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post #1 of 64 Old 10-21-2007, 07:19 AM - Thread Starter
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What do you think and why? Do no negative feedback amplifiers sound better than those with negative feedback?

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post #2 of 64 Old 10-21-2007, 10:04 AM
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They tend to, for reasons I illustrated in a thread on the DIY audio forum, the thread in the 'solid state' forum area, in a thread about page 208 in the recent Rec. Components of Stereophile. That is the thread where the gentleman from Ayre was attacked for trying to help out the understandings of some of the forum members. The same kind of fate I tend to suffer here, which is why I gave up after 7000 posts.

I outlined some basic points about the human hearing mechanism and how we measure and how the two have never met, at all, thus we seemingly have no comparative basis between the measurements and the listening.

I'm only here for a while, as I'll be involved in other companies fortunes, doing design work for them, on the side. Thus my opinions and positions cannot be public, the fortunes of others are then tied to such - and they simply cannot afford that.

Ken Hotte

"Never forget that only dead fish swim with the stream." -- Malcolm Muggeridge.
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post #3 of 64 Old 10-21-2007, 04:42 PM - Thread Starter
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The simplified Widipedia explanation for negative feedback in amplifiers:


http://en.wikipedia.org/wiki/Feedback_amplifier


"A negative feedback amplifier, or more commonly simply a feedback amplifier, is an amplifier which uses a negative feedback network, generally for improving performance (gain stability, linearity, frequency response etc.). Feedback can be negative or positive, but must be negative for overall stability of the system.

The negative feedback amplifier was invented by Harold Stephen Black (US patent 2,102,671 (issued in 1937) [1] ) while a passenger on the Lackawanna Ferry (from Hoboken Terminal to Manhatten) on his way to work at Bell Laboratories on August 2, 1927. Black had been toiling at reducing distortion in repeater amplifiers used for telephone transmission. On a blank space in his copy of The New York Times[2], he recorded the diagram and equations found in figure 1. [3]The idea of this invention is simple: If the loop gain is large, overall response of the whole system can be very close to inverted feedback network response. Fundamentally, all electronic devices (e.g. vacuum tubes, bipolar transistors, MOS transistors) are nonlinear devices. Negative feedback corrects this by trading unused gain for higher linearity (lower distortion). Though much more accurate, amplifiers with negative feedback can become unstable if not designed correctly, causing them to oscillate. Harry Nyquist of Bell Laboratories managed to work out a theory regarding how to make it stable.

Negative feedback is used in this way in many types of amplification systems to stabilize and improve their operating characteristics (see e.g., operational amplifiers). Note that negative feedback is used to "stabilize" the system, not in amplifying the signal, whereas positive feedback is used to amplify the signal itself (which may lead to instability)."

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post #4 of 64 Old 10-21-2007, 04:45 PM - Thread Starter
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Theta Digital about why they use no negative feedback in their amplifiers:

http://www.thetadigital.com/ThetaHistoryp6.html

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!

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post #5 of 64 Old 10-21-2007, 04:54 PM - Thread Starter
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From a Widescreen Review interview with Neil Sinclair of Theta Digital discussing Theta's design philosophies:

http://www.thetadigital.com/reviews/...ophy-wsr-3.htm


"WSR Reber: The amplification components in an audio system raise the low-level signal from the source components to a level loudspeakers can convert to acoustical energy. The preamplifier stages in surround processors or controllers provide switching, buffering, and volume control for six or more channels, along with some low-level signal amplification. The power amplifier receives the preamps' elevated signal and further "amplifies" it to a level that can drive loudspeakers. An accurate amplifier delivers an accurate replica of the pre-amplifier output to the loudspeakers. The amplification process occurs continuously as the signal passes from one stage to another, until it is sufficient to drive loudspeakers. The performance of the loudspeakers depends on the condition of the electrical signal they receive, after numerous stages of amplification through all the other components in the system. What is it about your amplifier approach that distinguishes Theta Digital?

Sinclair: Two things. As we mentioned with low-level amplification, we believe in differential balance, and our amplifiers are balanced input to output. This means there are really two channels of amplification for every channel. One channel for each phase angle, each one looking at the other, and doing differential correction, or common mode rejection, of any distortions that are there. The other key point is we believe in as little negative feedback as possible. None of our amplifiers use any overall negative feedback.
There are two reasons feedback is common in audio amplifiers. It's hard to stabilize an amp, to keep it from oscillating and blowing itself up without overall feedback. Also, while overall negative feedback can radically lower the measured distortion, we found that whenever overall negative feedback was used, it could be heard in a predictable way, in a more audible way than you could hear measured distortion. We worked hard to create stable designs that did not need overall feedback. To understand this you have to understand how feedback works.

(Continued from previous page)
WSR Reber: I was going to ask, how does feedback work? What is it exactly?

Sinclair: Basically you're comparing your input signal with your output signal.

WSR Reber: The idea being they should be exactly the same?

Sinclair: Yes, except for amplitude. The problem is that once you've compared them, you've lost some time and in so doing get time smear or phase shift, and you hear this. There's a sound I wish I could describe that tells you there's feedback in an amplifier. You get more of this sound the more feedback you have. It's an unpleasant sound. It manifests itself in the upper midrange for the most part and a little in the high frequencies. Would you go along with that, Mary?

