The amp/speaker match is why I dropped this link earlier: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpMapman, if you have speakers that your class D amps are really happy with, there is the chance that those speakers may not be compatible with tubes. Dracule1, 'tightness' in the bass is not a function of real music but is a function of an over-damped speaker. IOW its an audio system artifact. Kijanki, I usually do not include degenerative feedback when I make my comments about the negative effects of loop feedback. This is because degenerative feedback occurs simultaneously with the signal i.e. there are no propagation delay issues. I think it should be pointed out that the use of negative feedback to reduce IM distortion is a bad move. We get very low IM figures without using feedback- IM has a lot to do with power supply design, grounding and parts quality. I am of the opinion that is much better to design the amp to have low IM operating open loop, since IM will occur at the feedback nodes of the amplifier, resulting in a harmonic noise floor (rather than a natural hiss noise floor caused by component noise). The human ear can hear about 20 db into a natural hiss noise floor, but if there is a harmonic noise floor caused by IM (with harmonics and in-harmonc distortions extending to the 81st!), the ear will not hear below that point at all due to the way the masking principle works. This is one way to get more detail out of an amplifier, since a harmonic noise floor will block the ear's ability to hear detail below that point. |
Mapman,
I`d agree with your thoughts about appropriate amp for your Triangle monitors.My path was simplified by matching amp and speaker by the same builder. His objective was cbviously very good compatibility.
Regards, |
Charles,
I'd say pretty much the same thing with the ClassD Icepower BelCanto ref1000m amps in my system, used with any of my speakers, especially the OHMs and Dynaudios.
My small Triangle Titus XS monitors, which are more efficient and known to be more tube amp friendly, are the ones where I think I could improve the sound with another better matched amp. I do not think use of negative feedback or not alone would be the main factor to determine which amps would work best with those, though I am sure some that do not use extensive NF would sound very good indeed.
I have a pair of Tube Audio Design Hibachi monoblock amps that do a very competent job with the OHMs and Dynaudios (though I prefer the Bel Cantos with those overall), but are probably a better match to the Triangles on paper. |
Mapman,
In my experience the answer to your question is no. The one quality I`ve come to admire over time with my SET is honesty. What ever is the inherent sound character of the CD that`s what I`ll hear,flaws and all. Some SET amps could perhaps'pretty up' the sound but my does`nt do that. I have some recordings that are thin,brighter and in some cases edgy and that`s how they will sound.But those that are recorded well,oh my goodness.
My SET has less editorializing than all of my previous amplifiers. I reconized this aspect early on, It just seems to preserve the signal with minimal influence.The effect is heard as pure and unfiltered.
Regards, |
Can an amplifier with no or minimal negative feedback in play make a CD recording that might be irritating otherwise normally not irritating? Mapman, you basically asking if it is possible to make amplifier bad other way than NFB. I'm sure it can be done 100 ways from bad power supply to wrong bias current. NFB is producing TIM only when overused. We're talking global NFB since local ones are almost always there. Transistor amplifier output stage represent voltage source. For that it needs to be regulated = NFB. |
So here's a question.
Can an amplifier with no or minimal negative feedback in play make a CD recording that might be irritating otherwise normally not irritating? Not all recordings are created equal. SOme are more irritating than others for various reasons. I'd say only a very small % of my thousands of CDS, maybe a dozen or less, mainly in the hard rock/heavy metal or a few electronic dance genre MP3 tracks, sound as Atmasphere describes a negative feedback amplifier in practice. I listen to all my CD tracks queued up randomly, so I do not know what I will hear next, yet there is that occasional REM or MEtallica CD that will play and be hard to listen to, but most of the rest seem to sound more like what Atmasphere describes for non NF amps. SO I am not hearing the problem at least, FWIW.
