Euraudio PSMR101Sc assembly instructions

Welcome to the Euraudio PSMR101Sc assembly instructions. The PSMR101S is a symmetrical power supply with two single-pole mechanical relays, and is part of the Euraudio LDA 43S low distortion audio amplifier DiY kit, but may be used for other amplifiers as well. Please find the specifications of the PSMR101S here (to be supplied).

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

What the PSMR101Sc DiY kit contains and what it contains not

The kit contains the PSMR101S double-sided, fiberglass-reinforced epoxy PCB (fig. 1) and all the electronic and mechanical components to be mounted on it (fig. 2), except the filter capacitors, fuses, and cables/wires.


Component list

Please download the PSMR101Sc component list (BOM) from here.

Sizing the filter capacitors and fuses

The  filter capacitors and fuses are not part of the DiY kit, you must size these components according to the power and allowed load impedance (e.g. 4 ohms or 8 ohms) of the amplifier. The amplifier power and load impedance also largely determines the necessary transformer. The exact sizing procedure is not so simple, so I'd rather help with a table containing a few examples  below. These examples are valid for class B or class AB amplifiers that have an output signal that can approach the supply rail voltages within 3-5 Volts (such as the Euraudio LDA172mc, LDA172hc, LDA162mc or LDA162hc).

The LDA 43S DiY kits contains two LDA162mc's or two LDA172mc's, which are nominally 100 W power amplifiers, so unless you decide to make your amplifier into a lower power unit than that, the 1st or 4th row of the table applies.

Do not use capacitors with lower capacitance than shown in the table, if you want to exploit the full amplifier power, because the ripple current specification of smaller capacitors may be inadequate. You may choose higher capacitance value, but that translates to higher amplifier power, so that would call for a higher VA rated transformer or for a lower transformer voltage than that shown in the list. (The power supply rejection ratios of the LDA172 and LDA162 are excellent, so generally it makes no sense to use bigger filter capacitors with this amplifier.)

Amplifier power


Recommended fuse (4 pcs)

Recomm. filter cap (2 pcs)

2x100 W / 4 ohms

2x28 VAC, 380 VA


15,000uF / 50V

2x75 W / 4 ohms

2x25 VAC, 280 VA


12,000uF / 50V

2x50 W / 4 ohms

2x23 VAC, 220 VA


10,000uF / 40V

2x100 W / 8 ohms

2x38 VAC, 360 VA


10,000uF / 82V

2x75 W / 8 ohms

2x34 VAC, 280 VA


8,200uF / 63V

2x50 W / 8 ohms

2x30 VAC, 220 VA


6,800uF / 63V


PSMR101S is fully compatible with the Euraudio APBM and APBF protection boards. However, if you have to use PSMR101S with the Euraudio APBT or APB, then you'll have to reverse diodes D91 and D92 from their original direction shown in the PSMR101S PCB.

General assembly advice

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

Solder iron, solder wire, wire cutter, 10-conductor ribbon cable (the APBF/APBM kit contains it), suitable cables/wires, Phillips 1 (PH1) screwdriver, Phillips 2 (PH2) screwdriver, heatsink compound, drilling machine, 3.5 mm metal drill bit. A heavier object with solderable surface (e.g. a tin can). A PCB holder jig and/or a model-making vice may come handy. Cable insulation stripper is recommended for cables/thick wires. 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. Basic electronic component knowledge is necessary for identification.

D94 and D95

The original design contained a single 1N4148 diode in positions D94 and D95. The circuit works with it, but an improvement in the release speed of the relay can be realized by including a BZX85C24 or BZX85C27 Zener diode in series with the 1N4148, as shown in the images below. I recommend this modification for a better overcurrent protection. Please observe the diode polarities.


Assembling the IDC connector

The protection boards are connected through the J92 and J93 ribbon cable connectors (IDC). Observe the polarity: the red colored (marked) side of the ribbon cable should be aligned with the triangle sign on the connectors. Please crimp this connector carefully. If you're unsure how to assemble this type of connector correctly, please google "crimping IDC connectors".