Cardas: Yes. It's kind of like jitter.

Sinclair: It's aggravating stuff. You know it when you hear a system that does all the basic things right, but doesn't grab you because there's something that makes your muscles kind of twinge.

WSR Reber: But that could be in the source?

Cardas: Sure, it's all integral. That's the key. If you're amplifying a constant tone, feedback can be very useful, because what you're looking at is exactly the same at both ends of the circuit. And every time you com-pare it along the way, it can be exactly the same. Amplifiers like this are used for non-audio purposes in industry. However, no music you are going to listen to has a constant tone for any meaningful time; it's always changing. With feedback, you're taking this moving thing and comparing it to another signal some interval later-an interval that's always too long-and using this comparison to make changes to the reference signal, which has already moved along ... changing it to a reference that is no longer accurate.

Sinclair: Signals that are highly trenchant like a human voice, a plucked string, a percussion instrument--you hear the feedback because it doesn't allow for the proper trenchant response of the signal.

WSR Reber: You are saying feedback can never be really accurate because there's a time smear, and you use very minimum to non-existent feedback in your circuit?

Sinclair: Overall, non-existent."

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post #6 of 64 Old 10-21-2007, 04:58 PM - Thread Starter
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From an Audio Perfectionist Journal (Richard Hardesty) interview with David Reick of Theta Digital:

http://www.thetadigital.com/reviews/...terview-p2.htm


"Theta's amplifiers have no global feedback but some of your previous designs have used modest amounts of negative feedback. What's your current position on this controversial subject?


Right now, I have to say I love the way our amps sound without feedback. Feedback is, after all, a case of trying to correct for something that's already happened. There's a freedom from hash and grain without it that just seems to be saying, Hey, I'm running free, and I don't have to double back and cover my tracks. I'm not negating good sounding amplifiers that use some feedback. Hey, I reserve the right to do so in a future design if I see fit. I will even go out on a limb here and say, contrary to popular opinion, I think there are some excellent sounding op amps, and I have employed them successfully in past designs. Perhaps keeping the overall loop within chip size helps. But that said, zero feedback is where I am right now. And Theta's amps are very happy that way."

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post #7 of 64 Old 10-21-2007, 05:07 PM - Thread Starter
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I don't pretend to really much understand this engineering mumbo jumbo or to have an opinion one way or the other re use of negative feedback in amplifiers.

I can only tell ya that although I luved my Bryston 7B monoblocks, that my current Theta Enterprise monoblocks are so much more transparent, quick, real, dynamic. Just wonderful. This afternoon we did another extended listening session and my friend Allon,
who comes over about once a month and is a died in the wool audiophile from years back, again states that the Enterprises, on top of the Aerial 9s, have totally transformed my system, that whatever music I put on sounds transparent to the source and makes ya wanna tap your feet. Me, too. And this got me thinking about the merits of having no negative feedback in amplifiers.

As folks here at the 20K forum seem to have a lot of info and opinions re objective vs subjective, how components sound, engineering, etc., I thought this would be a perfect topic for this forum. And I appreciate opinions re use of negative feedback in amplifiers probably run both directions as amplifiers made by top designers may or may not have negagtive feedback.

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post #8 of 64 Old 10-21-2007, 05:25 PM - Thread Starter
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From Ayre Acoustics:

http://www.ayre.com/about.cfm


"Zero feedback circuitry delivers completely natural performance.
Conventional designs use negative feedback to compare the output signal to the input signal, which introduces time-domain errors and other artifacts. In contrast, Ayre's exclusive zero-feedback circuits are designed for intrinsically pure linearity, delivering performance that is coherent and natural."

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post #9 of 64 Old 10-21-2007, 05:27 PM - Thread Starter
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From noted amplifier designer John Curl:

http://www.diyaudio.com/forums/showt...pagenumber=104


"Fellow designers, for some reason, many of you cannot understand WHY we try to use as little feedback as possible, rather than as much feedback as possible. Could you possibly believe after 30-40 years of design that we have NOT tried high feedback? Did we not also attend college and learn the wonderful effects and the advantages of negative feedback?
What happened is that we were disappointed with the results of our high feedback amplifiers. That's why we cut back, ideally to no global feedback. However, to do this, we must keep our higher harmonics low and our overall distortion OK. We also have to fight off power supply noise, thermal modulation, high output impedance, and who knows what else! It is a worthwhile challenge for us, when people hear and love our efforts."

At the same forum, Charles Hansen, also a noted amplifier designer, states:

"quote: Originally posted by x-pro: for a budget mass produced amplifer this approach is very difficult if not impossible to implement so that the advantages of having no feedback would not be overrun by the disadvantages of a cheap implementation.

I agree. Fully balanced circuitry also drives up the cost considerably. I wish I knew how to make a fully balanced, zero feedback integrated amp for $1000, but I don't. (And I refuse to make stuff in China.) I'll keep working on it, though..."

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post #10 of 64 Old 10-21-2007, 07:03 PM
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Essay: "Negative Feedback - Fact and Fantasy"
Secrets of Home Theater & High Fidelity

Negative feedback, or NFB, has taken a bad rap in the high-end audio community. Misunderstood by much of the audio press, and occasionally abused by audio designers, many audiophiles shun the idea like anthrax. In response, marketing departments have made claims of products with "Zero Negative Feedback."

