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If you don't like to build the ComBox yourself: It can be bought from



1) Overview
2) Busstructure
3) ComBox
4) Mains
5) Microphones
6) Buscable
7) Basic Set
8) Instructions of use

1) Overview

The ComBox is a handy tool for LAN-parties, network games or other occasions, where a group of computer-users wants to communicate over two or more rooms. The ComBox is a hardware-solution, and therefore it's not software-dependant. The advantages are obvious:
Fig. 1: ComBox fully assembled

Dimensions: 50x38x14mm (2x1.5x0.6 inch)

First, the bus-structure of the ComBox will be explained. After that the constuction of each component and the use will be shown.
It is expected, that the reader is familiar with basics of electronic circuts and has access to all equipment, that is nessecairy for the construction and the test of electronic circuits in SMD.

2) Busstructure

Fig. 2  Bus-structure

The bus of the ComBox is very simple. On one end, a 9volts mains is connected which supplies each ComBox on the bus. The boxes themselfs are connected with a normal stereo 3.5mm headphone-connector and are attached to the soundcards microphone-in of the corresponding PC. The microphone of the headset is also connected to the ComBox and the headphone is plugged into the soundcard.

3) ComBox

For each PC a ComBox is required. It amplifies the microphone signal and injects it into the bus. It  also converts the bus signal from the other Boxes so that it is suitable for the soundcards mic-in.
A ComBox doesn't distinguish between the own microfone-signal and the signal from another Box, so you will hear yourself talking too. But this is not a disadvantage, because is simplifies communications with closed headphones and the calibration.

The Circuit

Figure 3 shows the schematic:

ComBox Schaltplan
Fig. 3: Schematic

The bus is connected at the connectors CON1 and CON2 and transfered through the device without modifications. D1 and R11 form a visible function control. The 9 volts-supply from the bus is low-pass-filtered trough R13 and C6 (D2 protects the circuit against reverse-polarity) and forms the supply for opamp IC1.
Another low-pass-filter is applied to this voltage to generate a stable supply for the pre-amp and surpress feedback. R5 and R6 generate the ground-potential for the opamps. The signal of the condensor-microphone reaces the circuit through CON3. R1 generates the supply for the microphone. The signal is decoupled through C1 and amplified by T1, R2, R3, and R4.
This amplified signal is decoupled by C2, amplified by IC1Gate2 with variable gain (R7, P1) and fed into the bus via R8 and C7. The bus-signal is decoupled by C4 and amplified with opamp IC1Gate1. After that, it is fed into the optocoupler OK1 which eleminates the galvanic connection between the computers.
The bus itself is terminated with a 470ohms resistor. The reason for this is, that with each connected ComBox the resistance of the bus and therefore the overall volume decreases rapidly. But with this termination, the much higher inner resistance of the boxes doesn't matter any more.


To construct a relatively complex circuit like this is a case like this it is nessecairy to use SMD-parts. Another advantage is, that a circuit with SMD-parts is more shock-resistant (this can be useful on rough LAN-parties...).
For the circuit a single layer board is suficent. The use of a epoxy-board is recommended beacuse connecting a plug causes mechanical forces. Figure 4 shows the layout (seen from the soldering side). To print the layout, use the version with higher resolution and print it with 600dpi so you get original size.

Layout der ComBox
Fig. 4: Layout

The parts are applied to both sides (fig. 5). The SMD-parts are places on ther soldering side and the wired parts are place on the placement-side. The LED has to be soldered so that its bottom has a distance of 6mm (0.23") from the board, so that it fits through a hole in the case.
The holes on the left and the right side of the board carry the pegs from the case and therefore have to be big enough. Note that the ground layer must not be cut.
Bestückungsplan Bestückungsseite
Fig. 5: Placement placement-side...
Bestückung Lötseite

...and soldering side


Bestückung von oben
Fig. 6: The wholly assembled board seen from top...
Bestückung von unten
...and bottom

PIN1 to PIN4 carry the mic-in cable to the soundcard. PIN1 and 2 are ground, PIN3 and 4 carry the signal.

