "64" and "150" Ohm drivers and am shooting for a DF of 4. A single channel output stage will need to be selectable to have a driving impedance of 16 and 37.5 Ohms.On a Roland digital keyboard, I would fully expect they are using a pre-canned headphone amplifier chip with an extremely low output impedance. You can measure the output with an oscilloscope by playing a note of around 1kHz and applying a 220 Ohm fixed resistor to one of the channels.First measure the unloaded RMS output voltage at 1kHz then, apply the 220 Ohm resistor and measure again. Zout will be 220 X [(Vhigh/Vlow) - 1). (You can use any resistor in the range of 64 to 330 Ohms).RegardsRay

author: Peabody
author_rank: Super Contributor
timestamp: July 10, 2023, 04:35:57 am
content: The AKG K240 Studios are 55 ohms impedance. How are they going to work with the 100 ohm output impedance of my Roland? I really would like to avoid having to buy a headphone amp if possible.

author: jonpaul
author_rank: Super Contributor
timestamp: July 10, 2023, 05:28:32 am
content: see Audio Cyclopedia to learn about impedance in Audio, headset driving etc.See SYN AUD CON for more tech info.Jon

author: Ray CI
author_rank: Contributor
timestamp: July 10, 2023, 11:35:57 am
content: Quote from: Peabody on July 10, 2023, 04:35:57 amRather than hit-or-miss suggestions, you might want to contact Roland support or check with their user group for suggestions about headphone recommendations.https://www.roland.com/us/support/I don't have much knowledge about digital keyboards but, I highly suspect the output impedance of the Roland is very low (probably well under 2 Ohms and possibly as low as 0.2 Ohms). Just to get a baseline of data, try some other headphones for comparison. When all is said and done, you might end-up needing to patch the Roland output to an equalizer (or PC with equalizer software) and then connect your earbuds to the PC. Don't run-out and buy a headphone amp until you know what problem you're solving.Ray

author: RoGeorge
author_rank: Super Contributor
timestamp: July 10, 2023, 12:04:17 pm
content: Quote from: Ray CI on July 10, 2023, 03:11:14 amFrom the link:"The Zero sport a neutral bright tuning, which is relatively uncommon in the ultra-budget segment. Most of their rivals are tuned V-shaped or to the Harman curve."which translates to me as "this is how the 7Hz earbuds are supposed to sound, flat, without fake bass/treble hyphened", like other headsets usually do. It is nothing wrong with the impedance or the amplifier.

author: Peabody
author_rank: Super Contributor
timestamp: July 10, 2023, 02:02:30 pm
content: Quote from: RoGeorge on July 10, 2023, 12:04:17 pmAs I decribed earlier, I recorded piano output to my computer using the headphone output as a line out, and it sounded perfect when listening on my stereo system or my MP3 player earbuds. So if the Zeros sound awful on the same output, I think they are the problem. They (at least my copy) are not at all flat. In any case, they aren't a good match for the piano.

author: switchabl
author_rank: Frequent Contributor
timestamp: July 10, 2023, 05:02:19 pm
content: It is possible that the piano doesn't have a dedicated headphone amplifier and just uses a simple resistor (or resistive divider) to derive the headphone output from the main power amp. Especially if it is an entry-level model. This is also often done on power amps/receivers, so these sometimes have output impedances >100 ohms on the headphone output as well. This is a very cheap option and provides some protection for both the amp and the headphones. But for 32 ohms headphones, like most in-ears, it can change the frequency response significantly (a variation of >10dB is possible in extreme cases, I don't know about your specific model).If you want to make sure, try the headphones with your PC, phone or MP3 player. Those are almost certain to have very low impedance outputs. For in-ears, fit also matters a lot. If they don't seal properly, bass response will be degraded. Then again, maybe they are just bad, broken or don't suit your taste.Also keep in mind that, unlike with speakers, there is no common, objective definition of what constitutes a "neutral" headphone. The sound from a speaker is strongly affected by your head/torso and ear shape before it enters your ear canal. A headphone has to mimic this effect to sound "right" but unfortunately the correct transfer function is not the same for everyone. So, even before accounting for taste, a headphone that works for someone else may not work for you.

