My reference headphone system consists of:
  • Onkyo DX-7555 CD player
  • Headroom Maxed Out Home amp
  • Sennheiser HD-600 cans

    • 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, quiet and ever so slightly on the warm side of neutrality. It has a liquid smooth midrange very much like a good SET amp without the slightest hint of harshness or grain. 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 tends to bloat 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.

    • As you might gather from the numbers, both of these are subtle. They can be noticed but it requires careful listening.

    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.

    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 > 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.

    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.

    Sennheiser HD-600 Cans

    These headphones have a balanced sound that is on the warm side of reality, with phenomenally low distortion. I've already described them elsewhere; see here for details.

    GENERAL OBSERVATIONS

    Overall this headphone system has some of the lowest noise and distortion currently available in any headphone system. The HD600s alone have lower distortion than many electronics, which is downright incredible for a transducer. The presentation errs on the warm side of reality, yet with resolution and detail that is often lost in a warm presentation. My big system with its Magneplanar 3.6/R speakers picks up just a tad more detail and resolution, but panel speakers are pretty much the ultimate in detail and resolution, and that system is also slightly brighter sounding (actually, it's more neutral but slightly brighter in comparison to the HD-600s). THe HD-600s have no sense of harshness or artificial brightness. If anything, on some recordings it tends to sound "dead" rather than "bright". I typically listen with the MOH process switch OFF, and the filter switch ON. The filter switch compensates for the HD-600s warmish tendencies so perfectly, I suspect it was designed with that in mind. The guys at Headroom who designed and built this amp are big fans of the Sennheiser HD580s and HD600s. Put the two response curves on top of each other and you have nearly flat response. I can listen to this system for hours at a time with no fatigue yet hearing every detail in the music. Indeed, I often do, since this the system I listen to daily when I'm in my office alone coding away.

    When listening to LP sources I tend to turn off the filter switch. My phono cartridge is an Ortofon MC-30 Super II which has a rise of about 2 dB at 12 kHz. This naturally counteracts the warm response of the HD-600s, making the MOH filter switch unnecessary.

    If I could change anything about this system here's what I would do (most urgent first):

  • Give the MOH flat frequency response. Even though I can't hear half a dB at 20 kHz, it is all additive so it tends to exaggerate whatever slight HF rolloff a CD player or headphone might have. For example I'd like to use my Onkyo with the digital filter in "slow" mode, but the extra 0.5 dB of attenuation at 20 kHz, which alone would not be a problem, can become audible when combined with the MOH's 0.5 dB rolloff. Also, no device can truly be considered high fidelity unless it has flat frequency response.
  • Fix the frequency response issues with the MOH's process switch. Either limit the process switch so it doesn't affect low frequencies, or add a high pass (subsonic) filter to reduce the bass bloom back to neutral. Also, reduce the effect of the filter switch or make it adjustable.
  • Give the MOH a gain selector switch - high or low. The MOH's gain setup is already well designed. Unity gain is just past 12:00 on the knob. Full blast is about 12 dB louder than unity. 9:00 on the knob (about 1/4 turn down from halfway) is about 15 dB quieter than unity, and the knob is linear through this range. This covers almost any situation. BUT - having a gain select switch would make it more suitable for headphones of varying efficiency and for source devices with varying output levels. A gain selector would enable one to use the middle most linear range of the volume knob in virtually any setup. For example, many headphones are so efficient that the MOH becomes unusably loud when using a full size source device with a standard output level of 2 V. A gain switch would resolve this.
  • Give the MOH a floating ground switch. Sometimes, when the MOH is on the same electrical outlet as other devices, an audible 60 Hz hum is audible in the headphones. This can be eliminated by detaching the ground pin on the MOH power cord. A switch on the back panel would be more convenient and spare the user the need to cut up a power cord.

    • All that said, my MOH is almost 10 years old. In the past 10 years Headroom has addressed everything except perhaps the first item on my list, which I'm not sure of because they no longer publish specs on their amps. Also, their prices have gone up significantly. I bought my MOH brand spankin' new for about $1 kilobuck. The latest version is about $2300. Over the past 8 years the neutral inflation adjusted price should have been around 25% higher or $1250. So it's basically doubled even accounting for inflation.

    Even $1 kilobuck might seem like a lot but it seems reasonable to me given the parts: a big, clean power supply with a toroidal transformer, four OPA627 opamps which cost about $15 each, a clean Spectrol volume pot, top quality low tolerace parts throughout, solid pro quality RCA and headphone jacks, and switches. Parts cost alone makes up about half its price. Add labor (each is hand built in small quantities) and there isn't much margin. One can see why their prices have doubled.

    That said, if I ever have to replace this amp I probably won't get another Headroom amp. They have gotten too pricey for me. Also, 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. I probably would get a Benchmark DAC-1 or a Grace m903 for a headphone amp. Then for the source, I would get a reference quality sound card like the E-MU 1212M, or use the digital output of a good CD player. I believe this is where the SOTA reference is now for headphone audio. And the total cost would be around $1500 for the entire system including the headphones. That is half what it would cost to build my current system today.