What does "Zero-Ohm Headphone Outputs" mean?

[olds1959special]

What does “Zero-Ohm Headphone Outputs” mean?

“Zero-Ohm” refers to the impedance of the headphone output circuit, which is not exactly 0 ohms, but really close (0.5 ohms).

knowledge.apogeedigital.com

Are there are any products the work like this and is this important for the best sound quality when using headphones?

 

[oleg87]

Most dedicated headphone amps made these days have zero-ohm outputs.

The exceptions tend to be audio interfaces, integrated amps, computer/laptop headphone jacks and the like.

 

[solderdude]

What does “Zero-Ohm Headphone Outputs” mean?

“Zero-Ohm” refers to the impedance of the headphone output circuit, which is not exactly 0 ohms, but really close (0.5 ohms).

knowledge.apogeedigital.com

Are there are any products the work like this and is this important for the best sound quality when using headphones?

The short answer is:
Sometimes output resistance doesn't really matter, sometimes it matters a little, sometimes it matters a lot.
It depends on the frequency dependent variance of the headphone's impedance relative to the output resistance of the amplifier/output.

The elaborate answer is:
It depends on the headphone itself whether or not the output resistance of an amplifier has any influence on the sound.
People think it has to do with 'damping current' but actually it is all about frequency dependent voltage division.

For speakers (woofers actually) the damping current is a factor because of the weight of the membrane.
Not so with headphones.

This means that some headphones (low ohmic ones) with a substantial varying impedance (this often includes MA IEMs) will sound different on an amplifier with a higher output resistance than on a lower output resistance.
There are plenty of headphones that do not have a varying impedance and these will sound the same on any output resistance source.

In the old days (very old) there even was a DIN standard for output resistance (120Ω).

The lower the impedance of the headphone the 'worse' the effect is when the impedance varies.

It is about the ratio between the impedance of the headphone and output resistance of the amplifier.
That's where the '1/8th or 1/10th rule' comes from. This rule often does NOT apply in reality though and depends on how much the impedance varies.

Some IEMs can be tuned with (additional) output resistors.

Most modern (and battery fed) devices are close to 0Ω (they never are actually zero in practice nor is that needed).
Some audio interfaces range between a few Ω and 50Ω or so.
Some OTL headphones amps can be between 10Ω and 100Ω or so.
Some headphone amps have selectable output resistance.
Some headphone amps have a resistance between 10Ωand 120Ω usually because the designers don't understand the theory or they need a resistor as 'current limiter' or to ensure stable operation.
A lot of 'headphone outputs' on integrated amps, receivers, AVR's use the power amp also for the headphone out and use a series resistor (or an attenuator with resistors). These devices can have output resistance between 50Ω and 400Ω (yep several hundred).

Some headphones actually might sound 'better' from a higher output resistance, some don't change the sound at all (or inaudible to very little), others might will sound worse.

Because for laymen the impedance plots are hard to understand (at least what influence it has on the sound) I measure the actual output on a fixture (the impedance rise on a fixture can be lower than when lying on a table) instead.

I then overlay traces to better gauge the tonal balance changes.
This is essential as there is voltage division (attenuation) as well.

Below the 55Ω HD595: on 0.2Ω, 10Ω, 32Ωand 120Ω amplifiers.

A substantial variance... due to the varying impedance of this dynamic headphone.

Not all (dynamic) headphones have a varying impedance so they don't care what they are connected to and sound the same from any source like the 32Ω OLLO S5X below.

The traces all overlay.

So not all dynamic headphones need to be driven from a low output resistance but some do.
To 'ensure' that the effect is minimal one can simply apply the '1/10th rule' just to be safe.
This means the output resistance of the amplifier must be 10x (or more) smaller than that of the headphone.

Here is an example.. below an impedance plot that might scare someone that does not look at the impedance scale:

That seems problematic doesn't it ?

However that same headphone's frequency response measured on 0.2Ω and 120Ω output resistance:

The output resistance has no influence on the sound. This is because of the relative low impedance change (15% in the lows) and high impedance of the headphone combined with the relatively low output resistance (voltage division) of the amplifier.
This means that higher impedance headphones are less likely to sound 'different' from higher output resistance sources than some low impedance headphones.

 

[mhardy6647]

from the reference posted above:​

to wit: https://knowledge.apogeedigital.com/what-does-zero-ohm-headphone-outputs-mean

“Zero-Ohm” refers to the impedance of the headphone output circuit, which is not exactly 0 ohms, but really close (0.5 ohms).​

well... it's infinitely higher than zero... so I guess it depends on what one considers close.

