In the noble fight against feedback, Mike Crofts is going Solo.

The ever-present problem of unwanted feedback is now no longer an unavoidable fact of life for sound engineers, thanks to the advent of digital filter technology. Now you can have a pocket-size device, which can be connected into any microphone feed for instant results — it's called the FBX-Solo 620 Feedback Exterminator.

The FBX-Solo 620 uses state-of-the-art digital filter technology to detect and notch out the frequencies which cause unwanted feedback, without noticeably affecting the signal being amplified — and all without any help from you or me. Too good to be true? I used to think so, until I first tried out the Solo's big brother (the Sabine FBX-2020, which was reviewed in the January 1997 issue of Sound on Stage) and was instantly converted to the faith. Consequently, I was very keen to try out this miniature single-channel version to see if it would be as effective and easy to use.

The Solo 620 is available in two versions: the SL-620 which has jack connectors for use with instruments and other high-impedance sources, such as mixer insert/patch points or high-Z microphones, and the SM-620 which has XLR connectors and (selectable) phantom power for use in balanced microphone lines.

CHECK THE SPEC

For the technically-inclined, here's a quick rundown of the functions and specification. Both models are provided with six digital notch filters, which are able to cover the frequency range from 51 Hz up to 17kHz. The filter width is selectable as either one-tenth or one-fifth octave, depending on the application and the degree of feedback suppression required — one-tenth is the normal setting for music applications, with the wider filter width normally reserved for speech or where the signal fidelity is not so critical. Each filter, once assigned automatically, is capable of 50dB of attenuation, which should be more than enough to knock down the most troublesome feedback, and this is typically accomplished in less than half a second! Any number of the filters can be used at the same time, and the user can decide how many fixed and how many dynamic filters are to be set, with the last setup automatically stored in the unit's memory (powered by an internal battery with a claimed seven-year life) each time it is switched off.

Frequency response is 20Hz to 17kHz at only ±0.75dB, and the overall signal-to-noise ratio is typically greater than 87dB. The physical size of the Solo 620 is, well, small — you could get six of them into a single rack unit space, and the depth of the box is about three golf balls.

An interesting feature of the Solo is its built-in noise gate, which has a threshold selectable in four steps between -60dB and -24dB — a very useful addition when connected to individual mic feeds. There is also a gain option, whereby you can select either unity gain or make use of the boost facility, which provides a gain range of up to 35dB depending on the model.

HOW I DID IT

The version I tried out was the SM-620, for use with balanced microphone sources via XLR connectors. The first thing to say is that the instructions are printed very clearly on top of the unit, so you can't really go wrong unless you're determined to. I was going to insert the Solo directly into the mic line before the mixer, so I didn't need any gain from the Solo which was left set at 'LO', ie. unity gain. As I was using a Shure SM58 microphone, I didn't need phantom power, so I left the back panel switch at 'OFF', which is how it is shipped. Having had a quick read of the instructions and checked the above settings, I connected up the power supply, which is provided by a wall wart with a special locking connector. This is a good idea especially if the Solo is placed anywhere other than at the desk end of the mic lead and may be moved about; it also prevents the unenlightened from trying to power a high-tech digital device with a transformer from a train set.

All very simple so far, although the power connector is — due to its locking design — a bit difficult until you get used to it. I had already set up the PA, so I turned all the levels down and patched in the Solo, placing it next to the mixer. When I switched it on, several LEDs lit up, and I pressed the reset button until all previous settings were cleared. Checking that the Solo was in active mode, I turned up the PA masters, then (slowly!) the mic channel in question and listened for the first feedback frequency to occur. Sure enough, the hall produced its usual whistle at a fairly low level, but in this instance, it only lasted for a moment before the Solo caught it and dealt with it — and I didn't move a muscle! One of the LEDs started flashing to show that filter number 1 had been assigned and set. I continued to increase the channel gain until another frequency started to take off, and the same thing happened, and again, until four fixed filters had been set. The system was sitting quietly, and the improvement was immediately apparent when someone walked up to the mic and said 'hello' or 'one, two' or something more original (now there's an idea for a competition).

With four filters set, I had stacks more gain than I could possibly use in that venue, and so the remaining two filters were left to roam, ready to lock on to spurious feedback frequencies and apply attenuation as needed. The system behaved itself impeccably all evening, even when the vocal mic took an unscheduled trip off the front of the stage and fell onto the hall floor — now that's impressive.

HANDLE WITH CARE

I've used these digital feedback eliminators several times now, and I am still surprised at how loud the system sounds after setting up; you do have to be careful as they place a lot of power in your hands, and the PA will be capable of producing much higher usable levels, so watch your hearing! The other important thing to remember is never, ever switch the unit to bypass without turning the system gain down first, as the potential for system (and people) damage is very real.

It's a lucky or very thick-skinned sound engineer who can ignore the potential of a device like the Solo Feedback Exterminator; they just don't seem to have a downside other than you miss them when they're not there. This cute little box took about a minute to set up, was worth its weight (about nine ounces in old money) in return bookings, and has got to be a 'must have' for any mobile sound rig. I reckon you could get more value from a couple of these than doubling the power of your main system, because it doesn't matter how many watts you've got if you can't get the sound out front to the customers.

SM-620 £429 inc VAT.
SL-620 £363 inc VAT.

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