This could be a ground loop hum/buzz or possibly a transformer hum through the speakers. In fact, large subwoofer transformers are more susceptible to a slight hum noise.

A speaker hum at your listening location is never normal. To determine if the sub amp is functioning correctly, the best thing to do is disconnect all of the connection inputs going into the subwoofer. Keeping the subwoofer plugged into the AC outlet, turn it on; if the hum still exists, there may be a ground loop issue or a problem in the sub amplifier. In that case, the amp should be serviced or replaced. If there is no hum at that point, the issue might involve the preamp or another ‘upstream’ source component. Some common causes of a ground loop hum are cable TV connections, digital recorders or satellite dish receivers.

Cables specifically designed for subwoofer use typically are better shielded than standard ‘patch cables’ and may offer possible noise reduction or cancellation from electrical devices the cable may come in contact with. Some subwoofer cables are directional, (one end always hooks up to receiver, the other to the sub), so always read any instructions that come with your specialty cabling.

It is generally believed that the bass you hear below approximately 80 Hz is non-directional. This means that you can point the loudspeaker in any direction and the sound will still reach the listener's ears. Since much of the bass that subwoofers produce is below that frequency, you can place the subwoofer almost anywhere in the room. This is the opposite of full-range speakers, which have just a few placement options in a room that allow them to sound good, since they must be positioned for the best combination of imaging and tonal balance. Putting a subwoofer in a corner of the room may cause the sub to sound louder. If your subwoofer is a ported design, keep it at least twice the diameter of the port exit (probably 6-12 inches) away from the nearest wall, so that air flowing out of the port is not obstructed. If the bass seems too boomy, you can fine-tune the sub by moving it farther from the wall until it sounds smooth to your ears.

The best way to find the ideal place for your subwoofer is to hook it up and put it right where you will be sitting in the room. Set the subwoofer to level, low pass and phase. Play something with consistent, deep bass and move around the room on your knees; this way your head is about where it would be when you are seated. The spot where the bass sounds best is a spot where you might put your subwoofer. You may find more than one location.

Corner placement of the subwoofer typically yields the loudest output (highest efficiency). This does not mean that it will always sound best in this position; experimenting with placement is always suggested. It's also important to have the sound from the sub reach the listener in sync with the sound from the main speakers; otherwise the sound may not blend properly. You should not be able to hear your subwoofer as a separate entity—it should seem that your main speakers go deeper with greater impact and authority.

The optimal connection is to use the RCA (Phono) "sub" or "woofer" line out found on Home Theater receivers and pre-amps. Some subwoofers offer High Level inputs (speaker wire connections), they are there for use with a receiver or pre-amp without a sub RCA connection. You don't need to use both and in most cases it's not recommended.

Just as receivers and preamplifiers are different across brands, subwoofer brands may be different as well. Every room may be unique in the way it supports or cancels low frequency information. The quantity of bass desired may differ for each listener. Additionally, hookups to the same amps or receivers and the same subwoofers can produce different results if "bass management" settings (speaker size selectors, etc.) or subwoofer settings are different. The best thing to do is to experiment until it sounds just right for your listening style.

In some rare cases, you may find an RCA connector marked LFE (Low Frequency Effects) on a receiver or pre-amp; hooking up the "LFE" jacks usually only sends bass special effects of a 5.1- encoded movie to the subwoofer. In that case, when music is played on a two-channel source (for example, a CD), the subwoofer may not receive any signal at all. That situation would call for a hookup of both the "LFE" jacks and the speaker terminal jacks to a subwoofer, in order for the sub to produce bass with all sources. You would need to set the main speakers to "large" in the receiver or pre-amp set-up menu. If you have both LFE and sub out connections on a receiver or preamp, your best choice would be to use the ‘sub out’ connector, because the LFE signal may still be mixed in with the sub channel.

Be sure to check the owner’s manual that came with the subwoofer about connecting to both LFE in and speaker wire inputs. With some older subwoofers, this was listed as "optional" and it's fine to connect to both; however with most subs today, this is not recommended, as it may damage the sub electronics. So double-check the manual or contact our tech support department to clarify any questions regarding Klipsch subs.

In the end, it comes down to what sounds best to you, given your equipment and your individual tastes. It takes some time and experimentation, but when you have listened to all your options, you’ll know which one is best for you.

