Series Multi-Stage Protection [SMP]

Let’s take a deeper dive into Furman Series Multi-Stage Protection

With SMP, there is virtually no downtime.

In fact, the circuit is tested to handle multiple 6000 volt or 3000 amp pulses without sustaining any damage. This is far beyond the demands placed on typical surge suppressors. But because of the extreme conditions and critical applications faced by Furman’s clients, the SMP circuit has been designed to pass this severe test and ensure that equipment damage or maintenance is extremely unlikely. Furman’s SMP relies on a network of components to slow down the impact of a potentially catastrophic surge by capturing it, dissipating it in the form of heat, and absorbing the remaining excess energy. When tested with multiple 6,000V/3,000A surges, the SMP circuit’s maximum let-through voltage is 188V Pk (133V RMS – only 11% higher than a nominal 120V line). Due to the design of the circuit, it will not degrade over time (unlike most standard surge suppressors) and will show minimal increase in line impedance (unlike many advanced surge suppressors).

The SMP circuit is not simply designed to protect from a catastrophic surge, such as a near-hit lightning strike – it is engineered to provide maximum life to connected equipment. This means it not only protects from devastating spikes and surges, but also offers protection from the dozens to hundreds of small spikes and surges your equipment is exposed to on a daily basis. These common voltage fluctuations, although small, can have a serious adverse effect over the long-term. Even when protected by a standard surge protector, digital circuits can see long-term damage due to exposure to voltage on the ground line, causing intermittent behavior, equipment lockups, and data loss. By absorbing these everyday surges without deterioration of the circuit or contamination of the ground line, Furman’s SMP maximizes the longevity of connected equipment and minimizes the risk of downtime or failure in mission-critical applications.

How SMP Works

Traditionally, surge suppression circuits have relied solely on components such as metal oxide varistors, silicone avalanche diodes, or active circuits with transistor devices (SCR’s). These robust components are designed to withstand the impact of an electrical surge or spike, and though they can do a credible job of clamping the rated voltage for which they have been designed, they have minimal immunity from large sustained electrical forces and can fail when tasked when presented with the challenge of a catastrophic surge.

Furman’s SMP circuit employs a hybrid design utilizing a non-sacrificial, multiple-stage inductance circuit of low-loss series inductors, DC rectifiers, and high voltage shunt capacitors to absorb the vast majority of energy in caused by a surge or spike, as well as a high-current MOV and thermal-fused varistor to reduce the remaining energy to a non-damaging level. With this topology, the SMP circuit is ensuring its own protection, while passing very little excess voltage to connected equipment – even when faced with a catastrophic surge. The circuit’s measured clamping voltage is 188 Vpk (this is equivalent to 133 Volts RMS, which is only 11% above an optimal 120VAC line) at 6000V/3000A input. For comparison, competing professional, non-sacrifical circuits have clamping voltages that we have measured in excess of 400 Vpk.

Furman SMP circuit measured with a 6,000V/3,000A pulse. Peak let through voltage = 188V.

Brand X non-sacrificial protection circuit measured with a 6,000V/3,000A pulse. Peak let through voltage = 461V.

Tests performed at World Products Laboratory with a Keytec #587 generator, measured with an Agilent Infiniium oscilloscope and associated current and voltage probes. Applied signal = 120VAC with a 6kV/3kA B3 pulse applied at the peak of the sine wave, direct connection to the SMP board.


SMP and EVS: Professional Protection for Professional Applications

Complementing SMP technology is EVS, or Extreme Voltage Shutdown. While SMP is ideal protection for transient spikes and surges, the circuit is susceptible to sustained overvoltage conditions, often caused by wiring faults within an electrical system (such as a disconnected neutral line). To protect against this occurrence, the SMP circuit includes Extreme Voltage Shutdown technology. This technology constantly monitors incoming voltage, and when voltage rises to an unsafe level for over a quarter of a second, it shuts off power to the SMP circuit and all connected equipment.

Since its introduction in 2001, Furman’s SMP and EVS protection technologies have proven themselves under the most demanding conditions for professional users around the world, whether on stage, in the studio, in the control room, at the front of house, in the backline, or in the theater. When downtime is not an option, professionals choose Furman


With this gear I can crank up the monitors without hearing line noise. It also helps to make the transmission pure by ensuring every piece of gear operates at its optimal level.”

— Maor Appelbaum| Mastering Engineer