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ElectroMix 842 (Part 2)

The second and final part of our superb professional quality studio mixer project

ElectroMix 842.

This month we continue our superb mixer project with construction and setting up details.


  • 8 Input channels
  • Mic or line inputs
  • Frequency Equalisation
  • Versatile Panning options
  • 2 Aux Send and Returns
  • Stereo Monitor output
  • Studio quality
  • Optional Meter Bridge

Although construction should be fairly straightforward, there are a few points worth noting. Before assembling any of the boards, slots for the buss wires have to cut in the edge using a hacksaw and/or small file. The buss wire should be 16swg, stretched to form straight lengths. IC sockets should be used although none were used in our prototype. A socket for IC1 of the Channel Module can be made by carefully cutting a 16 pin socket in half.

Insert and solder the required links on each board as shown in Figures 1, 2, and 3, with reference to the photographs of the completed boards. Next insert the veropins; on the component overlays these are shown with crosses when they protrude from the track side and with dots when they protrude from the component side. The resistors and capacitors can now be inserted and soldered, taking care with electrolytic capacitor polarities. If you are using IC sockets these can now be positioned and soldered. The switches and pots come next, not forgetting the pre-set on the monitor board. After this stage the IC's can be pushed into their sockets or soldered into place.

Figure 1. Channel Board component overlay (not to scale).
(Click image for higher resolution version)

Figure 2. Group Board component overlay (not to scale).
[Errata: Capacitors C3 and C8 are shown the wrong way round, refer to circuit diagram in Oct '82 Page 76, Figure 3.]
(Click image for higher resolution version)

Figure 3. Monitor Board component overlay (not to scale).
(Click image for higher resolution version)

As PCB mounting jack sockets seem almost unobtainable except to equipment manufacturers, normal types with solder tags are used which are fixed to the boards with short lengths of tinned copper wire, (soldered first and then inserted into the PCB's). The sockets and the switches must be seated squarely on the board to avoid problems when mating the PCB's to the front panel. There is a link to be added on the routing switches of each channel board and one on the monitor board which are soldered directly to the switch tags (see overlay diagrams). There is also an earthing link from each PCB to the back of one of its potentiometers. The LED's are bent and soldered to Veropins after the PCB's have been mounted to the face plate. The bulk of the wiring is to the Aux, Group and Monitor sockets which is made with screened wire. The returns of the Group faders should be screened although this is not necessary for the Channel faders (see wiring details). As the power supply is remote, miniature 3-core mains cable was used to feed the Channel supply busses. These should be linked to the Group and Monitor busses (see Figure 4).

Figure 4. Bus connection diagram.

Rear view of ElectroMix panel.

Checking and setting up

After checking wiring, IC, diode and LED orientation, power up and test supply voltages. If all is well, connect a suitable amplifier and speaker set-up to the Monitor outputs. Turn the Monitor level to maximum, little or no background noise should be heard. Plug a signal source (tape deck, radio etc.) into Channel 1 line input and select line input. Bring the Channel and Group faders to 0VU and the Panpot to central position. Press Group 1 and 2 switches on the Monitor module and adjust Monitor level. Run through all the functions on the Channel i.e. Input select (check Mic input is more sensitive), EQ, Routing, Pan, Aux sends, etc. This is repeated for the other Channels selecting the appropriate outputs on the Monitor select switches each time. If all inputs and outputs appear to operate correctly, disconnect the signal source and turn Monitor level to maximum. Run through all controls and switches which should operate quietly. Any thumps or clicks produced by switches indicates a DC level, which should be checked out with an oscilloscope or voltmeter. If any controls produce a small amount of noise this should be compared to normal listening levels to see if any further investigation is necessary.

Table 1.
Peak to peak voltages Signal level
880mV -10dBV
2.8V 0dBV
2.2V 0dBm
3.5V +4dBm
13.8V +16dBm*
22V +20dBm
*Recommended level for Clipping indicator.

