Understanding Feedback

What Is Negative Feedback?

Negative feedback occurs when a small portion of an amplifier's output signal is returned to its input in opposite phase.

Because this returned signal opposes the incoming signal, it reduces the amplifier's overall gain.

In exchange, several important characteristics improve.

Why Use Negative Feedback?

Properly designed negative feedback can provide several advantages.

• Reduced distortion

  Feedback corrects many of the small nonlinearities that naturally occur in electronic devices.

  The result is lower harmonic distortion and improved linearity.

• Wider frequency response

  Feedback tends to flatten the amplifier's frequency response, helping maintain consistent gain across the audio spectrum.

• Lower output impedance

  Reducing output impedance improves loudspeaker damping and gives the amplifier greater control over loudspeaker cone movement.

• Improved stability

  Carefully applied feedback makes amplifier performance less dependent on variations between individual valves or component tolerances.

Is More Feedback Always Better?

No.

As feedback is increased, amplifier stability becomes more critical.

Every amplifier contains small phase shifts caused by valves, transformers and circuit components.

If these phase shifts become excessive while large amounts of feedback are present, the feedback may become positive feedback at certain frequencies.

Instead of reducing errors, the amplifier may oscillate.

This is one reason why good amplifier design requires careful attention to stability as well as distortion.

Local and Global Feedback

Negative feedback may be applied in different ways.

• Local feedback

  Local feedback is applied around a single stage or component.

  It improves the performance of that stage while having relatively little influence on the rest of the amplifier.

• Global feedback

  Global feedback returns a portion of the signal from the amplifier output all the way back to an earlier stage, often the input stage.

  Global feedback can improve the performance of the entire amplifier but requires considerably more care to maintain stability.

  Many well-designed tube amplifiers use a combination of both approaches.

Feedback and Output Transformers

In tube amplifiers, the output transformer plays an important role in determining how much global feedback can be applied safely.

Every transformer introduces small phase shifts at both low and high frequencies.

A high-quality transformer allows greater feedback before stability becomes a concern.

This is one reason why output transformer design is so important.

Feedback and Sound Quality

One of the long-running debates in high-fidelity audio concerns whether negative feedback affects the perceived sound of an amplifier.

Poorly designed feedback systems can indeed produce undesirable behaviour.

However, well-designed feedback applied within the limits of circuit stability generally improves overall performance.

Good engineering is not about avoiding feedback.

One of the sonic effects of negative feedback arises because it reduces an amplifier's output impedance. Larger or more massive loudspeaker cones tend to continue moving after the driving signal is removed. As the voice coil continues to move through the loudspeaker's magnetic field, it acts as a generator, producing a voltage known as back electromotive force (back EMF).

If the amplifier has a very low output impedance, it effectively absorbs (almost short-circuits) this generated energy, damping the movement of the loudspeaker cone. If the amplifier has a higher output impedance, the cone is allowed to move more freely. This can influence the sound in ways that some listeners describe as fuller, more open or more "present".

This is one reason why the amplifier and loudspeaker should be considered as a complete system. Good amplifier design is not simply about applying as much negative feedback as possible; it is about understanding when, where and how much negative feedback should be used.

Modern Engineering

Modern measurement equipment allows amplifier designers to examine stability, phase margin and transient behaviour in far greater detail than was possible decades ago.

Computer modelling and precision test equipment have made it easier to optimise feedback networks while maintaining excellent stability.

Today's best amplifiers combine careful engineering with extensive measurement and listening evaluation.

From the Designer's Bench

One of the questions I am often asked is whether SpotFire amplifiers use negative feedback.

The answer is yes, but with the SE5 it can be switched-off so you can hear the difference and make a choice.

The objective has never been to avoid feedback simply because it has become unfashionable in some circles.

Instead, feedback is used where it provides genuine engineering benefits.

The amount is carefully chosen to achieve an appropriate balance between distortion, bandwidth, loudspeaker damping and stability.

Like every other aspect of amplifier design, it forms part of the complete system.

SpotFire Engineering Perspective

SpotFire amplifiers use carefully engineered negative feedback to improve measured performance while preserving excellent musical reproduction.

Rather than pursuing the lowest possible distortion figure, the objective is to achieve a balanced design that combines wide bandwidth, low noise, good stability and enjoyable listening.

The SpotFire SE5 amplifier uses a modest amount of negative feedback, which can be switched off using a rear-panel slide switch. This allows you to compare the sound with and without negative feedback. Depending on the recording, the loudspeakers and personal preference, some listeners may prefer the slightly fuller, more relaxed presentation with the feedback switched off, while others may prefer the tighter control and lower distortion provided by negative feedback.

Engineering is always about balancing competing requirements.

Key Points

• Negative feedback returns part of the output signal to the input in opposite phase.

• Properly applied feedback reduces distortion and improves linearity.

• Feedback extends frequency response and lowers output impedance.

• Excessive feedback can create stability problems if not carefully designed.

• Good engineering uses the appropriate amount of feedback—not simply the maximum amount.

• Negative feedback is one of many tools available to the amplifier designer, and its success depends on how well it is integrated into the overall design.