You want to connect a headphone amplifier to an audio interface or an external sound card, but you are not sure about the procedure to follow?
Well, rest assured, in a few minutes, once you finish reading this article, you will have no doubts on this subject.
Indeed, it is easy to make mistakes and buy the wrong cables to connect a headphone amplifier and a studio interface, but with the right information, it is ultimately quite easy.
Step 1: Identify the audio output
The first thing to do is to define the audio output you will use on your audio interface.
This may seem trivial, but there are a number of things to know.
The basic case
Normally, you should use a “line” output (line out in English) from your audio interface, that is to say an output emitting a signal at line level.
For example, in the image below, which shows the outputs of the Focusrite Scarlett 2i2 (top) and 4i4 (bottom) interfaces, you will find “outputs” that are line outputs. This is where you will connect your headphone amplifier.
Most of the time, these are outputs using female TRS jack (balanced) connections, but in some cases, they can be XLR connections or even RCA connections (which will then be unbalanced).
The headphone output
Many people plan to connect their headphone amplifier to the headphone output of their audio interface.
Electronically, this is not particularly a problem since the output impedance of headphone outputs is generally much lower than the input impedance of headphone amplifiers.
So yes, it will work.
However, it is not ideal, especially if you plan to connect a good headphone amplifier (by the way, I recommend you take a look at my headphone amplifier recommendations).
Indeed, in your external sound card, if you retrieve an audio signal from the headphone output, the audio signal will have already passed through a gain stage: that of the built-in headphone amplifier.
So it will have already, in one way or another, affected the signal.
And if the headphone output of your sound card is not of high quality, it is probably a bad idea to use it to retrieve an audio signal that you will amplify again with a dedicated headphone amplifier.
However — if it is just to power the headphones of a few musicians while they play, it is not a problem at all.
But if it is to power headphones used for mixing, for example, then I would not recommend connecting a headphone amplifier to the headphone output of your audio interface.
Step 2: Identify the audio input
Now that you know where your signal will come from, you need to check where it will arrive.
Generally, there are not many inputs on headphone amplifiers, but there can be several.
So take the time to check carefully which input of your amplifier you will send your audio signal to.
Generally, again, it involves female TRS jack connections, but you can sometimes find RCA, XLR… or even digital connections like TosLink if your headphone amplifier also serves as a DAC.
For example, in the photo below of an RME ADI-2 Pro FS R, you can choose to use digital connections (on the left, at the DIGITAL I/O text) or analog (on the right, ANALOG INPUTS).
But if it is a jack, make sure to check whether the connections are balanced or unbalanced. This should be indicated in the manual, but a little tip: if the input is a single jack that serves as a stereo input, then you can be sure that it is an unbalanced input.
Step 3: Choose the cable
As you have correctly identified the input and output connectors, you should now have no trouble choosing the right cable.
If you have TRS jack connectors, you just need to buy two TRS jack cables: one for the left channel, one for the right channel.
Feel free to check out my recommendations on jack cables.
However, if you have TRS jack connectors on one side and XLR connectors on the other side, then you will need an XLR-Jack cable.
Finally, another relatively common situation is when the connector emitting the audio OR receiving the audio is stereo.
For example, you have a single TRS jack that carries both the left (L) and right (R) signals.
In this case, you will need to use what is called a Y cable like this one from Cordial.
Be sure to check the connectors on each side again to ensure they match your setup exactly.
Step 4: Properly Adjust the System
It is now important to properly adjust the gains so that everything works under the best possible conditions.
The Basic Case
If you are using the line outputs (line out) of your audio interface, in this case, everything is generally well set by default.
That said, make sure that no gain or attenuation is applied at the level of the audio interface on the audio signal: the idea is really to send an audio signal that is at the expected “normal” level, so neither too loud nor too weak.
This normal level corresponds to a neutral gain setting on your audio interface.
With the Headphone Output
If you have now connected your headphone amplifier to a headphone output, the adjustment is a bit more complicated since the gain applied at the output of the audio interface is generally unknown.
My advice: make sure to have an intermediate gain on the built-in headphone amplifier of your audio interface. The idea is to send a signal that is strong enough without being at maximum.
Then, add gain at the level of the headphone amplifier you have added to reach a satisfactory listening level.
If it is too loud, of course, lower the gain on your audio interface.
In Conclusion
There you go, you now know how to connect a headphone amplifier to your audio interface.
► If you haven’t chosen your headphone amplifier yet, I recommend taking a look at my advice here.