Relay control (PROT), relay input (AOUT) and relay output (SPKHOT)

1st relay channel: Relay 1 control is via the PROT1 connector. Solder pad J111 marked "AOUT1" is the input of relay 1 (= amplifier output 1). Solder pad J110 marked "SPKHOT1" is the output of relay 1 (= loudspeaker hot signal 1).

2nd relay channel: Relay 2 control is via the PROT2 connector. Solder pad J109 marked "AOUT2" is the input of relay 2 (= amplifier output 2). Solder pad J108 marked "SPKHOT2" is the output of relay 2 (= loudspeaker hot signal 2).

Connecting the transformer

The center-tapped secondaries of the transformer must be soldered to the points marked "AC", "AC" és "COM", as shown in the image below:

Due to high current peaks and subsequent I2R heating, the wire cross section must be big enough to carry about 4 times the max. output current of the amplifier on the "COM" connection, and big enough to carry about 2 times the max. output current of the amplifier on the "AC" connections each. For home audio amplifiers, 0.75 mm2 is usually more than enough for both wires.

Securing the heatsink of the bridge rectifier

Drill a 3.5 mm diameter through hole into the heatsink, 12 mm from the bottom of it, as shown in the next image.

First spread some heatsink compound (thermal grease) on the back of the bridge rectifier (D93). Then fasten the bridge rectifier to the heatsink and on the other side to the metal corner bracket with the M3x12 screw, washers and nut. Put a flat washer both on the bridge rectifier surface and on the heatsink surface, and then put a spring lock washer on top of one of them. Insert the leads of the rectifier bridge into the corresponding holes in the PCB, and then fasten the whole thing to the PCB with the M4x8 screw, tooth lock washer and nut. Solder the diode bridge into the PCB only after all screws have been fully tightened.

Connecting the grounding points

There is star-like grounding scheme in the PSMR101S PCB to prevent hum resulting from ground loops. Of course, if you have created a ground loop somewhere else in your amplifier, then hum may still appear. Points "GND", "PGND", "ZGND", "SPKGND1" and "SPKGND2" are all run to the star point, and they all can be used as ground. It's recommended to connect the loudspeaker grounds to the "SPKGND1" and "SPKGND2" points, because they have the thickest PCB trace, allowing it to carry the highest currents. If you use PSMR101S with the LDA172mc or LDA162mc amplifier, then the ground points in the LDA172 and LDA162 PCB are identically marked as "GND", "PGND" and "ZGND".

Other high current connections

Besides "COM" and "AC", the supply voltage connections "+VCC", "-VCC", "SPKGND1" and "SPKGND2" also carry high currents. These need thick enough wires that can withstand the maximum amplifier output current. In most cases wires with 0.5 mm2 cross section are more than enough.

Connecting the chassis

For protection against electric shock, the metal chassis of the amplifier must be connected to the mains earth ground conductor (or alternatively an equivalently effective protection method against electric shock must be used, which we will not cover here). On amplifiers equipped with protective earthing, it's not recommended to connect the ground of the amplifier to the chassis as well, because hum will develop across the protective earthing conductor. Instead, connect the metal chassis at the same point where the earth ground is connected to the point marked "CHAS" in the PSMR101S PCB, by soldering the free end of the supplied wire with ring lug to this point. So the effect of this kind of ground loop will be minimized.

More information on earth grounding can be found at this page: Earthing Your Hi-Fi - Tricks and Techniques.

12V supply voltage connection

The power to the relay and protection circuits is supplied via connector J112. The pad beside the C103 capacitor is the regulated 12V, and the other pad is GND. If you ordered the Euraudio LDA 43S kit, then the MIB12 board in those kits is the 12V power supply. The PSMR101S takes max. 90 mA current from connector J112, when two APBM or two APBF protection boards are also connected to it.

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

Suitable wires of 0.25 mm2 cross section with red and black insulation are part of the DiY kit. After you finished the connector, the two wires should be twisted together (if they are separate).


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