Looks like there's no standard in supplying interface specs by manufacturers, me having no electronics knowledge whatsoever doesn't help.

So I have headphones with 32 ohms impedance and 112dB/mW sensitivity. They're nice and loud with Mackie interface, which has headphone out described as "max level 8mW into 600 ohms". They sound pretty weak even maxed out on Tascam, which has "18mW per channel into 32 ohms".

Sapphire 2i2 has headphone out described as 10 ohms, max level +10dBu. Can I guess from this how well would it work with phones like mine?

There should be some unified form of specs so us ignoramuses could easily compare devices.

Ideally manufacturers should state the maximum voltage, and the maximum current the amp can provide, and state the output impedance.

To be useful though, you need a standard for how much distortion an amp can have before being deemed to be at its voltage or current limits.

You need an agreed figure for a fair comparison, otherwise you could rate a crappy amp at 1W with 10% THD (Total Harmonic Distortion). Sure, you get volume, but it will sound like crap.

Also, output impedance is important. Generally you want at least 10 times lower output impedance on the amp relative to the impedance of the headphones so you get flat frequency response, and sufficient damping to prevent distortion. This is a different issue to amp power. As well as enough power, you also need an output impedance that is low enough relative to the impedance of your headphones.

Quote:

So I have headphones with 32 ohms impedance and 112dB/mW sensitivity. They're nice and loud with Mackie interface, which has headphone out described as "max level 8mW into 600 ohms". They sound pretty weak even maxed out on Tascam, which has "18mW per channel into 32 ohms".

Sapphire 2i2 has headphone out described as 10 ohms, max level +10dBu. Can I guess from this how well would it work with phones like mine?

The problem is incomplete specifications of the electrical performance of the amps.

You know how one amp performs into 600 ohms. You know how another performs into 32 ohms. But you don't know whether or not the one specified at 32 ohms is capable of producing extra voltage to keep the power up when connected to a 600 ohm load, or whether the one specified at 600 ohms can supply the extra current needed to keep the power up into a low impedance 32 ohm load.

Knowing that one amp can supply 8mW in to a 600 ohm load allows you to calculate the voltage the amp is applying to achieve that mW figure, and how many mA of current is flowing.

So, knowing the voltage, you can now calculate what the current and wattage of the amp would be into a 32 ohm load.

Bingo! Easy comparison, right? Of course it's not that simple. You don't actually know that the amp is really capable of supplying enough amps (same voltage as before, but only 32 ohms instead of 600, less resistance equals more current at the same voltage) to produce the wattage the calculation would suggest.

One solution would be to have everyone give specs for their amps at different loads, 16, 32, 64, 150, 300, 600 ohms. Then you could compare across a range.

A better solution would be if the maximum voltage and current capabilities of the amps were specified so that you could then calculate how many mW they could deliver into whatever the impedance of your headphones is.

In the case of the saphire, where it specifies +10dBu and 10 ohms output impedance, you can convert +10dBu in to volts, and if you know volts and the impedance of your headphones in ohms, you can now calculate amps and watts. But how much current is the amp capable of delivering? Can it produce it's full voltage with low impedance 32 ohm headphones without flowing too many amps and causing distortion, or can it only achieve that voltage figure into 600 ohm headphones where the impedance keeps the current flow lower?

At least it states the headphone output impedance at 10 ohms, unlike your other examples, allowing you to know based on the rough rule of 10 that your headphones should ideally be 100 ohms to have flat frequency response and good damping.

So, the first two tell you amp power, but at different impedance loads, and the last one tells you output impedance, and voltage, but doesn't specify what impedance the figure can be achieved at. Brilliant. And we're assuming that all of the specifications given are at an equal level of distortion, but we don't know that. Some may be more "optimistic" than others in the figures they produce.

Very hard to do a fair comparison without being given all the information, or actually measuring yourself the things they don't tell you.

The reasons for not stating all the necessary figures for a complete fair comparison include incompetence, laziness, lack of demand for info from the customer, a desire to hide just how average something is, or a mix of all those reasons.

It gets worse with consumer gear, like my Sound Blaster X-Fi Titanium Fatality Professional (yes, that is the real name of the card!), which only says it supports 300 ohm headphones, whatever that means.

Good reference for further reading, goes into more detail, and explains the math involved:

To be useful though, you need a standard for how much distortion an amp can have before being deemed to be at its voltage or current limits.

You need an agreed figure for a fair comparison, otherwise you could rate a crappy amp at 1W with 10% THD (Total Harmonic Distortion). Sure, you get volume, but it will sound like crap.

Complete description for Scarlett's headphone out (meant Scarlett instead of Sapphire, sorry):

Dynamic Range: 107 dB (A-Weighted)
THD+N: <0.002% (minimum gain, -1dBFS input with 22Hz/22kHz bandpass filter)
Maximum Output Level: > +10dBu
Impedance: 10 Ω

Assumed only impedance and max output would hint how loud the output will be, my limited knowledge tells me "dynamic range" would be just signal to noise ratio and THD will be, well, distortion.