In truth, every audio amplifier, from SET (Single-Ended Triode) to push/pull bipolar, relies on feedback to simply cover the bandwidth required by audio, as well as maintain linear operation. All amplifiers use feedback, period. Triode-based tube amplifiers don't require additional feedback circuits simply because that kind of tube inherently utilizes feedback within. But, because a triode doesn't need an external feedback circuit, you also can't control the feedback loop. Tetrode and Pentode tube-based devices, as well as MOSFETs, HEXFETs, and bipolar transistors, all require negative feedback circuits to achieve full audio bandwidth, low distortion, and in the case of bipolar transistor's negative temperature coefficient (as temperature rises, internal resistance falls), avoid thermal runaway.

And what does this term, negative feedback, bandied about like an overused hockey puck, mean anyway? As explained in our primer, negative feedback essentially creates a process which compares the output signal to the input signal, derives a difference signal, inverts it (hence the negative term) and applies that to the input of the device. Thus, the loop corrects differences in the waveform shape between the input and output as fast as the gain/bandwidth response of the device will allow. Used correctly, negative feedback has the following effects:

Lower distortion (THD)
Lower output impedance (increases damping factor) in the case of voltage feedback, higher output impedance in the case of current feedback
Greater bandwidth
Lower Gain (amplitude)
The benefits of lower distortion and lower output impedance seem rather obvious: more harmonic transparency, and a flatter frequency response into the reactive loads we've come to know and love as loudspeakers. While the excessive use of feedback can alter the proportion of harmonic distortion, tilting the ratio more towards higher order harmonics, which then might be more noticeable than 2nd or 3rd order harmonics of the same magnitude, both higher and lower order distortion will be lower in a carefully designed system.

What I find most interesting, though, is the relationship between gain and bandwidth. A device without negative feedback will have a lot of gain, but very poor bandwidth (rolloff in the high frequencies) - completely unsuitable for audio use. A device with a lot of negative feedback will have a high bandwidth and little gain. Gain can be increased with multiple stages, but bandwidth cannot. So, by necessity, a wide bandwidth amplifier with lots of gain will require more gain stages and/or feedback than an amplifier with lower gain and/or bandwidth. This flies in the flustered face of conventional audiophile wisdom which covets simple circuits above all else.

Most audio engineers would agree in theory that adding a gain stage which contributes neither current gain, voltage gain, nor a buffering function of any kind does nothing to improve the performance of a system. However, many don't grasp, or won't accept, that the number of gain stages is not necessarily inversely related to sound quality, so long as each stage carries out a valid function. After all, as one of our writers, John Busenitz, likes to point out, "If you think that a shorter signal path is always better, try driving speakers with a preamplifier."

Before it seems that I'm holding negative feedback up as some kind of holy water cooler, let me say that NFB can wreak some nasty havoc if abused. For example, if a designer was dead set on a limited number of gain stages for whatever concerns, but still wanted more gain and/or bandwidth than the devices he chose to use would allow, he might use negative feedback to push the bandwidth beyond the open loop (without feedback) response of the circuit. This "technique" results in a rising response that peaks before the final (and sharp) rolloff.

Although this amplifier would spec out well in terms of bandwidth and distortion into a resistive load, the sharp slope in the upper bandwidth would cause a very poor phase response (i.e., there would be "phase shift", meaning that the position of the high frequencies in time would be delayed from where they should be relative to the low frequencies). Regardless of whether the phase response itself is audible, phase shift creates a loop very prone to oscillating, making the correction very much after the fact, sometimes causing more error that in turn causes more correction, and so on. Not only does this promote ringing and IM distortion but, especially when driving capacitive loads which induce both a low impedance and heavy phase shift in the ultra sonic range, the oscillation can lead to catastrophic failure of the amplifier ("smoking" it).

In situations such as this, less feedback would have suited the design, but one could argue that it would also help to have a better engineer who knew the limitations of the technology and the chosen devices. Does it make sense to dull the knife because the cook cuts himself? I think not.

What then of these claims of amplifiers without negative feedback. Are they lies? No, not exactly. When an amplifier manufacturer or reviewer speaks of no negative feedback, what they're referring to is not simply negative feedback, but global feedback compared to local feedback. Also, amplifiers often have DC servo-feedback to keep the DC offset at the output as near to zero as possible, and power supplies can have feedback as well. These are not referred to when stating "no feedback" in the specification sheet or marketing information.

While local feedback compares the output of a single gain stage to the input of the same, global feedback loops the output stage of the entire amplifier to the input stage of the entire amplifier, so that the correction signal affects the entire chain in between and, likewise, the entire chain affects that correction.

Because of this, the total error which the feedback loop must address, including the high frequency cutoff phase error, is the sum of all the gain stages of the system, which gets a few designers in trouble for reasons already mentioned. In other words, all other things being equal, the correction speed of the global loop is lower than the local loop, because the bandwidth of the global loop is limited many times over by the multiple stages it must correct, and use to make that correction.

That does't make global feedback bad. Like anything else, you've just got to be careful about how you use it. Even though local feedback allows for more immediate correction, it also requires a separate loop for each device. Plus, there's no reason one couldn't use both (local and global) with great success, even at the same time.

