What is Feedback?
Feedback occurs when a portion of a system’s output is returned to its input and combined with the incoming signal. This principle appears widely in engineering and natural systems wherever outputs influence future behaviour. In electronics, this arrangement is known as a feedback loop.
Positive and Negative Feedback
If the feedback signal is in phase with the input, it reinforces the input and increases the output. This is known as positive feedback.
If the feedback signal is out of phase with the input, it opposes the input and reduces the output. This is known as negative feedback.
Both forms exist in electronic systems, but they serve very different purposes.
Feedback in Everyday Systems
Negative feedback is commonly used to maintain stability and control. For example:
vehicle cruise control systems adjust engine power based on speed feedback
biological systems regulate temperature, movement, and chemistry using feedback mechanisms
In each case, feedback helps correct deviations rather than amplify them.
Why Negative Feedback Is Used in Amplifiers
In audio amplifiers, negative feedback is widely used to:
reduce overall gain
extend and stabilise frequency response
lower output impedance (increasing loudspeaker damping)
reduce certain forms of distortion
Feedback can be applied around:
individual amplifier stages (local or nested feedback), or
the entire amplifier (global feedback)
Each approach has different technical and audible consequences.
Feedback Does Not Act in Isolation - the effect of negative feedback depends on:
circuit topology
available open-loop gain
bandwidth
phase behaviour
interaction with the loudspeaker load
For this reason, feedback cannot be evaluated as “good” or “bad” in isolation.
Its usefulness depends entirely on how and where it is applied.
Feedback and Audible Differences
Changing the amount of global feedback in an amplifier can alter:
overall gain
output impedance
distortion spectrum
behaviour near overload
Some listeners describe these changes in terms such as fullness, forwardness, or resonance. These perceptions are system-dependent and influenced by loudspeakers, room acoustics, and listening levels.
Selectable Feedback in Amplifiers
Some amplifiers provide a means of enabling or disabling global feedback to allow direct comparison within the same circuit. This is intended to help users understand how feedback affects system behaviour, rather than to suggest that one setting is universally superior.
In Summary
Negative feedback is a powerful and essential design tool in audio amplifiers. When applied with proper consideration of stability, bandwidth, and load interaction, it improves performance. When applied without sufficient understanding, it can introduce unintended side effects.
Understanding feedback helps explain why amplifiers with similar measured specifications can still behave — and sound — differently in real systems.
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