Mapman, That's the first thing I hated - The looks of two small coffins in glossy black (I hate glossy black). Overtime they grew-up on me and now I love them.
(It is a little bit like Eiffel Tower or glass Pyramid if front of the Louvre Museum, once called atrocity but loved today). |
Negative feedback reduces harmonic and intermodulation distortions, reduces output impedance and widens bandwidth. This can be easily measured. Negative feedback, especially a global one, can introduce Transient Intermodulation Distortion (discovered in 70's) that enhances odd order harmonics making sound bright and unpleasant. Loudness cues are in these harmonics and our ears are very sensitive to them. Transient Intermodulation can be completely avoided if bandwidth of the signal is limited at the input of the amplifier to value that amplifier had without feedback. For instance, if measured bandwidth is 30kHz without feedback then it has to be input limited to 30kHz after feedback is applied (that increased amps bandwidth). It is harder to design an amp that measures good with shallow feedback and often designer chooses to improve specifications at the cost of the sound. I would start width the best widest bandwidth most linear design and apply only enough feedback to reduce THD to 0.1%. After that I would temporarily disconnect feedback and measure bandwidth with very small signal. Finally I would limit bandwidth at the very input of the amp to this amount.
It is a tradeoff between THD, IMD and the bandwidth. If you want to lower THD, IMD by use of feedback then you need more gain and amp has lower bandwidth at the higher gain. Bandwidth will increase greatly with feedback but not limiting it at the input to original one will enable TIM.
Transient Intermodulation is practically an overshoot (in time domain) caused by the fact that amplifier, having limited bandwidth, introduces signal delay. This delayed signal is subtracted at the input reducing gain of the amplifier. For instance 40dB negative feedback means that amplifier has 100 times bigger gain without the feedback. Feedback lowers this gain to normal except for the moment of fast transition of the signal because feedback signal is subtracted a moment too late (being delayed) and amplifier has for this short moment 100x higher gain. It causes overshoot of transitions (enhanced odd harmonics) but doesn't show at sinewaves, normally used to test THD and IMD. |
Dracule1, I glanced over article and it seems to cover pretty much everything. I will read it in detail.
Stereophile also conducted experiment with adjustable feedback amplifier and they liked sound at minimum feedback. It doesn't mean that feedback at minimum was zero. It is virtually impossible to make SS amp without any feedback since even emitter resistors are form of feedback. The issue is not to use too much feedback. It comes to good very linear design to start with. Testing with sine waves doesn't show real picture (presence of TIM). You can have two amplifiers that measure identical but one is sounding very harsh/bright. Often amplifier that measures better sounds the worse. In early 70's before TIM was discovered SS amplifier's had huge negative feedback and THD in order of 0.001% (absolutely no need for that). In cases like that TIM can get so bad that output transistors choke on big overshoots (charge trapped at the junction) creating small gaps in music. Our brain fills them up but it makes us tired. I agree with Elizabeth that ZNFB is a catch phrase since amplifier with some amount of feedback will most likely sound better. NFB is a valuable tool that should not be used to cover poor design, cheap components etc. |
Many people like sound with some distortion that makes it more alive (distorted guitar vs clean guitar). My Benchmark DAC1 sounded too clean at first but it doesn't anymore (learned to listen). Many accused DAC1 of sounding sterile. When they compared different DACs in studio Benchmark was the closest to live sound but not favored by most of testers. One person expressed opinion that all instruments sound separately while he likes them more "together" (sound blob?). Reducing NFB increases distortion making sound more "alive". It also reduces bandwidth making bright systems sound more civilized. In addition it increases output impedance making impression of more bass on over-damped speakers with SS amp. All is a matter of taste. I prefer super clean highly resolving sound with very tight bass, while others might prefer some distortion or noise with rounder bass, calling sound more "organic". |
Mapman, the question here is if NFB makes reproduction MORE faithful. Some answered that there is no faithful reproduction, which doesn't answer this question, while others stated they prefer sound of ZNFB which doesn't answer this question either. The truth is, that there is no SS amp (and very few tube amps) with zero feedback. SS amp by nature is a voltage source, and as such needs some form of negative feedback, at least for the output stage. The question is how much and that might depend on the load, your preference etc. It is undisputed fact, that even small amount of NFB widens bandwidth, lowers output impedance and reduces THD/IMD distortions. Does it make reproduction more faithful on average? I think it might. What sound you like has nothing to do with original question.