Unfortunately for us audiophiles, there isn't a simple statement about the uses of feedback to evaluate the engineering merits of a particular piece of equipment. It's not simply a matter of stating that more or less is better, but rather, one must examine the specific context of the application. If one really wanted to, one could apply a battery of tests to filter out the best designs from the inept. Considering that some of the most highly touted amplifiers on the market fail miserably in something so basic as stability, it seems quite tempting.

But, to truly evaluate an amplifier purely on objective merits, one almost has to be an electrical engineer. As rewarding as that may be, not everyone can implement, let alone interpret such tests. Besides, it's so much more fun to spend the majority of the evaluation time listening to the equipment.

There is, though, a relatively easy way to get an indication of an amplifier's high frequency stability which relates directly to the implementation of negative feedback. Look at the rolloff curve. Since the phase error is proportional to the slope, an amplifier with a slowly tapered HF rolloff is more likely to be stable than the contrasting example of a rising peak followed by a sharp drop. Of course, this doesn't make any guarantees, as there are always more issues to consider, such as basic power supply characteristics for starters. But, if this stuff was easy, everyone would be designing audio equipment. Too bad, huh?
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I agree. Fully balanced circuitry also drives up the cost considerably. I wish I knew how to make a fully balanced, zero feedback integrated amp for $1000, but I don't. (And I refuse to make stuff in China.) I'll keep working on it, though..."

Does that mean the Chinese know how to make a fully balanced, zero feedback integrated amp for $1000?

"I've found that when you want to know the truth about someone that someone is probably the last person you should ask." - Gregory House
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post #12 of 64 Old 10-22-2007, 06:13 AM - Thread Starter
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Quote:
Originally Posted by Chu Gai View Post

Does that mean the Chinese know how to make a fully balanced, zero feedback integrated amp for $1000?

Chu, this is your opportunity to make a non-joke non-psychology objecive/substantive contribution. I am looking forward to to "hearing" your thoughts on this subject. Perhaps you don't think it makes any difference on way or the other???

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post #13 of 64 Old 10-22-2007, 06:24 AM - Thread Starter
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Quote:
Originally Posted by Philip Tan View Post

Essay: "Negative Feedback - Fact and Fantasy"
Secrets of Home Theater & High Fidelity

Negative feedback, or NFB, has taken a bad rap in the high-end audio community. Misunderstood by much of the audio press, and occasionally abused by audio designers, many audiophiles shun the idea like anthrax. In response, marketing departments have made claims of products with "Zero Negative Feedback."

In truth, every audio amplifier, from SET (Single-Ended Triode) to push/pull bipolar, relies on feedback to simply cover the bandwidth required by audio, as well as maintain linear operation. All amplifiers use feedback, period. Triode-based tube amplifiers don't require additional feedback circuits simply because that kind of tube inherently utilizes feedback within. But, because a triode doesn't need an external feedback circuit, you also can't control the feedback loop. Tetrode and Pentode tube-based devices, as well as MOSFETs, HEXFETs, and bipolar transistors, all require negative feedback circuits to achieve full audio bandwidth, low distortion, and in the case of bipolar transistor's negative temperature coefficient (as temperature rises, internal resistance falls), avoid thermal runaway.

And what does this term, negative feedback, bandied about like an overused hockey puck, mean anyway? As explained in our primer, negative feedback essentially creates a process which compares the output signal to the input signal, derives a difference signal, inverts it (hence the negative term) and applies that to the input of the device. Thus, the loop corrects differences in the waveform shape between the input and output as fast as the gain/bandwidth response of the device will allow. Used correctly, negative feedback has the following effects:

Lower distortion (THD)
Lower output impedance (increases damping factor) in the case of voltage feedback, higher output impedance in the case of current feedback
Greater bandwidth
Lower Gain (amplitude)
The benefits of lower distortion and lower output impedance seem rather obvious: more harmonic transparency, and a flatter frequency response into the reactive loads we've come to know and love as loudspeakers. While the excessive use of feedback can alter the proportion of harmonic distortion, tilting the ratio more towards higher order harmonics, which then might be more noticeable than 2nd or 3rd order harmonics of the same magnitude, both higher and lower order distortion will be lower in a carefully designed system.

What I find most interesting, though, is the relationship between gain and bandwidth. A device without negative feedback will have a lot of gain, but very poor bandwidth (rolloff in the high frequencies) - completely unsuitable for audio use. A device with a lot of negative feedback will have a high bandwidth and little gain. Gain can be increased with multiple stages, but bandwidth cannot. So, by necessity, a wide bandwidth amplifier with lots of gain will require more gain stages and/or feedback than an amplifier with lower gain and/or bandwidth. This flies in the flustered face of conventional audiophile wisdom which covets simple circuits above all else.

Most audio engineers would agree in theory that adding a gain stage which contributes neither current gain, voltage gain, nor a buffering function of any kind does nothing to improve the performance of a system. However, many don't grasp, or won't accept, that the number of gain stages is not necessarily inversely related to sound quality, so long as each stage carries out a valid function. After all, as one of our writers, John Busenitz, likes to point out, "If you think that a shorter signal path is always better, try driving speakers with a preamplifier."

