For years I've been mad when reading stuff like "this device is warm", or "those 2 amps really sound very different"... because I knew that half the time the guy was making a claim based on poor testing and even poorer understanding of what was going on. For this demo/advice/critic, I used only 2 IEMs and the sources I could directly plug into my laptop so I could get done with it rapidly, but obviously this applies to DAPs(digital audio players) and anything with an amplifier output. Here is an example with my laptop's headphone out, Odac/O2, Scarlett 2I2, and a portable DAC/amp, the UHA760. I measured them all at the same voltage straight into the input of the Scarlett(10kohm input). And as you can see even without clicking on the pic to expend it, they're all flat. I even pushed the evilness and used asio once, 44.1 one time, 48khz for others, different USB inputs(usb 3 or usb 2). Yet all we can really see is that one of them is going down just before 20khz( the one I set to 44.1khz). Else, super flat, super good! You see how meaningless those unloaded specs can be when it comes to guessing how DAPs or amps will sound with your headphone. The reason is simple, an unloaded measurement shows how the device behaves when it's not plugged into a headphone... Yup it's that silly. Once you plug your headphone into a device, the signature measured and what you're hearing can very much be different(and will be). Unloaded frequency response is given because it looks nice(marketing) and is easy to do, not because it's helpful for the consumer. It really doesn't help the consumer. If anything it makes many people think that measurements are wrong and pushes them to distrust graphs even more. That's a really sad side effect of having irrelevant graphs and people who don't know what they're looking at. "My DAP XXXX has a warm signature", "all amps sound different", and other such nonsense. One obvious possibility is that the specific headphone I'm using to listen to different gears is in part causing the differences I'm hearing. This is really at the heart of this post. I won't touch all the placebo, bias, and psychological stuff now. Real sound differences heard correctly are today's only cause for misleading conclusions. Someone will think: "I'm using the same headphone, so any difference I'm hearing must come from the source". So if that DAP sounds warmer, then it's warmer. Which seems intuitive. But it is a fallacy. Audio gears aren't Lego! They're electrical circuits and the headphone becomes part of the total circuit. meaning the amp specs are now interacting with the headphone/IEM. Here I'm showing the actual variations in signature as measured right out of a pair of JH13 IEMs with a microphone, all when using the different "super flat" sources from above: Oh boy! Already it's pretty clear that the unloaded graph from before was not helping in any way to guess the final signature I'm hearing out of my JH13. it's not that measurements don't "work". This one tells it like it is. It's just that if you measure the wrong stuff, that's what you get. Wrong stuff. ^_^ So what could be the cause of those changes in signature from device to device with the same IEM, given how they were apparently flat on their own? I measured the UHA760 somewhere around 0.4ohm. I get the O2 around 0.6ohm The headphone out of the scarlett 2I2 is close to 10ohm And my laptop's headphone out is in the area of 60ohm. could there be a correlation between my JH13 sounding brighter and the source's impedance. Indeed there is: This is the impedance curve of my JH13. it doesn't take an electrical engineer to notice the similarities between the frequency response variations I measured, and the shape of that impedance curve. For those who don't know, headphones and IEMs aren't perfect resistors so they can often show such fluctuations in impedance depending on the signal we send into them. The wildest curves are usually the result of using crossovers with multidriver IEMs or speakers. But just one driver can also show fluctuations over frequencies. Now let's go back to me making a review or a post on headfi and discussing the sound of gears. If I knew none of this, I would come and say that the UHA760 is a super warm device, and that my laptop sounds cold. Because of course it's very noticeable when I listen to music myself with the JH13. But this is true only when I'm using the JH13. So anytime I'm making claims about the signature of the UHA760 without being specific about how it is the sound I heard while using JH13 IEMs, I'm a fool misguiding the community with a false claim. It's hard to blame the ignorant, one cannot know what he doesn't know. But some make it easy for me by acting like they're audio experts while actually being absolute Jon Snows. The easy fix when you guys share your experience, is first to always mention all the elements in the playback chain you used to get your impressions. It's not hard and it changes everything. So please do it. Second is to try your best to make it clear that they are your impressions, as in what you felt instead of what factually is. That's a mighty important point too, because it makes the difference between sharing your honest impressions, or being a liar about the objective nature of a DAP or amplifier. When writing a post, the line is very thin and I'm sure I've crossed it myself many times. But let us all try to be more honest by describing impressions, as impressions. "I feel like", "what I'm hearing is", "it seemed to me that".... As long as you're sincere, you're