author: radiolistener
author_rank: Super Contributor
timestamp: July 10, 2023, 07:22:30 pm
content: Quote from: Peabody on July 09, 2023, 05:09:43 pmAt a glance yes - your measurement seems correct, but your circuit output can have not linear properties for very high impedance it may affect your impedance measurements, because open amplitude may be not twice higher.To check if your output really has 100 Ω impedance, try to measure amplitude for two known resistance near 100 Ω.For example, you can measure amplitude for R1=50 Ω resistor load and for R2=200 Ω resistor load.Then use the following equation to get output impedance:Z = R1*R2*(U1-U2) / (R1*U2-R2*U1)where U1 is amplitude at R1 and U2 is amplitude at R2 load. This way to measure output impedance is more reliable, because allows to measure output close to it's working load impedance to avoid non linear effects with very high impedance load for open output.You can use peak or RMS voltage measurement, it doesn't matters, just use the same voltage measurement type for both resistors.For audio frequency wave length is very large, so you can ignore wave reflections and ringing in the wires, because that effect will be too small. This is correct if you use short wires, about 2-3 meters of course. But impedance mismatch will affect amplitude and too low or too high impedance may lead to unexpected distortions because output amplifier cannot work in linear mode with any load.Audio amplifiers often is designed for very low impedance in order to support different impedance load, include very low one. But if your output has 100 Ω impedance it may lead to signal distortion with low impedance head phones which usually have about 32 Ω. In any case such low impedance headphones will have too weak signal level with such 100 Ω output.In your case it's better to use high impedance headphones which have >= 100 Ω impdeance in order to avoid distortions, for example you can use 250 Ω headphones.In order to use 8 Ω heads with 100 Ω output you're needs to add power amplifier between your 100 Ω output and 8 Ω heads, and this power amplifier output impedance needs to have 8 Ω impedance or lower than that.

author: bdunham7
author_rank: Super Contributor
timestamp: July 10, 2023, 07:48:22 pm
content: Quote from: Peabody on July 10, 2023, 04:35:57 amThat's not an ideal setup to be sure, and I think they'd be better with a low-impedance amp. That said, the K240 seems to be pretty tame and mine sound decent plugged into anything, as far as I remember at least. IEMs are notorious for wildly varying impedances and uneven frequency response when used with high-Z amplifiers. I can't predict how happy you would be with just the K240s plugged directly into the piano--that's going to be totally subjective. My own sense is that people that listen to live recordings and such will probably appreciate the K240s more than someone that listens to Moby or Daft Punk. But again, that's quite subjective.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 10, 2023, 07:52:08 pm
content: Since you are in the USI have picked up very expensive stereo headphones rather cheaply at my local supply:GOODWILLMaybe 5-10 bucks

author: switchabl
author_rank: Frequent Contributor
timestamp: July 10, 2023, 07:54:39 pm
content: Quote from: radiolistener on July 10, 2023, 07:22:30 pmThis shouldn't normally be the case. Low impedance headphones usually have high (voltage-)sensitivity. Also, as mentioned above, high impedance headphone outputs are often derived from speaker power amps, in which case they should have ample output level.

author: radiolistener
author_rank: Super Contributor
timestamp: July 10, 2023, 09:35:33 pm
content: Quote from: switchabl on July 10, 2023, 07:54:39 pmLet's get 1 Vpk @ 100 Ω source connected to a matched 100 Ω headphones, the power pushed into headphones = 6.9897 dBm.If you connect 32 Ω headphones to 1 Vpk @ 100 Ω source, the amplitude on headphones will be 0.485 Vpk @ 32 Ω = 5.653 dBm.So, the power pushed into headphones will be 6.9897 - 5.653 = 1.3367 dB lower. It means that sound loudness will be lower, because you push 2 dB less power energy into 32 Ω headphones in comparison with 100 Ω headphones.2 dB is not a big deal, but there is also another issue, since output amplifier is not designed for so low impedance load it may work outside of it's linear region of curve, and it may lead to a non linear distortions... As result you can hear distorted sound with intermodulation products which doesn't exists in a sound source.