Before it seems that I'm holding negative feedback up as some kind of holy water cooler, let me say that NFB can wreak some nasty havoc if abused. For example, if a designer was dead set on a limited number of gain stages for whatever concerns, but still wanted more gain and/or bandwidth than the devices he chose to use would allow, he might use negative feedback to push the bandwidth beyond the open loop (without feedback) response of the circuit. This "technique" results in a rising response that peaks before the final (and sharp) rolloff.

Although this amplifier would spec out well in terms of bandwidth and distortion into a resistive load, the sharp slope in the upper bandwidth would cause a very poor phase response (i.e., there would be "phase shift", meaning that the position of the high frequencies in time would be delayed from where they should be relative to the low frequencies). Regardless of whether the phase response itself is audible, phase shift creates a loop very prone to oscillating, making the correction very much after the fact, sometimes causing more error that in turn causes more correction, and so on. Not only does this promote ringing and IM distortion but, especially when driving capacitive loads which induce both a low impedance and heavy phase shift in the ultra sonic range, the oscillation can lead to catastrophic failure of the amplifier ("smoking" it).

In situations such as this, less feedback would have suited the design, but one could argue that it would also help to have a better engineer who knew the limitations of the technology and the chosen devices. Does it make sense to dull the knife because the cook cuts himself? I think not.

What then of these claims of amplifiers without negative feedback. Are they lies? No, not exactly. When an amplifier manufacturer or reviewer speaks of no negative feedback, what they're referring to is not simply negative feedback, but global feedback compared to local feedback. Also, amplifiers often have DC servo-feedback to keep the DC offset at the output as near to zero as possible, and power supplies can have feedback as well. These are not referred to when stating "no feedback" in the specification sheet or marketing information.

While local feedback compares the output of a single gain stage to the input of the same, global feedback loops the output stage of the entire amplifier to the input stage of the entire amplifier, so that the correction signal affects the entire chain in between and, likewise, the entire chain affects that correction.

Because of this, the total error which the feedback loop must address, including the high frequency cutoff phase error, is the sum of all the gain stages of the system, which gets a few designers in trouble for reasons already mentioned. In other words, all other things being equal, the correction speed of the global loop is lower than the local loop, because the bandwidth of the global loop is limited many times over by the multiple stages it must correct, and use to make that correction.

That does't make global feedback bad. Like anything else, you've just got to be careful about how you use it. Even though local feedback allows for more immediate correction, it also requires a separate loop for each device. Plus, there's no reason one couldn't use both (local and global) with great success, even at the same time.

Unfortunately for us audiophiles, there isn't a simple statement about the uses of feedback to evaluate the engineering merits of a particular piece of equipment. It's not simply a matter of stating that more or less is better, but rather, one must examine the specific context of the application. If one really wanted to, one could apply a battery of tests to filter out the best designs from the inept. Considering that some of the most highly touted amplifiers on the market fail miserably in something so basic as stability, it seems quite tempting.

But, to truly evaluate an amplifier purely on objective merits, one almost has to be an electrical engineer. As rewarding as that may be, not everyone can implement, let alone interpret such tests. Besides, it's so much more fun to spend the majority of the evaluation time listening to the equipment.

There is, though, a relatively easy way to get an indication of an amplifier's high frequency stability which relates directly to the implementation of negative feedback. Look at the rolloff curve. Since the phase error is proportional to the slope, an amplifier with a slowly tapered HF rolloff is more likely to be stable than the contrasting example of a rising peak followed by a sharp drop. Of course, this doesn't make any guarantees, as there are always more issues to consider, such as basic power supply characteristics for starters. But, if this stuff was easy, everyone would be designing audio equipment. Too bad, huh?

Good article!!!

Here'a another I found at the Secrets website:


http://www.hometheaterhifi.com/volum...gativeFeedback


"Harmonic Distortion

Negative Feedback

Natural sounds (not recorded) contain many harmonics, and primarily even ordered (second, fourth, sixth, etc.). It is the combination of harmonics that allows us to distinguish a trombone from a french horn when they are both playing the same note. Amplifiers produce an artifact called "harmonic distortion", which means that they create harmonics where they don't exist, or exaggerate harmonics that are already there. Even ordered harmonics (second, fourth, sixth, etc.) are pleasing (consonant) to the ear, while odd ordered harmonics (third, fifth, seventh, etc.) are irritating (dissonant). Think of it in terms of the sound of a barber shop quartet. If the three harmony parts are all singing in tune with each other and the soloist, all is well as far as the listener is concerned, no matter how loud the harmony parts are singing. This is consonance. However, if just one of the harmony parts sings out of tune, even quietly, the listener finds this irritating. This is dissonance.

Tube amplifiers, particularly the single ended type, produce even ordered harmonic distortion, primarily second order. Push pull tube amplifiers, particularly solid state amplifiers, tend to produce odd ordered harmonic distortion. If you are purchasing a single ended triode tube amplifier, 1% total harmonic distortion (THD), which means 1% of the sound is harmonically distorted, is actually pleasing to the ear, because it is even ordered. However, with a solid state amplifier, THD as low as 0.5% can be irritating, because it is odd ordered distortion. Therefore, when purchasing a solid state amplifier, make sure that the THD is no more than 0.5%. All high quality solid state amplifiers meet this specification, but you should note the fine print as to the THD in the rear channel of surround sound integrated amplifiers for the distortion factor. Often, it is larger than 0.5%. On the other hand, the bottom line in all cases, is whether the sound pleases you or not. "Specsmanship" as it is called (emphasizing the specifications on the technical data sheet supplied with a component) should not be the deciding factor. Decide with your ears, but ask to see the technical data sheet before you write the check.

