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Why Zero Feedback


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Hello all. It has been well documented by RV and others on this forum that the Vandersteens work best with a zero feedback design amp and at the suggestion of my dealer this is what I am using and love the sound. Could someone (RV?) please explain to me why this is so important? A somewhat technical explanation would be appreciated. I'm just trying to increase my knowledge of the principals involved.

Thank you.

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Nothing like a simple question!

Perhaps it would be worthwhile to review what feedback does before going into why zero loop feedback is a good thing.  If you really read the pro-feedback arguments, you can also see its limitations.

Here is a good source of links:

https://linearaudio.nl/feedback-feedforward-error-correction

And, here is a good book:

https://linearaudio.net/books/2220

Personally, I think there's three key aspects of this that are usually ignored.  (There's others, too.  But I'll only mention these.)

One is that what you generally need to do to get good measured results when using feedback is counter to retaining the original spectral content and balance of the signal you want to amplify. The big thing is adding loads and loads of open loop gain to the amplifier.  That is the gain you "throw away" when you use feedback as a corrective measure.  But, almost always this extra open loop gain requires circuit compromises that really aren't linear in nature.  This adds new kinds of distortion products.  It's tradeoff between low distortion levels and what the new distortion products are like.  You want lots of open loop gain?  Fine!  It won't always be pretty.

That might not be a good thing at all for replicating music and other sounds.

The second aspect is that a lot of designers really believe that feedback fixes everything.  For example, they use it to allow for poor power supply performance.  The problem with this is that the effect of feedback only works where there's lots of feedback.  There's very little in most amplifiers outside the audio band, but there sure is power supply junk outside the audio band that can and will affect the audio band.  (Intermodulation distortion and other effects.)  There's also the tendency to not work at making the basic open loop performance of an amplifier very linear. Feedback fixes everything, right?

Third, the distortion products that are generated within an amplifier with feedback are not always in the original time relationship as naturally occurring distortion products generated by the musical instruments and other sound generators.  You can even simulate this through computer modeling.

One of the big things about Vandersteen loudspeakers is that the harmonic content of recorded sound arrives to the listener in the same time relationship as was originally recorded.  You can't perform that magic with loudspeaker drivers that are breaking up and crossover networks that actually force content to not be in the same time/phase relationship as the original sound.  Adding in distortion products from the electronics that disrupt this may not be what you want.

I won't suggest that there's no place for feedback.  There's lots and lots of real world applications where feedback does a great job and makes the almost impossible possible. (Full disclosure - I've spent much of my working career employing feedback in circuits, whether it's amplifiers or control systems. )

My own philosophy when interrogated about this has always been that if you think absolute distortion levels are the big important part of an audio system, why give me, ahh, grief if I attain that by using good components and good design ideas that don't use feedback?  If I can get -100 dB distortion (to pick an arbitrary number) with zero feedback, why is that a bad thing?   

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32 minutes ago, BKDad said:

Nothing like a simple question!

Perhaps it would be worthwhile to review what feedback does before going into why zero loop feedback is a good thing.  If you really read the pro-feedback arguments, you can also see its limitations.

Here is a good source of links:

https://linearaudio.nl/feedback-feedforward-error-correction

And, here is a good book:

https://linearaudio.net/books/2220

Personally, I think there's three key aspects of this that are usually ignored.  (There's others, too.  But I'll only mention these.)

One is that what you generally need to do to get good measured results when using feedback is counter to retaining the original spectral content and balance of the signal you want to amplify. The big thing is adding loads and loads of open loop gain to the amplifier.  That is the gain you "throw away" when you use feedback as a corrective measure.  But, almost always this extra open loop gain requires circuit compromises that really aren't linear in nature.  This adds new kinds of distortion products.  It's tradeoff between low distortion levels and what the new distortion products are like.  You want lots of open loop gain?  Fine!  It won't always be pretty.

That might not be a good thing at all for replicating music and other sounds.

The second aspect is that a lot of designers really believe that feedback fixes everything.  For example, they use it to allow for poor power supply performance.  The problem with this is that the effect of feedback only works where there's lots of feedback.  There's very little in most amplifiers outside the audio band, but there sure is power supply junk outside the audio band that can and will affect the audio band.  (Intermodulation distortion and other effects.)  There's also the tendency to not work at making the basic open loop performance of an amplifier very linear. Feedback fixes everything, right?

Third, the distortion products that are generated within an amplifier with feedback are not always in the original time relationship as naturally occurring distortion products generated by the musical instruments and other sound generators.  You can even simulate this through computer modeling.

One of the big things about Vandersteen loudspeakers is that the harmonic content of recorded sound arrives to the listener in the same time relationship as was originally recorded.  You can't perform that magic with loudspeaker drivers that are breaking up and crossover networks that actually force content to not be in the same time/phase relationship as the original sound.  Adding in distortion products from the electronics that disrupt this may not be what you want.

