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The Infinite Sustain Feedback System™

For Musical Instruments

Developed by Al Jewer & Arthur Durkee

The ISFS is an acoustically-coupled feedback driver that was developed in collabotation with Al Jewer.

At the present time, the ISFS exists as a working model that I developed for my own Chapman Stick playing, but it's also a conceptual principle that's relatively simple, making it easy to build an ISFS on one's own.

Sample Track: Dry Sea/Farthest Shore, from my 1996 solo Stick CD, The Western Lands.

Here's how the Infinite Sustain Feedback System works:

Normally, sustained feedback is created by a stage monitor or other amplified speaker vibrating the air, which in turn vibrates the instrument's strings, which send a signal back to the speaker, thereby creating a feedback loop. The major problem with this system is that it's inefficient—it takes a lot of speaker volume to drive the air enough to drive the strings, meaning that most guitar-type feedback is only sustainable at relatively loud volume levels.

The inspiration for the ISFS was drawn from noticing how Adrian Belew has learned to "play" his guitar feedback by exactly positioning the guitar relative to his stage monitor. (I can't speak for Adrian, but that's what it looked to me like he was doing.) I started thinking about how I could generate Stick feedback to get some specific tones and sounds out of it.

The Infinite Sustain Feedback System consists of a BSR™ 60-watt contact driver mounted on the body of the electric stringed instrument in question, and driven by a secondary 50-watt amp. The BSR contact driver is essentially a speaker core with a magnetic reaction mass. Originally designed to be attached to the walls of an audiophile's listening room, the contact driver turns whatever it's attached to into a speaker by conductive acoustical/mechanical vibration. This could be a wall, a glass window, a wooden bench—those all work quite well, actually.

What the BSR contact driver does is cut out the middleman, so to speak. I split my Stick's signal so that it runs into a secondary 50-watt amp, which is then used to drive the BSR contact driver, which is plugged into the secondary amp's speaker output jack. Very important: With the BSR™ contact driver, do not use a high-power amp or you will blow the driver up! The contact driver directly vibrates the body of the instrument, transmitting sounds via conduction to the vibrating strings. The end result is acoustically-coupled feedback that does not require deafening stage volume to create some really insane feedback sounds!

I control the effect by spliting the Stick's output and sending the split signal to the secondary amp through a volume pedal; this allows me to bring in the sustain after a note has alrady been played, for a crossfade effect. So the signal chain looks like this:

Split the Stick's output (I have a custom DI box with multiple outputs, so I split it there). Run the output through a volume pedal to a secondary 50-watt amp. Plug the contact driver into the external speaker output of the secondary amp. I often use a Fender BandMaster for this; it's a reliable 50-watt tube amp with a great, warm tone. When you bring up the volume of the secondary output via the volume pedal, the feedback will kick in. You can use the amp's tone controls, if any, to shape the feedback tone, to emphasize which harmonic frequency the feedback will naturally gravitate towards and settle into. The contact driver is attached to the back of my Stick, behind the second dot from the top on the fretboard; this is the area on the fretboard, therefore, which provides the maximum sustain while playing. Once I get a string going with the ISFS, though, I slide around on the frets, without tapping again, which sustains the tone better. I also use a lot of string bends for subtle musical effect when playing with the ISFS. It works equally well with no other processing effect, or some delay, or distortion, compression, etc.

The key component of the Infinite Sustain Feedback System is obviously the BSR™ contact driver. This is also the component it's hardest to come by, I'm sorry to say. Several people have asked me if I can build an ISFS for them, but the limiting problem I have had is the relative rarity of the drivers. They were an audiophile fad of the 1970s and 1980s, when there was a tremendous amount of experimentation in speaker technologiess being undertaken, but which did not last. At the present time, I'm keeping my eyes open in the sound-design trade journals so that I can find somebody unloading another cache of drivers they found in a warehouse somewhere.

An important difference between the ISFS system and comparable systems such as the Ebow or the Fernades Sustainer is that, because the string vibration is mechanical/acoustical in nature, it will create feedback sustain with any kind of strings, not just metal strings. You could, for example, set it up to work on a nylon string guitar. Also, because it's driven by a separate amplifier outside the effects loop, you can create feedback sustain using no effects processing on, just a clean signal. I do this sometimes on the bass side of the Stick, when I want a sustained bass note that I can chord or melodize over. On other occasions, I have used some reverb and delay on the bass side to emulate humpback whale sounds, to good effect.