Negative feedback is the process of taking a portion of the output, electrically inverting it, and then feeding it back to the input. The purpose of negative feedback is to flatten the frequency response, reduce harmonic distortion, lower the output impedance, and also to reduce the effects of "parasitic oscillation" that can occur when parts of the circuitry cause an induced current to flow in other circuits where it is not wanted. However, too much negative feedback can sometimes be used to produce an artificially low harmonic distortion specification. Second order harmonics will be reduced, but fifth order harmonics will increase, and this odd ordered distortion is much more noticeable than the original second order distortion. A tell tale sign of too much negative feedback is an excess of sibilance in the human voice (the "s" is exaggerated)."

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post #14 of 64 Old 10-22-2007, 06:40 AM
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Next up: Steve posting about Class A vs Class AB amps...

I've never a/b'ed no neg f/b vs other amps, and I would wonder whether my 'class' post or f/b type of amps would make an audible difference, and whether it could be consistently picked up in a double blind test.

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post #15 of 64 Old 10-22-2007, 08:54 AM
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Originally Posted by Steve Bruzonsky View Post

Chu, this is your opportunity to make a non-joke non-psychology objecive/substantive contribution. I am looking forward to to "hearing" your thoughts on this subject. Perhaps you don't think it makes any difference on way or the other???

I think people who make amps ask consumers to focus specifically on some small aspect of how they made them which can often obscure other characteristics that can be important. Typically, they refer to some problematic amps that were made maybe in the 70's. By focussing on only one aspect, the consumer or reviewer may overlook other characteristics. For example, what about an amp with very wide bandwidth, neglible amounts of feedback, prodigious amounts of power that is susceptible to generating audible amounts of IM distortion depending upon the source that feeds it leading to what some may call 'detailed', sharp, edingess, etc.? Just using the term negative feedback without proper qualification can lead one to limit the playing field or include products that shouldn't be in in the first place. For example, Mac amps have relatively low output impedances but utilize local feedback. So do many others.

What I think Steve is that one chooses their speakers and then finds an amp that'll drive them adequately to the volumes they choose without difficulty. If that solution is QSC, Bryston, Theta, Classe or something else, is up to you and your wallet. If you're pleased with your current choice of amps - Theta - there's no need to either bolster your choice with select advertising copy or second guess your choise with select advertising copy.

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Quote:
Originally Posted by Chu Gai View Post

I think people who make amps ask consumers to focus specifically on some small aspect of how they made them which can often obscure other characteristics that can be important. Typically, they refer to some problematic amps that were made maybe in the 70's. By focussing on only one aspect, the consumer or reviewer may overlook other characteristics. For example, what about an amp with very wide bandwidth, neglible amounts of feedback, prodigious amounts of power that is susceptible to generating audible amounts of IM distortion depending upon the source that feeds it leading to what some may call 'detailed', sharp, edingess, etc.? Just using the term negative feedback without proper qualification can lead one to limit the playing field or include products that shouldn't be in in the first place. For example, Mac amps have relatively low output impedances but utilize local feedback. So do many others.

What I think Steve is that one chooses their speakers and then finds an amp that'll drive them adequately to the volumes they choose without difficulty. If that solution is QSC, Bryston, Theta, Classe or something else, is up to you and your wallet. If you're pleased with your current choice of amps - Theta - there's no need to either bolster your choice with select advertising copy or second guess your choise with select advertising copy.

I don't disagree with you. But I take it what you are saying is that ultimately, one needs to determine the price point they want to pay, listen to amps in that price range, and determine what they think sounds better - that specs alone, such as minimizing negative feedback, etc. don't tell the whole story.
Chu, am I correct that this is what you are essentially saying.

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post #17 of 64 Old 10-22-2007, 07:41 PM
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Yes, Steve. Look, everyone has their own personal preferences for things like amps whether they're grounded in some sort of absolute reality or not. You might be a class A kind of guy, a low distortion kind of guy, neglible feedback, high efficiency, whatever. Whether those preferences are firmly grounded or it's just something that resonates with you because of past experience or marketing, they're preferences. Morbius doesn't care for Class D's because he says the square wave waveforms aren't clean. At least they weren't that clean for the Tripath approach that Bel Canto once took. Problems occur when one tries to take their personal preferences and elevate them to absolutes. There's a lot more to a forest than focusing on a birch tree. Vendors want you to focus on some aspect because they're looking to find some way to differentiate themselves. IMO, that's short-sighted.

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post #18 of 64 Old 10-23-2007, 02:16 AM
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Originally Posted by Steve Bruzonsky View Post

I can only tell ya that although I luved my Bryston 7B monoblocks, that my current Theta Enterprise monoblocks are so much more transparent, quick, real, dynamic. Just wonderful.

Could it be that your Theta Enterprise monoblocks which sounds exceptionally transparent is actually achieving this by trading off a more natural tonal balance? Like lightening of the fundamental. An obvious effect of attenuating the fundmental is the loss of lower midrange fullness-sense of body.

