Feedback in amplifier is ultimately the driver of sound quality ?

[ash_dac]

Hi,

Pre-amplifier stage

Power amplifier stage:-

-Input stage
-Driver stage
-Power stage

From what I understand the input stage will be bandwidth limited with feedback often applied from power to the inverted input stage. The feedback is bandwidth limited to prevent oscillation of the power amplifier stage. The power stage should be linear in operation.

Does this mean that feedback in amplifier is ultimately the driver of sound quality ? (rather than esoteric parts)

[lumanauw]

Feedback amount and sound quality is not related. Linearity does.

[myhrrhleine]

Bandwidth is generally determined by the initial recording.
Tape, CD, Phono, Radio are all bandwidth limiting.

Feedback enhances overall linearity.
Some feel it can be detrimental.
Others believe it fixes more than it hurts.
YMMV.

[traderbam]

Oh, global feedback can definitely be detrimental to the sound. No doubt about it. Basic feedback theory doesn't mention this because basic feedback theory is based on linear systems.

The problem with audio amps is their non-linearities, of course.

Designers use feedback in audio amps as a technique for trading gain for linearity. But like any technique it must be used with care to avoid unwanted side effects. There are a lot of potential unwanted side-effects.

One solution is to avoid feedback wherever possible. This tends to lead to big, heavy, energy wasteful, expensive designs that are costly to manufacture as they require careful component matching across many parts.

Another solution is to use feedback with proper care. This tends to lead to small circuits with fewer components, relatively low power wastage, cheaper to buy and cheaper to manufacture because there are less parts.

Whether feedback is used or not don't lose sight of the fact that it is the non-linearities of the components that ultimately determines the sound.

[CBS240]

A perfectly linear transistor doesn't exist. This is why they are used so as the Q-point doesn't move very much. This gives them more linear operation, but still needs some quantity of error correction. For output devices the Q-point must move in order to have efficiency so non-linearity arises that must be corrected. Otherwise, you might as well go with good old vacuum tubes. A non-feedback transistor amp must be designed around the specific devices used, and are hard to duplicate for a production line and cheap mass production.

[wildswan]

I agree with Traderbam.
There is no free lunch.

Feedback trades even order harmonics (non linearity) for odd harmonics, which are distributed over the higher frequencies.

The types of distortion that remain after feedback (the odd higher order ones) are very objectionable. I suppose the question is, are these distortion components still audiable?

I think final quality has to depend on the initial design, and how much work the feedback is expected to do.

The effects of feedback are very noticable in musical instrument/studio applications, because things do clip very often, as the source signals have wide dynamic range.

HiFi amps should be able to avoid this type of input clipping, but the problem still remains- when you use feedback to correct for non linearity, you do create HF distortion.

I don't hate feedback, but I do hate high odd order distortion, no matter how well the amp measures

[CBS240]

Originally posted by wildswan


The effects of feedback are very noticable in musical instrument/studio applications, because things do clip very often, as the source signals have wide dynamic range.

This is why if you are interested in sound quality to allow lots of headroom for the actual average power you intend to listen to. Music isn't a single sine wave but has harmonics that produce spikes and peaks that can be way above the average power output. This is particularly true of clasical music. Of course most people who listen to clasical tunes don't listen at 1000Wrms.

[suzyj]

Originally posted by ash_dac
Does this mean that feedback in amplifier is ultimately the driver of sound quality ? (rather than esoteric parts)

Yes, in a nutshell. Transistors (and valves) are inherently non-linear. There's no getting around it. You can operate them around a ridiculously small range, to minimise the non-linearities, or else you can utilise feedback to linearise them.

Of course feedback doesn't always have to be global. each stage can be linearised a bit using emitter degeneration resistors as feedback elements, but the general principles always apply. There's no such thing as a free lunch.

Cheers,

Suzy

[PMA]

Suzy,

thank you for your post.

Best regards,
Pavel Macura

[traderbam]

Transistors (and valves) are inherently non-linear. There's no getting around it. You can operate them around a ridiculously small range, to minimise the non-linearities, or else you can utilise feedback to linearise them.

Or you can create a linear component by combining two conjugate non-linear components, or some combination of multiple components.