Headphone Output Impedance and Mastering

RedStone

Member
I've been researching buying a new interface since my old computer is aging and I've upgraded to a Macbook Pro M2.

I have an Apollo interface, which was a huge step up from what I had been using (Scarlett). For tracking and mixing, I'm Apollo all the way. I love those little tanks.

But for mastering, iI'm craving a more extreme level of linearity and exactness.

This has lead me down a rabbit hole of research, the results of which have surprised me (a lot!), Especially when it comes to headphone jack output impedance, dynamic range, and Total harmonic Distortion. This is where 95% of Prosumer audio interfaces seem to either lack or fail.

This YouTube channel by Julian Krauss has been revealing. Many of the interfaces I've wanted to buy because I thought they'd be great are no-gos now. Though, I'm looking for something pretty specific. This video in particular was really helpful (screenshot is below)

1720044311449.png

This is a readout of the headphone output impedance of a few common interfaces. What I've learned is that output impedance can be multiplied by 8 to roughly determine what headphone impedance will work well with the interface (it's not perfect, just a ballpark, but the closer to an exact match the better).

The closer the HP out is to zero ohm, as long as it has enough power, the more hifi headphones it is compatible with and can drive with a linear response instead of scooping the sound due to impedance mismatches.

A friend of mine loaned me a Steinberg UR22 Mk2 to use for recording on my Mac while I figured out what to do about getting a new audio interface. I'm thankful to have something, but man the UR22 mk2 is not great haha. The preamps are noisy, the channel balance is wonky on the outputs, and the HP output has an impedance of 91 ohm. That means the lowest impedance headphones that can it can supply with Flat frequency response and the tightest bass response is: 91ohm x 8 = 728 ohm.

Insane.

Now this doesn't mean that a 250 or 600-ohm pair of audiophile headphones won't sound good, it just means that the impedance mismatch will colour the audio signal. For mixing, it's not the worst if you're using 250 ohm headphones. But for mastering, I need the cleanest possible audio signal with no colouration, and inaudible levels of distortion across the frequency spectrum. As I've learned, this is why the likes of entry-level Lynx or Antelope interfaces start at over $2000 USD.

But I may have found a gem. The MOTU M2. It does have some harmonic distortion issues at certain volumes but overall doesn't seem to be all that noticeable. The frequency response is linear on all outputs, and the headphone out has an impedance of 0.2 ohms, near-perfect channel balance, excellent cross-talk, and transparently clean and quiet preamps. I've never used Motu Interfaces before, so I'm curious to know if anyone has experience with these, or what other interfaces people are using for mastering :)
 
I suspect it’s because few people really master on headphones, so the prime use of headphones is monitoring while tracking. If you are really concerned about tiny details then your room and your monitors will make far more difference to what other people hear? I’m also not really convinced that the scoops you mention are significant. As all those headphones we use have very different sounds anyway, so if the interface changes with impedance how is this balanced against headphones that are far from flat? Because they’re more affordable than speakers, we often have multiple pairs of headphones, and accept the different sounds they have. With speakers we have a more common standard as we don’t have lots we work with often. Headphones and speakers all sound different, so doesn’t that make the impedance thing just more eq changes we have to live with?
 
I have followed the vagaries of headphone amplifiers with regard to their output resistance* for a couple of years.

Historically HP amps had a physical 'build out' resistor in the OP circuit which performed three vital tasks.
1) It protected the amplifier from a short on its output (the amp of course had to be safe into just the resistor!).
2) It isolated the amplifier from load capacitance which can cause instability.
And 3) not vital but very useful, it helped to even the loudness into phones of a variety of nominal impedance.

Then came the "fashion" I shall call it, for HP amps to have an OP R of near zero Ohms or very much lower than the 33-100 Ohms that had been usual. Why? Well "better damping factor" was one cry...Hah!

"Damping Factor" is basically nonsense. What matters is the TOTAL resistance in the circuit and virtually all MC headphones have a DC resistance very close to their stated impedance. Check a few on your trusty DMM. So, it matters not very much if your HP amp OPR is 33R or zero, the current flow to effect damping is much the same with say 250R cans. Bit better for 32 R jobs but not much.

Then there is the fact that headphone manufacturers KNOW that most amplifiers have a significant output resistance and surely make their products accordingly? The number of near zero amplifiers is still quite a small percentage.

Lastly, to make an amplifier with an output of about a watt but with a very low internal resistance means you have to build in some other form of short protection. That is quite difficult to do without introducing glitching as the VI limiters kick in and all adds to the cost.

"Resistance not "impedance" because you don't want the output to be frequency dependent.

Dave.
 
I did this a while back because I got a set of Beyerdynamic 990s that are 250 ohm and settled on the UA Volt 276...here are there specs.

Maximum Output Power into 32 Ohms
Maximum Output Power into 300 Ohms
Maximum Output Power into 600 Ohms
84 mW per channel
22 mW per channel
12 mW per channel
 
I did this a while back because I got a set of Beyerdynamic 990s that are 250 ohm and settled on the UA Volt 276...here are there specs.

Maximum Output Power into 32 Ohms
Maximum Output Power into 300 Ohms
Maximum Output Power into 600 Ohms
84 mW per channel
22 mW per channel
12 mW per channel
Right! Back of envelope calculation gives that interface an output resistance for the headphone source of 18 Ohms. Lower than most but still very much above the near zero that the 'followers of fashion' say is needed. Since the 250 Ohm 990 pros have a sensitivity of 96dB/mW you should have no trouble getting enough, clean level.

Dave.
 
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