I use tube monoblocks and when I use NOS 12AX7 Mullard 10Ms input tubes with my Straightwire Maestro, the sound becomes so transparent and thin, but when I use the NOS GE 12AY7s with my Harmonic Tech Pro11s, the sound becomes softer at the top and richer in the midrange and there is body. There is less tension to the ears and the sound is more relaxed and open.

Anyway, I prefer tubes and I think many solid state amps are anti-euphonic No tube rolling to change the sound as you please and get stuck with what you have if it sounds overly transparent.

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Quote:
Originally Posted by Philip Tan View Post

Could it be that your Theta Enterprise monoblocks which sounds exceptionally transparent is actually achieving this by trading off a more natural tonal balance? Like lightening of the fundamental. An obvious effect of attenuating the fundmental is the loss of lower midrange fullness-sense of body.[/IMG]

You are starting to sound like Michael Fremer. Pretty soon you'll be writing for Stereophile and taking up the James Randi cable challenge!!!

More seriously, I appreciate what you are guessing at above. But you haven't heard the Enterprises, have you. so that's your guess. I'd have to tell you that the 3-4 folks who hear my system time to time and are familiar with it, all subjectively feel that my sonics are not only much more transparent than before (with the Bryston 7Bs), but simply that each audio area you could list has improved as well, across the board. Not a matter of trade offs. These amps are very, very detailed yet very natural. They don't put a glaze or mist over the detail, yet they sound natural and not overly analytical.

But as Chu pointed out, the no global feedback (Theta, in their literature posted above, accurately discusses the no global feedback, not saying no negative feedback, and simply lists it as one feature of the amps) is simply one feature in design/manufacture of the amps, you can't go on one feature to predict how good an amp will sound to you, but you must look at the price point you wanna pay and compare how amps in that range sound to determine what works best for you.

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post #20 of 64 Old 10-23-2007, 06:44 AM
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Quote:
Originally Posted by Chu Gai View Post

I think people who make amps ask consumers to focus specifically on some small aspect of how they made them which can often obscure other characteristics that can be important. Typically, they refer to some problematic amps that were made maybe in the 70's. By focussing on only one aspect, the consumer or reviewer may overlook other characteristics.

Chu Gai,

I would echo your sentiments above.

Unless you are an electrical engineer and really understand the entire design
of an amplifier; then I would just ignore these technical details as a basis for making
a purchasing decision.

As with many design techniques - it can be either helpful or harmful if used to excess.

Think of it like a medicine that your doctor prescribes for you. In some quantity, you may
require the medicine - but if you take it to excess and overdose - then that is bad.

Chu Gai is correct that in the '70s; amp manufacturers got consumers to focus on a
single spec - the Total Harmonic Distortion - or THD. One can get vanishingly small
amounts of THD by using large amounts of negative feedback. However, those low
THD figures come at a cost - large amounts of Transient Intermodulation Distortion or
TIM.

That's another example of the manufacturers getting the non-electrical engineer
consumers to focus on just one aspect of amplifier performance - to the detriment of
making a balanced decision.

If you are an electrical engineer, or someone who knows both the pros and cons and can
make an informed decision - then by all means, use that knowledge to better inform your
choices.

But if you are NOT conversant in electrical engineering and amplifier design; then do
yourself a favor and ignore the technicalities and just listen to the product.

Dr. Gregory Greenman
Physicist
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post #21 of 64 Old 10-23-2007, 06:49 AM
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Also to Chu's point, if I read him correctly, is that there is a system synergy issue with all amps, Theta included.

If you remember Steve, long before I got my Enterprises, I commented that I did not prefer my Dreadnaught when hooked it up to my main speakers - at that time Legacy Focus 20/20s. Specifically the deep bottom end seemed to be much weaker or disappear altogether. Having said that, vocals and piano and such were stunningly real. But in the end I want that deep bass that lends so much to the atmosphere and sense of space in many good acoustic recordings, so I put reinstalled my trusty Krell.

However, when I made that comment to you I hadn’t ever connected the Dreadnaught to my current main speakers, Legacy Whispers. When I did that – wow – no lack of bottom end at all, and I still got that incredible realness to boot. The Dreadnaught seemed to keep pace with the Krell in all important aspects. This is why I felt comfortable committing to the Enterprises.

So in the end, in my case especially, it was a matter of system synergy. What I observed early on with the Dreadnaught-Focus 20/20 combo was exactly what several critical people had commented on as Theta’s weakness. Yet what I observed with the Dreadnaught/Enterprise-Whisper combo is what so many people (Steve especially) have commented on as a Theta strength. Now with the Enterprises nicely settled into my system, I enjoy a natural and real presentation that I never experienced before with any combination before.

Just like Chu’s comment, I chose my speakers first, and then found an amp that worked with them.

And FWIW, when I had Brystons (either a pair of 3Bs or a single 4B-NRB) driving the Focus 20/20s, the bass was really anemic. But I believe this was more a bad speaker/amp combination than a reflection of Bryston’s lack of performance.

***EDIT*** - Amen Morbius - well said.

Dave
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post #22 of 64 Old 10-23-2007, 02:21 PM
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Quote:
Chu Gai is correct that in the '70s; amp manufacturers got consumers to focus on a
single spec - the Total Harmonic Distortion - or THD. One can get vanishingly small
amounts of THD by using large amounts of negative feedback. However, those low
THD figures come at a cost - large amounts of Transient Intermodulation Distortion or
TIM.

