The 8201 Line Mixer is a simple 8 into 2 mixer designed to deal with small mixing applications that do not require the facilities that a full scale mixer provides. Each channel will accept a line input, and has individual level and pan controls. Each channel is also provided with a 'Foldback' output, which may also be used to connect two mixers together for 8 into 4 operation. Alternatively, the outputs of one mixer may be fed into two of the inputs of a second to achieve 14 into 2 operation.

The unit will be found useful by the Electro-Musician or sound recordist in many applications. It is ideal for use in the home studio to complement recording equipment such as the Portastudio, or to produce a stereo mixed drum-machine signal (the mixed outputs of all the drum 'instruments' in these are usually mono and the line mixer can be used to combine all the individual instrument outputs). For live performance work, the mixer can be used as a keyboard sub-mixer and will also prove invaluable for providing a signal that is mixed differently from that fed to the P.A. for recording, foldback to musicians, extra effects send including echo and reverb.

There is nothing new about the concept of the 8201 Line Mixer; commercially made units are available. However, it goes without saying that this one costs dramatically less, and is designed to give low noise, studio quality mixing.

System Format

It was decided that a format suitable for 19 inch rack mounting was most versatile. The height of the panel used is 3½ inches (88 mm) which is a standard 2U size. The mixer requires an external power source of +15, 0, -15 volts. This has the advantage that many mixers may be operated from one PSU, which is not only economical but also helps to reduce the number of mains flexes you have to find a socket for. The absence of mains voltages within the mixer means that a complete case is not necessary (though you could of course use one), since a certain amount of screening is provided by the equipment above and below in the rack.

A single PCB design is used throughout which makes the system very easy to construct. A PCB is needed for each channel, plus two to buffer the mixing busses and to provide output level control. The PCB's are supported by the pots, which mount on them, and are interconnected by five continuous wires, which solder into small slots in each PCB. The wires need not be insulated, so interconnection is quick and easy to perform.

An eight way system is described here, since this is a standard mixer size and lends itself to eight track systems. More or less channels could be built into your system, depending upon your needs. You may prefer to use a different casing arrangement too if you do not use 19 inch racks. Because the PCB's are linked together with wires, they are versatile enough to find applications in other formats.

Circuit

Channel Inputs

The design consists of an op-amp based inverting amplifier driving a passive level control/pan network. See Figure 1a. The 'Foldback' output is taken from the amplifier output but is not affected by the level control. Note that this output is inverted with respect to the input which can be useful for bridge driving power amplifiers etc. The preset (RV3) adjusts the amplifier gain. With the values given, this ranges from 0 to 10. More gain could be achieved by reducing the value of R1, but this will result in poor signal to noise performance. Low level sources such as microphones are best used with a preamplifier to bring them to line level.

C1 and C5 provide a reduction in gain at high frequencies to prevent RF pick-up. C2 and C3 decouple the supply rails and C4 removes any DC bias at the amplifier output. (The polarity of C4 is not important — the DC bias may be positive or negative depending on the individual op-amp, but it will always be of such small voltage that wrong polarity will not affect the operation of the circuit nor will it damage the capacitor).

RV2 is the level control, with R2-5 and RV1 forming the pan network which feeds the two mixing busses.

Output Amp

The output stage is shown in Figure 1b. This is an inverting amplifier, similar to that in the channel inputs, which sums the signals present on its buss. RV2 adjusts the output level. The action of this amplifier causes the mixing busses to be 'virtual earths'. This means that the amplifier is in effect current operated. No signal voltage will be measurable on the buss lines because there is none, a point worth remembering if you are checking with an oscilloscope!

Power Supply

The mixer requires +15, 0, -15 volts at about 50mA. You may well have a suitable power supply available; if not, the one described will supply several mixers.

The circuit, see Figure 5, is a standard bridge rectifier/smoothing arrangement supplying three terminal fixed voltage regulators. You may notice that two separate bridge rectifiers are used where one would suffice; this is because two separate PCB's are used for the + and - rails. These are standard Maplin items (YQ40T, YQ41U), obtainable from Maplin Electronic Supplies, (Contact Details). The regulators do not require heatsinks. R1 provides a degree of earthing of the 0V rail whilst avoiding the possibility of earth loops when the mixer is used in conjunction with other equipment that is earthed. Refer to the component numbering list for the values of components used on the PSU PCB's. The identifier numbers correspond with those printed on the PCB's.

'Audioconn' connectors have been specified for the output connection since they are very reliable and provide a solid flex grip, but these could of course be replaced by DIN connectors to keep expense to a minimum.

Construction

Assembly is straightforward and should present no problem if care is taken with polarised components and semiconductors. Cut the pot shafts before mounting to the PCB's.

The PCB's used for the input and output stages are identical. However, eight of these are configured as inputs and two as outputs. When assembling as an input stage, all of the components are inserted and soldered. R6 should be fitted across AB, see Figure 3. When assembling as an output stage, omit the components R1-5, RV1, C1 and JK2. R6 should be fitted across AC.

Once the PCB's have been assembled, they can be mounted on the front panel. The boards are interconnected as shown in Figure 2. (The boards supplied by E&MM do not have the interconnection slots made, so it is advisable to cut these with a small hacksaw or file before mounting them, on the panel). If the mixer is to be mounted in a rack or similar enclosure, then bare tinned wire may be used to connect the boards. If the mixer isto be used as a free standing unit, then sleeving should be used to insulate the supply and busbar wires.

Finally, fit the power supply lead to the ends of the +15V, 0V and -15V wires. The spare holes on the output stage can be used to grip the supply cable with a nylon tie.

Testing

It is unlikely that there will be any problems, but if there are, it should be possible to check through the system with a signal source and an amplifier. Check each channel individually by inputting a signal and checking that it is returned through the foldback output. (Make sure the gain preset is not set to zero!) When the output amplifiers are operating correctly, no signal will be measurable on the mixing busses. (This was explained earlier). If the mixer does not operate first time, switch off and check for dry joints and solder bridges. Then check that electrolytic capacitors have been inserted with the correct polarity, IC's are correctly orientated and power supply is not reversed.

With the mixer operating correctly, the gain presets can be adjusted to suit the equipment you are using. Try to keep the levels on the busses fairly high as this means less gain will be required on the output amplifiers, minimising hum pickup on the busses. If hum is a real problem even when the mixer is mounted in the rack, an aluminium panel connected to 0V mounted under the busses will help. Connecting the front panel to 0V will also offer an improvement.