Euraudio APBFa/APBMa assembly instructions

Welcome to the Euraudio APBFa/APBMa assembly instructions. The APBFa and APBMa are protection circuit boards almost fully assembled. One of them is selectable as part of the Euraudio LDA 53DM, LDA53M, and LDA 43S DiY kits. APBF differs from APBM in that it can control a fan. Please find the specification of the APBF and APBM here.

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

What the APBFa and APBMa DiY kits contain and what they contain not

The kit contains the APBF or APBM double-sided, fiberglass-reinforced epoxy PCB with all the electronic components mounted on it (fig. 1.), except the overcurrent protection (OCP) LED and 3 function adjusting resistors. The unmounted components, the NTC thermistor, the connectors, the 10-conductor ribbon cable, and the other necessary wires are also supplied (fig. 2), as well as the mechanical components for vertical fastening (fig. 3).


The APBF kit doesn't contain the components needed for fan control, you must source them yourself, if you want to use APBF to control a fan.

General assembly advice

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

Solder iron, solder wire, wire cutter, Phillips 1 (PH1) screwdriver,  Phillips 2 (PH2) screwdriver, drilling machine, 3 mm, 3.5 mm and 4 mm metal drill bits, heat-resistant epoxy glue (e.g. CX80 Autoweld). A heavier object with solderable surface (e.g. a tin can). A PCB holder jig and/or a model-making vice may come handy. 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. 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 kit 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. The APBF and APBM boards can be fastened horizontally or vertically.

Horizontal alignment is straightforward. Put the PCB in its intended place within the amplifier enclosure, mark the 3 holes with a marker pen, and then drill the holes with a 3.5 mm drill bit. Note: The kit doesn't contain the mechanical components for horizontal fastening.

Vertical alignment is most useful when there is not enough space within the amplifier chassis. Please refer to the following image to see how the vertical alignment looks like.

Identifying components

The graphical markings and numbers printed on the PCB make it clear where the components belong and in what orientation.

The longer leg of the LED is the anode, this should be connected to the round pad (marked "A" in the image below). The shorter leg is the cathode, that connects to the rectangular pad (marked "C" in the image below). It's practical to secure the LED onto the front panel of the amplifier, so the functioning of the overcurrent protection is visible, then run cables from the LED to the APBF/APBM PCB.

Assembling the IDC connector

The Power supply & relay board is connected through J53, a ribbon cable connector (IDC). Observe the polarity: the side of the ribbon cable marked with red color 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".

Programming resistors

There are 3 unmounted resistors which adjust or select different functions. You must select the appropriate resistor value to match the specific needs of your amplifier or loudspeaker.

Overtemperature select

70°C (external heatsink)

82°C (internal heatsink)

APBF (R62)

R62 = 6k8

R62 = 4k7

APBM (R61)

R61 = 6k8

R61 = 4k7

Note: Always mount a 4k3 resistor in APBF position R62 when you use APBF to control a fan.

Loudspeaker protect speed

fast: 40‑70W/4ohm or 30‑45W/8ohm amplifier

medium: 70‑130W/4ohm or 45‑85W/8ohm amp.

slow: 130‑250W/4ohm or 85‑165W/8ohm amp.

APBF (R67)

R67 = 15k...33k (not supplied)

R67 = 33...51k (47k supplied)

R67 = 51...75k (68k supplied)

APBM (R69)

R69 = 15k...33k (not supplied)

R69 = 33...51k (47k supplied)

R69 = 51...75k (68k supplied)


Output pullup resistor

Solid state relay board (PSSR350, PSS701, PSS702)

Mechanical relay board (PSMR101S, PSM101S)

APBF (R68)

R68 = 11k

R68 = 6k8

APBM (R65)

R65 = 11k

R65 = 6k8

As you can see, the output pull-up resistor value depends on which kind of Power supply & relay board is connected to the protection board. Alternatively, you can mount the pull-up resistor on the Power supply & relay board instead of the APBF (APBM) board, except for the PSSR350. On PSSR350, there is no space for a pull-up resistor. If you mount the pull-up resistor on the Power supply & relay board, then you defeat all protections (the relay will always be on) when the ribbon cable connector is disconnected. I recommend that you mount the pull-up resistor on the APBF/APBM, in this way the relay will be off when the ribbon cable connector is disconnected.

Setting the overcurrent threshold value for a non-Euraudio amplifier

If you wish to use the overcurrent protection (OCP) function of APBF/APBM for your own amplifier (not Euraudio's LDA162 or LDA172), then you yourself should calculate and mount the Rs and Rb OCP resistors.

OCP resistors

OCP setting resistor Rs

Overcurrent boost resistor Rb


R54 (2 Watts)

R52 (0.6 Watt)


R53 (2 Watts)

R52 (0.6 Watt)

The OCP resistor values in Ohms can be calculated as follows:

Rs = (IthRm - 1.1) / 0.0115   (+/-5%),

Rb = Ri / 3   (+/-20%),

where Ith is the overcurrent threshold value you want to set in Amperes,

Rm is the output transistors' emitter resistor value in the amplifier in Ohms.