My general understanding of this matter, Morbius, is that the audio industry's initial transistor efforts were based on designs that had inadequate gain at high frequencies. The designs they worked with were simply narrow bandwidth that they forced to work outside of their design parameters. Feedback or not, they were phucked to begin with and never had enough phase margin. Couple that with the fact that the amps weren't being tested with realistic speaker loads, and all you had to have occur was a cable with a high enough total capacitance or a speaker which presented an increasing capacitive load at the right frequency (usually higher) and the amps turned into garbage. IMO, one way to examine an amp's robustness is to hook up a pair of Alpha Goertz cables to the amp and listen/observe what happens. Now, it's not like any of this was unknown nor did it take an Otala to describe what should've been known because it was common knowledge in the industry. So, it wasn't so much that feedback was the culprit here but poor engineering in other areas. Feedback was just the whipping boy du jour. That's why they work in audio instead of NASA.

dave7, I only have a quibble with the term synergy as it's much too San Francisco, Boulder, Colorado, new-wave-ish. Like you, I think the pragmatic approach is to pick your speakers, and figure out what you need to drive them to the volumes you want in the room you want with the program material you like to listen to. For those heretics that interpret 2 channel as 2.1 channel, the requirements on an amp may be considerably lessened.

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post #23 of 64 Old 10-23-2007, 02:45 PM
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Actually, I don't think TIM is caused by bandwidth limits but rather by slew limits. Arguably those are related, but they're not exactly the same thing. Slew limits effectively cap your bandwidth for large signals but not small ones---a decidedly nonlinear property---while bandwidth is a purely linear concept.

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post #24 of 64 Old 10-23-2007, 02:57 PM
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If you've got a slew limit, then you've also capped your maximum power over a particular range, no?

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Originally Posted by Michael Grant View Post

Slew limits effectively cap your bandwidth for large signals but not small ones---a decidedly nonlinear property---while bandwidth is a purely linear concept.

Michael,
Slew rate is closely related to unity gain large signal bandwidth...

S = 4*Pi*Vt*Ft ~ 0.3Ft
Where Ft = unity gain bandwidth and Vt = thermal voltage

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Quote:
Originally Posted by Chu Gai View Post

If you've got a slew limit, then you've also capped your maximum power over a particular range, no?

Slew rate is the rate by which the inherent capacitance of a circuit allows the voltage output of the amplifier stage to swing.
That's why it is interrelated to large signal bandwidth.
Small signal bandwidth does not depend on the slew rate.

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post #27 of 64 Old 10-23-2007, 03:55 PM
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You have it right, Andrikos. It affects large signals, not small signals. And because its effect depends on amplitude, it is a nonlinear effect, not unlike saturation/clipping.

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post #28 of 64 Old 10-23-2007, 05:12 PM
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Well let's see, slew rate is a measure of the maximum rate of change of the voltage which in turn determines the maximum frequency where the slew rate isn't limited at a particular power level. The slew rate at any point is given by...

dV/dt = 2*pi*F*V(peak)*cos(2pi*F*t)

The maximum value occurs where it crosses zero so that gives cos(0) = 1 which simplifies the above equation to...

dV/dt(maximum) = 2*pi*F*V(peak)

It is by definition a function of frequency whether that frequency is large or small but it must increase with increasing frequency for a given power into a given load. No?

I agree that slew rate is largely irrelevent for small signal bandwidth, but the amps in question weren't being driven lightly. They got into trouble when they were called upon to do their job with speakers over the 20-20K region. When limiting occured, the gain of the amp became effectively zero with the signal being modulated due to the limiting.

Are we saying the essentially the same thing here Michael or Andrikos? If not, what am I missing?

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post #29 of 64 Old 10-23-2007, 09:37 PM
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Quote:
Originally Posted by Steve Bruzonsky View Post

More seriously, I appreciate what you are guessing at above. But you haven't heard the Enterprises, have you. so that's your guess. I'd have to tell you that the 3-4 folks who hear my system time to time and are familiar with it, all subjectively feel that my sonics are not only much more transparent than before (with the Bryston 7Bs), but simply that each audio area you could list has improved as well, across the board. Not a matter of trade offs. These amps are very, very detailed yet very natural. They don't put a glaze or mist over the detail, yet they sound natural and not overly analytical.

Wow! the only SS amps that gave me goose bumps were the Mark Levinson no.33s. Must check out this Theta to see and hear how good it is.



Quote:
Originally Posted by Andrikos View Post

Slew rate is the rate by which the inherent capacitance of a circuit allows the voltage output of the amplifier stage to swing.
That's why it is interrelated to large signal bandwidth.

Is this the same as audio circuit designers talking about tuning the dynamic attack and recovery chracteristics (large signal pulse response) of both the audio circuit and the power supply as if they were inseparable?
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post #30 of 64 Old 10-24-2007, 06:15 AM
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Quote:
Originally Posted by Michael Grant View Post

Actually, I don't think TIM is caused by bandwidth limits but rather by slew limits.

Michael,

If I recall, you and I discussed this at length a couple years ago; and you are correct; the
fundamental cause of TIM, as I understand it, is slew limits.

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