My reference headphone system consists of:

  • Source Components from primary room system
  • Headroom Maxed Out Home amp
  • Audeze LCD-2 Headphones

    • I'll provide a brief description of each, followed by general observations.

    Headroom Maxed Out Home

    This is a headphone amplifier designed & built by Headroom in Montana. It has one set of RCA inputs, two headphone output jacks, one set of RCA outputs (so it can be used as a single-source preamp), a volume knob, and two switches that alter the sound. It resides in a dull black, utilitarian aluminum black box, with extremely high quality parts. This gives the impression of a piece of ruggedized military electronics. The impression is not inaccurate. The guts of this amp consist of an oversized power supply with a toroidal transformer that provides the juice for four Burr-Brown OPA627 op-amps configured in a symmetric class A topology. These are some of the fastest, cleanest, most linear and musical op-amps available on planet Earth.

    Subjectively, the MOH is clean and quiet with neutral tonal balance. It has a liquid smooth midrange very much like a good SET amp without the slightest hint of harshness or grain, but without any "tubilicious" euphonic distortion. It has awesome detail but it doesn't jump out and grab you. You have to listen for it, but when you do it's there. The noise floor is incredibly low, giving clean recordings a startling sense of realism. The bass, especially the extreme low bass, has great texture with total grip and control, yet naturally balanced without being emphasized or boomy. Overall it is a superbly clean, balanced, natural sound with plenty of detail yet without brightness.

    The first and most interesting of its two switches is the Headroom processor. It takes a portion of the R channel, knocks it slightly out phase and mixes it into the L - and vice versa. This simulates what happens when your ears hear normal sounds - each ear hears a little of what the other hears slightly out of phase due to each ear's different position in space. Simulating this helps eliminate the "blobs in the head" image that headphones can give when playing normal stereo recordings. It does significantly improve depth & imaging while reducing listening fatigue but has its limitations.

  • First, it bloats the low frequencies. This is due to the fact that bass often has a strong mono component, even in high quality stereo recordings, so you get a mono summing effect. To quantify this effect, it depends on the recording but raises the bass response anywhere from 1-3 dB @ 20 Hz, in a slope that tapers to zero by around 500 Hz.
  • Second, it slightly veils the upper midrange and treble. This is likely due to comb filter effects from the phase shift. This effect is more subtle, typically lowering response from 1.6 kHz to 8 kHz by around 1-2 dB.

    • The second switch is the filter. It raises the response by 2-3 dB at 5 kHz, tapering to zero around 2 kHz and 20 kHz. This is intended to counteract the treble attenuation caused by the process switch, but it does nothing to correct the bass response. Subjectively the effect is subtle yet easily discerned. It tends to overcorrect for the process switch; a slightly lower amount of boost would provide neutral response. But it is useful for correcting warm or dull sounding headphones or recordings.

    The net effect of the processor is to improve the image yet bloat the bass and veil the midrange. I find it useful for movies but don't use it for music. With one rare exception: if music is recorded in absolute stereo - one instrument or voice entirely in each channel. This impossible to listen to on headphones, and the process switch "fixes" it.

    Measured at unity gain, the MOH's THD+noise is unusually low. In a spectrum analysis of the THD+noise for a pure 500 Hz tone, there is only the barely perceptible slightest hint of 60Hz ripple, and the first 7 harmonics are not even visible above the noise floor which is around -90 dB or below.

    The MOH's frequency response provides a slight de-emphasis of the extreme high treble. It tracks ruler flat (<= 0.1 dB variation) from below 20 Hz to about 8 kHz, at which point it is attenuated down by about 0.25 dB. From there it gradually tapers to -0.5 dB @ 20 kHz. Of course the OPA627 bandwidth goes well into the MHz region, so this rolloff is by design. Few humans can hear half a dB at 20 kHz, so it is generally inaudible, but some claim that it smooths out the extreme highs making them sound more natural. There may be some truth to this, since, with sounds in the real world, ultra high frequencies are attenuated with distances as little as 10 feet away.

    The MOH has an input impedance of about 100 kOhms and an output impedance of < 1 Ohm. This makes it an easy load for any device to drive and gives it plenty of oomph on the output side. I tested and found its output remains clean even at high power output levels into low impedance loads.

    Onkyo DX-755 CD Player

    This player has a neutral, balanced sound with extremely low noise + distortion. I have already described it elsewhere; see here for details.

    Audeze LCD-2 Headphones

    These planar magnetic "ortho" headphones have wide, linear frequency response, neutral tonality, and phenomenally low distortion. I've already described them elsewhere.

    GENERAL OBSERVATIONS

    Overall this has some of the lowest noise and distortion currently available in any headphone system. It is as close as you can get to the sound of the live mic feed, or to an actual live performance of acoustic music. The sound overall is similar to my in-room system with its Magneplanar 3.6/R speakers. The differences are the in-room system is a hint more forward in the midrange and doesn't reproduce the bottom 1/2 octave (20 Hz - 30 Hz).

    It's a reference quality headphone system that enables me to listen to music in the evenings without disturbing my family, yet without giving up anything in terms of audio fidelity. In some ways, it's actually higher fidelity than my in-room system.

    If I ever have to replace this headphone amp what would I do? Headphone amps have gotten pricey. But over the years technology marches on... It is now possible to get reference quality headhpone sound for a fraction of what it used to cost.

    For a straight replacement (headphone amp only - no DAC) I'd look for a used Headroom amp on eBay. These are hard to find because Headroom made such great amps, nobody wants to part with them. I'd perhaps consider a Bryston BHA-1.

    But if I were going to do that, I might as well get a Benchmark DAC-1 because it would have comparable sound quality with a lot more features and capability.