Maximum allowable headphone amp output impedance.
 

Looking at some headphone impedance plots I find that a reasonably high output impedance in the headphone amp doesn't cause too much error in the ( electrical ) frequency response at the terminals of some headphones. But does it matter practically from damping point of view ?
Some headphones have a pretty flat impedance across the band and some are quite wavy with a lift at upper bass and at HF. So for the latter the Zout would matter a bit.
Are headphones designed to work with ideal voltage sources or are they designed not to be affected much by high Zout ( say 50 ohms or 100 ohms!).

there is a euro standard for 100 Ohm output resistance but I doubt many current headphones are really designed to that anymore

as you say a flat impedance curve means the frequency response of the headphone isn't very sensitive to the output Z of the amp

certainly the sensitive iem and ear buds for portable players are usually driven with very low Z source so as to use the most of their low 2-4V battery supply

although good sealing iems can be so efficient that they really need 10-20 dB step-down from even 1 Vrms drive

some headphone amp manufacturers have a output Z switch for people to tweak the response if they believe it makes a difference with their headphones

Actually 120 Ohm.

Reading between the lines it seems as if the 'standard' was introduced because there was such a wide range of 'phones impedances available. Setting the output Z to 120 Ohms means that the majority of 'phones could be driven by the majority of amplifiers to a reasonable level of loudness. In other words the 'standard' was set for bland harmonisation and not 'to set the standard for high performance'.

Ignore it is my recommendation, because that is what most headphone manufacturers seem to do.

My experience suggests that most headphones are not critical of low damping, but I've obviously not tried all combinations!

: )

I find even 10 ohms and 40 ohms makes a noticeable difference to the response at the headphones. 100 ohms affects most headphones. Some with almost 1.5db lift in the mid bass end and also some at the extreme HF end. Will surely alter the sound. A 2.2 ohm output impedance seems to be acceptable for most phones I have !

If anyone wants to see the response plots I can put them up later. Too much effort right now. I'm turning in for the night !

Most sets that I have measured are about 32R dc, so anything lower than 3 Ohms source is negligible. 120R will cause a treble lift with the inductance

Some interesting response plots.
 

I used a Sennheiser HD-580 ( 300 ohms ) and a Philips-805 ( 32 ohms).
I used a Zout of 2.2 Ohms, 10 ohms ,40 ohms, 100 ohms to check variations in the electrical response at the headphone terminals.The files starting with 580 are for the HD-580. Those without the phones number are for the SHP-805 . The HD-580-Z is the impedance of the Sennheiser and the SHP805-Z is the impedance of the Philips phones.
Both show visible response changes when the Zout is 10 ohms and gets worse as the Zout goes up. 2.2ohms Zout seems to be quite acceptable.
Maybe we can say that the Zout should not be higher than about 10 ohms (?). It might generally be hard to notice a 0.5 dB response change though on some tracks it may be audible.

Hi all,

I also vaguely recollect the 120 Ohms value as being some sort of standard value. It makes sense, because in many (cheaper) amplifiers, the headphone output is derived from the main speaker output by a voltage divider, which always presents a higher than zero impedance to the headphone jack, and 120 ohms is easily achievable with a wide range of division rations. See for example Headphone Amplifier

However, I don't think that this is still the case. I find no reference at all to this in datasheets of modern headphone amplifier chips. Websites of headphone manufacturers are also silent on this issue. I have posted the question to Sennheiser, hopefully they'll find some time to answer it. When I get any answer, Ill post it here.

you can see many headphone's impedance graphs at Headroom

Learning Center - Build a Headphone Graph | HeadRoom Audio

Sennheiser's comments on output resistance
 

My interest in headphone listening began three years ago with a gift of Sennheiser HD650. I built a ss Class A headphone amplifier based on John Linsley Hood circuit provided by Geoff Moss. The results were great and I really loved headphone listening despite (or because of) a rich and over-warm sound. I had built the amps with resistors in the output to the headphone socket and realised through reading forums that I could tune the sound by varying the resistor. I find with the HD650 that no resistor in the output is best.

I now use a very pure copper replacement cable and the HD650 is fast, detailed and punchy. Vocals in particular are very realistic. I also have a Borbely all FET amp which is very fast and clean, the HD650 is terrific with this.

I asked Sennheiser for their views on resistance and this is the reply from Germany:

“If a real headphone amplifier is used, the output impedance should be as low as possible. Therefore it should be 0 Ohms. If you add additional resistors at the AF it is more and more getting to go to current source mode and you are influencing the sound as experienced. In some cases this may be preferred - but the sound of the headphone with its frequency response gets influenced"

Not a very informative answer. They don't say if the 120 Ohm standard still has any meaning. I doubt it. If it had, one would expect manufacturers to design their headphones to provide the best sound quality at this impedance. Sennheiser at least seems to indicate that their products are optimized to be driven with a zero Ohm voltage source as provided by most dedicated headphone amps. This also means that Sennheiser phones are not sounding their best on the average integrated amp which provides an approx. 120 Ohm source. This, however, could be easily modded.

It would be interesting to know the positions of AKG, Beyerdynamic, Koss and others.

The 120 Ohm doesn't make sense at all for low impedance earphones in the 16 Ohm region, as are mostly used in portable gear. Most of the output power would just be wasted.