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!