author: bdunham7
author_rank: Super Contributor
timestamp: July 10, 2023, 10:09:57 pm
content: Quote from: radiolistener on July 10, 2023, 09:35:33 pmIn this specific case with those specific numbers, that would be a really terrible audio amplifier. Not that they don't exist, but I'd be pretty disappointed to find one that bad attached to a Roland piano. If it were a line output only, perhaps, since those expect 10K or so as a load.

author: Peabody
author_rank: Super Contributor
timestamp: July 10, 2023, 10:37:29 pm
content: So if I understand correctly, having the output impedance significantly higher than the phones' impedance means that there might not be enough power to drive the phones, particularly for something like the K240 which is pretty inefficient (I think they call it "sensitivity"). Moreover, that difference could also modify the frequency response such that you might particularly lose the bass.I don't know why Roland chose to do it this way, but it is complicating the choice of headphones. I only need the headphones so I can play at all hours without disturbing anyone, but I want them to sound good, and not tire me out listening to them.I've already checked with Guitar Center, and they don't have my piano on the floor. So there's no way I can go over there and see which phones work and which don't. And I don't want to wear out my welcome at Amazon by buying and returning a series of phones until I find one that works. I did find a guy on the PianoWorld forum who has my piano, and he strongly recommended the 7Hz Zeros, which he says sound great. These are the ones that sounded terrible to me, not just on the piano, but on my receiver and my computer, and that I returned this morning.I guess the only dependable solution is to build a heaphone amp. I undertand there's one that uses a single stage opamp per channel, and which fits inside an Altoids tin. But I'd really like to avoid that, but I'm not sure how.

author: bdunham7
author_rank: Super Contributor
timestamp: July 10, 2023, 10:43:23 pm
content: Quote from: Peabody on July 10, 2023, 10:37:29 pmIf you aren't that ambitious, you can buy them used/open-box/etc. This guy has some new EU-versions (you'll need to source a 12VAC wall wart) for only $50, you can't build it properly for that.https://www.ebay.com/itm/266320162847Your current IEMs might even sound good with it.

author: David_AVD
author_rank: Super Contributor
timestamp: July 10, 2023, 10:46:27 pm
content: My experience has been that a good pair of headphones sound good no matter what you plug them into.I've used my 55 Ohm K270 on a huge variety of sources; computers, mp3 players, hi-fi amps with dedicated HP driver, hi-fi amps with 330R series resistor feeds from the main amp output, keyboards, etc.Not once have I thought they sounded bad or low in level.Your first step is just buy a decent set of headphones. Then worry about any issues the Roland keyboard may have driving them. I'm guessing there will be none.

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2023, 12:18:08 am
content: Quote from: David_AVD on July 10, 2023, 10:46:27 pmI wouldn't say 'bad', but my K240s sound noticeably different with a low-impedance amp. I think it's better. The receivers and such that use the series resistor can usually produce plenty of power, so that hasn't been an issue either.

author: John B
author_rank: Frequent Contributor
timestamp: July 11, 2023, 12:31:32 am
content: Quote from: Peabody on July 10, 2023, 10:37:29 pmYou're overthinking this. You're driving headphones, not a broadcast tower.I use exclusively 250 ohm headphones and from memory only one of my devices was unable to sufficiently drive them to a loud enough volume for me. Most studio headphones will come with a couple of different drivers, ie 250 or 80 ohm. If you're concerned about volume try the 80 ohm versions.And yes, if you're wanting a project, a handful of NE5532 op amps configured as paralleled drivers will give you a studio grade headphone amp.

