Amp for my 2 channel

[RGA]

What do the others here think of this option? I like the idea of a tube amp and I see that they have a 7 day return policy.. Excuse my ignorance Poultrygeist, but what is S.E.T.?

Here are some links about SET (no feedback) amplifiers. Single ended amplifiers are the only truly linear amplifier devices that do not create errors to then need to be corrected by the amplifier - the damage has been done before the error correction circuit begins (called Feedback).

SET based systems typically always sound better than competing designs (however you pay a big price in amplifier power) The way to correct that is to use High efficient speakers. in other words speakers that can play loud with little power. The problem is high efficient speakers tend to cost more because efficient speakers use better quality drivers.

Whether they are Solid State or tube. Single ended no feedback amps do the least damage to the signal from input to output. Here are some links about it from several sources.

http://www.stereophile.com/reference/70/

"Negative feedback, quite simply, is the application of an inverted portion of an amplifier's output signal to its input terminals. This "extra" signal is subtracted from the input and serves to reduce the effective amplifier gain (as the input signal is then smaller). In addition, steady state distortion is thought to be reduced as the out-of-phase distortion components contained in the feedback signal cancels out some of the errors created by the amplifier circuitry.

This scheme presents two very obvious problems. Firstly, all amplifiers introduce some delay to passing a signal from its input, to its output and then back to its input. During this delay period, a feedback amplifier is operating at its natural (referred to as "open-loop") gain. It is not until this initial delay period is over, that the circuit begins to exhibit its intended operating ("closed loop") gain characteristics. There must be, by the very definition of a feedback system, some change in the gain factor G, during the transition from open to closed loop operation. This gain modulation would probably not be audible by itself, as the propagation delays of most good amplifiers are quite small, except that the increased gain of the amplifier during the initialization period results in a decreased maximum input capability before overload. Simply put, an amplifier which utilizes 20 dB of feedback (a relatively modest amount by modern standards) and requires an input of two volts to clip during closed loop operation, would overload with only two tenths of a volt input during the forward delay period. Once the amplifier is overdriven, it may take many times its delay period to become fully restored to normal operation. The distortion created by this condition has been commonly referred to as Transient Intermodulation Distortion (TIM), Dynamic Intermodulation Distortion (DIM), and Slew Induced Distortion (SID).

In addition to this obvious form of feedback induced distortion, there exists another more subtle effect of signal regeneration. Because all amplifiers have some forward propagation delay, the fed back portion of the output signal will always lag behind the input. There is therefore a constant introduction of "out of date" information into the amplifier. Under transient conditions (which is what music is; transients), this results in the presentation of an error correction signal intended to reduce the distortion of an input signal which has already passed through the amplifier and is either already out of the circuit or well on the way out of the circuit. The signal present at the input by the time the feedback has arrived may bear no relation to the previous signal and thus will not be properly acted upon by the regenerated information. The current input signal is then distorted once, through the subtraction of an erroneous feedback waveform, and again by the amplifier. Additionally, the error signal present in feedback is passed through the amplifier and again fed back, with all of the newly created distortions, to make yet another trip through the circuit, until it is allowed to decay through successive attenuation. Thus, a distortion signal which originally may have lasted only a few microseconds, can pass through the amplifier enough times for its effective duration to have exceeded the threshold of human audibility. The mechanism originally designed to reduce audible distortion, actually, under transient conditions, serves to regenerate, emphasize and, in fact, create distortion.
Negative effects of negative feedback.../...

[in a SET amp] such distortion regeneration does not take place. The circuits have been designed for maximum linearity without corrective mechanisms, and thus responds as easily to transient signals as it does to steady state waveforms. The amplifiers make no attempt to reverse the path of time in order to correct their own errors. Those distortions created by these circuits (which are almost entirely harmonic in nature) are allowed to pass only onto the loudspeaker, and not back to the input.