If the calculated Rs is lower than about 70 Ohms, then the OCP circuit of the APBF/APBM can't be used with your amplifier, unless you modify your amplifier, namely increase the emitter resistors of the output transistors in your amplifier. Do not install a resistor lower than 68 Ohms for Rs.

The ovecurrent threshold is specified within a certain tolerance at 25°C and at frequencies lower than 1kHz. You can read about the accuracy and frequency dependence of the OCP in the specification of the APBF/APBM.

Connecting OCP for a non-Euraudio amplifier

Connect the ERV connector (J51) in APBF/APBM to the output transistors' emitter resistors in the manner shown in the following image. Mind the polarity of J51. Reverse connection renders the OCP ineffective and may damage the protection circuit.

The J52 jumper

Install the J52 jumper to select the normal (lower) current threshold for the overcurrent protection, which is about 9 A peak / output transistor for LDA172hc and LDA162hc, 9 A peak for LDA162mc, and 11 A peak for LDA172mc (for frequencies lower than 1kHz). Normally, J52 should be installed at all times. However, if the overcurrent protection accidentally kicks in at high volume levels while a low impedance (4 Ohms) loudspeaker is hooked up to the amplifier, then remove the jumper for an approximately 20% boost in allowed output current.

Fan control circuit and fan connection (APBF only)

The APBF board is equipped with a DC fan control circuit, but the necessary components are not supplied in the DiY kit. These are commonly available components which are easy to purchase anywhere in the world. So if you want to use a DC fan, these are the additionally needed components:

R71    6k8  1% 0.4W

R72    11k  1% 0.4W (desolder the shorting wire and mount this resistor in place of it)

R73    750k  5% 0.25W

R74    47k  5% 0.25W

R75    22k  5% 0.25W

R79    47k  5% 0.25W

C60    220p or 100p  NP0 ceramic

Q58    BC546  or BC547

Q60    BC517  or BS170, see below (Caution! BS170 is an electrostatic sensitive device.)

J54     2-pin polarized pin header 2.54mm pitch

The fan will start to operate when the heatsink is at around 70 °C.

Important: If you use fan control, then check the value of R62. If it is 6k8, then change it to 4k7. With this the overtemperature protection will operate only at about 82°C heatsink temperature instead of 70°C, allowing headroom for the fan to operate between 70°C and 82°C.

You must make a suitable power supply for the fan, ensuring proper voltage and proper current sourcing capability at the intended operational temperature. The APBF board will not provide supply voltage, it only switches the fan into the power supply circuit by providing the path to ground, as shown in the next image.


The diode, denoted "D" in the image, is not needed for the majority of DC fans, for the so-called brushless fans that have internal switching electronics. However, for brushed fans it's compulsory, because in lack of the diode the inductive flyback will damage the switching transistor in the APBF board. This diode can be an 1A diode like the 1N4004 or 1N4007. If you're unsure about the type of the fan, please install this diode, it can't do any harm if it's unnecessary there.

The rated voltage and current of the fan must be below the following maximum ratings:

If Q60=BS170: max. 48V, max. 200mA; (Caution! BS170 is an electrostatic sensitive device.)

if Q60=BC517: max. 24V, max. 400mA.

Overcurrent and overtemperature protection connectors

J51 (ERV) is the overcurrent sensing connector, while TH51 connects the temperature sensor (thermistor) which is supplied in the kit.

The polarity of J51 is important, the pin marked "+" must be connected to the appropriate ER+ point, while the other pin must be connected to the appropriate ER- point in the LDA162 or LDA172 PCB. Supplied is a thin wire with red insulation for the ER+, and a thin wire with black insulation for the ER- connection.

TH51 is not polarized.

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

After you finished the connectors, the two wires of the overcurrent protection should be should be twisted together. Do the same with the two wires of the overtemperature protection.

Mounting the thermistor to heatsink

I recommend mounting the thermistor in the following way. Close to (<6 mm) one of the output transistors in the heatsink, drill a 3 mm diameter hole, and then widen the top of the hole to 4 mm in about 2-3 mm depth with a 4 mm drill bit. This is because the tip of the thermistor is 3 mm thick but it widens to about 3.5 mm near the legs. Take the heat resistant epoxi glue (e.g. CX80 Autoweld), mix its components, then apply it into the hole and spread it over the thermistor as well. Push the thermistor into the hole without making air voids. To "bubble out" possible air voids, you may push and pull the thermistor gently a few times. To hold the thermistor in position while the epoxi glue cures, you can thrust the legs of the thermistor through a sheet of paper, and then let the sheet rest on the horizontally positioned heatsink surface.

When the epoxi has hardened, shorten the thermistor legs, then solder the two thin wires to the thermistor legs, and twist the wires together. When it is finished, it looks like this:


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