When setting up a home theatre, there's often a need to run
fairly long video cables between your signal sources (DVD player, VCR and/or
laserdisc player) and your big screen display. The reason for this is simple -
it isn't always convenient to have the signal sources and the display at the
same end of the room.
Of course, there's no great problem feeding audio signals over
long cables, provided that the cables are of reasonable quality. However, that's
not the case with video signals due to their much greater bandwidth. Video
signal frequencies can range up to 5MHz or more (as against just 20kHz for
audio) and can suffer quite noticeable degradation when fed through cables
longer than about five metres.
This signal degradation is due mainly to cable capacitance.
This causes high-frequency losses and occurs even when you use high-quality
coaxial cable that has been correctly terminated at each end. The resulting
pictures lack contrast and colour saturation, and also become noticeably
"softer" (ie, lacking in fine detail) due to the lower
The best way to tackle this kind of problem is to use a video
"booster" every five metres or so. Basically, you take a 5-metre cable run and
plug it into the booster - essentially a wideband video amplifier. The booster
restores the incoming signal so that it is close to original before feeding it
to the next 5-metre cable run and so on.
A booster for conventional "composite" video signals needs just
a single wideband video amplifier channel. However, if you want to take
advantage of the higher quality available from the "S-video" output of your DVD
player, the booster needs two channels. That's because, in S-video, the
luminance ("Y") or black-and-white picture information is not combined with the
chrominance ("C") or colour information. Instead, the two signals are fed along
separate cables to prevent them interacting - see Fig.1(b).
Fig.1: this diagram shows how the video booster is connected for composite video signals (top), S-video signals centre and component vide signals (bottom). It's basically a matter of buying (or making up) the necessary cables.
The video booster described here can handle either composite or
S-video signals as required, because it uses an IC which actually contains four
wideband amplifier channels. This allows us to devote one channel to the
composite video input and output, while two more are dedicated to the S-video
input and output sockets. This means that there's no switching and the composite
video and S-video channels can even be used at the same time; eg, to pipe
composite signals to another room while you're watching S-video signals to your
home theatre display.
The fourth channel is spare and can only be accessed
What about handling the even higher quality "component video"
signal outputs? With this type of signal, as well as the luminance (Y) being
kept separate, the two "colour difference" signals (R-Y or "Cr" and B-Y or "Cb")
are also kept separate - ie, instead of being combined as the chrominance (C)
If your DVD player provides these outputs and your display can
also handle them, the video booster can help here too. All you need to do is buy
or make up some adaptor cables, so that the three component video signals can be
fed through the three main booster channels - see Fig.1(c).
1 PC board, code 02104031, 117 x 102mm
1 plastic instrument case, 140 x 110 x 35mm
2 PC-mount 4-pin mini-DIN sockets
6 PC-mount RCA sockets
1 PC-mount 2.5mm concentric male "DC" connector
1 9V AC plugpack (500mA) with 2.5mm female connector
1 MAX497 quad video amplifier (IC1)
1 LM833 dual op amp (IC2)
1 LM7809 +9V regulator (REG1)
1 LM7909 -9V regulator (REG2)
1 LM7805 +5V regulator (REG3)
1 LM7905 -5V regulator (REG4)
1 3mm green or red LED (LED1)
2 1N4004 1A diodes (D1,D2)
2 2200μF 16V RB electrolytic
2 100μF 16V RB electrolytic
2 10μF 10V RB electrolytic or tantalum
2 2.2μF 35V TAG tantalum
2 1.0μF MKT
2 220nF MKT
4 100nF monolithic ceramic
Resistors (0.25Ω, 1%)
Where to buy a kit
The design copyright for this project is owned by Jaycar Electronics.
Complete kits will be available from Jaycar Electronics by the time this article
appears in print.
The A/V output sockets are all accessible from the rear of the unit. They include a 4-pin mini-DIN socket for the S-video signals, plus three RCA sockets for the composite video and left & right channel audio output sockets. The socket at far right is the DC power connector.
As well as the video amplifier channels, the booster also
includes a pair of low-noise audio line amplifiers. This means that it can also
be used to handle any stereo audio signals which accompany the video, so these
too will reach the far end of the cables in good condition.
Probably the main use for the audio channels will be where
you're feeding the video and audio to a different room. They'll also come in
handy if you need to send one or more of the signals in a 5.1, 6.1 or
7.1-channel surround sound system to remote power amplifiers; eg, you might want
to send the SB (surround back) signals from your 6.1/7.1-channel decoder to the
rear of your home theatre room, to drive a power amplifier for the rear centre
Alternatively, you might want to drive an active subwoofer with
the LFE (low frequency effects) channel signals.
As you can see from the photos, the booster is very compact.
Everything fits in a small ABS instrument case measuring just 140 x 110 x 35mm.
Power comes from a 9V AC plugpack.
