====== Simple Loudspeaker DC Protector ======

**__WARNING__: This circuit requires mains wiring. If you are not confident around such wiring, __DO NOT__ attempt this project.**

This speaker protection circuit was originally designed by Randy Slone and published in "Audiophile Construction Manual". The circuit provides protection against DC faults, overheating, and unstable operating conditions by disconnecting the speakers whenever a fault is detected.

Speaker protection is extremely important in direct-coupled solid-state amplifiers because a failure in the output stage can place large DC voltages directly onto the speaker terminals. Unlike normal audio signals, DC current can rapidly overheat and destroy speaker voice coils. Protection circuits also help prevent damage caused by startup and shutdown transients, amplifier instability, or thermal overload conditions.

This design combines DC fault detection, thermal shutdown capability, relay-based speaker isolation, and visual fault indication into a relatively simple and effective protection system suitable for high-fidelity audio amplifiers.

{{::simple_speaker_protector:schematic.png}}

**Fig. 1: schematic of the speaker DC protector**

===== Circuit Description =====

The circuit in Fig. 1 above is designed to protect speakers from dangerous DC faults, overheating, and unstable amplifier operating conditions by disconnecting the speakers whenever a fault occurs. It also contains a low-voltage regulated supply for the control electronics and a flashing fault indicator based around an NE555 timer.

At the top of the schematic is the main power supply section. AC voltage enters through connector J1 and feeds bridge rectifier DB1, which converts the incoming AC into full-wave rectified DC. Capacitors C1 and C2 smooth the rectified voltage and create the positive and negative supply rails labeled Vcc and Vee. Because the capacitors are connected in series across the supply rails, their junction becomes the ground reference point for the amplifier system. Resistors R1 and R2 act as balancing and bleeder resistors, helping equalize voltage across the capacitors and safely discharge them after power is removed.

The protection electronics operate from a separate low-voltage supply derived from one of the transformer's AC secondary windings. Diode D1 half-wave rectifies part of the incoming supply and charges capacitor C3. Resistor R3 limits current into Zener diode ZD1, which clamps the voltage to approximately 12V. This creates a regulated supply for the NE555 timer and the low-level control circuitry. Capacitor C3 filters the supply voltage to reduce ripple and noise.

An important detail is that the relay coil itself is not powered from the 12V Zener regulator. Instead, relay RLA1 is powered directly from the main positive amplifier rail through resistor Rrla. The relay itself is a standard [[https://www.mouser.com/ProductDetail/Omron-Electronics/G2R-2-DC12?qs=OoIWmhSwP63apr9LY3Eisw%3D%3D|Omron PCB mount G2R-2 DC12]], for example. The resistor drops the excess voltage between the main supply rail and the relay coil operating voltage.

The resistor value is selected according to the relay coil voltage and current requirements using:

**Rrla = Vcc - Vrelay / Icoil**

where **Vcc** is the main amplifier supply voltage, **Vrelay** is the relay coil rated voltage, and **Icoil** is the relay coil current. This allows the relay to operate correctly even though the amplifier supply voltage is substantially higher than the relay's rated voltage.

Relay coil current can be calculated from it's coil resistance, and using the Omron G2R-2 DC12 as an example, the coil resistance is around 275 ohm. Using Ohm's law, we can calculate the coil current thus:

**I = V / R
Icoil = 12 / 275 = 0.043mA**

The [[https://www.mouser.com/catalog/specsheets/en_g2r-5532148.pdf|datasheet]] for this particular relay already mentions the coil current.

The power handling of the resistor is calculated thus:

**P = V * I
P = 12 * 0.043 = 0.516W**

Therefore, the power handling of the resistor needs to be at least 1W (which will be toasty at that dissipation), or 2W. 5W would be overkill, but a good recommendation.

The loudspeaker protection itself is based around relay RLA1, which connects or disconnects the speakers from the amplifier outputs. Under normal operating conditions, transistors Q2 and Q3 conduct, energizing the relay coil and closing the relay contacts. This connects the amplifier outputs to L-OUT and R-OUT.

Q2 functions as a small-signal control transistor while Q3, a BD139, supplies the current required to energize the relay coil. Diode D4 is the relay fly-back diode. When the relay is switched off, the collapsing magnetic field inside the coil generates a reverse voltage spike. D4 safely absorbs this inductive kickback and protects Q3 from damage.

