Over the past couple of years we’ve recommended connecting your outdoor speakers through a Panamax module (MOD-SPKP) designed to reduce the risk of connected equipment damage from outdoor surges. We’re not talking about surges created by anomalies in the AC electrical system or your power amplifier running amuck. We’re concerned here with nearby lighting strikes causing surges to travel into the home from the outdoor wiring and damage connected equipment.
“As part of a typical cloud to ground lightning strike, rapidly changing currents are generated that also create electromagnetic pulses (EMPs). These radiate outward from the ionic channel. This is a characteristic of all electrical sparks. The radiated pulses rapidly weaken as their distance from the origin increases. However if they pass over conductive elements; i.e., electrical wires, communication lines or metallic pipes, they may induce a current which travels outward to its termination. This is the “surge” that more often than not, results in the destruction of delicate electronics, electrical appliances or electric motors. Devices known as surge protectors (SPD) or transient voltage surge suppressors (TVSS) attached in series with these conductors are designed to detect the lightning flash’s transient [irregular] current, and through an alteration of its physical properties attempt to route the spike to an attached earthing ground, thereby protecting the equipment from damage.”
The SPKP is a handy device designed to do just that. It’s also useful for outdoor control devices like keypads, etc. Recently, however, it was brought to our attention by John Thall of Knightwing Audio, a Terra installer in Leesburg, FL, that the SPKP might not be designed for 70-volt distributed audio speaker systems. Initially, when we had looked into this unit, we were told it was usable with 70-volt systems. However, when we re-checked with Panamax tech support we were told that the SPKP isn’t designed to be used with such systems as the MOV’s used are only rated for a peak of 47 volts.
So by all means continue to use the SPKP on low impedance systems (2, 4, 8, 16 Ohms) but avoid using it on 70-volt installs. We’ve done some more research and found a couple of other units rated to work with 70-volt speaker lines for induced voltage surge suppression:
One is the JuiceGoose Model PHC-SP70:
Note that like all surge suppressors, these units can’t protect connected equipment from a direct lightning strike, but they can protect it to some extent from high voltage induced in the speaker line by a nearby lightning strike. And that’s OK because most equipment damage isn’t caused by direct lightning strikes.
If you’re looking to offer a modicum of speaker protection from internal system issues like overdrive, etc., (not surges or lightning) you can fuse the speaker lines using an in-line fuse holder. Note that if you’re too conservative with your fuse choices and the client cranks it, the possibility exists for the system to go down frequently. Remember, fuses aren’t designed to protect against lightning induced surges.
Below is a handy formula for determining fuse sizes for speakers. Please remember that a fuses’ rating means that the fuse will blow at some point above the stated value. So a 3 amp fuse won’t blow at 3 amps but at some value slightly above 3 amps (usually between 135% and 200% of the stated fuse value, for a given period of time). Therefore, you can consider starting by using a smaller fuse than the formula determines. Some recommend initially using a fuse rated at ½ the determined value to begin with and gradually increasing its rating if the fuse blows too early.
- Power (Watts) = current X current X resistance
- Current = the fuse rating, in amps
- Resistance = speaker rated impedance (2, 4, 8, 16 Ohms etc.)
- So the formula is fuse rated amps times fuse rated amps times load resistance in Ohms
- 1 Amp fuse with an 8 Ohm speaker P = 1 x 1 x 8 = 8 Watts
- 2 Amp fuse with an 8 Ohm speaker P = 2 x 2 x 8 = 32 Watts
- 3 Amp fuse with an 8 Ohm speaker P = 3 x 3 x 8 = 72 Watts
- 4 Amp Fuse with a 4 Ohm speaker P = 4 x 4 x 4 = 64 Watts
Using this formula you can determine an appropriate fuse value for the Wattage level you’d like to limit at for that speaker. Remember, the fuse will hold at these Wattages and blow slightly above them. This formula is for RMS power and not peak power ratings. Because of this we’d recommend considering “Slow-Blow” fuses for speaker protection.
Note that these fuses are installed in the speaker lines primarily to protect the speaker from overdrive and an amplifier that’s run amuck. We’ve already discussed amplifier clipping and its associated speaker damage in an earlier Newsletter, which won’t be prevented by any of these devices.
We believe this information gives you enhanced options and choices when designing your outdoor audio installs, while providing a modicum of security for your clients.