Incidentally, Jaycar Electronics will be making a complete kit
for the booster available, so you should be able to build it up very easily and
at an attractive price.
How it works
The booster's video amplifier channels are all provided by IC1,
a Maxim MAX497. This high-performance device is designed expressly for handling
video signals. It includes four closed-loop buffer amplifiers, each operating
with a fixed voltage gain of 2.0.
Other features of the MAX497 include a full-power -3dB
bandwidth of over 200MHz, exceptional gain flatness (
±0.1dB up to 120MHz), low
distortion, very low differential phase/gain error between the four channels and
the ability to drive four back-terminated 75Ω (or 50Ω) output
Fig.2: the booster circuit is based on a Maxim MAX497 quad video buffer IC (IC1). One amplifier in IC1 is used for the composite video channel, while another two are used for the Y and C channels for S-video. Op amps IC2a & IC2b (LM833) boost the left and right channel audio signals.
As shown in Fig.1, we're using one amplifier for the composite
video channel and another two amplifiers for the Y and C channels for S-video.
Each amplifier has a 75Ω resistor across its input and these ensure correct termination of the
cables from the video source. In addition, 75Ω resistors are used in series with each
output to give correct "back termination" of the output cables.
As mentioned, the amplifiers in the MAX497 have a
feedback-controlled gain of exactly two. This compensates for the attenuation
produced by the interaction between the back termination resistors and the
termination resistors at the far end of the output cables.
In effect, the Video Booster "restores" the incoming signal
before feeding it to the next cable segment.
The input and output connections to the composite video amp
channel are made via RCA sockets, as these are now standard for domestic
equipment. Similarly, the connections for the S-video channels are made via
4-pin "mini DIN" sockets, as these too are the accepted standard for
Note that the fourth "spare" amplifier in the MAX497 is also
provided with input and output termination resistors. This is done to ensure
that it doesn't interact with the three active channels. The resistors will also
make it easy to use the spare channel if you ever need it.
The two audio line amplifier channels are provided by the two
halves of an LM833 dual low-noise op amp (IC2). As shown, these two stages are
identically connected as non-inverting buffers, with the 100kΩ resistors providing
negative feedback for a gain of two.
The performance of these audio buffers is quite respectable.
They have a frequency response from 30Hz to 120kHz at the -1dB points, a THD
(total harmonic distortion) below .006% for 2V RMS output, a signal-to-noise
ratio of better than 91dB relative to 2V RMS output, and an output clipping
level of just on 14V peak-to-peak (5V RMS).
The audio buffers operate with a gain of two to ensure
sufficient signal to drive your remote power amplifiers, etc. However there may
be cases where even this small amount of gain could cause problems - producing
distortion due to input stage overloading, for example.
If that turns out to be the case with your particular
application, there's a simple modification which can be done to solve the
problem. All you need do is remove the 100kΩ resistors connecting pins 2 and 6 of
IC2 to ground. This turns the buffers into unity-gain voltage followers,
increasing the overload margin by 6dB.
Left: inside the completed booster unit. Keep all component leads as short as possible and be sure to solder the leads to both sides of the board where necessary, as indicated by the red dots on Fig.3.
This photo shows how the power indicator LED is mounted on the PC board and pushed through a matching hole in the front panel.
The power supply section is quite straightforward, even though
the video and audio amplifiers require four separate DC supply rails. The MAX497
±5V supply rails, while the LM833 require
Because the overall current drain is quite low (about 100mA
total), two simple half-wave rectifier circuits (D1 & D2) are used to derive
±12.8V DC rails from 9V AC plugpack. These rails are filtered using two
capacitors and then fed to 3-terminal regulators REG1 and REG2.
REG1 and REG2 produce the +9V and -9V rails respectively. They
also drive 3-terminals regulators REG3 and REG4 which produce the
LED1 is driven from the +9V rail via a 1kΩ current-limiting resistor and provides
power on/off indication.
The associated 100μF and 10μ capacitors are used to filter the regulator outputs. The
±9V supply rails are then further decoupled using 10Ω resistors and 2.2μF capacitors before being
fed to IC2. Four 100nF capacitors provide additional filtering for the
Table 1: Resistor Colour Codes
||4-Band Code (1%)
||5-Band Code (1%)
||brown black yellow brown
||brown black black orange brown
||yellow violet orange brown
||yellow violet black red brown
||brown black red brown
||brown black black brown brown
||violet green black brown
||violet green black gold brown
||brown black black brown
||brown black black gold brown
All the parts are mounted directly on a small PC board, so the
unit is easy to build. This includes all the connectors, so there's no off-board
wiring at all inside the booster box.