The only preset which has to be set on the mixer is the Reference for the clip LED's. Set all faders to 0VU and insert a signal. If using as a level indicator for recording adjust the LED's to come on at a 0VU reading on your Tape recorder. In a P.A. situation the LED's could indicate a signal level for full output power of the amplifiers used. Alternatively as a clipping indicator, reduce the level of signal with the fader and increase the gain until definite distortion is heard. Reduce the gain so the distortion is no longer audible and adjust the LED's to just light. If you have access to an oscilloscope Table 1 gives peak to peak values for various signal levels.

Channel board.

Group board.

Monitor board.

Figure 5. Power Supply circuit diagram.

Power Supply

The mixer requires a ±15V regulated supply at approx. 200mA. A supply designed specifically for the ElectroMix is shown in Figure 5. This circuit uses standard voltage regulators IC1 and 2 fitted with on board heatsinks. This supply will also power the meter bridge.

Construction is straightforward with components inserted in the usual order; veropins, diodes, capacitors and IC's. (— correct polarities are very important!). See Figure 6.

Figure 6. Power Supply component overlay.

[The component overlay for the power supply PCB, Figure 6 was incorrect, the track layout having been shown from the wrong side. A corrected component overlay is shown here: 842 Meter bridge]

Double check all connections before connecting to the mains supply.

Please note; LF351 and LF353 IC's were specified on the circuit diagrams last month. Certain samples of LF353 seemed to burst into high frequency oscillation, so alternative types TL071 and TL072 were tested and proved to be more suitable, with the added bonus of being a lot cheaper.

The Retro-fit input channel with an electronically balanced Mic input and the Meter Bridge will be described in a forthcoming issue.

All parts except panel and PCB's are available at most electronic component retailers. Details of complete kit suppliers will be published next month.

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.


Channel Module — Quantities For One Board — 8 Required
Resistors — 0.25W, 5%, Carbon except where marked.
R1,6,27 47k 3 off
R2 2k7
R4,5 100k Metal Film 2 off
R3 220R Metal Film
R7 82R
R8 22k
R9,10,16,19,20-24 100k 9 off
R11,14 1k5 2 off
R12,15 Ik0 2 off
R13,17,18 10k 3 off
R25,26 4k7 2 off

C1,12 220nF Siemens multilayer 2 off
C2,5 47pF Ceramic 2 off
C3 100pF Ceramic
C4 22pF Ceramic
C6 220uF 16V PCB electrolytic
C7,11 100uF 10v PCB Electrolytic 2 off
C8 47nF Siemens multilayer
C9 4n7 Siemens multilayer
C10 10pF Ceramic
C13,15 47uF 25v PCB Electrolytic 2 off
C14,16 10nF Ceramic 2 off

RV1 10k Reverse Log
RV2,3 22k Lin 2 off
RV4 4k7 Log Slider
RV5 10k Lin
RV6,7 100k Log 2 off

IC1 HA12017
IC2 TL071
IC3 741
D1 OA90

JK1,2 Mono switched jack socket 2 off
S1-4 2 Pole latchswitch 4 off
3mm Red Led 1 off
Grub screw knob 5 off
Knob cap 5 off
Slider knob 1 off
Switch cap 4 off

Group Module — Quantities For One Board — 2 Required
Resistors — 0.25W, 5%, Carbon.
R1,8 220k 2 off
R2,6,9,13,16 47R 5 off
R3,7,10,14,17-22 100k 10 off
R4,11 4k7 2 off
R5,12 10k 2 off
R15 330k

C1,6 10pF Ceramic 2 off
C2,5,7,10,12 100uF 10V PCB Electrolytic 5 off
C3,8 2u2 16v PCB Electrolytic 2 off
C4,9 150pF Ceramic 2 off
C11 4p7 Ceramic
C13,15 47uF 25v PCB Electrolytic 2 off
C14,16 10nF Ceramic 2 off

