"MI12" Mains input/distribution & +12V supply

The circuit named "MI12" is a proposal for the mains/line voltage input/distribution with two, separately grounded +12V power supplies. This PCB is not supplied within the LDA DMON and LDA MON kits, but is needed for them, so you must make it yourself.

"MI12" requirements

We deal with the "MI12" for the dual mono LDA DMON here (half of the circuit applies to the "MI12" for the mono LDA MON).

The "MI12" supplies +12V to the Power supply & relay board (PSS701/PSSR350) and via it to the Protection board (APBF/APBM or APBT/APB).

For the "MI12" used in the dual mono LDA DMON, two on/off power switches are recommended for the two channels when the two transformers are above approx. 175 VA each. This is to limit inrush currents that would trip your household circuit breaker. To avoid the need for inrush current limiters, the two separate switches should be operated one after the other. Up to about 2x175VA transformer power, the two switches can be within a single double pole switch (DPST or DPDT).

The "MI12" should include a mains voltage input connector (230VAC_IN), fuses for the power transformer primaries, line EMC filter capacitors, solder pads for on/off switches (ON/OFF) and solder pads for the power transformer primaries (230VAC).

The "MI12" should include two galvanically isolated +12V min. 40mA power supplies for LDA DMON (only one +12V min. 40mA power supply for LDA MON).

The proposed circuit

KiCAD schematic and PCB layout (use them at your own risk)

You can download the schematic and PCB layout file as a KiCAD project from here. (Rename the .zip1 extension to .zip after download and unpack.) The PCB material should be FR4 glass fiber reinforced epoxi with solder mask on the printed wiring side. You use the PCB layout at your own risk, I give no warranty that it complies with electrical safety regulations that are effective in your country.

The footprint of the EI30 core encapsulated small transformer used in the PCB layout:

The middle leg should be removed from the Hirose DF63 pin header that goes to position J81.

Although the simple X2 capacitor in the above "MI12" circuit is deemed sufficient in most cases, you may also equip your amplifier with a more sophisticated power line filter. There are many IEC sockets with built-in, effective power line EMC filters in the market.

Component list for the proposed "MI12" PCB layout

Please download the "MI12" component list (BOM) from here.

Sizing the power transformer fuses (F81, F82)

Note: Sizing the transformer primary fuse is an electrical safety related issue. I assume no responsibilty that the information contained in the sizing procedure below complies with applicable safety standards in your country. Euraudio assume no liability for any risk or damage, including injury or death resulting from your lack of adherence to applicable safety standards in your coountry.

(1) The fuse wire should not melt during the high rush-in current at switch-on, this sets a minimum on fuse rating (the higher the better). The rush-in current depends on the power transformer VA rating, the transformer construction, and the main filter capacitors in the power supply. (2) At the same time the fuse should blow when there is a short circuit on the transformer secondary to prevent fire. This sets a maximum on fuse rating (the lower the better). If the minimum and maximum gives a usable range, then a fuse amperage value can be selected.

Important: The fuse sizing procedure below is thought to be safe for transformers with 150 VA or higher rating. For smaller transformers, the procedure may yield fuse amperage values that are too high to protect the equipment in case of secondary coil short circuit, therefore smaller transformers should be measured to obtain the primary coil impedance under normal operating conditions. This measurement should be done using a variable autotransformer and a safety isolation transformer. The measurement procedure is described in the following article and the measurement setup is seen on figure 8: https://sound-au.com/articles/fusing.htm#s6. Then calculate the maximum current on the primary during secondary short circuit (Isc). The slow blow fuse must be lower sized than Isc/2.

Fuse sizing for transformers with 150 VA or higher rating: If you follow the power supply capacitor sizing recommendation that you find in the Euraudio PSSR350, PSS701/PSS702 or PS1S power supply assembly instructions, then a rule of thumb for 230/240 VAC primary voltage: calculate Af = sqrt(VA/27). For 110/115/120 VAC primary voltage, calculate Af = sqrt(VA/6.8). (VA is the transformer VA rating in Watts.) Then use a slow blow (time lag) fuse with standard amperage value closest to Af for a conventionally constructed (EI core) power transformer. Use the next higher standard size for a toroidal power transformer. If the equipment blows the fuse, then choose the next higher standard value.

Example: The transformer is a 300 VA toroidal one on 230 VAC mains, main filter capacitors in the power supply are as recommended in the applicable Euraudio power supply assembly instructions. sqrt(300/27)=3.33. Closest standard fuse value is 3.15 Amperes, so use a 4-Ampere time lag (T4A) fuse. If the equipment blows this fuse at switch-on, then use a T5A fuse.

You must use fuses that can withstand the mains voltage (e.g. a 250V fuse).

Advice on protection against electric shock

The following part of this documentation only contains some rudimentary guidelines on electrical safety. In each country, there are legally effective standards pertaining to the electrical safety of electronic equipment which are connected to mains/line voltage, which standards 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 your lack of adherence to applicable safety standards in your country.

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

Use a green-yellow wire of at least the same cross section as the wires in the mains cable. Onto one end of the wire, a connector (usually a spade lug) that matches the earth grounding tab of the mains socket should be properly bonded with crimping AND soldering. The other end should be connected to the chassis with a ring lug (crimped and soldered, not just crimped).

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 protective earth to the chassis

Drill a 4 mm hole on one of the metal panels (usually the back panel) and fasten the green/yellow protective earth conductor equipped with ring lug to the metal panel. You must use a ring lug, to which the cable is properly crimped and the crimping is fortified with soldering. (Not some other kind of lug terminal that can become separated more easily.) The sequence of the elements can be: internal surface of the chassis - tooth lock washer - flat washer - ring lug - nut - counter nut. Counter nuts are made of softer metal and are usually thinner than normal 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.

Technical data

At 25°C ambient temperature unless otherwise noted.

Mains/line input: max. 250 VAC, max. 4 A continuous

Recommended amplifier power: max. 2x250 W /  4 ohms or 8 ohms

Supply voltage outputs: 2x +12 V DC, max. 40 mA each (with recommended transformer)


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