At last, after months of careful thought and development E&MM have come up with a unit we consider to be ideal, both in quality and price, for the modern musician: The ElectroMix 842.

This studio quality mixer was designed to provide a versatile range of facilities, good specification and considerable cost saving over commercial units. It features an 8 input, 4 group format with a stereo monitoring system, making it ideal for 4 track recording. It is equally at home in live performance situations where instruments and vocals can be sub-grouped.

There are 11 PCB's, 8 channel, 2 group and one monitor board, which all mount directly on to a 19" x 10U panel, designed for rack mounting (or suitable enclosure). This considerably reduces the inter-wiring needed, thereby reducing the possibility of noise pick-up.

The inputs and outputs are connected via ¼" jack sockets and operate at -10dB (330mV) which is standard for the popular tape machines such as Teac, Fostex, Sony etc. Higher output levels could be used but would result in lower input sensitivity.

A block diagram of the system is shown in Figure 1.

Mic and Line inputs are high impedance (47k) although details are given to modify the mic input to 5k. A 'Retro-Fit' channel module with an electronically-balanced mic input will be available. Bass and Treble controls with a wide range of Boost and Cut are provided — further EQ can be added from external Parametrics or Graphics. The channel faders feed the four group mix busses via the Panpot and routing switches. Recording on one track (mono) is simply achieved by selecting a pair of groups and panning to one of them.

Aux 1 is switched pre or post fader, so it can be used as a Foldback send (pre fade) or an Effect send (post fade). Aux 2 is post-fade giving the option of a Foldback and Effect send or two Effect sends. There are two Aux returns with level controls that can be panned across Groups 1 and 2 or 3 and 4.

The monitor section consists of a series of switches and a level control to drive an external stereo amplifier and speaker set up. The stereo headphone amplifier follows the monitor selection with a separate level control.

Although there are no meters on the mixer there are fast acting clip indicator LED's for each channel. The threshold is common to all the LED's and can be preset over a wide range. Therefore they can be used as overload (clipping) indicators or set to operate when input signals reach a determined level i.e. simple metering. An add-on 19" unit to house 4 meters to monitor the group outputs (with one switchable to the Aux's) will be published in a future issue along with details of the 'retro-fit' channel module.

The power supply is housed in an external case to minimize hum pickup.

Circuit Description

Channel Module

The Input amplifier is based on a Hitachi Integrated Circuit, IC1 HA-12017, designed for use in low noise high performance Magnetic Cartridge preamplifiers. In this case, by modifying the feedback components, a high gain flat frequency response amplifier with low noise characteristics can be produced.

Mic and Line inputs are available selected by S1. With the component values shown in Figure 2, both inputs have a high (47K) impedance, which will suit low cost microphones, guitar pick-ups and most electronic equipment. Changing R2 to 5K6 and R4 to 4k7 gives input impedances for Mic and Line of 5k0 (suitable for low impedance microphones) and 50k respectively without affecting the gain. The Line input is attenuated by R1, R2 and then amplified by IC1. This arrangement may seem strange but the noise performance of a mixer is governed by the input stage and is worst case when using maximum gain on the most sensitive input, i.e. the Mic input. (The mixer specification is quoted for this condition.) It follows that attenuating Line level signals to a similar level to those seen at the Mic input will not increase the noise figure. Therefore, this method is both cost effective and easy to implement.

The amplifier IC1 is configured as Non-inverting with its gain set by RV1, R6 and R7;

Gain = (R6 + R7 + RV1) / (R6 + RV1)

As R6 is seen as a load on the output of IC1, keeping its value high will ensure maximum output swing. In this design R6 is a fixed value and the gain is adjusted by RV1, which has to be a Reverse log type to give an even rotation vs gain feel. Resistor R7 limits the maximum gain when RV1 is zero and C6 prevents any DC across RV1, R7 produced by the input current of IC1, causing an offset at IC1 output. R3, C2 and C3 provide a degree of RF rejection and C4, C5 are frequency compensating components. Capacitor C7 blocks any DC at IC1 output from the tone control stage which could cause the controls to be noisy in operation. The Tone controls follow the widely used Baxandal circuit and provides Boost and Cut at high and low frequency extremes. Resistors R10 and R16 were added to shape the response of the Boost/Cut curve and to limit the amount of Boost/Cut available. At this point IC3 monitors the signal level, where large amounts of EQ could cause the signal to clip. Using IC3 as a comparator, one input is fed by the rectified voltage on C12 with the other input fed by a reference voltage from R21, R22 and RV3 on the Monitor module. When the voltage on C12 exceeds the reference, IC3 output goes positive, lighting the LED. If the LED was connected to 0V, switching spikes produced as the LED lights would be picked up by the amplifiers and produce audible clicks. Returning the LED to the negative rail prevents this happening due to the good supply rejection of the Op-amps.