THis is the case even with my Dynaudio Contour monitors, which tend to have a significantly hotter sound running off my gear in their room than say my more laid back OHM Walshes in others. |
I have two push-pull class AB amplifiers. One is 40 watt/ch with EL34 tubes and the other is 100 watt/60 watt(UL/Triode).The bigger amp is one of the best PP amps I`ve heard it uses 6550/KT 88/KT90 etc. This was my main amp,top choice for driving my Coincident speakers. Three years ago I purchased my 300b SET 8 watt amplifier.From day one fresh out of the shippng carton this SET put the others into storage. It was simply better across the board.
The 100 watt amp has more bass weight and impact(the gap is`nt large however).In terms of tone,harmonics,noise floor,presence,3 dimensionally,nuance and dynamic ebb and flow the SET is superior.When playing high energy big band jazz ,the ability to clearly seperate and hear the individual instruments is extraordinary.The scale,energy and dynamics completely fill my room, it really as if the walls vanish(I`m right there at the venue,spooky real). In the sense of realism,believability and natural sound it`s the best I`ve heard so far.Some SET amps are better than others.The better quality DHT SETs are the most live-like,realistic and trueful amplifiers I`ve ever experienced. This was a profound moment for me as it put me into a different sonic realm,there`s no going back.I`ve own SS (Symphonic Line amplifier) it was`nt close to this.My 100 watt PP was better than it in terms of live flesh and blood presence.The 300B SET is a new standard however.
Just an example of how people have different experiences and impressions. The good news is there`s an amplifier topology out there for everone. SET,PP,OTL,SS,class D, take your pick.What I have discovered with SET I know others have experienced with other types of amplifiers also.
Regards, |
Thanks everyone for the informative responses. In regards, to speaker/amp pairing, I have three sets of speakers, two pairs are dynamic and one pair electrostatic. In all three of these, overall I liked feedback over none.
As for DHT amp using no feedback, I have yet to hear one that has the dynamics, bass extension and tightness of a KT88/6550 using feedback...that is after taking account power output of the amp. But good DHT no feedback amps have glorious midrange, though not accurate IMO.
Regarding odd order harmonics being irritating and having increased perceived loudness, this is very difficult to pin down in one's audio system among all the other forms of distortions inherent down the chain, unless you are willing to deliberately increase the odd order harmonics in your amp and listen for the change. What I thought was irritating odd order harmonics arising from my amp using feedback was actually distortion arising from the source and boundary effect from room acoustics. |
"I have found amps with less or no feedback sound loose and diffuse with less dynamics... I know you should design am amp with excellent open loop gain before applying feedback. I can see the use of no negative feedback for low level amplification (eg, preamp and gain stage of CDP or DAC). So why this myth perpetuated by audiophiles and even many manufacturers?"
I don't see why you would say that 0 feedback components better SQ is a myth. If you can, try to audition some Ayre products. That will probably change your mind. (There's other great sounding 0 feedback electronics other than Ayre. I recommend Ayre because I feel that they are the best example. |
Kijanki,
I believe your right in that the vast majority of amplifiers use some degree of NFB.I think DHT SET amps are a small niche that can be the exception.
Regards, |
Charles1dad, It is possible but perhaps not very common. Everything depends on definition since even single unbypassed cathode resistor is NFB. |
I don`t agree that 'every' amplifier has NFB. There are some DHT SET amplifiers that lack NFB in their circuit.Some of these types of tubes are linear enough so that it is`nt necessary.My amp is one example(unless Israel Blume told me a fib,and I doubt that).
Regards, |
It is not only 2nd harmonic, but also Intermodulation Distortion, Bandwidth, Output Impedance - practically everything. The key to avoid TIM is to limit bandwidth at the input to one that amp had before feedback (that extended bandwidth) was applied. This will be OK if amplifier had wide bandwidth to start with, otherwise reduced bandwidth will cause phase shift at higher frequencies and wrong summing of harmonics. Also, as I mentioned before, every amplifier has some form of NFB. |
Dracule1,
I don`t agree with the myth description.It just depends on the amplifier`s chosen output devices and the particular speaker being driven.What good is it to reduce 2nd order harmonics to extreme levels and then replace it with odd high order harmonic distortion(worse),that`s a bad trade off.A tiny amount of this NFB odd order distortion is not natural to human hearing and is processed as irritating and artificial.
