Okay. Give me an example of how I can set up
my own matrix surround sound system.
First, check out your existing stereo system to see if it already accommodates surround sound. Many systems, even some low cost mini units, already have connections for matrix surround. If you see an additional pair of speaker jacks (usually labelled "rear") in the back of the unit, then you most likely have surround sound capabilities built-in to your current gear. Many such systems also have a switch labelled "two-by-two matrix" or "matrix surround". This has to be switched on, after the additional speakers have been attached, to activate the matrix circuit.
Of course, even if your current system does have a "matrix surround" output, unless you can specifically control the surround speaker volume, you can often get better results by building your own surround decoder. The circuits I show below use only "speaker level" wiring. No internal modifications are needed to your existing equipment.
Diagram #1 - Passive Matrix Decoder Utilizing A
Phantom Center Channel
If your existing stereo does not have surround matrix capabilities, the diagram shown (immediately below) illustrates a very simple way to set up a good basic surround sound system for very little cost. For parts, all you need are an extra pair of speakers, additional speaker wire, and a potentiometer to use as a volume control. The rear speakers should be reasonably efficient. Preferably with an impedance rating of 8 to 16 ohms. The potentiometer should be rated for at least 3 watts and can have a value anywhere from 25 to 200 ohms.
In the diagram above, the black box represents your stereo receiver or amplifier. The brown boxes denote your speakers, and the potentiometer (volume control) is shown near the top. Of course, you can locate your volume control anywhere in the surround portion of the circuit. It is shown on the top of this diagram purely for convenience.
To summarize the above circuit, the left front and right front speakers are attached normally. A wire also goes from the right front "+" (positive) speaker terminal to the right rear "+" (positive) speaker terminal. Another wire connects the left front "+"(positive) speaker terminal to one end of the surround volume control. The other end of the volume control is then connected to the left rear "+" (positive) speaker terminal. Finally, a speaker wire connects the right rear "-" (negative) speaker terminal with the left rear "-" (negative) speaker terminal.
Features of this circuit
This simple surround system provides you with a left, phantom center, right, and rear surround channels. (Even though you have two speakers in the rear of the room, they both generate the same surround channel.) This is probably the most common matrix surround system in use today. Note, it is, in a sense, only a "3 channel" system. (In that that you do not have a physical center channel.) However, if you are placed relatively close to the center of the listening area, you will experience a "phantom" center channel. That is, sounds that are in phase in both the left and right front speakers will appear as coming from directly between the two front speakers.
Indeed many fans, of passive matrix surround sound decoding, believe that you are better off without a physical center channel when using your surround system for music playback. This is because, in general music listening, a physical center channel tends to "narrow" the soundstage. (In fact, many people with Dolby Pro-Logic units, turn off the center channel when listening to music for this very reason.)
A word of caution
Some very low cost stereo systems may have amplifiers that cannot handle the additional load of attaching two additional speakers in this manner. If your stereo receiver (or amplifier) is rated at 50 watts or more per channel, you will unlikely have a problem. Even if your stereo is rated for just a few watts per channel, this surround sound circuit should not present a problem as long as you use reasonably efficient rear speakers and do not play the system at extreme levels. (For extra protection, one can always add a two amp speaker fuse to the circuit if desired.)
Obtaining a potentiometer
Any local electronics parts supply house or a TV/electronics repair outfit should be able to obtain this item for you. (I do not see Radio Shack currently carrying potentiometers with sufficient wattage handling for this circuit, but you may want to check there to be sure.) A volume control is essential in order to achieve the proper surround effect. Without it, your surround channel is likely to be too loud. Also, though my circuit diagram does not show it, I highly recommend adding a switch to the circuit as it is sometimes desirable to completely turn off the surround sound speakers. (The surround channel can be very noisy when receiving a weak FM or TV signal.) Additionally, a switch gives you a quick way to compare the effectiveness of your surround sound system. With a switch you'll be able to do quick surround on/surround off comparisons.
Rear speaker selection
The rear speakers do not need to be the same size as your front speakers. In fact, it is very common to use a smaller, less expensive pair of speakers in the rear. If you already have rear speakers as good as your front speakers, great! But you can get by with smaller speakers because, 95% of the time, the surround channels are producing only the ambient, reflective sounds which are much less demanding of loudspeakers. In my home setup, I have very efficient bass-reflex speakers as my front pair. (They feature a 10" woofer and 1" tweeter.) I bought a pair of Sony (acoustic suspension) speakers for $100. They feature a 6 inch woofer and 1 inch tweeter. I have found them quite adequate in reproducing both the thunderous effects found in surround sound movies as well as the ambient sounds in music reproduction.
