Field Report: Aphex Model 454 Head Pod
Jun 1, 2009 12:00 PM, By Bill Eisenhamer
I never thought I would be writing about a headphone amplifier � a device we use every day and never give any thought. We use headphones in our business; it is our business, and we can be very particular about them as we know the quality of what we hear is reproduced by them. But do we give much thought on how we drive these headphones? I know I am a victim of �using what has been successful in the past.� I learned my lesson after listening and comparing our existing headphone amplifiers with the Aphex Model 454 Head Pod.
The Head Pod is a compact headphone amplifier designed for those who need a loud and clean feed. The controls are cleanly laid out and what one expects in a headphone amp. The front has four �" TRS output jacks fed by their own independent stereo amplifier, each with a separate gain control on top. Along with the individual gain controls is a master gain control on top. The back has the power connection for an external power supply, an input selector switch and two types of inputs.
It features one unbalanced 1/4" TRS jack for a left, right, ground feed and a second pair of balanced 1/4" TRS jacks for discrete left (mono) and right channel feeds. Though marked as balanced inputs, the discrete inputs accept unbalanced connections as stated in the easy-to-read manual. The external power supply is critical and the manual stresses the need for an ac supply in the 12-16V range. It is also recommended that the supply have at least a 6W capacity. This is no problem as a wall-wart type power supply is provided. The bottom provides a wiring diagram for each input and a graph depicting output power versus headphone impedance.
Plenty of headroom
Performance at a glanceHigh gain, up to 35dB
Stereo input, four outputs
Operates on 16Vac
As clearly stated on the top of the unit, this is a high-output headphone box. As advertised, it provides a high output with very low distortion. The specifications state a maximum gain of 35dB and a THD of <0.001 percent. I need to obtain some better test equipment because my Potomac peaked at +30dB and a THD of <0.1 percent. A better instrument is required to know how much better the spec can be. Likewise, I attempted to verify the frequency response of 10Hz-120kHz �1dB. I was able to measure 20Hz-80kHz �1dB with the Potomac before the meter's response broke down. This is an amazing frequency response considering our hearing extends to 20kHz when we are young. I measured a signal-to-noise of greater than 90dB and the stated typical SNR is 100dB. I would say the unit passed all tests with flying colors. The low 10 ohm output impedance is capable of driving 8 ohm through 1kohm devices.
Back view of unit showing connections
The clarity of this headphone amp is superb. I fed a number or sources into the box ranging from a typical air monitor tuner and straight program feed to HD Radio off-air feeds. I even used different tuners for the air monitor feeds because I had them available, and in one case I took a feed from a Day Sequerra M2.2R modulation monitor. With this headphone amp I learned quickly that two same-model tuners can have different output characteristics. I was able to clearly hear the audible difference between an off-air analog feed versus an off-air HD Radio feed. Even the differences between program sources were clear and make me wonder how some songs ever passed a quality mastering process. Even at high level listening, which I do not recommend for long periods of time, the music was clean and distortion-free. I listened using a pair of Sony 7506 headphones and my own Ultrasone Pro 650 headphones.
I highly recommend this device for critical listening environments, and I plan to install these for our main operator positions. Given such a high output level, the box is perfect for a loud environment such as concert events where the extra gain is necessary. The ability to drive higher impedance headphones is a plus for those using 600q headsets as found in sports broadcasting. The higher gain makes a huge difference. Many headphone amps do not make the grade when driving these higher impedance devices. The Head Pod's small footprint makes it a great addition to a small computer-based production environment where multiple headphone feeds are lacking. For the price, this little box is a monster performer.
Eisenhamer is the staff engineer for Lincoln Financial Media Company of California.
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