I'm working on this equation, you asked for, but it can take some time. |
Edwyun, Yes, we have to realize that when they talk about zero feedback they mean global feedback. Amplifier can have many local feedbacks and they might still call it ZNFB. Why? Because ZNFB became catch phrase and sells amplifiers.
You stated the you would buy amplifier with zero output impedance, infinite bandwidth zero distortions etc. but I'm not sure everybody would. Some people would complain that it doesn't have good bass with their speakers (zero output impedance), others would call sound analytical and sterile (lack of distortion and zero noise) while some would call it bright (infinite bandwidth) since their system has tendency for it. In addition many people like warm sound (enhanced even harmonics) and would not be happy with this amplifier.
Shallow global feedback, properly used, can reduce distortions, widen bandwidth and lower output impedance WITHOUT introducing Transient Intermodulation.
As for getting rid of the equipment, I think you're right. For the money some people constantly spend on audio they could hire Symphony Orchestra to play for them. |
Dracule1, What I'm getting from this is that some amount of imperfection can sound to many people more natural. Difference between natural, pleasing and faithful is fluid. Article was very interesting and thank you for posting it. I would only disagree with notion that deep feedback has no negative effects if amplifier is stable and input limited to slew rates equivalent to 20kHz. In ideal world it might be true but bandwidth never ends at perfectly 20kHz (TT often go to 50kHz) while high gain amplifiers, including op-amps have, (in spite of compensation) tendency to ring when presented with capacitive loads (speaker). It is big temptation for designer when he can improve everything 10-fold just by introducing more gain. I know that top designers like Nelson Pass or Jeff Rowland don't take shortcuts and that's why I bought Rowland class D amp. I know that modules were designed and made by B&O but Jeff Rowland name is enough for me to get interested. Some amplifiers are very expensive but you get what you pay for (Brand name). My amazing Hyperion HPS-938 speakers were a bargain but company doesn't exist anymore (so it seems). Now I worry what to do if something happens to these exotic drivers since nobody else makes them. Next time I'll pay more. |
Edwyun, this particular design (Icepower) uses Nielsen Karsten multiple feedback. One feedback comes from modulator while the other is from the speaker output. Class D is relatively linear to start with, while output impedance is inherently low hence it doesn't require a lot of feedback. In addition it is only one stage (modulator) making for small signal delay - less chances of TIM. Early class D consisted of primitive sawtooth modulator while this resembles sigma-delta A/D converter's modulator. There is nothing wrong with class D (SACD,DSD are class D) but carrier frequencies are still too low (Mosfets are getting faster every year). For the money it is bargain IMHO. |
Edwyun, this particular design (Icepower) uses Nielsen Karsten multiple feedback. One feedback comes from modulator while the other is from the speaker output. Class D is relatively linear to start with, while output impedance is inherently low hence it doesn't require a lot of feedback. In addition it is only one stage (modulator) making for small signal delay - less chances of TIM. Early class D consisted of primitive sawtooth modulator while this resembles sigma-delta A/D converter's modulator. There is nothing wrong with class D (SACD,DSD are class D) but carrier frequencies are still too low (Mosfets are getting faster every year). For the money it is bargain IMHO. |
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Mapman, Woofers have only one invention - lack of suspension (spiderweb) compensated by large diameter voice coil mounted to flat disc in place of dustcap. It makes membranes lighter, faster and stiffer. Midrange (6.5") uses, in addition ferrofluid as a suspension. Midrange is absolutely breath taking. Tweeter is so well integrated that I cannot really tell transition from midrange.
http://hyperionsound.com/Images/HPS-938%20review.pdf |
Atmasphere, It make little sense to seek very high DF for better damping (assuming that speaker needs it) when inductor in series with the woofer has most likely resistance of 0.1 ohm limiting DF to 80 even with perfect amplifier. |