Answers to some Frequently Asked Questions:
(culled from various emails; please excuse some redundancy)

As for what a contact driver itself consists of, it's basically the guts of a traditional speaker, but without the cone. Like a speaker, it converts electrical signals into mechanical vibration that in turn move the air (in the case of a speaker) or the body of an instrument (in the case of the driver). It looks like a circular speaker core, about 4 inches in diameter and 2.5 inches thick, and has a thin speaker wire trailing off that ends in a standard 1/4-inch jack. The other commmercial name that contact drivers go by is surface transducer.

Here's the data from the BSR's manual you might find useful. I originally bought my unit thru DAK Industries, a mail-order audiophile catalog company that I believe is now defunct.

BSR Model SFT-1 Surface Transducer

Specifications (per the manual):

Type = MC-dynamic, full-range
Nominal Impedance = 8 ohm
Rated Power Input = 20 watt
Maximum Input Power = 50 watt (which is in actuality closer to 60 watt; the 50-watt amps I use don't blow it up, though)
Dimensions = 3.3 x 1.25 inches
Weight = circa 1 pound

"The transducer can be connected to any amplifier speaker terminal that has an impedance of 4ohm-16ohm."

One very important ting to remmember when using a contact driver: Do not use an amplifier of higher rating than the driver's own wattage rating. If you do this, you will burn out the driver. Since the BSR is rated at 60 watts, I never use an amp of higher power than 50 watts, for example, a Fender BandMaster. By very careful about this.

However, experience has shown that 50 watts, due to the efficient nature of the ISFS's non-air acoustic-coupling system, is more than adequate. Efficiency is more useful than raw power, in this case. You may judge for yourself by listening to an excerpt from this piece that showcases the ISFS: Dry Sea/Farthest Shore.

I don't know of any smaller contact drivers. In fact, the BSR is the lightest, smallest contact driver that I have encountered so far. Really, it only weighs about a pound or two, and I have found that with mine semi-permanently attached to my polycarb Stick, which itself weighs about 5 or 6 punds, I have gotten used to the weight. (Of course, I use a wide martial arts belt to wear my Stick when playing, and nowadays usually lean or sit on a chair or stool that is just the right height to give me back support.)

Probably a smaller contact driver would work for the Infinite Sustain, though. The system is very efficient, so there isn't a lot of wasted sonic energy going into the air or into heat-loss. If you ever hear of a smaller driver, please let me know.

Remember, though, that the key to the contact driver's efficiency is that magnetic core on the back that serves as a reaction mass. A regular small speaker would not work as well, because the magnet in that case is driving a paper cone designed to move air, rather than to transmit sound vibrations into a physical entity such as the body of my Stick. So, even a smaller contact driver still must have a certain amount of reaction mass in order to be effective. Probably nothing smaller than 1/2 the size of the BSR would give enough volume to generate the kind of feedback I want.

My BSR is mounted on the back of the fretboard itself, behind the 8th fret (or second "dot" from the top), where I find myself playing most often. The driver does create a "wart" on the back of the instrument, but I have gotten used to it being there, and rarely have a problem with it.

Mounting the entire driver/amp configuration on your Stick, even with a custom-built MOSFET-type mini-amp (the lightest technology I can think of), would probably add 5 pounds minimum, not to mention a significant "wart."

With regard to feedback tone:

The BSR obviously plays whatever you feed into it. If I am using a clean signal or just a little chorus, that's what I get out of it. If I'm using distortion, that's what I get out of it. Because the Feedback driver comes at the end of my effects chain (a split signal of my actual output from the DI, as described above), it has the same tone quality as whatever effects I'm using. Some of my distortion patches are EQed a little louder, too, so I get more volume coming from the BSR.

Feel free to email me if you have any comments or questions:

Before you email, however, please be aware that I am not a commercial producer or manufacturer. This is a custom solution to a specific musical problem. I am happy to help you figure things out for yourself, however I am not in a position to produce an ISFS unit for you. I am happy to answer your questions, however almost everything I could tell you is already given on this page.


Additional Sample Tracks, for your listening pleasure:

Ancient Girl, with Dangerous Odds

Begging Bowl, with Dangerous Odds


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