A Multiroom System
Using Existing Components
One rainy March evening a number of years ago, I
inadvertently bought almost a thousand feet of wire. It was attached to a house my wife
and I had fallen in love with, and as we were admiring the window casements and asking
what we hoped were pertinent questions about the plumbing, the owner casually mentioned
that he had had the whole place wired for audio.
We bought the house, of course, and one of my projects was
to find out just how it had been wired -- there were bits of zipcord protruding from holes
in every room, but nothing was labeled. Even the former owner had no idea what was
connected to what; a nephew had strung the wires, but since he had failed to provide any
indication as to where any of them came from or went, the system had never been used.
Like many households, ours had audio of some sort in
several rooms. There was a full-blown system in the living room, the family room housed an
audio/video setup, while an old receiver and pair of speakers in the bedroom served as a
clock radio, thanks to a handy little electronic timer. And occasionally we listened to a
boombox on the patio. Quietly, of course.
Those were all independent systems, with different
functions, and normally we used only one at a time. There were occasions, however, when it
was useful to tie all of them together. The idea of occasionally piping music from the
main system to the patio or the bedroom was attractive, especially since a built-in
network already existed.
Obviously the original purpose of all that wire was to feed
a number of extension speakers from a central amplifier, as 16-gauge lamp cord had been
used throughout the house, but there are good reasons not to do it that way. Hooking a
bunch of speakers together in parallel could result in an overall impedance low enough to
damage most amplifiers. And the distances from the main system to the outlets in other
parts of the house were as much as 100 feet; runs of that length require much heavier
cable than had been installed.
An even bigger drawback to driving all the speakers from a
central amplifier, however, is that such an arrangement offered no opportunity to control
the sound at the remote locations. Speakers vary in efficiency and balance, rooms vary in
size and acoustic characteristics, and listeners have different preferences, so it was
essential to be able to set both level and tone controls in each room.
The challenge, then, was somehow to present the inputs of
each remote system with a flat, line-level signal fed from the central system, so that it
could be selected and adjusted on the spot. One way to do this would have been to replace
the existing wiring with balanced, shielded cable that simply took a flat, line-level
signal from the main preamplifier and fed it to the other systems.
I was unwilling to rip my walls apart to replace wiring
that was already there, however, so I decided to devise a system in which the
less-than-ideal nature of my built-in cable would not matter, but which would still
deliver a clean signal to each remote location. In the process, I thought I might be able
to economize by employing some old audio equipment.
Two things were required to coax a high-fidelity signal
through my zipcord labyrinth. First, the system's impedance had to be very low to prevent
line losses; second, the signal level between the master system and the remote locations
had to be high enough to override any interference or hum and yet not overload the
high-level inputs of the local amplifiers.
The solution was to use a small power amplifier to drive
the system, and I pressed an ancient 15W receiver into service for this purpose. While
that amount of power would not be enough to drive very many speakers satisfactorily, which
is why the unit was retired, it would be more than ample for my purpose; even with the
output kept to the lower part of its operating range, the signal would be many decibels
higher than any spurious noise likely to be picked up along the way.
Also, because the receiver's output would rarely exceed a
watt or two, the system's performance would not be adversely affected by the thinness of
the wires between the main system and the other rooms. The output impedance, designed to
drive 8-ohm speakers would be appropriately low as well.
To connect this "house system" to the individual
remote amplifiers, additional resistance would be required to drop the few watts produced
by the central amplifier to a level acceptable to the local system's line inputs. I could
have experimented with numerous fixed resistors to find one with the right value, but
using a variable resistor (a "volume control") would not only have the advantage
of cutting the signal to a manageable level, but would also allow it to be matched to
other input levels at each location. The specific rating of such a control would be fairly
arbitrary because it could easily be adjusted to an appropriate value.
The first step in turning my theoretical circuit into
reality was simply to find all the outlets. Then, once I had located all the wires, I
untangled them to make sure there were no visible short circuits, and stripped those ends
that required it.
In the living room, one of the main system's tape monitor
outputs was connected to a high-level input on the distribution amplifier. The volume
control was set to its minimum so that the system would in effect be fed silence, and the
speaker terminals were connected to the wires leading to the rest of the house. Then, the
power was switched on. Each remote amplifier was connected to the system in turn, with the
proper input selected but the level down. Gradually, this was turned up, and any peculiar
noises -- hum, crackles, radio interference -- noted. Fortunately, no disturbing sounds
were heard. Finally, it came time to feed an actual signal through the system.
Very gingerly, I advanced the level on the distribution
amplifier a notch or two, then checked at each remote location to see what was coming
through.
It is almost axiomatic that an audio system as complicated
as this will initially be riddled with bugs, glitches, and other similarly unpleasant
things. To my great relief -- and surprise -- everything worked perfectly right off the
bat.
...Ian G. Masters
ian@mastersonaudio.com
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