Despite the absence of feedback, the forward propagation delay of all our amplifiers has received much attention. All our output transformers have been designed using this criterion, obviously with a keen eye on cost. It is obvious that if this delay is not absolutely invariant, for all conditions, the DeltaT component of the input signal will not be accurately preserved. Thus, those factors which determine delay have been carefully observed and stabilized. In addition, the operation of all amplification stages at nearly constant power, independent of signal conditions, i.e. Class A operation at every stage, greatly contributes to the symmetry and linearity of our circuits.

It is, however, not enough for an amplifier to operate linearly by itself. In order to minimize audible distortions, the device must be able to operate as well into a real loudspeaker as it does into a laboratory resistive load.

In order to adequately control the cone excursions of the loudspeaker and to optimize power transfer, the effective output impedance of the amplifier should be as far below the impedance of the load as possible. The ratio of these two impedances is referred to a damping factor, usually referenced to an eight ohm speaker. Thus, a damping factor of eighty reflects an amplifier output impedance of one tenth of one ohm. The design of the output transformer is extremely critical, and taps on the output are normally provided to match the load impedance best possible.

A problem in the normal expression of damping factor is that its measurement is performed using steady state signals. This results in a factor relying quite heavily on the action of an amplifier's feedback. The damping ability of an amplifier under transient conditions, before the feedback mechanism has been able to reach, is only accurately expressed as the steady state damping factor divided by the feedback factor. Thus, an amplifier with twenty decibels of feedback and specified damping factor of one hundred, has a damping value of only ten under transient conditions. This not only reduces the amplifier's ability to control the cone movement, but allows voltages created in the speaker voice coil to mix with the output signal and enter the amplifier's feedback system. In this condition, distortions created by the speaker's motion are not only unattenuated, but are emphasized through feedback regeneration.


http://www.audionote.co.uk/articles/...feedback.shtml

Interview with one of the biggest SET manufacturers (starts at about 2:40) http://www.youtube.com/watch?v=zEUW3...eature=related

http://www.enjoythemusic.com/Magazin...amplifiers.htm

http://www.stereophile.com/interviews/399/

[E-Stat]

Single ended amplifiers are the only truly linear amplifier devices that do not create errors to then need to be corrected by the amplifier - the damage has been done before the error correction circuit begins (called Feedback).

You are mistaken. Other feedback-free designs exist that are not such as the Ayre MX-R.

rw

[RGA]

You are mistaken. Other feedback-free designs exist that are not such as the Ayre MX-R.

rw

Can you clear this up - John Atkinson claims that it [MX-R's] circuit uses a low level of overall loop negative feedback. Which is more than zero.

There is more to the game than this anyway since class D amps have yet to impress me and some or maybe all have no feedback and run class A. So there is more to the game than either or element. No feedback is one, although even there I have liked amps with a little feedback more than others with zero. For instance I love the Sugden A21a which is both a SET and no feedback - but I like the AN OTO more which is a SEP and uses a little bit of feedback.

I have not auditioned Ayre extensively - Soundhounds did pick up the line and I did like their conversion from streamed sources quite a lot.

Wes Phillips was the big reviewer of this amp and ran his Dynaudio's on it. But since then he's heard what a big time SET amp can do. So I am not convinced that the sonic aesthetic offered up by Ayre is going to do it for me. As the Ayre dealer notes - "it sounds great...for solid state." I'd like to try a complete Ayre system at some point - Charles seems like an interesting fellow and has a passionate viewpoint on audio. And his prices are not insane for the most part.

[Feanor]

...
There is more to the game than this anyway since class D amps have yet to impress me and some or maybe all have no feedback and run class A. So there is more to the game than either or element. No feedback is one, although even there I have liked amps with a little feedback more than others with zero. For instance I love the Sugden A21a which is both a SET and no feedback - but I like the AN OTO more which is a SEP and uses a little bit of feedback.
...

More research is called for, however I believe most or all class D amps use feedback.

However the issue of class A is irrelevant in case of class D amps. Class A means that the operation of the transistor (or tube) is "biased" so, simply put, it runs in the middle of its power range thus avoiding distortion that occurs at very low operation levels. I.e. "crossover distortion" in case of push-pull amps. However class D amps always work the transistors at maximum output (or maximum for lowest distortion).

Local feedback is typically innocuous because the delay times, which cause the problems, are extremely short.