author: Peabody
author_rank: Super Contributor
timestamp: July 11, 2023, 04:57:43 am
content: Yes, I guess I'm overthinking it. Perhaps I should just order the K240 phones and be done with it. And if it isn't loud enough, it looks like one of these modules could be the amp:https://www.amazon.com/gp/product/B08D7KCNX9/It looks like a low-noise dual rail power supply would be the hard and expensive part.Anyway, as I understand it, if I do an amp, I don't need any voltage gain. I just need the low impedance drive.

author: jasonRF
author_rank: Regular Contributor
timestamp: July 11, 2023, 05:47:10 am
content: Quote from: Peabody on July 11, 2023, 04:57:43 amI agree that it makes sense to get the headphones you really want, then try them to see if you like the way they sound through the piano headphone jack. That is the only way to know. I also agree that if it isn't loud enough then you may indeed only need a unity-gain buffer since the kh240 are 55 Ohms and have a sensitivity of 104 dB/V. I typically listen < 80 dB, so even for loud dynamic peaks 1V would be plenty for me with those phones. A simple amp or buffer can be easily built using a suitable opamp (eg njm4556) and a few resistors and capacitors. You might even be able to just use a single rechargeable 9V battery to power it if you use a single-supply design and an output capacitor; the output cap also protects the headphones in case there is extra DC on the amp output. Or you could justbuyan amp.jason

author: John B
author_rank: Frequent Contributor
timestamp: July 11, 2023, 10:46:38 am
content: Quote from: Peabody on July 11, 2023, 04:57:43 amThat would be a design decision depending on whether you want to be able to drive both high and low impedance headphones. I think the shift towards low impedance headphones is due to the use of portable devices with batteries that give you very little voltage swing to utilise, but if you make a mains powered device with a transformer you can easily generate a clean +/-10V supply. In that type of device you would add an output resistance in the region of 100-120 ohm, probably with some voltage gain. As I mentioned this evens the response between high and low Z headphones, and adds a little safety margin when you forget to turn the device before plugging in your low Z phones.

author: switchabl
author_rank: Frequent Contributor
timestamp: July 11, 2023, 11:56:48 am
content: Quote from: John B on July 11, 2023, 10:46:38 amThis seems to be what the OP already has. And IMHO for a dedicated headphone amp it is a bad idea.(Almost?) all modern heaphones, no matter if they are 16 ohms or 600 ohms, are designed to be voltage driven, from a low-impedance source.If you want to build a universal headphone amp, aim for something like <1 ohms.There are a lot of audio-related myths and the idea that you should somehow match the impedance of the amp to that of the headphones is unfortunately a particulary persistent one. Therearesome reasons why you would add an output resistor:- cost-cuttting: if you already have a speaker amp, a high impedance headphone output is very, very cheap to add, without having a dedicated headphone amp circuit- some protection against accidentally turning the level too high on sensitive headphones (and cheap short-circuit protection if you the amp doesn't have it already); an alternative can be to have a low gain and a high gain mode (with a switch at the back/bottom)- "audiophile" amplifiers that intentionally change the soundBut it is a compromise that results in a frequency response different from the intended one (may be acceptable or not depending on headphone model and taste).It also wastes power. This sometimes seems to confuse people. If you already have a 100 ohm source, then a 100 ohm load will indeed extract maximum power from it. But if you have a 100 ohm load, a voltage source will drive that perfectly fine, adding 100 ohm output resistor to "match" it reduces the power delivered to 1/4 (and burns another 1/4 in the resistor). [Yes, for high frequencies you often need to match impedance to prevent reflections but not for audio.]

