"MIA" Mains input/distribution & auxiliary AC supply
The circuit named "MIA" is a proposal for the mains/line voltage input/distribution with an auxiliary AC power supply. This PCB is not supplied within the LDA MIN and CTA MIN kits, but is needed, so you must make it yourself.
"MIA" requirements
The "MIA" should include a mains voltage input connector (230VAC_IN), a fuse for the power transformer primary (F81), solder pads for the on/off switch (ON/OFF) and solder pads for the power transformer primary (230VAC), and optionally a line EMI filter capacitor (C81).
The "MIA" should include a ground independent 12...15 VAC, min. 110 mA power supply for the SMRS2 (TR81).
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 then 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, Euraudio gives no warranty that it complies with electrical safety regulations that are effective in your country.
Component list for the proposed "MIA" PCB layout
Caution! In order to avoid fire hazard, it is mandatory to use class X2 capacitors, where the part list indicates this with the text "X2".
Please download the "MIA" component list (BOM) from here.
Auxiliary transformer
The footprint of the EI30 core encapsulated auxiliary transformer used in the PCB layout:
The auxiliary transformer must be short-circuit proof, i.e. of a design that can be operated without a primary fuse.
The mains input connector (J81)
The mains input connector is a 2-pin connector with 7.92 mm lead spacing with appropriate current rating. If you use the 3-pin Hirose DF63 connector with 3.96 mm lead spacing, proposed in the BOM list, then the middle leg should be removed (e.g. by heating with a solder iron and pulling it off).
EMI filter capacitor (C81)
Although the simple X2 capacitor in the above "MIA" 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 EMI filters in the market.
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 the "MIA" circuit. 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
"MIA" PCB was developed in good faith that it presents no risk of
electric shock, nevertheless it is the sole responsibility of the
builder of the circuit to determine that the "MIA" circuitry and PCB
layout complies with the 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 non-conductive 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 prevents 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 - 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.
Sizing the power transformer fuse (F81)
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 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 a fuse that can withstand the mains voltage (e.g. a 250V fuse).
ON/OFF switch debounce capacitor
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 a 0.01uF 310VAC X2 capacitor directly onto the ON/OFF switch contacts. To avoid the risk of fire, class X2 capacitors must be used here. This should eliminate any disturbing sound when you switch off the amplifier.
Technical data
At 25°C ambient temperature unless otherwise noted.
Mains/line input: max.
250 VAC, max. 4 A continuous
Recommended maximum audio amplifier power (class B or AB amplifiers): 1x250 W or 2x125 W / 4 ohms or 8 ohms
Supply voltage output: 12 VAC, 125 mA (with recommended auxiliary transformer)