Rear speaker placement
Feel free to experiment with the placement of the rear speakers. My own setup has the rear speakers mounted on my rear wall next to the ceiling. They could just as easily have been located on the floor along the rear wall or placed (or mounted) along the side walls of my listening area. There are many different philosophies regarding rear speaker placement. It generally comes down to a matter of personal preference and what works in your listening environment.
If you do mount your speakers along the rear or side walls, you will, of course, have the back of the speakers against the wall with the sound firing out across the room. However, if your rear speakers are going to be located on the floor, and not permanently mounted, you may wish to experiment with facing these speakers in different directions.
Some matrix surround listeners, even end up positioning the rear speakers a few feet from the rear wall--with the front of the speakers facing the rear wall! This forces much of the rear channel sound to bounce off the rear wall prior to arriving at your ears. This technique accomplishes two goals. First, it provides a chance for the rear channel sound to spread out a bit more prior to arriving to your ears. Thus enhancing the ambiance effect. Secondly, it provides a means of "delaying" the rear channel sound by a few milliseconds.
Calibrating the system
In order for this circuit to work properly, you'll want your left and right channels as equally balanced as possible. An easy way to adjust your balance is to play a mono recording (or other mono source such as AM radio) and adjust the balance control on your stereo until there is no sound coming thru the rear speakers.
You can then play a stereo recording. Adjust the volume of your rear speakers to a point just below where you are conscious of hearing them as distinct sound sources. (Make sure you are adjusting the volume at your usual listening location and position.) The paragraph immediately below further discusses the rear volume setting.
Adjusting your surround (rear channel)
After you've completed installation of your surround sound system, your first temptation will be to play your surround speakers rather loudly. While this may provide a dramatic effect for movies and music, eventually you will want to adjust your system so that the surround effect is much more subtle. In fact, one of the interesting things about a surround sound system as that you normally don't hear the rear speakers as a distinct sound source at all. (But switch them off and you will notice an abrupt collapse of the sound field towards the front wall.)
I've also noticed that different material often requires a different surround volume setting. This is especially true for surround sound encoded movies. Some movies require a larger rear speaker volume setting as their surround effects are very subtle. Other movies have an incredible amount of music and sound in the rear channels and require a reduced setting for the rear speaker volume. If you have trouble hearing the movie dialogue it may be due to your rear speaker volume being set too high.
Audiophiles often refer to a room or listening environment as being "alive" (more reverberent than normal) or "dead" (very little room reverberation). In two channel stereo listening, audiophiles often try to achieve that delicate balance between too little or too much room echo. Too much echo and the imaging suffers. Too little echo and the sound lacks "life".
With surround sound you definitely do not want an "alive" room. Why? Because the surround speakers provide the "room" sound that comes from the original performance. A recording of a symphony in a large hall will sound like it's being played in a large hall. Play back a CD of a pipe organ being played in a large cathedral and you'll get the reverberation of the large cathedral in your own listening environment! So if "treating" a room for surround sound, error on the side of a more absorbent room for your surround sound listening environment. If your room is too "alive", hang up large drapes around the windows, or put up some large canvas paintings for the walls. Let the surround speakers provide the "you are there" experience as you play back your favorite recordings.
Modification to the above circuit
As the circuit is shown above, each rear speaker will be in phase with its corresponding front speaker. Some listeners of matrix surround prefer to have the rear speakers out of phase with their front counterparts. This can be done by wiring the right front speaker "+" (positive) terminal to the right rear "-" (negative) terminal. Then go from the right rear "+" (positive) speaker terminal to the left rear "+" (positive) terminal. To complete the circuit, attach a wire from the left rear "-" (negative) speaker terminal to the left front "+" (positive) speaker terminal.
Are there other variations of this "passive
matrix decoder"? Such as one with a center speaker?
Yes. You can do that too with a passive decoder. However, the addition of a center speaker will have a tendency to "narrow" the soundstage as the center speaker will contain the left and right signal information as well. Nevertheless, a center speaker can improve dialogue imaging for off-axis viewers. The diagram below represents a passive matrix surround sound system in a typical "5 speaker" home theater layout arrangement.
Diagram #2 - Passive Matrix Decoder Utilizing a
Center Channel Speaker
In the above circuit, the"-" (negative) terminals of both the left and right amplifier channels are shorted together (see "A note of caution" below). This short is then attached to the"-" (negative) terminal of the center front speaker. The "+" (positive) terminal of the front center speaker has two wires attached to it. One goes to the "-" (negative) terminals of the left front speaker and the other wire goes to the "-" (negative) terminal of the right front speaker. The right channel "+" (positive) output of the amplifier is wired to the right front speaker "+" (positive) terminal.. The left channel "+" (positive) output of the amplifier is wired to the left front "+" (positive) speaker terminal. As in the first diagram, a wire goes from the right front "+" (positive) speaker terminal to the right rear "+" (positive) speaker terminal. An additional wire goes from the left front "+"(positive) speaker terminal to a potentiometer (volume control) which is then connected to the left rear "+" (positive) speaker terminal. Finally, a speaker wire connects the right rear "-" (negative) speaker terminal with the left rear "-" (negative) speaker terminal.