During recording, to cater for variations in signal level, the Channel Fader is operated at a nominal level below maximum. This position is marked 0VU on the fader scale and gives an output of -10dB relative to fader maximum. As -10dB is the final output level required this is ideal and gives +10dB signal adjustment without disturbing gain settings. From the fader the signal is split by R17 and R18 across the Panpot, RV5. Values are calculated to give a -3dB output from either channel with the Pan-pot central and 0dB at each end of travel. Audibly this gives a signal equal loudness as it is panned across a pair of speakers. In achieving this law there is some signal attenuation which has to be made up in Group amplifiers. S3 and S4 route the Pan-pot to the four Group mix busses, with the pan-pot operating across each pair of selected busses i.e. 1-2, 3-4.

Aux 1 (VR6, S2, R24) and Aux 2 (VR7, R23) feed onto their own mix busses to provide the foldback and effects send.

Group Module

Each Group board contains the summing amplifier for a pair of Groups and one Aux, the appropriate buss is selected with links on the circuit board. The summing amplifiers use the Virtual earth principle produced by grounding the non-inverting +ve input of an inverting amplifier stage and applying signals via input resistors to the inverting -ve input. The -ve input is held at near ground potential (virtual earth) by feedback around the amplifier, which in effect forms a current summing point. Due to the low impedance at the summing point, there is very little interaction between inputs and as the mix busses are fairly long, less chance of interference pick-up. After the Groups are summed by IC1 and associated components, see figure 3, they are fed via the Group Faders to IC3, a +10dB amplifier. This restores the 10dB of signal held in the faders which operate in a similar manner to those in the Channels. This ensures a constant signal level through the mixer when the faders are at 0VU. These amplifiers buffer the Group faders so fairly low impedance loads can be driven.

The Aux returns are simply a level control, pan-network and a switch to route the return to Groups 1 and 2, or 3 and 4. By keeping the resistor values high, a reasonable input impedance (30k) is achieved, which should not load most effect units.

Monitor Module

The Monitor section has been kept simple to keep cost down and in many cases make it easier to use. The outputs of the Groups and Aux's are routed through S1-6 to summing amplifier IC1 and out via Monitor level RV1. The Output level should be suitable to drive most power amplifiers. Pressing any of the Group switches will select the appropriate Group to both outputs and is heard in mono. If Groups 1 and 2 are selected together, Group 1 will be heard on the Left and Group 2 on the Right. Similarly with Groups 3 and 4. The Aux switches have priority over the Groups and select the Aux to both outputs. This means the Aux's can be checked with the minimum of button pushing.

The Monitor output is tapped off via RV2, Headphone level, to a stereo headphone amplifier, IC2 and 3. LM380 power amplifiers are used because they require so few external components to operate. They do however tend to be noisy which appears to be due to the high internal gain of the device. R13 on IC2 (R16 on IC3) are added to reduce this gain and C8 (C11) helps prevent RF pick-up and further reduces high frequency noise. To prevent ear and headphone damage, (the LM380 can put out 2 Watts,) R14 (R17) limit the power available. If you are using low impedance headphones (8 ohm) the level is reduced more than if using high impedance (120 or 600 ohm) types, which gives suitable listening levels for both types. The headphone amplifiers require relatively large amounts of current which could lead to instability and cross-talk problems. The supply to the amplifiers is therefore heavily decoupled by R19 and C18. Careful track layout on the circuit board also helps to overcome any problems.

The chain RV3, R21 and R22 forms a potential divider to supply a reference voltage for the Clip comparators on the Channel modules. LED1 and R20 provide a 'Power On' indication.

Next month we will move on to the construction and setting-up of the ElectroMix. However, to allow you time to order, the front panel and PCB's are available from us now.

The fully finished panel and a set of PCB's are available from Electronics and Music Maker, (Contact Details). The prices are £24.95 for the drilled, sprayed and screen printed panel and £24.95 for a full set of PCB's inc. p&p and VAT. Order as ElectroMix 842 Panel and ElectroMix 842 PCB set. Please allow 28 days for delivery.