 

[egellings]

Many amplifiers implement a headphone amplifier output by putting a small resistor in series with the power amp's output and the headphone jack. The resistor is usually in the 100's of ohms in value. The resistor attenuates the power amp output to a safe level for the headphones through voltage divider action between the resistor and the phones' internal resistance. A zero ohm output would be a small dedicated power amplifier, likely a chip amp, to power the 'phones, and like the main power amp, these amplifiers have a low, near zero ohms output resistance.

 

[Mnyb]

The short answer is:
Sometimes output resistance doesn't really matter, sometimes it matters a little, sometimes it matters a lot.
It depends on the frequency dependent variance of the headphone's impedance relative to the output resistance of the amplifier/output.

The elaborate answer is:
It depends on the headphone itself whether or not the output resistance of an amplifier has any influence on the sound.
People think it has to do with 'damping current' but actually it is all about frequency dependent voltage division.

For speakers (woofers actually) the damping current is a factor because of the weight of the membrane.
Not so with headphones.

This means that some headphones (low ohmic ones) with a substantial varying impedance (this often includes MA IEMs) will sound different on an amplifier with a higher output resistance than on a lower output resistance.
There are plenty of headphones that do not have a varying impedance and these will sound the same on any output resistance source.

In the old days (very old) there even was a DIN standard for output resistance (120Ω).

The lower the impedance of the headphone the 'worse' the effect is when the impedance varies.

It is about the ratio between the impedance of the headphone and output resistance of the amplifier.
That's where the '1/8th or 1/10th rule' comes from. This rule often does NOT apply in reality though and depends on how much the impedance varies.

Some IEMs can be tuned with (additional) output resistors.

Most modern (and battery fed) devices are close to 0Ω (they never are actually zero in practice nor is that needed).
Some audio interfaces range between a few Ω and 50Ω or so.
Some OTL headphones amps can be between 10Ω and 100Ω or so.
Some headphone amps have selectable output resistance.
Some headphone amps have a resistance between 10Ωand 120Ω usually because the designers don't understand the theory or they need a resistor as 'current limiter' or to ensure stable operation.
A lot of 'headphone outputs' on integrated amps, receivers, AVR's use the power amp also for the headphone out and use a series resistor (or an attenuator with resistors). These devices can have output resistance between 50Ω and 400Ω (yep several hundred).

Some headphones actually might sound 'better' from a higher output resistance, some don't change the sound at all (or inaudible to very little), others might will sound worse.

Because for laymen the impedance plots are hard to understand (at least what influence it has on the sound) I measure the actual output on a fixture (the impedance rise on a fixture can be lower than when lying on a table) instead.

I then overlay traces to better gauge the tonal balance changes.
This is essential as there is voltage division (attenuation) as well.

Below the 55Ω HD595: on 0.2Ω, 10Ω, 32Ωand 120Ω amplifiers.

A substantial variance... due to the varying impedance of this dynamic headphone.

Not all (dynamic) headphones have a varying impedance so they don't care what they are connected to and sound the same from any source like the 32Ω OLLO S5X below.

The traces all overlay.

So not all dynamic headphones need to be driven from a low output resistance but some do.
To 'ensure' that the effect is minimal one can simply apply the '1/10th rule' just to be safe.
This means the output resistance of the amplifier must be 10x (or more) smaller than that of the headphone.

Here is an example.. below an impedance plot that might scare someone that does not look at the impedance scale:
View attachment 470399
That seems problematic doesn't it ?

However that same headphone's frequency response measured on 0.2Ω and 120Ω output resistance:

The output resistance has no influence on the sound. This is because of the relative low impedance change (15% in the lows) and high impedance of the headphone combined with the relatively low output resistance (voltage division) of the amplifier.
This means that higher impedance headphones are less likely to sound 'different' from higher output resistance sources than some low impedance headphones.

Wov a very thorough write up for future reference nicely done sir

 

[DVDdoug]

I've never seen such a claim before...

And we're not trying to match impedance so we usually wouldn't try to drive 8-Ohm speakers with a "zero ohm" headphone output.

There's rarely zero anything in the analog world. Sometimes a value is too small to be measured ("A drop in the ocean"). Even something that crosses from positive-to-negative it's hard to find exactly zero.

 

[Mnyb]

Practically zero does it for me regarding output impedance of amplifiers I think that how we should think about it . Its often much lower than the wires to the headphones or speakers .

It’s practically zero for me when it’s very small refered to the intended load.