A note of caution:
Note that this circuit requires you to "bridge across" (essentially short) the two negative terminals of your left and right amplifier outputs. Some systems tolerate this. Others don't. If at all possible, check with the manufacturer before trying this particular circuit. Also, to save money, some people have implemented the above circuit using only one rear speaker with excellent results. (Note, to reduce the potential for stressing your amplifier, if only one rear speaker is used, ensure that the impedance rating for that speaker is at least 8 ohms.)
Calibration of the above circuit
The balancing and rear speaker adjustment for this 2nd circuit is the same as for the first circuit listed on this page.
Is there any way I can derive 4 channels
without a center speaker?
Yes. 4 channel matrix sound can be decoded using the same speaker layout as in the first diagram. The wiring is just a bit more complicated and you'll want two potentiometers and an 8 ohm resistor for this circuit. In the diagram below, you'll see where the left and right negative amplifier channels have been bridge and connected with an 8 ohm resistor to the wire connecting the two rear speakers together.
Diagram #3 - 4 Channel Passive Matrix System With Phantom Center Channel
To summarize the above circuit, the left front and right front speakers are attached normally. A wire goes from the right front "+" (positive) speaker terminal to a potentiometer with is then connected to the the right rear "+" (positive) speaker terminal. A wire goes from the left front "+"(positive) speaker terminal to a potentiometer which is then connected to the left rear "+" (positive) speaker terminal. A wire connects the right rear "-" (negative) speaker terminal with the left rear "-" (negative) speaker terminal. Finally, a wire goes from both the "-" amplifier output channels to an 8 ohm/10 watt resistor. The other end of this resistor is then attached to (either one) of the rear speaker "-" (negative) terminals.
A note of caution
Note the 8 ohm resistor should be rated for at least 10 watts. Radio Shack sells an excellent resistor for this circuit (part# 271-120). Keep in mind that, like the 5 speaker arrangement shown above, this circuit requires you to bridge across the negative terminals of your stereo receiver/amplifier. Use extreme caution when first attempting this as some amplifiers cannot handle the bridging of their negative speaker terminals.
Features of this circuit
This particular circuit (taken directly from the days of quadraphonic "4 channel" stereo) provides you with the following:
A normal left front speaker channel. A normal right front speaker channel.
A left rear speaker channel containing the difference of 75% of the left channel and 25% of the right channel.
A right rear speaker channel containing the difference of 75% of the right channel and 25% of the left channel.
With the right material, you could actually derive 4 channels
of information from the above circuit. Note, however, that
channel separation will be pretty minimal. It takes careful
adjustment of the potentiometers and proper placement within the
sound field to truly experience a "4 channel stereo"
This seems a bit too easy. Surround
sound without a new receiver. Does this stuff really work?
Indeed it does. Of course, it's not as refined or as sophisticated as a Dolby Pro-Logic system. Dolby surround systems provide a short delay to the rear speakers and Pro-Logic units use a form of channel steering to increase the separation between channels. However, the above systems do provide excellent ambiance extraction and deliver excellent rear channel effects at a very low cost. You can always add new equipment later should you decide to obtain a more "refined" surround sound decoder.
Are many people using these circuits?
These matrix surround circuits have been in widespread use since the mid 1970's. They first surfaced in the days of Quad Stereo but now have become especially popular as a means of setting up surround sound systems with a very minimal investment.
In fact, many stereo systems that feature matrix surround, utilize one or more of the above circuits in their systems. I once had a "high end" Zenith stereo (built in the early 70's) that utilized a variation of the circuit shown in diagram #3 (above).
For those buying a new receiver...
If you're going to be buying a new stereo receiver, I highly recommend that you purchase one with a built-in Dolby Digital decoder. Prices are dropping every year making Dolby Digital more and more affordable. Dolby Digital units will work with DVD and High Definition TV in a way that is much better than what current matrix systems can provide. (Dolby Digital receivers also offer Pro-Logic and usually one or more basic surround modes for music listening.)
In the meantime, why not buy some nice additional speakers and set up one of the above systems. A good set of speakers can easily last 30 years or more and will most likely be perfectly suited to any future surround equipment you purchase.
I'm no longer actively working with surround sound. Still, if you have a question relating to this material, feel free to send me an email. My name is Chris and I can be reached at: firstname.lastname@example.org (Or just click on the envelope icon below to activate your email software.)
This web page was last updated on: 02/10/11 12:46 PM Pacific Time.