It is always dangerous to connect a DC-coupled amplifier to a loudspeaker without proper protection against a DC fault. Such faults can not only destroy an expensive loudspeaker in a second, but may also present a safety hazard as the speaker coil suffers catastrophic overheating and possible fire.
The Amplifier/Speaker Protection PCB.
The Signal Transfer Company Protection PCB can be added simply to almost any amplifier system, providing safety against excess DC-offsets, and also over-temperature conditions and turn-on/turn-off transients. The PCB is designed to run directly from the power amplifier supply rails, accepting a very wide range of voltages; no additional power supply is required. Special circuitry is used to accelerate relay drop-out when an error occurs.
- Two channels of relay protection against excessive DC offsets. Both positive and negative error voltages are detected symmetrically. Separate relays are provided for each channel so high-current single-changeover types can be used.
- Slow turn-on to allow amplifier to settle before the speakers are connected.
- Fast-off on mains failure or switch-off, to prevent turn-off transients reaching the speakers. This can be disabled if your mains supply is subject to serious dips or brief interruptions.
- Two channels of over-temperature detection
- Immediate shutdown input
- Output for status LED
- Has on-board supply regulation and is suitable for any HT voltage between +/-25 V and +/-45 V
C O N N E C T I O N S
- The amplifier outputs are simply connected through the relays on this card to the output terminals
This also makes the connections to the DC offset detect circuitry
- Two connections to the low-voltage AC supply are made to drive the loss-of-AC detector
- Two 2-way connectors are provided for wiring to two thermistors for over-temperature detection
If only one temp detect point is required, the unused connector is shorted
- The shutdown pin is simply grounded to 0V to give immediate muting
- One 2-way connector for status LED
- Three power supply connections
S P E C I F I C A T I O N S
Most of the operational parameters can be modified by changing component values. For example, the turn-on delay can be altered by using a different value of timing capacitor. Full details are given in the constructional notes supplied with the PCB.
- Supply voltage: dual rail, from +/-25V to +/-45V.
- Turn-on delay: 3.0 Seconds +/-10%
- Turn-off time: Less than 1 msec (not including relay mechanical drop-out time)
- DC offset detect threshold: +/-2.0V
- Temperature cutout threshold: 87 degC (using recommended PTC thermistor EPCOS B59901D90A40)
- Relay type: 24V 1C/O
- Max relay coil current: 35 mA at 24V
- High-quality double-sided PCB, roller-tinned with solder mask and full screen print. Board size 78 by 89 mm
- Four fixing holes provided for simple mounting
Note: This unit does not give protection against overload or short-circuiting of a power amplifier output. In the case of Signal Transfer designs, overload protection is provided on the power amplifier card.