Negative feedback, voltage and output impedance

I think that simply saying no feedback means nothing since there are many different kinds of feedback that manufacturers may describe in plain English just like Eldartford did.

Feedback circuitries is the whole separate topic and must be clarified in terms of what feedback is present and what feedback is not. Simply saying "no feedback" lacks the helluva information and knowing just from words on paper that amp has no feedback doesn't mean that it will sound better than the one with.

Amp's input and driving stages do definitely have a feedback so the statement "no feedback" isn't truthfull in any case. All the author of the post wants is just a truth about feedback and why it's "so bad and negative".

Sean's leaky boat analogy is not appropriate. He is all wet :)

If we must have an analogy, consider the task of steering your car around a curvy road. First you turn the steering wheel. Then your eyes notice that you are drifting into the opposite lane or into a tree. You apply a correction to the steering wheel. (Is that "negative"?)

This analogy also illustrates the limitation of a feedback control loop. If the car goes fast, and the road has sharp curves the driver may not be able to respond quickly enough to prevent a crash. In fact, if the required corrective inputs are too quick, the driver may become confused and apply inputs of the wrong polarity, and the car "spins out". For similar reasons. an audio amplifier with too much feedback is not only bad sounding...it becomes an oscillator.

Extending the analogy even further, some cars need less corrective feedback than others. A Ferrari corners better than Chevy. It also costs a lot more.

In a unity gain operational amplifier circuit (usually, but not always implemented by an integrated circuit device) the feedback is 100 percent. The FULL output signal is applied to the negative input of the differential amplifier. Since the desired signal is applied to the positive input, what drives the output is the DIFFERENCE between what you are getting for output and what you want. Important characteristics of the amplifier are slew rate and absurdly high gain. Linearity does not matter. (Low noise is nice).

In a normal linear amplifier some gain is desired, so 100 percent feedback is out of the question. Our audio amplifiers have bandwidth to at least 20KHz, and often to 100KHz. Circuitry like this does not delay the signal very much. Delay is bad for feedback because you need the feedback to be properly phased with respect to the signal. A given delay (microseconds) corresponds to more phase angle for high frequency signals. When the phase shift gets near 180 degrees your negative feedback becomes positive feedback, and you oscillate. The feedback signal is usually run through a filter to remove ultra-high (inaudible) frequencies where delay would induce oscillation. "Local" feedback (around just one stage of circuitry) is less affected by delay, and can be used to higher frequency than "global" feedback which is around the multistage amplifier circuitry.

When negative feedback is used, the forward amplification needs to "work harder". An analogy here might be a turboprop engine. The turbine of a 1000 HP engine needs to develop 8000 HP, because 7000HP is needed to run the compressor. (The exact numbers are my guess). This requirement, rather than the delay issue, probably accounts for sound quality degradation in very high feedback amplifiers.

An amplifier intended for use only with a subwoofer can have a very high amount of feedback, as long as high frequencies are removed from the feedback to avoid oscillation. Of course, in the case of subwoofers of the servo variety (eg: Velodyne) the driver cone is included inside the feedback loop, by developing a signal from a tiny accelerometer mounted on the cone. Here delay is a big time issue, which is why this technology has never been successful except in subwoofers.

Greg: I should have clarified that my "NO" feedback was in regards to "global" feedback. The use of small amounts of local feedback is not a big deal and results in less time lag. Sorry for my lack of clarification.

Tom: That was a good find. I'm sure that Theta won't mind the re-post here either : )

Marakanetz: I agree with your basic statements. See my response above to Greg.

El: My analogy applies more to "global feedback" than to "local feedback". Distortion and the correction of that distortion is like gossip. It can be quelled much faster and in a more effective manner "locally" than it can when trying to fix the problem "globally".

Think of "local feedback" as having a dedicated supervisor for each stage of amplification. If everything is good in section A, B & C but section D is a mess, only section D's supervisor has to get involved.

On the other hand, global feedback is kind of like having one supervisor by himself trying to keep track of what is going on in the entire "plant". The potential for more problems to slip through the cracks un-noticed is far greater. Once those problems are noticed, it is possible that the amount and type of correction applied may not be directly to the source of the problem, resulting in other problems. This is why global feedback is less desirable. Sean
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Sean...Analogies can get you in trouble! One popular management approach today is the "self-directed workforce" where little or no supervision is exercised at the day to day level, and the only thing that is supervised is the overall product going out the door, ("global supervision").

Whatever floats your boat :)

The idea that:

>Amp's input and driving stages do definitely have a feedback so the statement "no feedback" isn't truthful in any case.

-is incorrect. It is quite possible to build an entire amplifier without feedback of any kind.

There are two types of feedback that are both considered 'negative' (in that the feedback can be used to lower distortion): Loop and degenerative.

Usually Loop feedback is the type that is considered malfesent, as it is the type that increases high odd-ordered distortion. This occurs because there is a very finite time that it takes for the input signal to propogate to the output of the amp, where the signal is then tapped off back to the input. What this means is that as frequency increases, the feedback signal becomes increasingly out of sync with the input signal. Hence the addition of odd-ordered harmonic content. Esentially at higher frequencies the loop feedback results in ringing unless steps are taken to control it (usually by tailoring the feedback circuitry).

Degenerative feedback occurs in real time and so does not contribute to odd ordered harmonics, however the output impedance of the circuit is increased.

Audio Note (UK) made amps that lacked either type of feedback, back in the early 90s, at least according to the designer whom I met at CES.