No. I'm not engineer, but what I believe is that NFB is a tool that can be used by good engineers to do very good things in an amplifier design. That doesn't make it a panacea, but it doesn't make it a bad thing either.
Tim
Measurements and the Correlation of What We Hear
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No. I'm not engineer, but what I believe is that NFB is a tool that can be used by good engineers to do very good things in an amplifier design. That doesn't make it a panacea, but it doesn't make it a bad thing either.
Tim
We can achieve all that without feedback,
"The other disadvantage of a negative feedback amplifier is that the transition from clean to distorted is much more abrupt, because the negative feedback tends to keep the amp distortion to a minimum until the output stage clips, at which point there is no "excess gain" available to keep the feedback loop operating properly. At this point, the feedback loop is broken, and the amp transitions to the full non-feedback forward gain, which means that the clipping occurs very abruptly. The non-negative feedback amp transitions much more smoothly into distortion, making it better for players who like to use their volume control to change from a clean to a distorted tone."
When properly used NFB can be useful.
"One of the reason tubes are best for voltage amplification is they’re natural high-voltage devices that act in a linear fashion. The circuit itself is a simple, 2-stage, class-A triode driver stage. The first stage is open-loop with no negative feedback, just like in many preamplifiers. The output of the first stage, summed with negative feedback from the output, feeds the second driver stage capacitively-coupled to the MOSFET power module. The two-stage design provides the voltage swing needed for the powerful 200W/ channel 402Au."
You get in trouble when you use excessive amounts to cure a bad design.
All of that is interesting enough, but I don't know what it has to do with your answering a quote about negative feedback flattening and extending FR and lowering distortion with "And the point would be that excessive feedback colors the sound." It may or may not color the sound, but the certainly wasn't the point.
Tim
I tried to give an an example of how NFB helps and hurts. Hard clipping and overdamping the bass were two examples of "hurt". I think either I mistataed or you misunderstood.
Sorry, talking as an engineer that's a ridiculous way to deal with providing a design for some mechanism. It's equivalent to being asked for a road bridge concept, and offering up one which slowly sagged in the middle as more and more traffic crossed it, until the road deck slowly made contact with earth when the maximum allowable weight was reached. The only intelligent way of designing an accurate amplifier is to have it distort as little as possible until design limits are reached, and then all bets are off. If you deliberately exceed these constantly it only indicates that you should have bought a more powerful amp in the first place.
The major problem that people have with accurate amps is that it exposes weaknesses elsewhere in the system. If you hear these, then it's a case of either fixing up the weaknesses, or hiding them ...
Frank
Tom-What my friend said and what the RDH said are in 100% agreement. The cathode follower is not a source of feedback to the preamp circuit that has to be accounted for. Like I told you, pick out one tube preamp that uses a cathode follower and the designer is still alive and I will write him and ask him how much feedback his preamp circuit contains. You would think that the guy who designed his circuit would know how much feedback is there.
No, I think I just disagree. I have a problem with clipping, period. I don't want my hifi amp to "clip soft," I want sufficient headroom so it won't clip at all. If it is designed to extend that headroom far enough, "how" it clips is of no consequence to me. I'm not going there. Similarly, what is "over-damped" in a guitar amp doesn't relate well to speaker control in a hifi system. Many players, myself included, want guitar amps that play "loose." They clip slow but easy. They don't have particularly good speaker control. They distort, a lot, on purpose, and they do so in such a way that you can control the amount of distortion by playing them on the edge of clipping where just a harder pick attack can drive them into a bit of crunch, and a little of the guitar's volume control can push them into overdrive. Transient attacks don't get louder, they just get gnarlier.
It's exactly the opposite of what you want from a hifi amp.
Alternatively...and it hurts me to say this
...what Frank said.Tim
Thats not s design philosophy. It's a description of clipping with feedback vs NFB amplifiers. Lot's of bridges have weight limits. Get out and drive the back roads. Yes a bridge that gradually sags when it weight limit is ececeeded is infinitely prefrable to one that fails castostrophically.
Sorry, Greg, you misunderstood: the bridge starts sagging, in my example, well BEFORE the weight limit is reached. Unless I misunderstood you, that is the implication of what you stated, a quote which, as Tim clarified, is a description of how guitar players can prefer their amps to behave, as a music CREATING mechanism. We want (perhaps!!) our audiophile system to perfectly reproduce how Tim made his guitar amp gracefully distort ...
Frank
Well, you probably want it to reproduce a better guitarist, but the point is well taken. The objective of a reproduction system should be to never over-stress that bridge in the first place. Unless you like the sound it makes as it slowly fails.
Tim
By definition a bridge that "slowly sags to the ground" has exceeded its weight limit the moment it starts to sag. Leaving our bridge analogy aside and my apology for forcing you to agree with Tim..
As for Tim's claim that an amp should have sufficient power to drive a speaker. That is as valid as the claim that a" competent amplifier design should not require NFB." See the Rogers Sanders discussion of amplifier clipping in the technical section. In the world of mix-match speakers and amplifiers, they do clip. Certainly the concept of soft versus hard clipping has been widely discussed in audio.
For example Steve's Lamm with it's 3% distortion and 32 watts. With the right load could be driven to clip. It has variable feedback. Vladimir thinks it degrades the sound.
Also See Myles Post #379
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And, an amplifier intended for accurate reproduction has started to distort, it is no longer correctly doing its job.
Is that somewhat like a bridge failing catastrophically?
I just don't see it this way. My experiences have led me to believe that real clipping is very rare unless you're using your system in party mode. And that's not a smart thing to do, especially if you value your tweeters. For normal listening, the vast majority of audio enthusiasts will never experience clipping, and if it happens it will be so transitory you won't notice it, assuming decent design of the power supply.
As a comparison with Steve's setup, his speakers are about 6dB more sensitive than mine, and his amp is twice as powerful. Yet I've had my HT system effectively flat chat, and the volume is subjectively enormous, but there is no sense of any clipping. I know on many, many normal systems compression comes in very early, much too early, but that's a different thing. The Lamm appears to have a good power supply, so I'm sure he never hears it suffering any problems, of course if he tried to drive some electrostatics he would hear the strain, but you don't try to drive a tank over a footbridge ...
Frank
Frank, I'm a bit shocked that someone so focused on the small stuff would ever come to such a conclusion. You think transitory clipping is inaudible but you can hear a power cord?
Tim
Yes, because the key word in my previous post was "transitory", the actual clipping is a mere blip, a momentary glitch in the replay. There are 2 analogies to this: firstly, there is a lot of music out there that has clipping recorded as part of the track, I have one of Beethoven's Ninth, recorded by a highly respected organisation, with quite severe, momentary clipping in a number of places, and I have never picked the point of overload in replay; and also at one point looked at a track posted by Bruce, of what he considered to be high quality jazz: same story, there was clipping at 2 points. This is using Audacity, an easy to use software tool. So, clipping of the recording vs. clipping of the amplifier: there should be almost no difference in the perceived effect, IF the amp is engineered properly.
Secondly, scratched CDs clip a fair bit, as in they have a split second dropout of sound at a number of places. Does that get in the way of enjoying the sound of the album in an overall way?
The power cord thing is completely different, because the effect it has is constant, the slightly debilitating influence it casts on the system remains there throughout the track, it persists, on and on and on: the "distortion" it adds just grinds you down, because it never goes away. Much better a short, sharp pain than a persistent low level headache ...
Frank
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