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XM4 General FAQ

Why would I need a Headphone Amplifier?
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 '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' XM4, with its maximum 23dB of gain, and full 9-volt output swing, are able to drive even very high-impedance headphones.

XM4 Technical FAQ

What kind of headphones will the XM4 drive?
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 range, you may need to install the gain jumpers for the extra +10dB of gain.

Can I use the XM4 to drive small speakers?
Usually, no. The XM4 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.

Can I use the XM4 to amplify a Line-Level signal, such as from a CD player or VCR?

Can you send me the schematics?
Sorry, we do not release our schematics. Please don't ask.

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?
Most dual amp chips in an 8-pin dip package will work. A popular alternative amp is the OPA2227. We don't stock these other amp chips; however, you can get them from many electronic supply stores (,

Can I run the XM4 from my car's (or motorcycle's) +12volt cigarette lighter?
Yes. However, the +12v on cars is notorious for having a lot of alternator noise; this shows up as a high-pitched whine whose frequency varies with engine RPM. If you hear this whine, any local car stereo shop can set you up with a filter for cheap.

What external power supply should I use?
If your XM4 came equipped with the Lithium FastCharge® system, just use the included supply. If your XM4 does not have the FastCharge system, then the power supply must have a "Barrel-type connector which takes a 1.3mm center pin, center pin positive". The milliamp (mA) rating of the supply does not matter. Power supply must be 6 to 15V DC (if you want to use the supply to recharge a NiMH or MiCd battery, then the supply must be Regulated 12VDC ? 5%). One such 12VDC regulated supply is RadioShack PN#273-1773 (make sure you ask for Adaptaplug "H" pn#273-1711, which is included with the adapter). Note: If you purchased an XM4 before Feb1,2006, or you have an XM3, then your power supply must have a "2.5mm mono Audio plug" on the end (RadioShack Adaptaplug "E" 2.5mm connector pn#273-1708)

What rechargeable battery should I use?
We recommend any 8.4volt Nickel Metal-Hydride (NiMH) battery. An example is RadioShack#23-529.

What are the dimensions of the XM4?
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 XM4?
The XM4 is capable of 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 XM4 has plenty of power to spare.

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