I won't suggest that there's no place for feedback.  There's lots and lots of real world applications where feedback does a great job and makes the almost impossible possible. (Full disclosure - I've spent much of my working career employing feedback in circuits, whether it's amplifiers or control systems. )

My own philosophy when interrogated about this has always been that if you think absolute distortion levels are the big important part of an audio system, why give me, ahh, grief if I attain that by using good components and good design ideas that don't use feedback?  If I can get -100 dB distortion (to pick an arbitrary number) with zero feedback, why is that a bad thing?   

Seriously glad you are here.  Man do you bring up salient points for us to think about when looking at various aspects of the system etc...  It's great that we can all discuss pros and cons and that Richard will chime in etc... I've learned so much more about my system and audio in general that I never got on Audiogon or other sites.  Too many bullies and trolls.  

The feedback question always comes up in my conversations with many about time and phase correctness (usually among my Wilson buddies as they claim time and phase correct and put neat adjustments into their speakers on the top end, but they aren't.   That doesn't always matter to some folks who don't hear the difference, but it's probably why 90% of Vandersteen owners have them). 

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The thing is, except for idiots like me who like to tinker and the good folks who invent these products, in the end I personally think people should just purchase and listen to gear they like the sound of.  OK - they should also be interested in the reliability and service practices of the products they buy.  The rest is Inside Baseball kinda stuff.

That whole thing about informed consumers often gets manipulated by people who want to sell stuff!  Just yesterday I was reading a "white paper" about some fancy cabling.  Utter rubbish!  The thing is, lots of sane people have reported that this cabling really does work well.  So, why offer up explanations that make the products appear to be snake oil?  I'll never get that.

Again, I know that this sounds like apple polishing, but one of the aspects I respect about Vandersteen Audio is that the product explanations are based on good principles and are straight forward.  I don't feel queasy after reading them.  At all.

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So true.  It's why it was so easy to spend the money on a Basis TT.  AJ was the same way as was Charlie (Ayre).  Physics can't be changed (that I know of)...Oh, and Garth at AQ is the exact mindset.  it's not difficult to part with my funds on products like these and most here seem to agree to some degree.  

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21 hours ago, BKDad said:

The thing is, except for idiots like me who like to tinker and the good folks who invent these products, in the end I personally think people should just purchase and listen to gear they like the sound of.  OK - they should also be interested in the reliability and service practices of the products they buy.  The rest is Inside Baseball kinda stuff.

That whole thing about informed consumers often gets manipulated by people who want to sell stuff!  Just yesterday I was reading a "white paper" about some fancy cabling.  Utter rubbish!  The thing is, lots of sane people have reported that this cabling really does work well.  So, why offer up explanations that make the products appear to be snake oil?  I'll never get that.

 

An interesting and quite valid point. It's hard to envision an audiophile world, however, where this would be the rule. Common sense is a rare commodity.

 

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21 hours ago, ctsooner said:

 It's great that we can all discuss pros and cons and that Richard will chime in etc... I've learned so much more about my system and audio in general that I never got on Audiogon or other sites.  Too many bullies and trolls.  

 

Agreed. The Vandersteen Forum is the best.

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22 hours ago, Richard Vandersteen said:

It's not complicated.  Feedback is time distortion.  Vandersteen's are Time and Phase Correct.  Having said that tube amps typically use moderate feedback so maybe that is a key.  RV

Makes perfect sense! Thank you. I thought it might be more complicated than that involving impedance curves, voltage, current etc.

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Well, there is at least one dude on Audiogon that thinks I am a troll 😉 

I was super fortunate early on to work at a hifi shop that carried a low NFB SS amp designed using new research on Transient Intermodulation Distortion ( TIM ) by Dr Otola. It was a small 70 wpc affordable amp that could run circles around a LOT of amplifiers of the day that measured better…here is the kicker…. with SINE waves, music is NOT a sine wave.

In my simpleton mind, NFB is where you take something that already happened, flip it around out of phase, reduce it somewhat ( otherwise you get no gain ) and then feed it back aka mix it with a signal happening NOW. Since with sine waves then = now… no harm…. Ah but music is not a sine wave. 

Recently I had a nice 2 year run with a variable NFB tube amp the Music Reference RM-9. It has three is NFB ( aka gain ) settings that w careful level matching allowed for an excellent learning experience and a LOT of discernment about sonic differences between the 3 levels of NFB. Image depth was greatly impacted. Now imagine running that same experiment with a brand X speaker w the midrange wired out of phase… argh, no wonder that many believe time and phase don’t matter… 

but don’t overthink it….most of your great Vandy dealers can demo low NFB amps AND time and phase and pistonic speakers…. from the ONLY company on the planet doing that 😉

ok, back to trolling….. for salmon….

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4 hours ago, TomicTime said:

 

Recently I had a nice 2 year run with a variable NFB tube amp the Music Reference RM-9. It has three is NFB ( aka gain ) settings that w careful level matching allowed for an excellent learning experience and a LOT of discernment about sonic differences between the 3 levels of NFB. Image depth was greatly impacted. Now imagine running that same experiment with a brand X speaker w the midrange wired out of phase… argh, no wonder that many believe time and phase don’t  matter.

