Negative feedback, voltage and output impedance

Hi Marakanetz,

Just FWIW, as a manufacturer I have to be careful what I say as this forum is moderated. At the same time what I am expressing is actually the way I see it and that has served me pretty good so far for the first 26 years or so anyway :)

I hope you don't consider it a sin that I try to walk what I talk. OTOH, our early amps for the early 80s did use feedback, so its not as if I've not given it a fair shake. Every now and then we re-try some of the stuff we abandoned, just to see if we are still on target. FWIW, amps without feedback are harder to set up as you have to be more careful about the speaker choice. This means our market is probably a bit more limited. As a result it doesn't take the ability to download and chew gum at the same time to figure out that I'm not in it for the money...

Another difference between us and Mac is that our warranty is transferable and we will reactive that warranty on any product that we have fully updated.

Negative feedback aka "error correction circuitry" is kind of like trying to bail water out of a boat that has a major leak. Plug the hole i.e. build a linear circuit first, then worry about corrective measures later ( if needed at all ). After all, you can't respond to an error until it's already happened, making error correction introduce it's own errors due to the lag time involved. As such, designing the circuit for maximum speed and linearity negates much of the need for such "band-aids" as feedback. This is because the circuit is both fast and stable enough to keep up with any of the demands placed upon it without introducing its' own non-linearities into the equation.

As a side note, high levels of negative feedback is what makes a large percentage of SS designs sound hard and sterile. Most of the SS amps that offer excellent high frequency "air" while retaining midrange "liquidity" are of a low or no feedback design.

For those of you that have never heard a low / no feedback SS design that is fast with wide bandwidth, i would suggest checking it out. For those that have never heard an Atmasphere OTL amp with suitable speakers, i also suggest checking that out. Both of these types of products are what "accurate musicality" sounds like. Sean
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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".