RV1,2 4k7 Log Slider 2 off
RV3 10k Log
RV4 100k Log
RV5 100k Lin

IC1,3 TL072 2 off
IC2 TL071

Mono unswitched jack socket 4 off
Grub screw knob 3 off
Knob cap 3 off
S1 2 Pole latchswitch
Switch cap 1 off
Slider knob 1 Off

Monitor Module — Quantities For One Board — 1 Required
Resistors — 0.25W, 5%, Carbon
R1-8 100k 8 off
R9,11 220k 2 off
R10,12,1417 47R
R13,16,21 10k 3 off
R15,18,20,22 1k0 4 off
R19 100R

C1,3 10pF Ceramic 2 off
C2,4 100uF lOv PCB Electrolytic 2 off
C5 Not used
C6,7 2u2 16v PCB Electrolytic 2 off
C8,11 150pF Ceramic 2 off
C9,12 100nF Siemens multilayer 2 off
C10,13 220uF 16v PCB Electrolytic 2 off
C14,16 47uF25v PCB Electrolytic 2 off
C15,17 10nF Ceramic 2 off
C18 470uF 16v PCB Electrolytic

IC1 TL072
IC2,3 LM380 2 off

RV1 10k + 10k Log Dual
RV2 100k + 100k Log Dual
RV3 22k Min. Vertical Preset

S1-6 2 Pole latchswitch 6 off
JK1 Stereo switched jack socket 1 off
JK2,3 Mono unswitched lack socket 2 off
3mm Red Led 1 off
Grubscrew knob 1 off
Knob cap 1 off
Switch cap 6 off

Power Supply
C1,4 2200uF 25v Electrolytic 2 off
C2,5 100nF Siemens multilayer 2 off
C3,6 47uF 25V PCB Electrolytic 2 off

D1,4 1N4001 4 off
IC1 7815
IC2 7915

T1 15-015v, 30VA Transformer
S1 Mains rated switch
FS1 500mA 20mm Fuse
20mm Fuse holder
Case, to suit transformer used.


(*Screened Wire)
Channel Board
1 RV4/2 -
2 RV4/2 -
3 RV4/3 -

Group Board
1 RV1/3 -
2 RV1/1- Screen
3 RV1/2- *Conductor
4 RV2/3 -
5 RV2/1- Screen
6 RV2/2- *Conductor
7 JK1/2 Screen
8 JK1/1 *Conductor
9 JK2/2 Screen
10 JK2/1 *Conductor
11 JK7/1 *Conductor
12 JK7/2, JK5/2 Screen
13 JK5/1 *Conductor
Links:- 14 to 15, 17 to 19, 20 to 22

Group Board 2
1 RV1/3 -
2 RV1/1- Screen
3 RV1/2 *Conductor
4 RV2/3 -
5 RV2/1- Screen
6 RV2/2- *Conductor
7 JK3/2 Screen
8 JK3/1 *Conductor
9 JK4/2 Screen
10 JK4/1 *Conductor
11 JK8/1 *Conductor
12 JK8/2, JK6/2 Screen
13 JK6/1 *Conductor
Links:- 14 to 16, 17 to 18, 20 to 21.

Monitor Board
1 Screen
2 Group Bd. 1/13 *Conductor
3 Screen
4 Group Bd. 2/13 *Conductor
5 Screen
6 Group Bd. 1/8 *Conductor
7 Screen
8 Group Bd. 1/10 *Conductor
9 Screen
10 Group Bd. 2/8 *Conductor
11 Screen
12 Group Bd. 2/10 *Conductor
13 JK2/1 *Conductor
14 JK2/2, JK3/2 Screen
15 JK3/1 *Conductor

Previous Article in this issue

Amdek Chorus Kit

Next article in this issue

Allen & Heath 1221

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Nov 1982


Electronics / Build



Part 1 | Part 2 (Viewing)

Feature by Paul Bird

Previous article in this issue:

> Amdek Chorus Kit

Next article in this issue:

> Allen & Heath 1221

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