How cool an experiment that must have been.

On a somewhat related topic (time and correction) I have often wondered about some of the turntable designs that utilize feedback to correct for speed anomalies. To my mind the speed corrections that are made based on the feedback must take place at a different point in time and would therefore be incorrect at that point. Hmmm.

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7 hours ago, TomicTime said:

Well, there is at least one dude on Audiogon that thinks I am a troll 😉 

I almost feel sorry for the fellow.

a workmate back in the 90s was asking for advice in dealing with the fairer sex. 
I told him, “If you find yourself talking more than listening, then that is a sign to shut up and start listening.”

One could p’rolly shift that into a forum discussion with experts in their field… 

 

7 hours ago, TomicTime said:

In my simpleton mind, NFB is where you take something that already happened, flip it around out of phase, reduce it somewhat ( otherwise you get no gain ) and then feed it back aka mix it with a signal happening NOW. Since with sine waves then = now… no harm…. Ah but music is not a sine wave. 

Ralph talks about the gain bandwidth product and the “Flip it round, and add it back in” part is very effective a low frequency… the phase angle error is close to zero.
But at higher frequencies it can not work out as well

the modern class-D seem to be really a lot better with feedback than the legacy Class-AB topology.

 

7 hours ago, TomicTime said:

but don’t overthink it….most of your great Vandy dealers can demo low NFB amps AND time and phase and pistonic speakers…. from the ONLY company on the planet doing that 

There is time/phase, pistonic drivers, and some of the modern super linear motors.
On the simpleton mind front, I would like to see all three coincide. It seems like it could be a trifecta where the speaker distortion could be low enough to make the amplifier really important.

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good systems engineering thinks of the speaker and speaker wire and high pass amplifier and subwoofer restoration amplifier also known by RV as “ unfair advantage “ , as a SYSTEM….. ponder this for a bit.  Throw in the room below 120 HZ, and nobody else is doing that in the analog domain….

 

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@gsal it was fun and Steve Edwards on this forum continues the RM-9 experiment to this day. 🙂

Great thought on speed, i am biased and feel a high mass rotational system w a direct drive motor with a very light touch on voltage correction sounds pretty darn good with a minimum of maintenance….. hence my choice of a Brinkmann Bardo, but there are obviously MANY other great sounding tables and approaches…… God give us tge time, money, and inclination to muck about with them ALL in the service of music 😉

Jim

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I'm going to find that link (Jim, please send me a link via text if you can). I have to read this guys stuff.  I dont' feel badly for folks who try to start crap behind a keyboard.  He was sure full of himself and made sure we all knew he did 2 years of engineering and is a spinal surgeon.  I asked a buddy who is also a spinal surgeon and he said most aren't such jerks (my Dr actually has a really nice system).

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11 hours ago, Gsal said:

How cool an experiment that must have been.

On a somewhat related topic (time and correction) I have often wondered about some of the turntable designs that utilize feedback to correct for speed anomalies. To my mind the speed corrections that are made based on the feedback must take place at a different point in time and would therefore be incorrect at that point. Hmmm.

The PID controller was essentially developed by Sperry in 1911 for yaw control of ship steering.
It is very similar in concept to race car driving, where one wants to adjust the steering to the yaw rate, and not the yaw position… and also to flying.

I don’t know if TT use a PID but they are ubiquitous, and deemed to be the best controller if one has no model of the system.
(But they are sub optimal if one does have a physical mathematical model of the system.)

 

 

1 hour ago, ctsooner said:

… He was sure full of himself and made sure we all knew he did 2 years of engineering and is a spinal surgeon…

I thought he wrote that he was a chiropractor?

and… hence my comment about him being skilled at getting people noses out of joint. 😝

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10 hours ago, Holmz said:

 

I don’t know if TT use a PID but they are ubiquitous, and deemed to be the best controller if one has no model of the system.
(But they are sub optimal if one does have a physical mathematical model of the system.)

 

 

 

I was thinking of the Clearaudio tables which utilize an Optical Speed Controller. Just musing on the concept of time and correction in general.

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BTW, if you take away the output protection and use slightly more modern transistors, you find that the original Krell KSA-50  and KSA-100 are both pretty much a clone of the CC2.  Just add a fan and occasional flames...  (The fan was a nice touch in that it added real meaning to "fanning the flames.")

And, if you use even more modern devices, cascode the input stage and add emitter followers to drive the VAS stage, you get an Aragon 4004.  No fan or flames, though.

None of those really got the phase margin right, at least compared to the CC2.  Yet another hidden reason why feedback, especially in a power amplifier, might not always be the best idea.  To be really fair, the designers back then didn't have access to modern computer simulation (available for free! now, even) nor to good instrumentation (close to free).  But, as that article explained, they did get to take a lot of abuse from some contemporary audio entrepreneurs.  So, they had that going for them, which may have been nice.

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