Feedback affects Soundstage, Imaging, Transients ?

[ChPuls]

Hello,

I have experienced that if you have big feedback and therefor low distortion you have lost good soundstage or imaging reproduction. I think the keyword is transients. I think feedback destroys the ability for transients.
The german test magazine Stereoplay I think current issue July 2011 shows an ampflifier without feedback but with another trick to lower distortion.

Does somebody know more about this amp?

What do you think, guys?

[EWorkshop1708]

IMO if the amp is fast enough, it should reproduce ALL transients.

[AndrewT]

My opinion states that the amplifier must be capable of reproducing everything that gets past the amp's input filters.

[forr]

I think the keyword is transients. I think feedback destroys the ability for transients.

The transient response is what happens between two stable states.
If feedback destroys the ability to reproduce transients, it would be preferable to try demonstrate it (it would be quite easy) than to stay at a stable belief state.

[ontoaba]

The transient (if really did) should affect high freq response, and less in bass, but most differences of amplifiers sound quality is also in low (bass).
Often, oscillation start when I connect independent VAS to output buffer (with no global feedback) just because the input of buffer disturb the VAS output.
Stability is very crucial here, but the key isn't just transient, but something else, may be don't know, natural perhaps.

[WuYit]

Hi,

I have experienced that if you have big feedback and therefor low distortion you have lost good soundstage or imaging reproduction

A 100% correct observation, that´s due to time errors and disrupted phase relationships.

I think the keyword is transients. I think feedback destroys the ability for transients.

Large amplitude-high frequency-short duration signals are not transients, just ordinary music signals. Transients are distortion of oscillatory nature, often as a result of insufficient circuit response, originated from various mechanisms, some of them intimately associated with global feedback, which is the main cause of instability, reduced bandwidth, compressed dynamic range, degraded signal handling capability, slew-limiting, decreased dynamic margins, increased susceptibility to overloading, overdrive, overshoots, clipping, voltage peaks, spikes, surges and so on...giving the proper hard, cold, lifeless, unmusical low-THD-sound.

[Miles Prower]

Originally Posted by ChPuls

I have experienced that if you have big feedback and therefor low distortion you have lost good soundstage or imaging reproduction.

I've seen the same thing, but more with hollow state designs. I did a project using PP 807s for the final. Without any NFB connected, the 807s made lots of nasty higher order harmonics that became difficult to listen to for any length of time. However, the soundstage was wide open. Connecting both local and global NFB fixed the nastiness, but at the cost of soundstage. So it looks like a design trade-off here.

I think the keyword is transients. I think feedback destroys the ability for transients.

No idea if that's right or not, and not guessing. There's definitely more going on than the simplistic answer of just throwing as much NFB at the distortion problem as you can manage.

[tiefbassuebertr]

Originally Posted by ChPuls

Hello,

I have experienced that if you have big feedback and therefor low distortion you have lost good soundstage or imaging reproduction. I think the keyword is transients. I think feedback destroys the ability for transients.
The german test magazine Stereoplay I think current issue July 2011 shows an ampflifier without feedback but with another trick to lower distortion.

Does somebody know more about this amp?

What do you think, guys?

Which kind of feedback we talk about? - go to
http://www.eecs.berkeley.edu/~bora/p...8-Feedback.pdf
And how much gain stages are in the negative feedback (NFB) loop?
Read also the paper about
http://www.eecs.berkeley.edu/~bora/p...8-Feedback.pdf
and post #2097 about
Bob Cordell's Power amplifier book

[forr]

Quote

>I have experienced that if you have big feedback
>and therefor low distortion you have lost
>good soundstage or imaging reproduction

A 100% correct observation, that´s due
to time errors and disrupted phase relationships.

>I think the keyword is transients.
>I think feedback destroys the ability for transients.

Large amplitude-high frequency-short duration signals are not transients, just ordinary music signals. Transients are distortion of oscillatory nature, often as a result of insufficient circuit response, originated from various mechanisms, some of them intimately associated with global feedback, which is the main cause of instability, reduced bandwidth, compressed dynamic range, degraded signal handling capability, slew-limiting, decreased dynamic margins, increased susceptibility to overloading, overdrive, overshoots, clipping, voltage peaks, spikes, surges and so on...giving the proper hard, cold, lifeless, unmusical low-THD-sound.


As at the recording stage, the music signal already passes through some amp-ops having high global NFB, must not we understand that the soundstage is lost for ever ?

All what is underlined above is related to observable facts which could be seen even with a modest apparatus. Can you provide some convincing pictures showing them ? If not, the good soundstage and image reproduction in the absence of global negative feedback must be correlated to something else, most probably, the presence of high distorsions of all kinds.

[Bob Cordell]

Originally Posted by WuYit

Hi,

A 100% correct observation, that´s due to time errors and disrupted phase relationships.

Large amplitude-high frequency-short duration signals are not transients, just ordinary music signals. Transients are distortion of oscillatory nature, often as a result of insufficient circuit response, originated from various mechanisms, some of them intimately associated with global feedback, which is the main cause of instability, reduced bandwidth, compressed dynamic range, degraded signal handling capability, slew-limiting, decreased dynamic margins, increased susceptibility to overloading, overdrive, overshoots, clipping, voltage peaks, spikes, surges and so on...giving the proper hard, cold, lifeless, unmusical low-THD-sound.

This is just a bucketful of silly generalizations, a blind repitition of what many mis-guided souls have said and repeated over and over again. As I have pointed out in my book "Designing Audio Power Amplifiers", negative feedback has gotten an undeserved bad rap over the years. Much of this is from guilt by association - with early solid-state amplifiers that were pretty bad in many respects, but which happened to typically have more negative feedback than tube amplifiers.

It is important to recognize that there are very bad amplifier examples of every genre. People who do not know how to apply feedback properly will indeed be fully capable of producing a poor-sounding amplifier. People who start with a junk design and then expect to make it sound good by applying negative feedback will also produce a poor-sounding amplifier.

There are many things about the sound of amplifiers that we do not fully understand or are not usually measured. However, most of the things mentioned above are readily measurable and those measurements on a good feedback amplifier show that these statements are just generalizations.

Cheers,
Bob