Negative Feedback a deal killer?

"...Negative feed back is just like anything else. It can help or it can hurt. It depends on implementation and how it is integrated with all other technical aspects of a design. "

Absolutely true !!!!!!!!!!!!

Still, I wish to add that NF is dangerous weapon and many nmanufacturers simply don;t know how to use it and it took decades to learn its positive and negative contributions to the sound.

My favorite amplifier and I own it, Spectron bases its design on "control theory" where they treat amplifier as a "control system". They do not use large amount of negative feedback but theirs - about 10 times faster then typical amplifier...and they can control speaker with the load of 0.1 Ohm...and reproduce music, particualrly dynamic peaks more realistically then any other amp I owned or auditioned. For hard rock - ain;t better.

Hello Unsound,

I think that H2Oaudio has the full right to participate as a "private user". The reason is that he is not a designer but a person who buys mass production B&W Icepower module and installs them in his chassis (I bet makes some modifications too) and place the label "H2O" on it. Incidently ihe same is with Bel Canto, Wyred4Sound, D-Sonic and bazzilion of others buyers of ICEpower modules.

The single exception, IMO, is Jeff Rowland who while uses B&W modules yet developed his own highly sophisticated power supply and with PFC at it. My hat off to him!

Athmosphere designed his equipment form A to Z and his post is 100% accurate. He is particualrly right in: " It has also been shown that feedback, due to the time delay, actually **increases** certain distortions, namely the 5th 7th and 9th harmonics."

In his 2nd post he acknowldge that "The faster the amplifier, the less this is a problem!" He correctly noted that "... faster is not the same as 'real time' " and he is right here as well - otherwise Spectron for example would list its distortion level as zero which they are not (but very small and even order of magnitude smaller in their monoblocks but its different mechanism in force).

Also, you are mistaken, not all class D amplifiers use the same fast negative feedback as you believe. Spectron explains its method on its web-site but, for example, Bruno Putzeys (formerly of Philips) uses exactly the opposite approach and his powerful class D amplifiers (under "Kharma" name) while expensive at $25k - $30k, are said to be excellent.

Regarding what H2Oaudio wrote in relation to the capabilities of Spectron amplifiers - I copied his post and e-mailed it to them. If they want they will answer.

I believe that B&O recommends to the users of their Icepower modules not to apply them to the loads below 2 Ohms - at least until very recently so I can understand this concern... Surely, Athmosphere amplifiers, particularly since he explained the "badness" of negative feedback, cam drive such loads with ease....otherwise I will start to believe that even Athpmosphere designs are not "perfect" and he is only human ... :--) :--):--)

Cheers, life is gorgeous!

Hello H20Audio,

I have received reply from John Ulrick of Spectron - their chief designer:
============================================
Issue #1, stable driving a 0.1 ohm load. We can prove a Spectron amp will drive a 0.1 ohm load by providing lab data in the form of a oscilloscope graph of the amplifier driving a 0.1 ohm load. I will send a scope screen shot tomorrow [ I don;t need it - Dob].

Issue #2, driving the Apogee Scintilla speaker load. This is the issue of driving a low value complex load. The Spectron understand customers have chosen the Spectron amp because, for them we do drive their Scintilla speakers. There is a guy in, I think, Australia, who refurbishes Apogees that claims we do drive the Apogee speakers.
============================================
From me - Stereo Times reviewer Don Schaulis own Apogee speakers (don;t remember the model) and he gave Spectron spectacular review -http://www.stereotimes.com/amp013008.shtml

Cheers

Aljordan,

In my post I expressed NO JUDGMENT of the sound of H2O and other amplifiers which use B&O ICE Module whatsoever. I knew in moment I will do it people like you will attack me and I like to be attacked exactly the same as next guy. If you still feel offended then my sincere apologies - we really talked about differences in designs and their effect on even/odd order of ditortions as well as capability to drive difficult loads.

"...they sound different from other ICE module amps"

Here, I must take issue this this statement. Firstly, if linear power supplies used are BEEFY then most certianly this amplifier will have better dynamic range then all others based on tiny switching power supplies (again - huge exception is Jeff Rowland). So here I agree with you but in regard of the dynamic range only.

Secondly, if your amplifier is reasonably neutral and B&O modules certainly are then any change (say in PS wire or input capacitor) will produce somewhat different texture, so obviosuly each of them will sound slightly different but their main sonic signature is determined by their input/output stage...which are identical or nearly indentical (with some mods) in all of them.

Cheers

Hello Unsound,

You wrote:

"As I understand it great deals of negative feedback are often used in Class D amplifiers. One poster has already commented that since it is done at much greater speeds, the lag is inconsequential "

- which I understood that you generalize all negative feedbacks in class D amlifier. However, since you said that you do not believe that then my sincere apologies for misunderstanding.

Cheers

Hello Athmaspere,

I have received reply from John Ulrick of Spectron - their chief designer. I sent him your first post as well:
==================================
"The above comments are only relative to analog circuits. It’s true that negative feedback in slow circuits with a –Z term in their transfer function have inherent ringing and do not enjoy the benefits of negative feedback. Spectron’s forward loop does not have any analog circuits, it’s digital with a propagation delay of .2uS. Engineering requires quantitative analysis. The first order look reveals some insight. Applying super-position theorem: The period of a 20KHz sine wave is 50uS. The ratio of the wave period to the forward loop propagation delays is 50uS/.2uS = 500 In simple terms, this means that the control loop can imitate 500 control vectors at shortest wave period of the input function."
===================================
Now, if you look midrange then the ratio would be about 5000:1 and for the bass 50000:1. So, while you are correct (in your 2nd post) that fast feedback is still not in real time - I would submit to you that in practical terms you can call it "nearly real time speed" it so fast!. While John mentioned here that the control loops used in Spectron are not analog but DIGITAL - you can read it in much greater details on Spectron web site.

I will not work as a mailman here anymore - if you want to discuss NF with John or Simon - take it to them, directly.

Cheers,

Hello Kijanki,

To talk, as you did above, about Jeff Rowland (switching) power supplies and not to mention that he is first in the high fidelity audio world (to the best of my knowledge) who produced probably the most technologically advanced power supplies with fully or partially regulated PFC (power factor correction)and DC:DC converter (allowing safe high voltage headroom - thus his dynamic range is "exception" from all other SMPS in audio,I mentioned) is the same as to visit a zoo and miss the elephant there.

I hate to go personal but I feel that many of your posts are not for the discussion or observation or sharing experience but for "Gotha" purposes. Here, you missed and missed by miles.