This is really crappy when spec sheets on manufacturer are so useless (or at least pretty damn obscure and requiring doing assumptions and own calculations).

Loved Ethan Winers talk on audio myths - will try to watch this one, but there's a chance I won't understand much from it

Complete description for Scarlett's headphone out (meant Scarlett instead of Sapphire, sorry):

Dynamic Range: 107 dB (A-Weighted)
THD+N: <0.002% (minimum gain, -1dBFS input with 22Hz/22kHz bandpass filter)
Maximum Output Level: > +10dBu
Impedance: 10 Ω

Assumed only impedance and max output would hint how loud the output will be, my limited knowledge tells me "dynamic range" would be just signal to noise ratio and THD will be, well, distortion.

This is really crappy when spec sheets on manufacturer are so useless (or at least pretty damn obscure and requiring doing assumptions and own calculations).

Loved Ethan Winers talk on audio myths - will try to watch this one, but there's a chance I won't understand much from it

OK, we know voltage. To figure out current, and then be able to multiply the voltage by the current to determine power in W rms, we can use this calculator:

So we punch in 2.449489742 V rms, and then input the specific impedance you want to test with.

This will give us figures for current and watts.

At 32 ohms current I=76.5 mAmps rms.

Power is W=0.1875 W rms. 0.1875 W rms = 187 mW rms.

If you were using 600 Ohm headphones the figures would look like this:

current I=4.08 mAmps rms.

watts W=0.01 W rms. 0.01 W rms= 10mW rms.

Not much compared to serious headphone amps that often have 600mW or more power.

But, reality check time.

We're taking it on faith that the headphone amp can actually produce 2.449489742 V without distorting.

OK, to keep it simple, lets assume it can do what it says on the box.

We did two calculations, for 32 ohms, and for 600 ohms. The 600 ohm calculation has a much greater probability of being accurate than the calculation done at 32 ohms.

This is because at 600 ohms, the chance of having too much current flowing for the amp to handle without distorting is low. We can be pretty certain that if the stated voltage capability is correct, that the power calculations will reflect reality.

It is much less certain at 32 ohms. There is 19 times a much current flowing as there is into a 600 ohm load. Can the amp do it? No idea, because they don't specify maximum current capabilities for the amp.

Impedance matters for other reasons besides allowing you to calculate current and watts.

The relationship between the output impedance of the amp and the impedance of the headphones affects frequency response, distortion levels and volume in headphones.

WHY DOES OUTPUT IMPEDANCE MATTER? It matters for at least three reasons:

The greater the output impedance the greater the voltage drop with lower impedance loads. This drop can be large to enough to prevent driving low impedance headphones to sufficiently loud levels. A real world example is the Behringer UCA202 with a 50 ohm output impedance. It struggles with some 16 - 32 ohm headphones.

Headphone impedance changes with frequency. If the output impedance is much above zero this means the voltage delivered to the headphones will also change with frequency. The greater the output impedance, the greater the frequency response deviations. Different headphones will interact in different, and typically unpredictable, ways with the source. Sometimes these variations can be large and plainly audible.

As output impedance increases electrical damping is reduced. The bass performance of the headphones, as designed by the manufacture, may be audibly compromised if there’s insufficient damping. The bass might become more “boomy” and less controlled. The transient response becomes worse and the deep bass performance is compromised (the headphones will roll off sooner at low frequencies). A few, such as those who like a very warm “tube like” sound, might enjoy this sort of under damped bass. But it’s almost always less accurate compared to using a low impedance source.

The guy who wrote that is the guy who designed the O2 headphone amp, and released the design to the world under a creative commons type license. Anyone can build them and sell them.

One of the best designs around. The guy who designed never it revealed his identity, but it is assumed he is an electrical engineer working for some company, who got sick of the terrible designs going into headphone amps everywhere, and decided to do somethings about it.

If you want a really good headphone amp, get one of those.

Well worth checking out the rest of the stuff on his site. He goes into a lot of detail on the technicalities of building a headphone amp properly.

OK, to your headphones:

32 ohm... That's unfortunate. To not run into the problems mentioned in the quote above, you want at least 8 to 1, or more ideally 10 to 1 or greater ratio between the input impedance of the headphones and the output impedance of the headphone amp.

So, for 32 ohm headphones, you'd want a headphone amp with an output impedance of 3.2 ohms.

The scarlett has about three times that output impedance at 10 ohms. I'd be willing to bet that the mackie and tascam are much worse. Possibly over 100 ohms. 60 if you're lucky.

I wonder how much of your troubles are related to impedance and how much is related to the power of the amps? I mean, the amps are all pretty weak, but the impedance issue isn't helping, and degrades both tone and volume.

The O2 headphone amp would actually drive your headphone properly. It has an output impedance of 0.54 ohms! It can output 613 mW into 32 ohms, 355 mW into 150 ohms, and 88mW into 600 ohms.

But realistically, few 32 ohm headphones are very good, and if you've going bother to get an O2 headphone amp, you're probably going to want better headphones anyway.