The PC board measures 117 x 102mm and is coded 02104031. It's
double sided, with copper tracks on both top and bottom, but the top pattern is
mainly an earthed ground plane. Only a handful of component leads are soldered
to this top pattern, so we don't need a board with expensive plated-through
Fig.3 shows the assembly details. Begin by fitting all the
input and output connectors, as they often need a small amount of juggling and
pin straightening before they'll mount without stress. Make sure that they're
pushed down hard against the board, while you make the solder connections
Next, fit the two PC board terminal pins (for the input and
output of the spare video channel), followed by the resistors and the diodes D1
and D2. Be sure to fit each diode the correct way around as shown on
Note that some of the resistors have their "earthy" ends
soldered to the top copper pattern as well as to the pad underneath. The leads
concerned are shown with a red dot on the board overlay diagram.
The four voltage regulators can go in next. These are all
TO-220 packages and are mounted horizontally on the top of the board. Be sure to
fit each one in the correct position, as all four are different and mixing them
up could result in component damage when you apply power.
All regulator leads are bent downwards 6mm from the package
body. This allows you to mount them by pushing the leads down through the mating
holes and then fastening their tabs down against the copper using 6mm x M3
machine screws and nuts. The leads are then soldered to the pads underneath and,
in some cases, to the top pads as well - see Fig.3.
The two 2200μF capacitors and the two 100μF capacitors adjacent to REG1 and REG2
can go in next. Make sure you fit all of these polarised parts the correct way
around, as shown in Fig.3.
LED1 is fitted with its "flat" cathode side to the left (ie,
furthest away from CON4). To install it, first bend both its leads bent down 90
degrees, 6mm away from the LED body. That done, it can then be soldered into
place with its axis exactly 8mm above the board.
Table 2: Capacitor Codes
Power supply checks
At this stage, it's a good idea to check all of the power
supply wiring by plugging the lead from your 9V AC plugpack into CON9 and
turning on the power. LED1 should immediately light and you can now check the
regulator outputs. You should get +9V from REG1, -9V from REG2, +5V from REG3
and -5V from REG4. These voltages are all measured relative to board earth and
at the righthand pin of each regulator, as indicated on Fig.3.
If everything is correct, you can switch off and continue
fitting the remaining parts to the PC board. Conversely, if one or more of the
regulator outputs is incorrect, switch off immediately and check for wiring
errors. Most likely, you'll have made a mistake fitting D1 or D2, one of the
electrolytic capacitors or one of the regulators. With a bit of luck, you'll be
able to fix the problem and not have to replace any parts.
Completing the PC board
Fig.3: install the parts on the double-sided PC board as shown here. The red dots indicate where component leads must be soldered to the copper tracks on the top of the board (and usually underneath as well).
The remaining parts can now be fitted to the board, starting
with the MKT audio coupling capacitors, the 2.2μF tantalum bypass capacitors and the
electrolytic capacitors. The two ICs can then be installed, taking care that you
fit each one the correct way around.
Note that the pins for IC2 (the LM833) are only soldered to the
copper pads underneath, while some of the pins for IC1 (the MAX497) are soldered
to the top copper pattern as well. This applies to pins 1, 3, 5, 7, 9, 11 &
The next components to fit are the two 100nF bypass capacitors,
which are at each end of IC1. These mount with their "earthy" leads soldered to
the top copper pattern as well as the pads underneath. That done, fit the two
remaining 100nF bypass capacitors for IC1 and the remaining 10mF electrolytic capacitor for
the -5V rail. As before their leads are soldered to pads on the top of the
board, with their "earthy" leads soldered to the bottom pads as
All that remains now is to fit the booster board to the
First, you have to fit the front and rear panels over their
respective RCA connectors, before lowering the three items together into the
bottom of the case. That done, LED1 can be pushed into its 3mm mating hole on
the front panel and the board secured to integral pillars in the bottom of the
case using eight 6mm self-tapping screws.
Be sure to use all eight screws to secure the board. These give
the board added support in the vicinity of the various input and output
The final step of all is to fit the top of the case, using the
two long countersink-head self tappers provided. Don't lose these screws by the
way, because they're a special size and surprisingly hard to get.
Your Video & Audio Booster is now be finished and ready for
Component video cables
Before we end up, let's take a look at the adaptor cables
required if you want to use the booster for component video signals.
There's nothing terribly complicated about this. All you need
to do is buy or make up four cables - two for the luminance (Y) signals and two
for the chrominance (Cb and Cr) signals.
The cables for the Y signals each consist of single lengths of
coax with an RCA plug at each end. These connect to the booster's composite
video channel, as shown in Fig.1(c).
The other two cables are each of double coax, with a mini-DIN
plug connected at one end and a pair of RCA plugs at the other. They are used to
carry the Cb and Cr chrominance signals and are connected to the booster's
Note that both RCA-RCA and 2xRCA-miniDIN video cables are
available from many suppliers. However, you may want to make up your own using
high quality coaxial cable, to ensure lower signal degradation - especially if
you have fairly long cable runs. Some prewired cables leave a bit to be desired
in this respect.
By using the correct adaptor cables, the booster will operate
just as effectively with component video as it does with composite video or
Happy home theatre viewing!