Transcript example of positive feedback

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.