Euraudio MIB12dc, MIB12mc, and MIB12sc assembly instructions

Welcome to the Euraudio MIB12dc, MIB12mc, and  MIB12sc assembly instructions. The MIB12 is a mains input and distribution board with two auxiliary 12VDC supplies, that can be configured for dual supply (for dual mono) or single supply (mono or stereo) setup. MIB12dc is part of the Euraudio LDA 53DM dual mono low distortion audio amplifier DiY kit, the MIB12mc is part of the LDA 53M mono low distortion audio amplifier DiY kit, while MIB12sc is part of the LDA 43S stereo low distortion audio amplifier DiY kit. Please find the specifications of the MIB12 here.

Please read through these instructions before doing anything with the DiY kit.

What the MIB12dc, MIB12mc, and MIB12sc DiY kits contain and what they contain not

The kits contain the MIB12 fiberglass-reinforced epoxy PCB (fig. 1) plus all the configuration specific electronic and mechanical components to be mounted on it: MIB12dc (fig. 2), MIB12mc (fig. 3) MIB12sc (fig. 4), except the cables carrying line voltage. The small transfomer(s) are part of the kits, but are not shown in the images.


Component list

Please download the MIB12dc, MIB12mc and MIB12sc component list (BOM) from here. Note that a this single spreadsheet file file contains the dc, mc and sc BOMs as well.

The protection against electric shock

The following part of this documentation only contains some overall guidelines on safety. In each country, there are legally effective standards on the electrical safety of electronic equipment which are connected to mains/line voltage, which standards which may differ from country to country.

The protection against electric shock is the sole responsibility of the builder of this DiY kit, and Euraudio assume no liability for any risk or damage, including injury or death resulting from the lack of adherence to applicable safety standards.

The general rule is that for protection against electric shock, the metal cabinet/enclosure/chassis of the amplifier must be connected to the protective earth conductor (or alternatively an equivalently effective protection method against electric shock must be used, which we will not cover here).

Connecting the chassis to the protective earth conductor when using an IEC mains input socket

Please use the supplied green-yellow wire with ring lug. Onto the free end of the wire a connector (usually a spade lug, which is not part of the DiY kit) that matches the earth grounding tab of the mains socket should be properly bonded with crimping AND soldering.

The MIB12 board has a 47nF X2 line filtering capacitor. The IEC socket could incorporate a more elaborate filter to keep mains noise from entering the cabinet.

Connecting the chassis to the protective earth conductor when using the power cord with strain relief bushing

When wiring the line/mains voltage, the cord should be strain relieved with a bushing (e.g. Heyco), and where the hot and neutral were wired should have service loops of about 3 cm (1 inch) diameter. The earth ground wire should have an 8 cm (3 inch) service loop, and should be connected to the chassis with a ring lug (crimped and soldered, not just crimped) with the ring lug connection near the entry point of the cable. This is important for safety reasons, such as, if the power cord got snagged on something that could pull it out, the hot and neutral wires would break loose first, and the ground wire would remain connected to the chassis for a longer time than the other two. If the hot wire happened to short against the chassis, it would short direct to the earth ground (while it's still earthed). This would prevent the chassis from becoming a shock hazard.

Fastening the ear lug to the chassis

Drill a 4 mm hole on one of the metal panels (usually the back panel) and fasten the ear lug with the M4 screw. The sequence of the elements is: internal surface of the chassis - tooth lock washer - flat washer - ring lug - nut - counter nut. The counter nut is the thinner of the two M4 nuts.

Connecting detachable panels to the protective earthing point

All the externally touchable and detachable metal panels of the amplifier cabinet/enclosure/chassis should maintain a high conductivity, high current capable, solid metallic contact to the panel on which the earth grounding point was established. Any detachable panels on the cabinet/enclosure/chassis should be bonded to the earth connection with a wire of at least the same cross section as the wires in the mains cable.

General assembly advice

What you'll need and is not in the DiY kit

Solder iron, solder wire, wire cutter, suitable cables, Phillips 1 (PH1) screwdriver, drilling machine, 3.5 mm metal drill bit. A PCB holder jig and/or a model-making vice may come handy. Cable insulation stripper is recommended for thick cables. A solder suction pump and/ or desoldering wick may be needed if you make soldering mistakes.


Please click this link, if you need soldering tips.

Where polarity matters

There are electronic components which are polarized, these have to be soldered in the right orientation. The polarity is shown in the PCB with appropriate marking, so you need knowledge of the electronic component markings. If you solder any of the polarized components not in the correct orientation, that causes trouble when you power up the circuit; either the circuit will malfunction or even some of the components may be damaged.

Detailed help for assembling

Note: I can't help in troubleshooting assembly faults, I can only resend the whole component set on request.

Drilling the fastening holes

It's practical as a first step to drill the holes in the amplifier chassis that will hold the PCB in place. Put the PCB in its intended place within the amplifier enclosure, mark the 4 holes with a marker pen, and then drill the holes with a 3.5 mm drill bit.

Identifying components

The graphical markings and numbers printed on the PCB make it clear where the components belong and in what orientation. Though basic electronic component knowledge is necessary for identification.

The mains voltage connections

For the dual supply (MIB12dc) config, all components are mounted, and there are separate on/off switches for the left and right side. You normally switch on the left and right side one after the other. This is to limit the inrush current when 2 high power (e.g. 2 pcs of 350VA) transformers are used. If you use two transformers less than 175VA each, then in most cases you can use a single DPST or DPDT on/off switch without the risk of tripping your household circuit breaker.

For the single supply (MIB12mc or MIB12sc) config the components of only one side (either left or right) are mounted.

J81, the mains input connector is part of all 3 configurations. J82 (left) and J83 (right) are the fused & switched mains voltages to which the main transformer(s) primary should be connected. SW81 and SW82 are the left and right side ON/OFF switches. Please review the mains connections in the image below:

The mains input connector

The middle leg should be removed from the pin header that goes to position J81.

Please click this link, if you don't have a crimp tool suitable for the contacts of the connector.

Sizing the fuses

The sizing of fuses is dependent on inrush current, which is in turn dependent on the power rating and construction of the transformers as well as on the size of the filter capacitors in the power supply. You must use slow-blow fuses, practical values range from T3.15A to T6.3A. For most of the 100W amplifiers T3.15A or T4A is OK, and for most 200W amplifiers T5A is OK. Toroidal transformers may need higher amperage fuses. The doubled fuses on the left and right side is only an extra safety feature. If a fuse is blown, please change the other fuse on the same side, even if it still seems to be working.

The 12VDC supply voltage output

The 12VDC outputs are J84 (left side) and J85 (right side). The allowed maximum load current is 45mA for each side of the MIB12dc, 45mA for the MIB12mc and 90mA for the MIB12sc. The ground points and +12VDC output points are as shown in the next image. The two gnd points (gnd1, gnd2) are separate (galvanically isolated). There are suitable wires with red and black insulation supplied with the kit.

Securing the board within the chassis

The MIB12 carries hazardous voltage: please take extra care to fasten all the 4 screws properly, e.g. use the supplied spring washers together with the flat washers to ensure the board can't get loose.

The ON/OFF switch debounce capacitor

I supply one or two 0.01uF 310VAC X2 capacitors. Switches may bounce when switched on or off. If you hear a click or pop sound from the speakers when you switch off the amplifier, please solder this capacitor directly onto the ON/OFF switch contacts. This should eliminate any disturbing sound when you switch off the amplifier.


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