Some speakers do require amplifiers with NFB. I`d prefer speakers designed that don`t 'need' these types of amplifiers in the first place.Amps without NFB sound more natural and realistic in my own experience, YMMV.
Regards, |
Well there's a couple years of intense study in electronics engineering in a nutshell! I'm looking forward to a lot more analyses geared toward practical utility. Am I asking too much? Lets keep this ball rollin guys. Nice change. Is that a lyric? |
You might say that the use of global negative feedback causes the amplifier to violate one of the fundamental rules of human hearing/perception: how we determine how loud a sound is. We do that through analyzing the odd ordered harmonics rather than processing fundamental tones. So if the amplifier has trace amounts of the 5th, 7th and 9th harmonics added, it will not only sound louder than real music of the same volume, but bright/less relaxed as well. This is one reason why two amps can measure perfectly flat with the same bandwidth on the bench, but one will sound bright and the other won't. As Kijanki pointed out, global feedback reduces 'output impedance' (I put the term in quotes because it is a definition that is only used that way in audio!). So amps without feedback will have a higher output impedance. If coupled with a speaker that demands a lower output impedance of the amp, tonal aberrations may result. So you can see that a more ideal combination might be such an amplifier with speakers designed to work with higher output impedances. Then you get proper tonality coupled with no violation of human hearing rules. For more information see: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php |
Kijanki explained it pretty well as I understand it.
The argument against negative feedback is based on it being an imperfect process that introduces time delays and artificial harmonics accordingly as explained and that this inherent negative can outweigh advantages of applying negative feedback otherwise.
Audiophiles hate anything that is not perfect so being inherently impossible to implement perfectly, negative feedback is a common target of ire. |
Dracule1, feedback is a very good think if you know how to use it. It improves practically everything - reduces distortions (THD, IMD), lowers output impedance*, widens bandwidth. The only problem is that amplifier presents delay to signal. When you feed this delayed signal back to input stages it doesn't subtract properly - especially when signal is changing fast. It causes overshoot after transition since gain for tiny moment was higher (feedback was late = open). It is called TIM (Transient Intermodulation) This overshoot in time domain is equal to extra odd harmonics in frequency domain. Since odd harmonics carry loudness clues we're very sensitive to them and hear it as bright unpleasant sound. In extreme case when amplifier has very deep global feedback fast transitions might even choke output transistors that stay that way for a moment (charge trapped at the junction) resulting in tiny gaps that are inaudible since brain "fills the gaps" but makes us tired. It sounds unlikely but they designed SS amps like that in 70s before TIM was discovered.
Every amplifier has some form of feedback but it is better if it is local (around one stage) in few places than one global going from the back to front. Also sane designer would make amplifier as fast as possible to reduce delay and would limit bandwidth at the input. I would also be very careful with amount of feedback - tiny overshoot might become 10x higher with 20dB deeper feedback. Reducing harmonic distortions, by the feedback, below 0.1% might not be audible but brings risk of TIM. Also reduction in output impedance might not be necessarily a good thing if your speaker is already over-damped. Class AB amplifiers often have a lot of feedback to linearize output transistors and keep gain constant when both transistors are conducting (gm doubling).
Preamps most likely have stages with local feedbacks. Bandwidth is also easier to obtain with practically no gain and no power stage.
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* - Some time ago I made example of feedback reducing output impedance:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1315855364&openflup&21&4 |
I have found amps with less or no feedback sound loose and diffuse with less dynamics... So why this myth perpetuated by audiophiles and even many manufacturers?
Dracule1 (Reviews | Threads | Answers | This Thread)
You did not hear the amp sound loose, you heard the amp paired with a specific pair of speakers sound loose. With a different pair of speakers you may have heard something different. |
