Euraudio SPRSc assembly instructions

Welcome to the Euraudio SPRSc assembly instructions. The SPRS is a turn-on thump muting circuit, which is part of the Euraudio LDA MIN kit, but can be used for other DiY amplifiers. Please find the specification of the SPRS here.

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

What the SPRSc DiY kit contains and what it contains not

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


General assembly advice

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

Solder iron, solder wire, wire cutter, suitable wires, Phillips 1 (PH1) screwdriver, 3x0.6 mm-es flat-head screwdriver, drilling machine, 3.5 mm metal drill bit, a PCB holder jig and/or a model-making vice. A heavier object with solderable surface (e.g. a tin can). Cable insulation stripper is recommended for 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. 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.

The resistor color-codes can be found on many sites on the internet, search it, if you don't know it from memory.

In the SPRS PCB, you have to observe the polarity of the following components: diode and LED, transistor, polarized electrolytic capacitor, bridge rectifier, voltage regulator.

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 muting is visible, then run the wires from the LED to the APBF PCB.

The rectifier bridge is the circular, 4-lead component. The longest lead of the component that is marked with the "+" sign should be soldered to the rectangular pad on the PCB, also marked "+" in the PCB.

The biggest electrolytic capacitor should be soldered into the position indicated by the yellow arrow in the image below, the 3 smaller elcos go to the remaining 3 positions.

Mount resistors with the value indicated in the table below into the positions shown with red arrows. Two 30k and two 47k resistors are supplied. (In case of a bridging, half both the power of the bridged amplifier and the connected impedance.)

Loudspeaker protection speed

fast: <60W/4ohm or <40W/8ohm amplifier

medium: 60‑130W/4ohm or 40‑85W/8ohm amplifier

Resistor value




Connecting the ground (GND) points

The grounding of the SPRS PCB is implemented so that hum is avoided because ground loops are not formed. (Of course, if you created a ground loop somewhere else in your amplifier, then you may have a hum problem.) To avoid hum, there are three GND points, which need to be connected via separate wires to the point in the amplifier where the ground reference is established. This is in most cases in the main power supply of the device. The ground reference ideally has a star-like implementation, a power supply board so designed is for example the Euraudio PS1S, but of course you can use your own power supply, if you wish. In some amplifiers, the power supply is located in the amplifier PCB, and in this case you have no option.

You need to connect the "GND(L)" and "GND(R)" points marked yellow in the image below to the amplifier ground with wires thick enough to carry the maximum current of the loudspeaker; in most cases wires with 0.5 mm2 cross section are sufficient. The wires can be connected through the supplied terminal blocks, or simply by soldering them into the PCB (without using the terminal blocks). The "GND" point (J122) marked yellow is the grounding point of the muting circuit, solder a thin wire there and connect it to the ground reference of the amplifier.

Making the amplifier and loudspeaker connections

The outputs of the two amplifier channels (e.g. left and right output of a stereo amplifier), or in case of bridged amplifier channels the outputs of the two out-of-phase channels should be connected to the respective (L, R) "IN" points.

In case of non-bridged channels, the loudspeakers (loudspeaker connectors) of the two channels should be connected to the respective (L, R) "OUT" points (red loudspeaker connectors), and to the "GND" points next to them (black loudspeaker connectors). In case of bridged channels, the loudspeaker should be connected between the two "OUT" points.

The amplifier and loudspeaker connections can be made by fastening wires to the terminal blocks that were soldered into the PCB, or by soldering the wires directly into the PCB (without using the terminal blocks). The wires must be thick enough to withstand the maximum output current of the amplifier; in most cases wires with 0.5 mm2 cross section are sufficient.

Supply voltage connector

You have to ensure the power supply to the muting circuits through the connector marked "12VAC". Here a ground independent alternating voltage is acceptable; ground independent practically means that the power  must be supplied either from a separate small transformer or from a separate secondary coil of the main transformer of the amplifier. The transformer rating is 12...15 VAC min. 125 mA.

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

The necessary wires are not part of the DiY kit, and any kind of thin, insulated wire can be used up to 0.35 mm2 cross section. After you finished the connector, the two wires should be twisted together (if they are separate).


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