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.