Euraudio SPRS specifications
Welcome to the Euraudio SPRS specifications, in which you can read about the system requirements (application criteria), operation and technical data of the SPRS.
The SPRS is a simple two-channel analog protection circuit, which is recommended for home-made audio power amplifiers. Its function is limited to turn-on thump muting and loudspeaker protection. It prevents the pop sound which often arises when amplifiers turn-on, and it may protect your loudspeaker from damage, if your amplifier fails in a way that a dangerous DC voltage appears at its output(s). Please find the SPRSc assembly instructions here.
It is recommended for amplifiers with dual or in other words symmetric (+/-) supply voltage, because amplifiers with asymmetric (single) supply voltage (though not many are around in this genre these days) there is an output coupling capacitor, which prevents any dangerous DC voltage from appearing at the output terminal of the amplifier, even if the amplifier becomes defective. Of course, the coupling capacitor may also fail, at least in theory... Note: This circuit is not able to eliminate the turn-on thump on amplifiers with asymmetric supply voltage.
One piece of SPRS 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 SPRS units = number of amplifier channels per two, and the result rounded up to the next integer.
One SPRS unit requires a 12...15VAC @ 100mA, min. 40mA, ground independent power supply, which is not part of the SPRS. 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.
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 SPRS is recommended only for proportionately lower amplifier power (the 2x100 W or 1x200W power derated in proportion to the impedance).
The SPRS 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 protection is active, that is when the loudspeaker(s) are disconnected.
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 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 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 seconds, and it is increasing with increasing supply voltage. 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 SPRS kit.
The loudspeaker protection speed should be adjusted to the amplifier power by sizing a resistor, which procedure is found in the assembly instructions.
At 25°C ambient temperature unless otherwise noted.
Allowed supply voltage range at 100 mA load
current: 12...15 VAC (ground independent)
Supply current: max. 40 mA
Relay maximum switchable current: 2x 8 A (independent amplifier channels) or 1x 8 A (bridged amplifier channels)
Turn-on muting time: 1,8 s ±0,8 s
Maximum tolerated DC voltage: +1.2/-1.6 V ±0.6 V (fast); +1.6/-2.2 V ±0.7 V (medium speed)
Typical loudspeaker protection actuation time at +35 or -35 VDC step voltage: 40 ms (fast); 60 ms (medium speed)
Recommended amplifier power: max. 2x100 W / 4 ohms or 8 ohms; or max. 1x200 W / 8 ohms (bridged mono)