Euraudio APBF/APBM specifications

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

The APBF (APBM) is a single channel analog protection circuit without relay. It is part of the Euraudio LDA DMON and LDA MON DiY kits. Its function is switch-on/switch-off muting, loudspeaker protection, heatsink overtemperature protection, and overcurrent protection. Please find the APBFa/APBMa assembly instructions here.

APBF/APBM system requirements

What kind of systems can the APBF/APBM be used in?

Normally, the APBF/APBM board connects to one of the Euraudio Power supply & relay boards (PSSR350, PSS701, or PSS702) and receives the overcurrent signal from an Euraudio Ultra low distortion power amplifier board (LDA17 or LDA172). These boards are part of the Euraudio LDA DMON and LDA MON DiY kits.

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

Power supply & relay board compatibility

The APBF/APBM is fully compatible with the Euraudio PSSR350 Power supply & relay board. If you have to use the APBF/APBM with the Euraudio PSS701 or PSS702, then two components need to be reversely mounted on the PSS701/PSS702 PCB, which is covered in the assembly instructions of the PSS701/PSS702.

If you wish to use the APBF/APBM with a non-Euraudio Power supply & relay board, then please contact me for the pinout of connector J53 through which you may be able to connect your own Power supply & relay board(s).

Audio amplifier requirements

The APBF/APBM boards were specifically designed for the Euraudio LDA17 and LDA172 power amplifiers. 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 APBF/APBM overcurrent protection (OCP) monitors the current flowing through the output transistors. The APBF/APBM 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 LDA17hc, LDA17mc, LDA172hc and LDA172mc amplifiers. However, by modifying the OCP setting resistor (R54 in the APBF, R53 in the APBM), 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 APBF/APBM, please refer to this document, if you wish to use the APBF/APBM for your own amplifier.

The overcurrent protection of APBF/APBM 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 APBF/APBM is a fully analog, 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 +12V supply voltage 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 50 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 to 47nF 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 APBF/APBM kit.

Loudspeaker protection

If, in the amplifier output, a DC voltage higher in absolute value than about 2...3 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 LDA172mc connected, while the second image was taken with an LDA172hc connected. They depict how the sinusoidal peak current at which the OCP activates depends on frequency. Typical curves.


In the LDA172hc (or LDA17hc), 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 J52 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 red OCP LED is lighted when the OCP is active, and you must turn off the amplifier to reset the OCP circuit. Caution: The OCP is activated when there was an overload condition that could damage many 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 J51, the ERV connector.

Overtemperature protection

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 APBF/APBM deactivates the relay control can be programmed via a resistor value. Refer to the assembly instructions for information about this programming resistor.

The usual overtemperature threshold value is 70°C for external heatsinks, and 82°C for internal heatsinks. If you wish to temporarily deactivate the overtemperature protection, then disconnect TH51, the thermistor connector.

Output pull-up resistor

The relay control is an open collector output that requires a pull-up resistor. That resistor can be mounted either in the APBF/APBM board, or in the connected Power supply & relay board. The exception is PSSR350 in which there is no place for a pull-up resistor. If you mount the pull-up resistor in the Power supply & relay board, then you can defeat all the protection board functions by disconnecting the J53 (PROT) connector: all protections and all muting features will be bypassed. Note: The author prefers mounting the pull-up resistor in the protection board.

Fan control

The APBF 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: +12 VDC

Current consumption: max. 15 mA

Turn-on muting time: 2.0 s ±0.8 s

Maximum tolerated DC voltage:  +1.8/-2.0 V ±0.8 V (fast);  +2.3/-2.7 V ±1.0 V (medium speed);  +2.9/-3.5 V ±1.2 V (slow)

Typical loudspeaker protection actuation time after a +30 or -30 VDC step voltage: 37 ms (fast);  54 ms (medium speed);  75 ms (slow)

OCP activation speed at a current step 110% of the low frequency threshold current: <500 us

OCP overcurrent threshold accuracy: ±30%


Main page