author: Peabody
author_rank: Super Contributor
timestamp: July 11, 2023, 04:55:15 pm
content: Yes, I think I already have a 100-ohm output impedance amp with volume control, powered in this case from 12VDC. If that's loud enough with the chosen headphones, and sounds pretty normal EQ-wise, then that should be enough. But if not, then I think impedance reduction would be the goal. I guess it's technically possible that reducing the impedance to 8 ohms or whatever without increasing voltage still wouldn't be loud enough, but I think that's unlikely. And if that happens, I could of course add some gain.Based on the discussion here and elsewhere, it seems that the lower the drive impedance the better your chances of getting the "true" frequency response of the phones, along with better damping, etc. But then why would you switch to a higher impedance to drive, say, 250-ohm phones? In other words, why do some of these headphone amps have switches for low and high output impedance? Why wouldn't 'low' work for everything?Edit: That module uses the TI TPA6120A2 chip, which sure looks good. And I think two rechargeable 9V batteries would be the way to power it. I'm just not sure how to do ground. Ground could be the junction of the two batteries, or it could be divided down from the rails. Not sure about that.

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2023, 05:06:26 pm
content: Quote from: Peabody on July 11, 2023, 04:55:15 pmThis is a very good question. IMO the correct answer is that low impedance does work for everything. However, anytime there is an audible difference between two configurations, there will always be some that prefer the technically worse choice. So if high impedance sounds different, someone will like it that way.http://nwavguy.blogspot.com/2011/02/headphone-amp-impedance.html

author: donlisms
author_rank: Frequent Contributor
timestamp: July 11, 2023, 05:20:58 pm
content: Doesn't it seem that the impedances of the source would combine with the impedances of the input to form a filter? For sure, there are inductances (it's a coil, for goodness sake!) as well as well as dc resistances in the phones, and resistance and quite probably noticeable capacitance at the output of the amp. I don't know the specifics; it just seems like they'd all form a reactive, somewhat practically unpredictable mess!Similarly, why the same phones will sound different in different amps -- it's not just power; the qualitative differences would be in the frequency domain... the reactive bits - capacitance and inductance and filters, oh my!Or so it seems to me.In a practical sense, obvious changes in audio tend to be powers of ten; more subtle changes powers of 2. So going from 50 to 100 anything's - no big deal. 5 to 50... now we're talking about obvious. Whether that's ohms or amps or Hz or dB or dollars or euros... it just seems to work that way.Also practically, in my lifetime of (mostly live) sound systems, I've abused headphone outputs and headphones with whatever connections and combinations I needed in the moment, and EVERY SINGLE ONE worked out fine!Well, mostly. It's forgiving, especially if we're looking for practical solutions rather than audiophoolery.Those zeros seemed wrong, or wrong for your ears, or something. I predict everything's going to be fine with the path you're on.

author: radiolistener
author_rank: Super Contributor
timestamp: July 11, 2023, 06:08:25 pm
content: Quote from: Peabody on July 11, 2023, 04:55:15 pmin order to use low impedance headphones you're needs high quality amplifier which supports very low impedance load with no distortions. Cheap electronics doesn't have it due to component economy reason. So, there is balance between cheap and quality electronics which doesn't allows to add high quality amplifier with powerful low impedance output into every device.If you overload amplifier with low impedance load it can leads to a non-linear distortions in amplifier. It can produce non linear intermodulation products and you will hear clicking, crackling, buzzing and other sound distortion effects. Also it can affect frequency response.

author: Yuu
author_rank: Regular Contributor
timestamp: July 11, 2023, 09:12:45 pm
content: Quote from: radiolistener on July 10, 2023, 09:35:33 pmSure you are supplying less electrical power but how much is converted into acoustic power? I think that's the point switchabl is making. That although the impedance is lower, you usually have higher sensitivity (which I'm assuming is directly related to efficiency of electrical to acoustic power conversion).

author: switchabl
author_rank: Frequent Contributor
timestamp: July 11, 2023, 09:46:35 pm
content: Quote from: bdunham7 on July 11, 2023, 05:06:26 pmThis, basically (btw, that blog has a lot of good info on audio design).Also, once upon a time... when amplifiers all used vacuum tubes, it was hard to get low output impedance. So, in order to optimize power transfer, it was actually common to have matched audio connections (input impedance = output impedance = 600 ohm). Headphones had 600 ohm impedance as well and could just be plugged into a normal line output. Those systems have been obsolete for a long, long time. But I guess it is still at the back of people's minds, so you always get someone saying "aren't headphones supposed to be impedance matched?".

