Negative feedback, voltage and output impedance

How can there be a ss amp without ANY feedback?? I.e., Isn't some local feedback necessary?
I'm not savvy on these subjects: which can explain my stupid question:)

Theta Digital amps employ a no feedback design. Here is a quote from their webpage:

"Feedback

What is feedback?

Like a snake biting its tail, a negative feedback loop sends some of its output signal back to its input.

To cancel out the errors that have crept in during the amplification process, a compensation signal is applied at the input. Obviously, this correction can not actually take place instantaneously. There are two basic categories of this sort of negative feedback. The impossibility of instantaneous correction is one factor that makes this distinction important.

Local Feedback:

Local feedback is very common in almost all analog circuits. It stabilizes, sets operating points, limits unwanted oscillation, reduces distortion, and protects delicate devices from potential damage. Local feedback is applied almost immediately back to the input, with very little delay.

Global feedback:

Global feedback is also very common in circuit design. It is usually applied to reduce distortions and lower output impedance. It can be used to stabilize circuits that are unstable on their own. There is significant time delay between the input signal and the feedback signal, due to the number of stages the input signal must pass before being applied back to the input in the form of feedback. Additional circuits must also be used in the feedback path to make sure the negative feedback never becomes positive feedback at any frequency. Because of the significant time delay, global feedback can cause a smearing of imaging and an upper midrange with harsh or glare. The audible effects of global feedback vary, mostly depending on the amount of feedback but also on the circuit they are correcting. Nearly all power amplifiers use global feedback in large proportions.

Theta’s goal is to create very sonically accurate components. Measurements typically published as "specifications" do not reflect some of the most important aspects of sonic performance. It is quite possible to design circuits that measure well but sound bad. In fact, it’s done all the time.

Time delay created by global feedback creates audible problems. The "envelope" is too big, resulting in serious phase shift and intermodulation of the signal with its own error products. This fantastic complex of distortions goes unmeasured (in all the usual specifications), and is not correctable. Since Theta is able to offer circuitry that is inherently stable, there is little incentive to trade actual performance for measurements.

The decision was easy, if radical: Theta’s amplifiers use no global feedback!"

I am very, very happy with my Enterprises and Intrepid. To my ears, Theta amps sound nicely balanced, wonderfully detailed, with great soundstage, and above all, are very musical. Though to be honest, I do not know how much of this is attributable to the zero feedback design, and how much is the product of other characteristics.

Tom.

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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