# Output impedance and loudness

I've a DAC for headphones (GeekOut 720) which has two output jacks with different impedances: 47 Ohm (with a large headphone icon next to it) and 0.47 Ohm (with a small headphone icon next to it).

I'm currently using Sony MDREX110AP headphones, with 16 ohm impedance according to its specs. When I connect them to the 0.47 Ohm output I get much louder output than with the 47 Ohm jack, which makes me wonder, shouldn't the smaller impedance output drive my headphones with less power than the larger one? If such is the case, why is the response louder for the larger impedance output? It seems to me that I'm not quite grasping the output impedance concept.

Bonus question: what output should I use to get better sound quality and protect my headphones?

I'm currently using Sony MDREX110AP headphones, with 16 ohm impedance according to its specs. When I connect them to the 0.47 Ohm output I get much louder output than with the 47 Ohm jack

Possible explanation.....


The socket labelled 47 ohm has an output impedance of 47 ohms. This means that with a 16 ohm load and, using the voltage potential divider equation, the output will be reduced from an open-circuit voltage of (say) X to a value that is about 0.254 X.

The socket labelled 0.47 ohm has virtually zero output impedance compared to a load of 16 ohms hence the output is barely attenuated.

A reduction in signal voltage level to 0.254 is an attenuation of nearly 12 dB which is more than halving the loudness level (based on 10 dB being a halving or doubling of perceived loudness at 1 kHz).

• this answers my question clearly. a follow up question is why not always go for the lowest output impedance jack regardless of my headphones in order to maximize the power transfer to the load? – jimijazz Nov 28 '18 at 15:56
• If power consumption was a major consideration then using a lower output impedance would be sensible but I often worry about acoustic shock (headphone problem) and having a significant resistive output might alleviate this problem. Of course the final word has to go to what the headphone manufacturer recommends for the equipment and, as another answer points out, you may get more "hi-fi" with a lower output impedance. – Andy aka Nov 28 '18 at 16:07

Bonus question: what output should I use to get better sound quality and protect my headphones?

For "better" protection use the 47 Ohm output as that has a more limited output current (due to the internal 47 Ohm resistors).

The 0.47 Ohm outputs will also have some maximum output current but that maximum current will very likely have a much higher value than the 47 Ohm output.

For "better" sound quality: that depends on the headphone you're using.

In practice the best protection for your headphones is not to drive them louder than your ears can cope with. When connected to the 0.47 Ohm output you might be able to damage some headphones but the sound level would need to be way beyond what your ears can deal with. So then you would not be using the headphones correctly anyway.

• thanks for addressing that. for the better sound quality I was asking specifically for my 16-Ohms headphones. – jimijazz Nov 28 '18 at 15:56
• You will just have to try out what sounds better to you. It is possible that you could hear some noise or hiss (when there is no music) when using the 0.47 Ohm output. It might be that the 47 Ohm output will not go loud enough. You will just have to try and see what suits you best. – Bimpelrekkie Nov 28 '18 at 16:04

The ratio of the load impedance to the output impedance of the amplifier is known as the damping factor, and since the drivers have mass, and the voice coils generate a back EMF as the coil moves in the magnetic field, the relationship between the voltage applied to the voice coil, the current through it and the displacement resulting isn't linear, but has a time response, and so affects the frequency response.

The best (flattest) frequency response is generally achieved with a high damping factor (lowest output impedance). On large speakers this is significant, but with the small drivers in headphones this may not be even noticeable.

Since a significant proportion of the signal will be dropped across the 47 ohm output impedance the loudness will be significantly different between the two, so it may not be easy to do A-B testing to see of you can tell the difference.

• if damping factor = z_output / z_load, wouldn't a high damping factor be achieved with a high output impedance? – jimijazz Nov 28 '18 at 15:54
• It's the other way round - edited for clarity. – Phil G Nov 28 '18 at 19:36