Euraudio DPB specifications

Welcome to the Euraudio DPB specifications, in which you can read about the system requirements (application criteria), operation and technical data of the DPB.

The DPB is a microcontroller controlled single channel protection circuit without relay. It is part of the Euraudio LDA MID, LDA DMx, and LDA MON DiY kits. Its function is switch-on/switch-off muting, loudspeaker protection, heatsink overtemperature protection, and overcurrent protection. Please find the DPBsc assembly instructions here.

DPB system requirements

What kind of systems can the DPB be used in?

Normally, the DPB board connects to one of the Euraudio Power supply & relay boards (PSSR2S, PSSR1, PSSR2, or PSS701) and receives the overcurrent signal from an Euraudio Ultra low distortion power amplifier board (LDA17). These boards are part of the Euraudio LDA MID,  LDA DMx, and LDA MON DiY kits.

The DPB is a mono protection circuit, for dual mono or stereo systems you'll need two of it.

Non-Euraudio Power supply & relay boards

If you wish to use the DPB with a non-Euraudio Power supply & relay board, then please contact me for the pinout of connector J54 through which you may be able to connect your own Power supply & relay board.

Audio amplifier requirements

The DPB boards were specifically designed for the Euraudio LDA17 power amplifier. However, the switch-on/switch-off muting, loudspeaker protection, and heatsink overtemperature protection should work fine on all class B or class AB amplifiers with symmetric power supplies having at least +/-30 V supply rail voltages.

The DPB overcurrent protection (OCP) monitors the current flowing through the output transistors. The DPB overcurrent protection is suited to class B or class AB transistor amplifiers with output transistors that have low ohmic value resistors in their emitters (emitter follower output) or low ohmic value resistors in their collectors (CFP output). It's overcurrent protection input network was customized for the LDA17xc, LDA17hc, and LDA17mc amplifiers. However, by modifying the OCP setting resistor (R54), it might be tailored to output stages of other amplifiers. Please note that the given emitter/collector resistor value in the amplifier sets a lower limit on the overcurrent threshold value below which you can't go. The calculations to determine the OCP resistor values are found in the assembly instructions of the DPB, please refer to this document, if you wish to use the DPB for your own amplifier.

The overcurrent protection of DPB can't be used with those kind of MOSFET output stages that don't have low ohmic value resistors in the sources or drains of the output transistors.


The DPB is a mono protection circuit that provides a control signal for the switching transistor of the relay that connects the amplifier output to the loudspeaker. When there is a condition while the loudspeaker shouldn't be connected (mute or protect), then the low level control signal deactivates the relay.

Turn-on/turn-off muting

If there is no dangerous DC voltage at the amplifier output, then the relay control goes active in approx. 2 seconds after the supply voltage to the DPB appears (turn-on thump muting). This time is assumed to be sufficient for any turn-on transients in the amplifier to die away. When the power (main transformer) is turned off, the relay control goes inactive very quickly, in about 35 milliseconds (turn-off thump muting).

If during switching off the amplifier, a disturbing, acute sound is generated right when you trip the on-off switch, that may be caused by the bounce of the switch. Such problems can usually be cured by soldering a 10nF class X2 capacitor directly onto the leads of the on/off switch. To avoid the risk of fire, only class X2 capacitors are allowed for line voltage applications. This capacitor is not part of the DPB kit. (The Euraudio MID2A Mains input/distribution & auxiliary supply PCB has a place for this X2 capacitor.)

Loudspeaker protection (SP)

If, at the amplifier output, a DC voltage higher in absolute value than about 1.5...2 Volts appears, then the relay control goes inactive after some delay time. That time needs to be allowed so that there should be no false trigger for high amplitude, low frequency amplifier signals. The higher the absolute value of the DC voltage is, the faster the relay control is deactivated.

Like the other loudspeaker protection circuits, neither is this one suitable to protect against loudspeaker overload due to excessive loudness, it only intervenes if the amplifier fails in a way that a dangerous DC voltage appears at its output that would damage the loudspeaker.

The loudspeaker protection speed can be programmed via a resistor value to suit your loudspeaker wattage (high speed, medium speed, low speed). Refer to the assembly instructions for information about this programming resistor.

Overcurrent protection (OCP)

When measured with sinusoidal amplifier output signals, the OCP threshold current has a frequency dependent behavior, as seen in the following images. The first image was taken with an LDA17hc connected, while the second image was taken with an LDA17xc connected. They depict how the sinusoidal peak current at which the OCP activates depends on frequency. Typical curves.


In the LDA17xc, there are two output transistor pairs, and since only one pair is monitored, the actual peak output current of the amplifier at which the OCP intervenes is approx. 2 times as much as shown in the 2nd image.

For the guaranteed accuracy of the threshold current at 25°C, refer to the Technical data section. It is also temperature dependent.

Images were taken with jumper J53 installed. By removing the jumper, the threshold current can be increased by about 20%, which may be necessary if the OCP falsely triggers at very high volume levels with low load impedance (4 ohms).

When the amplifier output current exceeds the maximum allowed value, the OCP activates, latching the control signal in the low level state, thus the relay disconnects the load (loudspeaker) permanently.

The OCP LED is lighted when the OCP is active, and you must turn off the amplifier to reset the OCP. Caution: The OCP is activated when there was an overload condition that could damage components in the amplifier. Never retry turning on the amplifier until the fault that activated the OCP is cleared (e.g. check loudspeaker connector for short circuit). Repeated retries may very probably result in more and more damage to the amplifier (or even to the protection circuit).

If you wish to temporarily deactivate the OCP, then disconnect J52, the ERV connector.

Overtemperature protection (OTP)

At the TH51 connector, a thermistor is connected that had to be glued to the main heatsink of the amplifier, near an output transistor. The temperature at which the DPB deactivates the relay control is selectable from 2 values: a lower value (70°C) for external heatsink, or a higher value (81°C) for internal heatsink.

The DPB is factory set to the higher overtemperature (OT) threshold. This is how you can verify and change the OT setting:

1. Hold down the push-button on the DPB PCB while turning on the power to the DPB.

After the FAN, OCP, OTP, and SP LEDs are flashed for checking, the selected OT value is displayed as follows: If the OT threshold is set to the lower value, then only the OTP LED is lit. If the OT threshold is set to the higher value, then both the OTP and OCP LEDs are lit.

2. Release the push-button.

3. If the setting is appropriate for you, then push the push-button shortly.

The OT threshold setting will not be changed, and the DPB starts to operate.

4. If the setting is not appropriate for you, push and hold down the push-button until the OCP LED changes.

The OT threshold setting will be changed, which is reflected in the change of the OCP LED, then the DPB starts to operate.

If you wish to temporarily deactivate the overtemperature protection, then disconnect TH51, the thermistor connector.

Fan control

The DPB has an optional temperature dependent fan control circuit. Please refer to the assembly instructions for more information.

Technical data

At 25°C ambient temperature unless otherwise noted.

Supply voltage: regulated +9 VDC or +12 VDC

Current consumption: max. 22 mA (when all LEDs are lit)

Turn-on muting time: 2.0 s ±0.8 s

Maximum tolerated DC voltage:  +1.2/-1.3 V ±0.5 V (fast);  +1.5/-1.6 V ±0.7 V (medium speed);  +1.9/-2.1 V ±0.9 V (slow)

Typical loudspeaker protection actuation time after a +30 or -30 VDC step voltage: 34 ms (fast);  52 ms (medium speed);  71 ms (slow)

OCP activation after a 120% step of the 1 kHz threshold current: <500 us

OCP overcurrent threshold guaranteed accuracy (1kHz): ±30%


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