Hi,
Where can we observe this Thorsten? I would be most interested in seeing evidence of this. Any links to studies backing this up with real world examples?
You can observe this using a spectrum analyser of suitable design (EMU 1616M and for Example RMAA will do) and with a suitable circuit. Or you could read Crowhursts 1950's AES Articles, which can be obtained from
www.aes.com
Sorry but that is pure conjecture surely?
Well, if you consider Otala's, Crowhursts and others articles in JAES (all peer reviewed and backed by measurements) conjecture, be my guest.
Again, without some case studies, it is purely conjecture.
Annecdotal evidence is legion, including Martin Colloms Article, among many.
I'd love to read something that verifies your statements Thorsten.
Clearly Martin Colloms fails to convince you. The link between Inverse Feedback and TIM as demonstrated by Dr. Otala, as well his demonstrations of PIM as result of the use of NFB fail to convince you, I won't add much more, sorry. If you wish to reject what has been basically established for many decades as fact, be my guest.
I think you will agree that we have been discussing the impact on speakers,
To discuss amplifiers alone, without considering the speaker they drive is ranking in irelevance right up with certain current political figures.
but pointing out that this could well be merely the result of overdamping transients because of ultra low output impedences rather then because of the NFB loop itself. It would be interesting to rig up a variable output impedence buffer and observe the effects.
Actually, I worked extensively with variable output Impedance in the 80's, including the use of negative impedance. I found that using open loop, current output (high output Impedance) amplification to drive dynamic treble and midrange units was by far the best solution, however it demands an active speaker.
I struggeled long and hard to get a bass section that matched the midrange and treble, ending up with Inverse Feedback around the woofers. Using inverse feedback there tought me much about how it works, what it's limitations are and so on.
I agree about listening but I don't think you've presented a viable case for Antonio's removal of the NFB loop as being anything other than a wild gamble that may fortuitously result in subjectively preferable results.
I never presented any arguments for it being other than a gamble. In his case (and in many others) the gamble worked out, but there is no unversal case for removing NFB, nor is there any for having it.
I noticed however that after Antonio presented his experience ("I removed NFB and liked the results") load of people jumped on to declare that thuis should never be done and cannot work and pretty much that Antonio should put it back and not speak about it again.
That is an approach I have noticed here on several occasions when views and experiences are presented that seem unpalatable to some.
And without measurements highlighting the before and after performance, it is impossible to say whether Antonio is hearing the increase in distortion. It is only possible to say that distortion from the amplifier will undoubtedly have increased considerably, and that it most certainly will not have reduced!
First, if the open loop amplifier produces 1.2% THD @ 10W I would consider it pretty low Distrotion, at least for normal listening levels using 96db/W/m efficient Speakers. As in a PP Output stage 3rd HD dominates and this goes down with the square of the reduction of level we can expect around 0.12% 3rd HD from the amplifier at an SPL from the speaker of 96db and comparably or more 3rd HD from the Speaker.
Further, the levels of this distrotion are well below anything suggested as audible by current research (eg. Geddes/Lee) which I would recommend to you for study, so I think we are reasonably safe to conclude that harmonic distortion levels are sufficiently low in Antonio's case not to play a material part in the equation.
As to reduced/increased, usually when adjsuting an amplifiers feedback levels towards zero we find HD to go up and TIM and PIM to go down.
So we trade an increase in one form or type of distortion (harmonic), which we have good evidence is in fairly high levels at low orders quite inaudible, for a reduction in other types of distortion (PIM, TIM, FIM) for which we have some formal and much annecdotal evidence as to them being quite objectionable.
In the case of Antonio the differences overall only amount to 4.8db, which is not much, but as MC found himself, as little NFB as that can cause audible consequences that are percieved negatively.
Ciao T