Atmasphere, It depends how tight is tight. Acoustic bass is the tightest type because of the very longest scale. On the other side of the spectrum is Paul McCartney's Hofner Violin bass that has very short scale and horrible definition. I believe McCartney started using longer scale Rickenbacker first time in "Paperback Writer" and it shows - much tighter bass. The best recorded bass (very tight) I have is on Chick Corea "Akoustic Band" and it is upright bass. Perhaps at 7th grade you couldn't afford good instrument? |
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 |
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. |
Charles1dad, It is possible but perhaps not very common. Everything depends on definition since even single unbypassed cathode resistor is NFB. |
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. |
Learsfool, Upright bass has better "definition" related to fact that strings are, being long scale, at high tension. Sound of plucked string instrument can be defined by factors like Presence, Projection, Sustain, Separation and Tone. Upright bass has huge projection and great separation making for punchy tight sound, but at the same time has good presence and long sustain. Sound depends entirely on the player, that can play it to use projection and shorten the notes to kill sustain or can play softer because of good presence and use sustain "filling" the room with bass that reverberates. All I'm saying is that short scale bass guitar like McCartney's Hofner will sound flabby no matter how you play it, because it is extremely short scale bass with very low string tension. For the same reason Strat electric guitars sound punchier than shorter scale Les Pauls. |
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). Atmasphere, I'm not sure I understand it. IM is caused by the presence of two frequencies on non-linear element resulting in "harmonic noise floor" meaning that there will be new frequencies of small amplitude harmonically related (sum and the difference) to original frequencies. NFB reduces IM by linearizing circuit - therefore REDUCES harmonic noise floor. Very deep NFB can make it almost perfect practically eliminating IM at the cost of introducing TIM. Eliminating NFB might be possible with the inherently linear tubes but not with transistors that require linearization and regulation. Interaction with the speaker might have more to do with the type of the amplifier - SS representing Voltage Source and Tube amp representing Power Source. |
Dracule1, Read this:
http://www.stereophile.com/reference/70/index.html |
Atmasphere, Now I got it. Thanks! |
Ralph, My previous speakers had smaller woofers and smaller cabinets but bass extension was 5Hz lower. Bass quality wise it is much better now with more natural attack and decay. It sounds more even, resonating less with the room (same amplifier). I read that speaker bass can be tuned for max extension (it sells!) or for the lowest distortion. Bass now seems to be very melodic, natural and effortless while before it was a little congested. As for damping, my amp has DF=4000 at low frequencies, but it doesn't make much difference since xover inductor in series with the woofer is usually about 0.1ohm limiting DF to 80.
Dracule1, on p4 of the Stereophile article they test Cary amp with adjustable feedback and describe sound change. At the end no feedback sounded the best. It doesn't mean that amp had zero feedback. There could be still a lot of local and perhaps even some global feedback left. |
Learsfool, Term "Scale" applies to all string instruments. Please read here: http://en.wikipedia.org/wiki/Scale_%28string_instruments%29
Upright basses have 10" longer scale than bass guitar. It makes for much higher string tension* resulting in (as Dracule1 called it) "initial fast transient attack followed by natural decay and rich harmonics".
*Since string tension goes in square of length 43.3" upright bass has twice the string tension of McCartney's 30.25" Hofner bass that in comparison sounds flabby with lack of definition - poor attack, muddy overtones. . |
