Burning the Midnight Monitor Oil
by Eddie Cilletti, continued from page 1


Powered (active) monitors typically have some sort of driver protection either in the form of a peak limiter or a safety relay. Passive monitors are further down on the food chain, and the installer should include not only a safety valve (a fuse), but also a healthy gain structure between source and power amp. Your system should be calibrated so that a comfortable listening level occurs when the control-room level pot “lives” between straight up and the 3 o’clock position.

Figure 2: A tweeter bobbin distorted by voice coil overheating

Once upon a time, I was fast-winding an analog tape past what should have been the end of any recorded material. A sudden screech and then an eerie silence jolted me back to consciousness too late to pull my finger away from the heads. A 1-amp fast-blow fuse did not react in time to protect all of my beloved Radio Shack Minimus 7s—four in a passive-surround configuration. At least they were fused. Fig. 2 shows the distorted bobbin after the voice coil overheated. The two surviving tweeters, which were made in Japan, didn’t sound anything like the two new replacements from Malaysia. Yeah, I know these are cheap consumer-grade speakers. My “real” monitors are by Dynaudio Acoustics, modelBM15A.

Monitor fatigue gradually occurs over time, so you may not notice the change until a driver fails and is replaced. The difference may also be related to manufacturing tolerances. If you read no further, at the very least, keep two spares and always replace in pairs. If one tweeter dies, replace both, dating the new and labeling the used, working driver. Some near-field monitors are cheap enough that an extra pair of monitors can be less expensive than buying spare drivers. You can always use them in a passive surround configuration, which will be discussed in a future article.

Monitor selection is not an easy task. The terms “accurate” and “pleasing” can be dissimilar qualities. An accurate speaker may appear unremarkable at first audition, while a more pleasing model might provide instant gratification suitable for those moments of “easy listening,” when the ear is off duty. Great-sounding speaker systems are easy to come by, but the path toward greater accuracy requires many deliberate steps.

The natural, free-air resonance of a woofer changes when installed in a cabinet, sealed or ported. Disconnect one wire from a passive monitor and tap on the woofer with your finger, listening closely for its natural resonance. Next, alternate between connected and disconnected, with the power amp on. You should notice more resonance with the amp disconnected. This demonstrates Damping Factor, the relationship between the power amp’s output impedance and the speaker’s nominal impedance. DF is undermined by cable resistance, hence the hunger for cable a la Monster.

Powered monitors have made Damping Factor less variable by reducing the cable length from the power amp to the driver to about 12 inches. In addition, an active crossover network before the power amps (rather than a post-amp passive network) results in better damping for both woofer and tweeter. An active system may seem a little less exciting than its passive cousin, but it is now more accurate, with tighter bass and smoother treble.


Figure 3: Adding Ferrofluid to the magnetic gap

All wire has resistance (stated in ohms per foot and referenced to room temperature) that increases when hot and decreases when cold. This applies to voice-coil windings as well, so when you continuously blast those boom boxes, heat makes the coil less effective at delivering transients. If you listen at loud levels for an extended period of time, the monitors will sound dull and floppy from fatigue. Your ears get tired, too.

Figure 4a: Oscilloscope trace of a 500Hz square wave through an unmodified tweeter.
Figure 4b: Same tweeter, same 500Hz square wave after Ferrofluid treatment.

There is one, quite remarkable, “solution,” called Ferrofluid (www.ferrofluidics.com), a magnetic material suspended in a viscous liquid designed to fill the magnetic air gap in which the voice coil is suspended. The fluid provides three benefits: greater transfer of magnetic energy, “hydraulic” damping of the resonant diaphragm and the transfer of heat from the coil to the magnet assembly. It’s not hard to see, and hear, that Ferrofluid makes a driver more efficient, less colored and better equipped to deliver transients even when driven hard.

The manufacturer’s sample pack included fluids for tweeters through subwoofers, including one optimized for midrange and high-frequency compression drivers. I was willing to sacrifice a Minimus 7 or two to see how much magic I could conjure out of them.

Fig. 3 shows the powerfully focused magnet assembly attracting the fluid from its container. It took some experimenting to achieve an optimum, repeatable mic placement.

Fig. 4a shows a 500Hz square wave through an unmodified tweeter, captured with an AKG C-300 with an omni capsule and a Great River transformerless mic preamp. Fig. 4b shows the same tweeter’s response after Ferrofluid treatment. Notice that the amount of overshoot decreases with treatment.

That’s all the room I have for this month. Look for Part Two in the next issue. Ciao!

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Visit Eddie at www.tangible-technology.com.

Reprinted with permission from Mix Magazine, May 2000

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