How to Connect a Headphone Amplifier to an Audio Interface

We often hear from musicians and recording engineers who need to boost listening volume of their headphones. I recently tried a set of Sennheiser HD-650s using the headphone output of a Focusrite Scarlett 2i2 interface and completely agree. Volume on the interface can be turned to 100% without threatening my ears. A headphone amplifier is the answer.

Alas, mating pro audio gear with typical headphone amplifiers is less than straightforward. Audio interfaces are built to accept balanced inputs and outputs. Headphones and most headphone amplifiers are designed for unbalanced connections.

What are Balanced and Unbalanced Connections?

Unbalanced audio consists of a signal conductor and a ground conductor. Thus, an unbalanced stereo audio signal requires only three wires: +L, +R, and ground.

Balanced audio contains an extra inverting signal for each channel. Balanced, stereo audio requires at least five wires (excluding shielding): +L, -L, +R, -R, and ground.

The equal and opposite signals in a balanced connection are subtracted at the receiving end, so that the signal level doubles and any symmetrical noise cancels out. Noise cancellation is why balanced output is standard in pro audio. Long cable runs and a plethora of devices need all the help they can get to keep a mix noise free.

Connecting Headphones to a Balanced Interface

You have a few options to drive headphones from your interface. Diehard audiophiles will hold strong opinions as to which method is best. In no specific order, you can:

  • Make peace with your interface’s headphone output – It may not be loud, but it is free.
  • Double amp to an unbalanced amplifier – Connect a stereo patch cable from your interface’s headphone output to the input of an ordinary headphone amp. This will boost listening volume, but is generally frowned upon by the HiFi community.
  • Connect balanced line-outputs to a balanced amp – Depending on the model, a balanced headphone amplifier may require a balanced headphone cable as well. JDS Labs currently produces no such design.
  • Connect balanced line-outputs to an unbalanced amp – The majority of interface owners we hear from wish to use the balanced line-outputs of their interface with a popular headphone amp, such as Objective2. To do so, you will need two TRS to RCA adapters (one per channel). Remember, balanced wiring differs from unbalanced wiring, so pay careful attention to the drawing below! Notice that only the positive (+) portion of the adapter is accessed, and we do not connect the inverting (-) signal. After connecting the TRS adapters to your interface’s Left and Right TRS outputs jacks, simply connect an RCA cable between the adapters and the RCA inputs of your amplifier as shown:

 

 

Selecting Maximum Gain

To achieve more volume than your interface’s own headphone output, it’s important to customize your new headphone amp with optimal gain. You will need to refer to your interface’s specifications. Assuming you use its balanced outputs with Objective2, use the following equation:

Maximum Gain = 14/(VRMS of Balanced Output)

For example, Focusrite’s Scarlet 2i2 is listed with a Maximum Line-Output of +10dBu, which converts to 2.45VRMS. Your new Objective2 should be configured with high gain of 14/2.45 = 5.6x. Low gain can be left at 1.0x. Since a configuration of 1.0/5.6x is unlisted at jdslabs.com, please mention your special request in the Order Notes field during checkout.

Feel free to reply below or reach out if you need more help!

15 thoughts to “How to Connect a Headphone Amplifier to an Audio Interface”

    1. I use cables similar to this, which I think it’s better:
      https://www.amazon.com/Hosa-CPR-201-Stereo-Interconnect-Cable/dp/B000068O16/ref=pd_sim_267_2?_encoding=UTF8&pd_rd_i=B000068O16&pd_rd_r=3B510QNBCMKZ14ET2PPF&pd_rd_w=LXfBw&pd_rd_wg=hO3qc&psc=1&refRID=3B510QNBCMKZ14ET2PPF

      However, I think there can be a THD+N penalty with the 2i2 when using unbalanced connectors. An active balanced to unbalanced solution might be better. However, most off the shelf ones are worse than the cable solution. There are some ebay bare board solutions though.

      1. That is true, and we have an active balanced-to-unbalanced design in the works. @Jude will be seeking beta testers ASAP.

  1. Hi, thanks for the article it’s teally helpful.

    I’m a little confused however on whether to buy the leads as suggested in the article or whether to pick up the kind as suggested by ‘ULTRABIKE’ above?

    I’m not technically minded enough to understand the talk re the ‘penalty’ he referred to.

    Thank you

  2. Hey, thank you for this amazing article but my knowledge in this area is limited.

    I have the Topping Dx3pro amp/dac for my Sennheiser HD 58X and Behringer UMC22 audio interface to record from Audio-Technica AT2035. Can i connect the amp/dac with the audio interface ? If so, is there any advantages of doing that ?

    Thanks !

    1. If I am understanding your question correctly, no, there is no ideal way to use your amp/DAC and interface simultaneously as an *output* device driving headphones. You must pick one or the other. An interface is an amp/DAC with ADC. You can rely on an interface for input, and a separate amp/DAC for output, if you wish.

  3. Hi there,

    This was very informative! I have a 2i2 that I currently use to drive my studio monitors through the line outputs on the back of the interface. How would I connect a headphone amp to my interface while still using my monitors?

    1. @Andrew – While it may be considered ‘double amping’, you can use the interface’s line outputs for your monitors and rely on the interface’s headphone output to drive your external headphone amp. This works well for interfaces with good noise performance.

  4. I am not very technically inclined when it comes to this and I am just getting started so pardon my ignorance. I have a scarlet 2i2 interface and ah400 behringer. I wanted to be able to use multiple headphones but the output from my interface isn’t putting out a proper signal as I can only hear the left side on the headphone. It was also my understanding that the signal should be much cleaner but i am hearing quite a bit of distortion. Any suggestions please?

    1. Raza – Hearing only Left channel output suggests wrong cabling or connectivity between your interface and amp. If the above article does not bring you clarity, you may reach out to Focusrite. It’s probably easiest to run a TRS 1/4″ cable between the interface’s headphone output to the input of your amp. Using an incorrect TS cable may create the problem you’ve described.

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