Euraudio SMRS specifications

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

The SMRS is a turn-on thump muting circuit, which is recommended for home-made audio power amplifiers. Its function is to inhibit the pop sound which often arises when amplifiers turn on, but it may also protect your loudspeaker from DC offset. This latter function is not guaranteed. Please find the SMRSc assembly instructions here.

The loudspeaker protection function is not guaranteed in case of high supply voltage and low loudspeaker impedance, because amplifiers usually fail in a way that one of the supply voltages (that is, DC voltage) appears at the output. The normal mechanical relays arc when they try to break a high DC current/voltage and unfortunately, the special mechanical relays with arc blowout coil are expensive. The commercial amplifiers (Sony, Denon, Yamaha, Onkyo, etc.) all have normal mechanical relays for loudspeaker protection, so this circuit is just as capable to protect a loudspeaker as that of those renowned manufacturers'.

SMRS system requirements

What kind of amplifiers can the SMRS be used for?

It can be used for amplifiers with dual or in other words symmetric (+/-) supply voltage, the circuit is not able to eliminate the turn-on thump on amplifiers with asymmetric supply voltage.

One piece of SMRS can be used for amplifiers containing no more than two amplifier channels, that is e.g. for a non-bridged stereo or e.g. for a bridged mono amplifier. In a system containing more than two channels, the necessary number of SMRS units = number of amplifier channels per two, and the result rounded up to the next integer.

What else does the SMRS demand?

One SMRS unit requires a 12...15 VAC, min. 120 mA, 50...60 Hz ground independent power supply, which is not part of the SMRS. Ground independent practically means that this alternating voltage must be supplied either from a separate secondary coil of the main transformer of the amplifier or from a separate small transformer. The latter is advisable, because in this way the loudspeaker protection will remain intact whenever the main transformer of the amplifier becomes defective or gets overloaded.

Up to what amplifier power can it be used?

It is recommended up to 2x100 W / 8 ohms or 4 ohms non-bridged, or up to 1x200 W / 8 ohms bridged amplifier power. If a normal (non-bridged) amplifier connects to a loudspeaker impedance lower than 4 ohms, or if a bridged amplifier connects to a loudspeaker impedance lower than 8 ohms, then the SMRS is recommended only for proportionately lower amplifier power (the 2x100 W or 1x200W power derated in proportion to the impedance).


The SMRS connects the loudspeaker(s) to the amplifier output(s) in approx. 2 seconds after its supply voltage appears. This time is assumed to be sufficient for any turn-on transients in the amplifier to die away. If any channel exhibits a DC voltage higher in absolute value than about 1.5 Volts, then it disconnects the loudspeaker(s) via a mechanical relay. The higher the DC voltage is, the faster it disconnects the load. A red LED is lighted when the relay is deenergized.

Like the other loudspeaker protection circuits, neither is this one suitable to protect against loudspeaker overload due to excessive loudness. The DC offset protection intervenes if the amplifier fails in a way that a dangerous DC voltage appears at its output that would damage the loudspeaker, but in case of breaking a high DC voltage the relay contacts may arc, therefore the loudspeaker protection is not guaranteed.

The grounding of the SMRS 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 still have a hum problem.)

Turn-off is not specifically muted by this circuit. The time after which the relay disconnects the output following the removal of its supply voltage is not specified, but for information only: it is usually about 0.5...1 seconds. 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 on-off switch. Such problems can usually be cured by soldering a 10nF to 47nF class X2 capacitor directly onto the leads of the power switch. To avoid the risk of fire, only class X2 capacitors are allowed for line voltage applications. This capacitor is not part of the SMRS kit.

The loudspeaker protection speed should be adjusted to the amplifier power by sizing a resistor, which procedure is found in the assembly instructions.

Technical data

At 25°C ambient temperature unless otherwise noted.

Necessary power supplying transformer: 12...15 VAC, (ground independent), min. 120 mA, 50...60 Hz

Relay maximum switchable AC current: 2x 8 A (independent amplifier channels) or 1x 8 A (bridged amplifier channels), resistive load

Turn-on muting time: 1,3 s ±0,7 s

Maximum tolerated DC voltage: +1.1/-2.7 V ±0.6 V (fast); +1.3/-3.6 V ±0.8 V (medium speed)

Typical loudspeaker protection actuation time after a +30(-30) VDC step voltage: 38 (55) ms (fast); 53 (75) ms (medium speed)

Recommended amplifier power: max. 2x100 W /  4 ohms or 8 ohms;   or max. 1x200 W / 8 ohms (bridged mono)


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