To protect the cable against pulling-forces, the cable can be glued to the board with super-glue.


You will need the following parts:
IC 1 TL 072  SMD (SO8)
OK1 Optocoupler PC817
T1 BC847C
D1 LED 3mm green (or whatever you like)
D2 LL4148 SMD
R1 5,6k SMD 0805
R2, R10 1M SMD 0805
R3, R5, R6 10k SMD 0805
R4 100R SMD 0805
R7 100k SMD 0805
R8 4,7k SMD 0805
R9 82k SMD 0805
R11 820R SMD 0805
R12 470R SMD 0805
R13 150R SMD 0805
R15 22k SMD 0805
C1 220nF ceramic SMD 1208/1206
C2, C3, C4 100nF ceramic SMD 1206
C5, C6 ELKO 100µF 10V RM 2 max. 7mm high
C7 ELKO 4,7µF 50V RM 1,5 max. 7mm high
C9 1nF ceramic SMD 0805
CON1, CON2, CON3 3,5mm headphone-plug
P1 trimmer 500k / 470k
PIN1...PIN4 3,5mm-headphone-connector with cable
short axis for trimmer


My suggestion for frontside design is:

Fig. 7: Frontside

The frontside-design is available in higher resolution for printing. When you print it with 300dpi, you will get original size. The printout is a little bit larger than the case and has to be cut to size. This printout can also help drilling the frontside-holes.

4) Mains

As mains you can use a normal stabilized 9V supply, but a modification of the plug is required before connecting it to the bus.

Fig. 8: Modified mains-plug

Therefor the old plug is removed and a 3.5mm-headphone-plug with an additional 470Ohms resistor is connected. The wiring-diagram is shown in fig. 8.
One ComBox needs about 15mA and more than 300mA should not be transmitted through the cable, so about 20 ComBoxes can be connected to one supply, as long as the supply can source the required current.

You will need:
1   Mains 9volts min. 300mA
1   3,5mm-stereo-headphone-plug
1   470 Ohms-resistor 1/4 W

5) Mikrophones

As microphone a normal headset (for PC usage, not for telephone-applications), a computer-microphone or a self-made microphone can be used.
If you decide to build one, you will need a condenser-microphone, a shielded one-wired cable, a microphone-connector and some design material. The design s left to the reader. For the circuit, you connect the shield of the cable to the microphones ground and the ground of the connector. The signal wire is connected to the signal-pad of the microphone and the left and the right channel of the microphone-connector.

Mikrofon zum Anklemmen
Fig. 9: Microphone for an existing headphone

You will need:
1   elektret condensormicrophone
1   3,5mm-headphone-connector
2m one-wired shielded cable
some design-stuff

6) Buscable

For the connection of two ComBoxes you can use a normal cable with a 3,5mm stereo-headphone-connector on each end. If you build one yourself, don't swap signal- and supply-wire!
15m (50 feet) have already been successfully tested, 50 (150 feet) should not be a problem.

7) Basic Set

A complete basic set consists of zweo ComBoxes, two microphones one mains and one bus-cable.

Komplettes Grundset für zwei Computer
Fig. 10: Complete basic-set for four users

8) Instructions of use

First you connect all ComBoxes with the bus-cable and then you connect the first box to the powered down mains. After that, the microphones are connected to the boxes and the line-cables are plugged into the microphone ins of the soundcards. A headphone is connected to each soundcard.
Set the mixer of each soundcard to low and activate the microphone in.
Now switch the mains on and adjust the volume carefully. The poti on each combox selects how loud the connected microphone is fed into the bus. With the mixer of each soundcard you adjust how loud you hear yourself and all others.

Warning: Don't change connections while powered on. These plan is provided "as is". No warranties! Use at your own risk!

You can get the whole page as Zip-File!

Have a lot of fun!

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last change 2001-08-28 Archi