Introduction

In the basic Dolby Surround, four audio channels are encoded into two, which are equivalent to the left and right channels of a stereo system. The center channel is the sum of left and right signals, and the rear channel is the difference.

This means that it is easy to add a rear channel to any two-channel (stereo) amplifier, by connecting a third speaker between the positive leads of the main speakers. This kind of setup is not intended to replace a proper Dolby system, but it can bring some new life into your existing gear. Therefore I recommend using something simple and inexpensive for the rear speaker. As there is hardly any bass in the rear channel, the surround speaker(s) can be quite small.

For a more complete system you can add two rear speakers in series. There are several possible configurations, none of which is the proper or the best one, so feel free to experiment. My personal setup is the one below, also known as

The Poor Man's Surround Sound System

 left main                                right main
  speaker                                   speaker
   ,---.          ,--------------.           ,---. 
   |   |          |  STEREO AMP  |           |   |
   |+ -|          | - +      + - |           |- +|
   `---'          `--------------'           `---'
    | |             | |      | |              | |
    | `-------------' |      | `--------------' |
    |                 |      |                  |
    +-----------------'      `------------------+
    |                                           |
    |                                           |
    |    ,---.                         ,---.    |
    |    |   |                         |   |    |
    `----|+ -|-------------------------|- +|----'
         |   |                         |   |
         `---'                         `---'
       left rear                     right rear
        speaker                        speaker
   
   

Technical remarks

Notice how the rear speakers are connected in series, in opposite polarities. This may seem quite weird, as if the two would cancel each other's sound. In practice it sounds quite pleasant, and the sound will be spread around well. You can also try with the same polarities for a different effect, which is closer to the original Dolby Surround. In either case, there is really only one rear channel (L-R), so there is no single perfect way of distributing it over the speakers.

In some cases the rear channel is too loud, and it can be corrected with a series resistor of about 10 ohms; try different values for the best overall sound. It depends on how the the impedances and sensitivities differ between the main and rear speakers. The resistor is more likely needed if you only use one speaker.

While Dolby Surround is usually associated with movie soundtracks, it seems that many pieces of music are also produced with surround systems in mind. Instruments can be panned between front and rear, as well as left and right. However, many classical recordings are better off without the surround, as they are specifically recorded for 2-channel stereo.

Shortcomings

In this setup the rear channel gets a voltage difference, whereas the current difference would be more appropriate. However, the result can still be good for such a simple hack.

Real surround amplifiers usually add some delay into the rear channel, to make your room sound larger. In addition they have the center channel, but it is not always necessary; if you listen at roughly equal distances to the main speakers, you get the same center-channel sound anyway. The center speaker is more important for bigger audiences.


Source: This is an old trick of audio enthusiasts. I think I first found it in the Finnish Hifi magazine some years ago.

Disclaimer: I know how technically incorrect the setup is (I have an M.Sci. degree in physics), but it sounds better than it seems. Just try it and listen.


Update on April 23, 2003:

Adding the center channel

The surround (difference) channel is effectively in parallel with the main speakers. It may not be surprising that the center (sum) signal can be obtained by adding yet another speaker in series with the main ones.

Unlike the surround boxes, the center speaker should be at least as good as the left and right ones; it will play back most of the bass and lead voices. You will have to consider if you really need it, as it may introduce further problems.

If you already have the surround speakers, they will need more attenuation when the center speaker is added. This is due to the increased impedance in the front channels. I am currently using 23.5 Ω (2x 47 Ω in parallel) in series with the surround pair, and my speakers are between 6 and 8 Ω. Here is the full system diagram:


         left                          right
         ,---.                         ,---.     
         |   |                         |   |     
    ,----|+ -|-----------+-------------|- +|----. 
    |    |   |           |             |   |    |
    |    `---'           |             `---'    |
    |                  ,---.                    |
    |                  | + |                    |
    |                  |   |center              |
    |                  | - |                    |
    |                  `---'                    |
    |                    |                      |
    |                    |                      |
    +---------------+ +--+---+ +----------------+
    |               | |      | |                |
    |             ,--------------.              |
    |             | + -      - + |              |
    |             |  L        R  |              |
    |             |              |              |
    |             |  STEREO AMP  |              |
    |             `--------------'              |
    |                                           |
    |                                           |
    |                                           |
    |    ,---.                         ,---.    |
    |    |   |                         |   |    |
    `----|+ -|----------####-----------|- +|----'
         |   |        resistor         |   |
         `---'                         `---'
       left rear                     right rear
   
   

Notes and potential problems

  • Do not run the cables in big loops, it's just to make the ASCII diagrams clearer! Use paired cables like you normally would with speakers, and connect them up accordingly. You can use a single pair between the amp and the rear set. (Loops will cause trouble via electromagnetic induction.)

  • Make sure your amplifier can handle the extra load. The increased impedances mean that you need a higher volume setting for the same physical sound level. However, you'll need less power per speaker to achieve satisfactory levels, so the increase in power consumption is not that big.

  • The diagram assumes common negative (ground) at the amplifier poles (which is why the minus poles can be connected together). This is not universally true, although it is very often so.

  • Sounds mixed to the center will be played from all three of the front speakers. The result is a rather complex diffraction pattern which is slightly detrimental to the soundscape, particularly bass which has the longest wavelength. The problem is further complicated if the center speaker is different from the L/R speakers, because they may have different phase responses. Also, such diffraction is not limited to this simple setup; it comes in any surround system where the center signal equals L+R.

As goes for hi-fi in general, do not worry too much about the technical shortcomings. Trust your ears, experiment and enjoy!


Idea adapted from Building Hi-Fi speaker systems by M. D. Hull.

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