The second method applies to amps and receivers that have a separate subwoofer out jack or a separate LFE (Low Frequency Effects) jack. The LFE output is the .1 in the Dolby® Digital 5.1 designation and applies to those amps or receivers that have that feature. Remember that 2-channel recordings never have any LFE signal. These connections use an RCA jack, so a cable with those jacks is necessary (special "subwoofer cables" use these jacks and may be purchased separately). Connect the "line out" jack of the amp/receiver to the "line in" jack of the subwoofer (or the "LFE" out to the "LFE" in); two-channel signals will have “sub out” but NEVER have LFE out. This connection allows the amp or receiver to control the low frequencies going to the subwoofer. These frequencies are directed by the "bass management function" in the amp or receiver. Examples of "bass management" include, but are not limited to, speaker size settings (i.e. Large or Small) or subwoofer output controls located in the amp/receiver.

The third method of hookup involves some combination of all of the hookups listed above

• To connect a soundcard's digital output to the GMX system, you may use the included dual RCA to mini plug y adapter.     
• For Soundblaster cards, plug the mini plug end into the S/PDIF output of the card, and the white RCA plug into the Coax In of the GMX decoder.     
• For Turtle Beach Santa Cruz, plug the mini plug end into the S/PDIF output of the card, and the red RCA plug into the Coax In of the GMX decoder.

Note: When playing an analog signal (mp3s, music CD, PC sounds, etc) through the S/PDIF output, you will hear the front two speakers and subwoofer when the decoder is in stereo mode. Changing the Surround mode to 5CH Stereo or PLII will allow you to listen through all 5 speakers.

• The input display is correct (coax or optical)        
• The DVD disc is capable and DVD menu is set to Dolby or DTS        
• The device playing the DVD is properly configured for Dolby or DTS (check player menu)

All recordings have noise, since all signals in a recording studio must pass through electronics.  GMX D-5.1 speakers have tweeters, which in a Klipsch way of thinking, improves the sound quality over the majority of multimedia speakers that don't.  The wider bandwidth, in addition to the Klipsch mantra of high sensitivity and high efficiency, forces us to pay special attention to the noise floor of the electronics.  If the sensitivity of the speakers goes up by 3dB, the noise in the electronics has to go down 3dB to keep the same noise floor

Although the GMX D-5.1 decoder is a complex piece, it has a relatively simple signal path, which is typical of other systems in its price category.  There is a digital receiver which takes the bitstream (PCM or S/PDIF) from an optical or coaxial digital cable and routes it to the DSP (digital signal processor).  There is an ADC (analog to digital converter) that takes the analog input signal and routes it to the DSP.  The DSP itself takes the bitstream and decodes it into multiple digital signals, which are then sent to the DAC (digital to analog converter).  The ADC and DAC are integrated into a CODEC (coder/decoder), which also does the volume control in the digital domain.  The DSP performs bass management duties through the use of digital filters, specifically tailored to the needs of the GMX speakers (no canned “large” and “small” setting here).  After the DAC is a low-noise buffer/gain stage which sends the now analog signals to the power amplifier.  The GMX uses high quality Crystal 493xx DSPs and 24 bit CODECs, as well as 5532 op amps.

The typical ProMedia system has about 30dB of gain.  This allows a wide variety of signal sources to be connected and still produce full system output.  If there is any noise in the input signal, it is multiplied by the same gain factor.  The difference between a sound card's “line level” output and that of an MP3 player can be as much as 18dB.  If the input signal has noise, the output signal will have 30dB more noise at full volume, and there is nothing anyone can do about it.

One of the decisions in a system like the GMX is where to set the gain.  As mentioned before there is a wide variation in the level of the analog input signals.  The gain has to be high enough so that people using MP3 player are not disappointed, and still allow the higher level inputs of sound cards without causing problems from overdriving the inputs.  With the system architecture of the GMX, if you add 6dB of gain you add 6dB of noise, and the analog input is like an “open mic.”  There is no way for the DSP to know whether a signal is present or not, so it applies gain to the signal, even when no “signal” is present.

If you connect the digital output of a sound card, you would expect that when there is nothing “playing” that the system would mute in the same way, but that is not the case.  The sound card can behave like an “open mic” as well.  Look at the sliders on the Windows volume control.  The sound card takes all these inputs, mixes them, and puts out a digital bitstream based on the sum off all the inputs.  Even with all the sources muted, the soundcard spits out a digital bitstream.  When the DSP in the GMX decoder detects a digital bitstream it un-mutes.  Usually this isn't much to worry about, but it can add up, and some sound cards are better than others. On the other hand, with the digital inputs, if the DSP does not detect a bitstream, then we have the option of “muting” the outputs of the decoder, which we do.  So if you connect the GMX to a DVD player through the optical connector and pause or stop the movie, the GMX recognizes the loss of the digital bitstream and the system mutes.  

During times when your not listening to your system, like at night when the hiss might bother you, the mute or power button can be used to silence the speakers.