Sound Principle:
Active vs. Passive

By Jay Frigoletto

While exploring music technology, one will come across the terms “active” and “passive” rather frequently. The precise meaning of these words can seem elusive, as they can apply to everything from the pickups in your guitar to your studio monitors to your equalizers. The most basic distinction is that active electronic circuits require power, while passive electronics do not. However, that deceptively simple definition only begins to describe the myriad of possibilities and combinations.

The first place most musicians are likely to come across the subject of active vs. passive is in their guitar pickups. The basic pickup consists of a magnet and a coil of wire. When the guitar string vibrates in the magnetic field generated by the pickup, an electrical current is generated. This changing magnetic field is all that is needed to create the current. No outside source of power is required, though the current generated is rather small. A passive tone control is also usually included. This is simply a knob controlling variable capacitance and resistance that attenuates the treble frequencies as you turn it.



Active pickups include a small amplifier (or “pre-amp” if you prefer, since it will be further amplified once it hits your stack) that boosts the signal from the pickup. This allows the pickup to use smaller coils of wire making it less susceptible to external noise, yet also offering a “hotter” output due to the amplification stage. It also allows for two or three band equalizers with boost and cut instead of the simple cut-only passive tone control. These active electronics require power, usually in the form of one or two 9-volt batteries.

As was introduced above, passive EQ uses resistors and capacitors to attenuate certain frequencies. It can only reduce, not increase. Active EQ adds amplifiers to the mix, usually in the form of op-amps, or operational amplifiers. This offers gain, additional control, and flexibility. Some passive EQs offer boost as well as cut. This feature is accomplished by having an active makeup-gain stage after the passive EQ circuit. The classic Pultec EQ is a good example. Passive EQ is often said to be smoother, or to have more character, but this could have as much to do with transformers and tubes in the output gain section as with the passive method of equalization.

Studio monitoring is another place where you will see active and passive options. Passive speakers require outboard amplifiers to produce sound, while active speakers have amplification built in. Passive speakers employ crossover networks using pre-set filters similar to the passive EQ described above. The signal from the amplifier is split into bass and treble components. Bass frequencies are sent to the woofer while the treble is directed to the tweeter. Active or self-powered speakers accept line level signals and utilize an active crossover (using active filters) to split the signal into bands, sending each to its own built-in amplifier, powering the woofer and tweeter separately. This process is known as bi-amplification. It allows the manufacturer to match the amplifier to the driver, improve efficiency, and remove the variable of an unknown amplifier, which is required to power a passive speaker. Many companies offer both active and passive versions of their speakers, like Event, Quested, and Dynaudio, while others specialize in one or the other, like the Genelec active range, or passive speakers from Earthworks and many other audiophile manufacturers.

An active or passive design each has advantages and disadvantages, and in the end, you need to use your ears to tell you which one works best for your particular application. Neither is right nor wrong, and both can perform beautifully given the right circumstance and a quality design.

Question for Jay? Visit him online at www.promastering.com or www.myspace.com/sslmixer.