Why do audiophiles shun feedback in amplifiers?
The amp/speaker match is why I dropped this link earlier: http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php Mapman, 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. |
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. |
Atmasphere, Your explanation of the natural noise floor compared to the 'harmonic noise floor' sounds absolutely right,here`s why. I know my former Symphonic Line and my PP tube amplifiers will measure lower distortion than the 300b SET amp. Yet when I listen to familiar music with these amps the SET in reality had the lowest noise floor for actual listening. It teveals more nuance,inner detail,ambience clues, all the subtle sounds that were either buried or not heard at all with the other amplifiers.This contrast is very apparent. Thanks for this explanation. I could hear all of this easily but did`nt understand why.This is likely a major factor why evrything sounds substantially more real and convincing with the DHT SET in place.It all makes more sense now. Regards, |