telling the truth with such sentences.because you're telling how you feel. But when writing, "the DAP is warm", "that amp has rolled off bass", and other objective claims presented out of your very specific listening context, you're now making false claims. Here is a different example, in case the correlation with the IEM wasn't obvious enough. Now I'm using a pair of Shure SE215 with the same sources: We go from a signature shift of almost 8dB with the JH13 between my laptop and the UHA760, to less than 0.5dB variations here while using the SE215 with the same sources. I'd call that different ^_^. So imagine a second reviewer with his SE215 who's going to argue that all those sources sound pretty much the same. What happens when he meets me and my JH13? Noise between 2 wrong people is what usually happens. And because neither knows about all this, they can only rely on how they really felt a certain way and conclude that the other guy is wrong. When we have too little information, we still draw conclusions, it's in our nature. But our nature leads to a good deal of stupid stuff. And by now you should be able to guess why the SE215 doesn't show much variation. The impedance of the SE215 is almost completely flat: So depending on the IEM or headphone used, different amps and DAPs might give you different impressions. Call that synergy if you don't care to understand the electrical implications, but remember that it is real and that your anecdotal experience of a device does not define the device under all circumstances. Remember it next time you want to claim that you know how some device sounds and how that other guy must be wrong because he experienced something different with a different playback chain. again here I'm not talking about personal taste or differences in our hearing abilities which are obviously relevant but a story for another time. I'm only talking about people who have the right impressions about the sound but come to false conclusions anyway. basically because of impedance and the habit to turn anecdotal experience into rules. If I had something like a Shure SE846 with me, I could show the reverse effect compared to the JH13. With the 846 my laptop would have sounded super bassy, and my UHA760 would have sounded the "coldest". From a practical angle we don't always have the impedance curve of the IEM/headphone we use, so that can make it difficult to predict the sound you'll get with a given source. But always keep in mind that if you use IEMs and headphones with an impedance much bigger than that of your amp or DAP, then the signature changes will stay small(less than 1dB if at any frequency the ratio is at least 1/8). To make reviews it is a great idea to procure an IEM/headphone with a flat impedance curve and ideally not too low impedance. That way you can expect most DAPs and amps to sound the way they should when used correctly(cf: impedance bridging). That's one easy way to improve the accuracy of a review, by testing the gear under nominal conditions. Having other IEMs with extreme electrical specs can be of value too, very sensitive to notice background noise, crazy impedance to notice sources with high impedance even if you don't know how to measure that(like if the bass is a little weak on my JH13 I know the source is above 1ohm. extreme gears can be very helpful. just not as the main source of impressions. Because then you describe an exception as if if was the norm. Also some device might have protection caps at the output, in that case, a low impedance IEM will roll off the low end, and the lower the IEM's impedance the bigger the roll off. so that can be another way to get technical information from a subjective impression with the right IEM. Otherwise, low impedance sources and not too low impedance IEM/headphone are what you should aim for. The "at least 1/8" damping ratio rule of thumb is your friend most of the time. And can really help a review hit closer to home. Of course I focused this post on the amplifier section, but it's just as true when you decide to tell people how a multidriver IEM sounds but forget to mention that you're using some weirdo cable into the high impedance balanced output of some fancy DAP. The electrical interaction obviously goes both ways. and of course because I'm focusing on one frequent cause for signature change, doesn't mean there cannot be other causes. but taking care of this and having matched levels tends to take care of a great deal of the actual sound differences that people notice. so it's certainly worth knowing it exists. Tomorrow I'll post about how eating Pringles while reviewing gears was kind of an issue. (OK maybe I won't) Addendum(stuff nobody cares about): All those measurements were done rapidly by noob me. I used rather cheap measurement gears and didn't wait for the IEMs to settle in the coupler(the se215 had foam tips but I'm crazy I don't wait^_^). I moved the cables around a lot from device to device between measurements, so some stuff aren't as flat as they should be. But I feel that the general points are visible enough to make those graphs relevant. if you don't think so, shoot me and tell me what I absolutely have to change. I used: Behringer ECM8000 microphone with a cheap coupler. Scarlett 2I2 as ADC The output of all gears were set to reach close to 90dB SPL out of the IEMs, and then fine tuned with digital attenuation(again, way faster). No smoothing on any graph. The impedance is measured with a worthless 100ohm resistor so I won't claim utmost accuracy down to 0.1ohm.