EXAMPLE OF POSITIVE FEEDBACK

WHAT IS FEEDBACK Feedback- The high-pitched squeal or ringing caused by sound finding its way out of the loudspeaker back to the mic and out of the speakers again at a resonant frequency dictated by the characteristics of the environment, system, and equipment. It can be lessened by lowering the volume and/or evening out the peaks in the frequency response of the system. Directional mics and speakers are fundamental in helping to overcome this. Frequency shifters can also help. In Electronics, a circuit arrangement in which a portion of the output of an amplifier is fed back into the input. Negative feedback reduces amplifier gain but also decreases distortion; positive feedback increases the gain and may lead to self-oscillation.

A circuit that allows a portion of the signal from a later stage in an amplifier to be "fed back" to an earlier stage, or within the same stage. Feedback can be voltage or current, negative or positive. Negative voltage feedback decreases gain, and is used to reduce distortion, flatten frequency response, increase input impedance, decrease output impedance. Negative current feedback increases output impedance, and is used in some solid-state amplifiers to obtain a more "tubelike" response. Positive feedback will increase gain, but can make a circuit oscillate if too much is applied. Sometimes a small amount of positive feedback is used to offset the reduction in gain caused by application of negative feedback.

Feedback Circuit

Positive Feedback Positive feedback can be exploited for gain enhancement and stability concerns can be eliminated by embedding the positive feedback gain stage in a standard negative feedback configuration as shown in Figure 3. In this nested structure, the inner loop provides positive feedback for gain enhancement while the outer loop ensures the stability of the overall system by providing negative feedback.

Feedback in music Feedback happens when a signal is regenerated or re-amplified over and over causing an uncontrolled oscillation. Most people think of feedback as a high frequency squealing or ringing, but feedback can happen at just about any frequency. Typical "stage" feedback in the pro sound arena is caused by the amplified signal from the speakers or monitors being picked up again by the microphones and then being re-amplified. This creates a feedback "loop".

Most Common Stage Feedback The most common stage feedback loops happen with vocal mics and are in the upper midrange to high frequencies, and result in the previously mentioned squealing and ringing. This is because, for one thing, many stage mics are designed to easily reproduce and even accentuate these frequencies to begin with. Also, being highly directional, these frequencies tend to leave your very efficient horns and bounce off of the ceiling, floor, back wall, amplifiers, etc., right back into the on-stage mics.

Types of Stage feedback Squealing and ringing may not be the only type of feedback problems you will experience, however. Acoustic instruments are notorious for midrange or low-midrange "howling". This occurs when the amplified signal of the acoustic instrument is loud enough to cause a sympathetic vibration in the instrument which is then sent back to the pickup and re-amplified. That's why you may hear of an instrument or a pickup being referred to as "microphonic" in a negative context, meaning it feeds back too easily.

Controlled Feedback You will also hear of an instrument or pickup yielding "controlled" feedback in a positive context. This is when a performer such as an electric guitar player purposely causes a feedback loop by standing at just the right distance from the amplifier, with just the right amount volume to be able to control it, yielding a desirable harmonic event, as opposed to an uncontrolled squeal or howl.

Acoustic Coupling Low frequency feedback can occur from micing bass instruments such as bass guitar or kick drum and having the amplified signal from the main PA or monitors picked up by the same mic and re-amplified, similar to vocal mic feedback. Low frequency feedback can also be caused by "acoustic coupling". This is when the low frequencies being reproduced by the Main or monitor speakers, physically vibrate the stage and then these vibrations are picked up and re-amplified by the on-stage mics and instrument pickups. This is also sometimes referred to as "stage rumble".

How to deal with Feedback The easiest answer is the one that most people are unwilling or unable to comply with, and that is simply: Lower your stage volume. You can have the biggest, baddest main PA system, blasting your audience into heart palpitations, and still keep your stage volume reasonable. Let the main system do the work. You will reduce the chance of feedback, and may actually perform better because you can hear yourself more clearly. Unfortunately, most performer's stage volume exceeds their system's "potential acoustic gain" (PAG) before feedback. This means there is a finite volume threshold in any given stage set-up that you can either stay below and avoid feedback, or exceed and then have to deal with feedback.