If you own a reasonably-good quality set of headphones, you either need, or could probably benefit from, a headphone amplifier. Here's why: Quality headphones generally have an impedance of 64ohms or higher; some high-end studio 'phones have 600ohm impendence. Most portable audio devices, and a disappointingly-high number of home stereo amps, have output amps at their headphone jack which aren't capable of supplying enough voltage swing to drive the higher-ohmed headphones. The end result is your headphones are too quiet, and/or clipping occurs.

Practical Devices' line of amplifiers, with their high gain, and full-voltage output swing, are able to drive even very high-impedance headphones.

The XM4 and the XM5 are the same physical dimensions; The XM6 is slightly smaller. All three use the same Lithium Charger. The major differences are shown:

The amplifier will drive any headphones with an impedance of anywhere between 24 and 600 ohms. This includes Sennheiser, AKG, Grado, Shure, Sony, Beyerdynamic, and many other headphone brands. For those of you who own headphones towards the higher end of that impedance range, you may need to enable the gain boost, for the extra +8dB of gain.

Usually, no. The XM6 was designed as a headphone amp, and will usually not do a good job of driving speakers, even small ones (most speakers have impedances of around eight ohms). What you are looking for is a power amplifier, not a headphone amplifier.

With the main volume knob at full volume, the overal gain for the XM6 is:

Configuration	Gain
With gain boost switch disabled:	+10.3dB
With gain boost switch enabled:	+18.4dB

The bass and treble buttons on the front of the XM6 add the following boosts:

Configuration	Approximate Gain
Bass boost button IN:	+6dB @ 100Hz
Bass boost button OUT:	+0dB @ 100Hz
Treble boost button IN:	+3dB @ 10kHz
Treble boost button OUT:	+0dB @ 10kHz

The XM6 is equipped with Active Variable Crossfeed. To use Crossfeed, the Crossfeed Enable switch (on the front of the XM6) must be in the IN position. Then, you can adjust the amount of crossfeed using the Crossfeed Adjuster potentiometer (the blue knob on the back of the XM6).

Why You Want Crossfeed: Most music is mixed to be listened to on loudspeakers. The studio technicians mixing the recording assume there will be alot of crosstalk, since, with speakers, the listener's left ear clearly hears the right speaker's output, and vice versa. Thus, most music is mixed with a very wide stereo soundstage.

With headphones, however, the left ear only hears the left channel, and the right ear only hears the right channel, with nearly zero crosstalk. This can, for many recordings, cause the soundstage to appear to be "in the center of your head". Further, some recordings (early Beatles' recordings are infamous for this) have each instrument and vocal track panned 100% to either left or right, which can quickly cause listener fatigue.

The XM6's CrossFeed feature feeds some of the left channel into the right channel, and vice versa. In order to properly simulate the forward soundstage of loudspeakers, the amount of crossfeed needs to vary with frequency, which the XM6 does. Low frequencies (bass) bend around corners (including the corners of your head - bass from the left speaker will bend around your head and enter your right ear) much better than higher treble frequencies (treble is more directional); thus more crossfeed must occur in the lower bass frequencies.

Further, the XM6 allows the user to adjust the overall level of crossfeed, from near-stereo to complete mono. This is adjusted using the crossfeed controller potentiometer on the back of the unit (don't forget to turn on the crossfeed with the CrossFeed Bypass Switch).

The crossfeed vs frequency plot, shown for various crossfeed potentiometer positions, is shown below: Don't confuse this graph with a Frequency Response (also known as Amplitude Response) graph; This Crossfeed graph shows how much one channel is fed into the other, whereas the Frequency Response of the XM6 is always near-perfectly flat (except when the Bass or Treble Boost is enabled, of course). The Frequency Response graph is shown in the "Specifications" section.

Can I use the XM6 to amplify a Line-Level signal, such as from a CD player or VCR?

Yes.

Can you send me the schematics?

Sorry, we do not release our schematics.

What does "Socketed amp" mean?

Normally the amp chip is soldered onto the board. "Socketed amp" means a socket is soldered to the board, and the amp chip plugs into the socket. This allows the amp chip to be removed and replaced easily, without soldering and desoldering. Some folks like to try out different amp chips until they find one pleasing to their particular ear. This is, of course, something that only the harder-core audiophile types will do; for most of us, the stock OPA2134 chip is more than adequate.

What other amp chips will fit the XM6?

Please see this page for details on what amp chips and buffer chips can be used in the XM6.

Can I directly power the XM6 from my car's (or motorcycle's) +12volt cigarette lighter?

No. If you want to permanently power the XM6 from a vehicle, you need an inverter and use the charger/adapter supplied with the XM6.

What are the dimensions of the XM6?

Click here to see a dimensioned drawing.

Do you have a 2-output version?

Sorry, we currently do not.

Do you have a version that takes 1/4" plugs, instead of the 1/8" (3.5mm)?

No, we don't (they physically wouldn't fit within the case). Your only option is use an adapter.

Do you do custom work?

Sorry, we do not do custom work or modifications.

What is the power output of the XM6?

The XM6, when equipped with BUF634 output buffers, is capable of outputting on the order of 1 full watt per channel. When using copper busbars in place of output buffers, the XM6 can output a maximum of 75mW (milliwatts) per channel, when operating into an ideal load of 120 ohms. Since most headphones usually have a sensitivity somewhere around 100dB per 1 milliwatt, the XM6 has plenty of power to spare.

Why would you want 75Ω output impedance?

The XM6 is equipped with a switchable 75Ω output impedence. There are several headphone systems which can benefit. In particular, Etymotic Research's ER-4P in-ear headphones, when used with the 75Ω feature enabled, perform nearly identically to the generally-regarded-as-much-better, and much more expensive, ER-4S's. Other headphones, especially lower-impedance headphones, can also benefit. It is a matter of personal preference.

What is the typical battery life of the XM6?

Your XM6's battery life is dependant on which amp chips/buffering is used, and what kind of load it is driving. Here are approximate numbers:

Configuration	Approximate Battery Life (Hours)
OPA134 + Busbars	35
OPA134 + BUF634	28
AD8397 DoubleCaT	21
AD8610 + Busbars	44

What is the Aux Connector on the back of the XM6?

The auxiliary connector contains LineOut audio signals, +5V USB power, and digital signals to control USB playback (provides the same functionality as the multimedia controller stick). We currently do not have a product available to interface to the connector, but hope to have one "soon". We have released the pinout of the connector into the public domain; the pinout can be found in your User's Manual (download here).

---

All contents Copyright © 2015 Practical Devices Corporation