Euraudio PSSR350c assembly instructions

Welcome to the Euraudio PSSR350c assembly instructions. The PSSR350 is a symmetrical power supply with an ultra low distortion SPST solid state relay, and is part of the Euraudio LDA 53M and LDA 53DM low distortion audio amplifier DiY kits, but may be used for other amplifiers. The Euraudio LDA 53M contains one PSSR350c kit, while the LDA 53DM contains two. Please find the specifications of the PSSR350 here.

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

What the PSSR350c DiY kit contains and what it contains not

The kit contains the PSSR350 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 PSSR350c 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 LDA172 and LDA162).

The LDA 53M and the LDA 53DM DiY kits contain LDA172hc's, which  is nominally 200 W power amplifier, so unless you decide to make your amplifier into a lower power unit than that, the 1st or 4th row of the table applies.

Only 2 filter capacitors are necessary altogether, so you don't have to fill in all the 4 capacitor positions in the PCB. 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 (2 pcs)

Recomm. filter caps (2 pcs)

200 W / 4 ohms

2x37 VAC, 350 VA


10,000...12,000uF / 63V

150 W / 4 ohms

2x34 VAC, 275 VA


8,200...10,000uF / 63V

100 W / 4 ohms

2x30 VAC, 200 VA


6,800...8,200uF / 63V

150 W / 8 ohms

2x44 VAC, 300 VA*


6,800...8,200uF / 82V

100 W / 8 ohms

2x38 VAC, 220 VA


5,600...6,800uF / 63V

* The unloaded power supply voltage should not exceed +/-70V in any circumstances, which is the absolute maximum that the PSS701/PSS702 can withstand.


PSSR350 is fully compatible with the Euraudio APBM and APBF protection boards. However, if you have to use PSSR350 with the Euraudio APBT or APB, then you'll have to reverse diodes D91 and D92 from their original direction shown in the PSSR350 PCB.

General assembly advice

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

Solder iron, solder wire, wire cutter, technician's tweezers,  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. Anti-static wristband and anti-static (ESD) gloves. 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. Though basic electronic component knowledge is necessary for identification.

Q92 and Q93

Q92 and Q93 are electrostatic sensitive devices, which can be damaged unknowingly by electrostatic voltages as low as 20...25 Volts. You MUST use anti-static gloves and an anti-static wristband connected to an earthed point while you handle and solder these components. Such an earthed point can be the metal chassis of an earthed appliance plugged into an earthed wall outlet, e.g. the metal back plate of a desktop PC. The wristband and gloves in themselvesdo not eliminate all the risks of electrostatic discharge. For complete protection, anti-static desk pad and floor mat, anti-static clothes and anti-static shoes are recommended, but if you don't have them, by all means wear clothes from 100% cotton or other natural fabrics and not synthetic. Touch the packaging of Q92 and Q93 (pink or gray anti-static plastic packet) for a few seconds before you take these transistors out of the packaging. Please install Q92 and Q93 as the last step of the assembly process and handle the PCB with care afterwards.

You must "straddle" the leads of these transistors to allow soldering them into the PCB.

Assembling the IDC connector

The protection board is connected through the J92 ribbon cable connector (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".

U91 and associated components

U91 can be one of two kind of components. It's either marked with a white "V" or with a red "I". Some other components depend on which of them is installed in the U91 position, as follows:

Component "V":  do not mount Q99, mount Q100 and R100 (470k or 510k), short R99 (wire link or 0R)

Component "I":  do not mount Q99 and Q100, short R99 (wire link or 0R), and short R100 (wire link or 0R)

Soldering Q100

When component "V" is installed in position U91, a small SMD component is needed in position Q100. This tiny SMD can be easily overheated if you are not careful with soldering. However, this component is essential for the reliable operation of this circuit with component "V". To solder Q100, I recommend to use a controlled temperature solder iron and technician's tweezers. Do not heat the leads for more than 2 second each, and after soldering one lead, let the component cool down before you go on with the other leads. Check the solder joints after soldering.

D96 and D97

If you intend to use the PSSR350 for amplifiers with more than 200 W output power, then I recommend using bigger diodes in positions D96 and D97 instead of what is supplied, e.g. use P600M diodes.

Solid state relay input and output (AOUT and SPKHOT)

Solder pad J102 marked "AOUT" is the input of the solid state relay (= amplifier output), while J101 marked "SPKHOT" is the output of the solid state relay (= loudspeaker hot signal).

Ground lifting diodes

PSSR350 is the first PCB version, and I changed a component in it to make it somewhat cheaper. Instead of the D94 bridge rectifier, please solder the two supplied diodes into the D94 position, as shown in the next image. The orientation of the diodes must be such that one diode cathode and one diode anode is soldered to the two pads in the middle of D94. So it's either done the way as shown in the image (A=anode, C=cathode) or the other possibility is to turn both diodes half a turn (A --> C, C --> A). (Note: The actual diodes supplied may be bigger than what is shown in the image.)

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 PSSR350 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" and "SPKGND" are all run to the star point, and they all can be used as ground. It's recommended to connect the loudspeaker ground to the "SPKGND" point, because it has the thickest PCB trace, allowing it to carry the highest currents. If you use PSSR350 with the LDA172hc, LDA172mc, LDA162hc 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" and "SPKGND" 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 for "+VCC", "-VCC" and a wire with 1.0 mm2 cross section is more than enough for "SPKGND".

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 PSSR350 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 connector

The power to the solid state relay and protection circuits is supplied via connector J103. The pad next to the "+12V" marking is the regulated 12V, and the other outside pad is GND. If you ordered the Euraudio LDA 53M or LDA 53DM DiY kit, then the MIB12 board in those kits is the 12V power supply. The PSSR350 card takes max. 40 mA current from here when an APBM or APBF protection board is also connected.

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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