The DC fault detection circuitry monitors both amplifier outputs through L-IN and R-IN. Resistors R5 and R6 isolate and combine the two channels into the sensing network. Capacitor C4 blocks normal AC audio content so the protection system mainly responds to sustained DC conditions rather than music signals.

Under normal operation, the average DC voltage at a power amplifier output is very close to zero volts. However, if an amplifier output stage fails, large positive or negative DC voltage may appear at the speaker terminals. This is dangerous because loudspeakers are designed to reproduce AC signals, not continuous DC current.

Bridge rectifier DB2 acts as a polarity-independent detector. Whether the DC fault is positive or negative, the bridge rectifier converts it into the correct polarity needed to drive transistor Q1. This allows the circuit to detect both positive and negative DC faults equally well.

When a DC fault occurs, transistor Q1 turns on and pulls the base of Q2 to ground, "stealing" its base current. This removes drive from Q3, causing relay RLA1 to de-energize and disconnect the speakers from the amplifier outputs before damaging DC current can reach the speaker voice coils.

**Note: most speaker voice coils can sustain a DC voltage for a short period of time before irreparable damage occurs; relay drop-out time is in the real world around 100ms.**

Capacitor C6 and resistor R11 create a time constant in the fault detection circuitry. This prevents false triggering from brief transients or asymmetrical low-frequency program material and ensures the protection system mainly responds to sustained DC fault conditions.

Thermal protection is provided through connector J3, which connects to a normally-closed thermal switch mounted on the amplifier heatsink. Under normal operating temperatures the switch remains closed and the relay operates normally. If the amplifier overheats, the thermal switch opens and interrupts the relay drive path. This causes the relay to release and disconnect the speakers exactly as though a DC fault had occurred. If the switch was not to be included, J3 should be a link.

The NE555 section in this circuit does not provide a startup delay. Instead, it functions as a fault-condition oscillator and mute indicator. Under normal conditions, the relay remains energized and the 555 output remains inactive. However, when a protection condition occurs — either a DC fault or thermal shutdown — the control circuitry enables the 555 oscillator.

The 555 is configured as an astable multivibrator using R9, R10, C5, and D3. Capacitor C5 repeatedly charges and discharges through the resistor network, causing the 555 output at pin 3 to oscillate between high and low states. This oscillating output drives LED1 through resistor R8, causing the LED to flash whenever the amplifier enters protect mode or during turn-on delay muting.

When the relay is energized during normal operation, current flowing through Q3 is coupled through diode D3 into pin 7 of the NE555. This forces the discharge transistor inside the 555 into a state that prevents capacitor C5 from charging and discharging normally, effectively stopping the astable oscillator operation. As a result, the output at pin 3 stops oscillating and LED1 no longer flashes. Instead, the LED remains continuously illuminated as a normal power-on indication.

When a fault condition occurs and the relay de-energizes, current through Q3 stops. Diode D3 no longer inhibits the 555, allowing the oscillator to run normally. Capacitor C5 then repeatedly charges and discharges through R9 and R10, causing the 555 output to oscillate and making LED1 flash to indicate that the amplifier has entered protection mode.

Finally, the relay will drop out almost instantly (around 100ms) after the AC power is removed, because D2 and R4 are no longer supplying current to the base of Q2. This will case C6 to discharge through the base-emitter junction of Q2 and the relay to drop out. This will prevent any loud thumps, or other rude noises that may come from the power amplifier(s) as the main supply rails collapse.

Overall, this circuit continuously monitors the amplifier outputs for dangerous DC conditions and overheating. During normal operation, the relay remains energized and the speakers are connected normally. If a fault occurs, the relay immediately disconnects the speakers while the NE555 activates the flashing LED warning indicator to show that the amplifier has entered protection mode.

===== Conclusion =====

This basic circuit has been built and tested and has proved to work as described. It will protect expensive loudspeakers from a catastrophic amplifier fault condition - mainly DC on the output, which is **never** good for any loudspeaker.

This was the power supply/speaker protector chosen to power both the [[2011|AEL-2011]] and the [[2026|AEL-2026]] power amplifiers.