author: TimFox
author_rank: Super Contributor
timestamp: July 11, 2023, 09:50:46 pm
content: A vacuum-tube amplifier transformer coupled to a loudspeaker or medium-impedance headphone is designed to drive that impedance, but its actualsource impedancedoes not equal the optimal load impedance.Audio-generator test equipment, in order to have a defined impedance, usually has a physical resistor after a low-impedance output (usually into a matched attenuator) to have a 600 ohm output impedance.Such equipment can be used to drive a high impedance or matched 600 ohm impedance, or even a lower impedance, where the known 600 ohm output impedance allows you to calculate the voltage across the load.Equipment designed to drive very long twisted-pair audio transmission lines (like telephone trunk lines) often did the same thing in order to double-terminate the transmission line.

author: radiolistener
author_rank: Super Contributor
timestamp: July 12, 2023, 10:17:14 am
content: Quote from: Yuu on July 11, 2023, 09:12:45 pmPower consumed by headphones is transformed into energy of acoustic waves. In other words - more power consumed by headphones lead to more powerful sound. Of course, some fraction of power will be loss for heating, but that part is relative to a total power and that fraction will be about the same for both - matched and non matched connection (on a high frequency with a small wavelength it may be different due to HF energy accumulation and amplitude grow). Since headphones with matched connection consume higher power, they put more energy into acoustic waves.It means that headphones connected with impedance match is more sensitive and produce more loud sound than one connected with impedance mismatch.The reason why low impedance headphones are named "more sensitive" is because their impedance is better matched with low impedance output of amplifiers. But if their impedance will be too low for amplifier (much smaller than amplifier impedance), they will loss their sensitivity the same as high impedance headphones...This is because the best power transfer efficiency happens when power source and power load impedances are the same. So the best headphones sensitivity happens when their impedance is equal to amplifier impedance.But since headphone power is too small, their sensitivity is not critical parameter, because you can easily set up higher sound volume on your amplifier in order to compensate headphones sensitivity. It will be critical for a high power speakers, because if you attach high impedance speakers to a low impedance amplifier (or low impedance speakers to a high impedance amplifier), it will lead to a high power loss and it may even damage your amplifier.

author: Peabody
author_rank: Super Contributor
timestamp: July 12, 2023, 08:07:13 pm
content: I spent some time at Guitar Center trying out their headphones. The only ones that were big enough for my ears were the Sennheiser HD280s, but they pressed too hard against my head, and were quite uncomfortable. The AKG K240s were nowhere near big enough. Well, better to find out that way than after buying them.So it looks like I'll be using some kind of in-ear monitors. I'm still a bit perplexed by my experience with the 7Hz Zeros. I watched all the reviews of them again, and what I heard was nothing like what they described, or what the graphs showed. And that wasn't just on the piano. I also tested them on my computer, my MP3 player, and my stereo system, and they sounded like Rice Krispies on all sources, whereas my cheap MP3 player earbuds sounded ok on all. But if something was defective, how would that affect both sides? So it's a mystery.I did order the headphone amp module from Amazon. It was only $5.50, and could be fun to play with even if I don't end up needing it.

author: Yuu
author_rank: Regular Contributor
timestamp: July 12, 2023, 09:08:02 pm
content: Quote from: radiolistener on July 12, 2023, 10:17:14 amHmmm, are you sure? If I look up headphone sensitivity, the definition is something like "decibels of sound pressure level per milliwatt." This sounds to me like a useful characterization of transducer efficiency and not a marketing gimmick. If it was a marketing gimmick then all that would matter to characterize the headphones would be headphone impedance as a function of frequency. Maybe it's the case sensitivity ends up being somewhat an irrelevant factor compared to impedance matching? Just curious