Euraudio MID2 and MID2A assembly instructions
Welcome to the Euraudio MID2 and MID2A assembly instructions. The MID2(A) is a mains input and distribution board with two galvanically isolated auxiliary DC power supplies, that can be configured for dual, ground-independent auxiliary supply (for dual mono amplifier) or single auxiliary supply (for stereo or mono amplifier). MID2(A) is part of the stereo Euraudio CTA MID and LDA MID, dual mono LDA DMx, and mono LDA MON ultra low distortion audio amplifier DiY kits. Please find the specifications of the MID2(A) here.
The assembly of the MID2 will be covered in this documentation. The assembly of the MID2A is very similar, the difference is that it contains debounce capacitors for the ON/OFF switches.
Please read through these instructions before doing anything with the DiY kit.
What the MID2(A) kit contains and what it contains not
The kit contains only the MID2(A) fiberglass-reinforced epoxy PCB. You have to source the configuration specific electronic and mechanical components to be mounted on it.
Component list
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 MID2 part list (BOM) from here.
Please download the MID2A part list (BOM) from here.
Note that a single spreadsheet file file contains the BOMs for CTA MID and LDA MID (stereo), LDA DMx (dual mono), and LDA MON (mono) configurations.
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 MID2 or MID2A DiY kit. 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
MID2(A) PCB was developed in good faith that it presents no risk of
electric shock, nevertheless it is the sole responsibility of the
builder of this kit to determine that the MID2(A) circuitry and PCB
layout complies with the applicable safety standards in your country.
The MID2(A) circuitry is presented in the specifications of the
MID2(A).
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 fuses (F81, F82)
Note: Sizing the transformer primary fuse is an electrical safety related issue. I assume no responsibility 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 country.
(1) The fuse wire should not melt during the high rush-in current at switch-on, this sets a minimum on fuse current 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 current 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 sized lower 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 PSS701/PSS702, PSSR2S, 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 nominal power 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 chosen 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).
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.
Soldering
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
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 auxiliary supply configuration, 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 350 VA) transformers are used. If you use two transformers less than about 200 VA 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 auxiliary supply configuration the components of only one side (either left or right) are mounted.
J81, the mains input connector (230VAC_IN) is part of all 3 configurations. J82 (XFR_PRI1, left side) and J83 (XFR_PRI2, right side) 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 (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).
Please click this link, if you don't have a crimp tool suitable for the contacts of the connector.
Linear regulator for CTA MID and LDA MID
The L7809 linear regulator used in the CTA MID and LDA MID needs a small radiator plate. This can be a 15x24x2 mm aluminum piece mounted to the TO-220 case with thermal paste applied.
Footprint of the auxiliary transformer(s)
The footprint of the EI30 core encapsulated small transformer used in the PCB layout:
The auxiliary transformers must be short-circuit proof, i.e. of a design that can be operated without a primary fuse.
The allowed maximum load current depends on the auxiliary transformer. Recommended transformers: CTA MID and LDA MID: 9VAC, 3VA. LDA DMx: 12VAC, 1.5VA (2 pcs). LDA MON: 12VAC, 1.5VA.
The auxiliary supply voltage output
The auxiliary supply voltage outputs are J86 (left side) and J87 (right side). The two GND points (GND1, GND2) are galvanically isolated to allow dual mono construction.
Securing the PCB within the chassis
The MID2(A) carries hazardous voltage: please take extra care to fasten all the 4 mounting screws properly, e.g. use spring washers together with flat washers to ensure that the board can't get loose.
The ON/OFF switch debounce capacitor
Switches may bounce when switched on or off. The MID2A PCB has place for the bypass capacitors of the ON/OFF switches, but the MID2 PCB does not. If you hear a click or pop sound from the speakers when you switch off the amplifier, please solder 0.01uF 310VAC X2 capacitor(s) directly onto the ON/OFF switch contacts when using MID2. 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.