Can a Amp be "timeless" and compete with todays amps?

@arcticdeth   They are going directly back to Kevin Hayes at VAC in Sarasota FL. Where they were built.

I’ve tried the three options and 8 is the one (and only one) with both amps on the Soundlabs, also what Soundlabs states.

Both amps sound excellent dynamically with a subtle tonal balance difference. 6550 vs KT150 and 1995 vs 2017 or so.

@williambf , your result would imply a difference in output resistance between the two models. Have you tried using different taps to see how you prefer the sound? I gleamed this from Stereophile, not sure they are your exact amplifiers.

This is 2012, may be similar.

As expected, the Ref150's output impedance varied according to the transformer tap selected. The 16 ohm tap measured 1.4 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband. The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm

This appears to be a 1994 review:

The VT-150's output impedance was nearly 1 ohm (0.99 ohms) at any audio frequency measured at the 16 ohm tap. This value decreased to 0.56 ohms at the 8 ohm tap, and 0.3 ohms at the 4 ohm tap.

@fthompson251 

where are located? Don’t just send them to any old shop, research a lot!

we drove many hours to Rockford,IL TO A PLACE CALLED : https://soundsclassic.com

i refuse to ship my gear, I will drive my,amps to the headquarters if needed.

I was reading through this thread and decided to post here what I had posted not too long ago on the SoundLab Owners Forum:

Audio Research VT-150SE mono amplifiers (1994) vs. Audio Research Reference 150SE stereo amplifier (2016).

"I recently purchased a Audio Research Reference 150SE stereo amplifier that uses eight (four per channel) of the (relatively new designed) KT-150 power tubes (150W pc) and the input tubes are four 6H30s.  All of the tubes are new and matched from Audio Research. Power supply energy storage: 1040 joules total for both of the channels.

The SL A3PX full-range electrostatic speakers sound excellent when driven by it using the 8 ohm taps.

Surprisingly or not, it is not “better” (whatever that means) than the two 1994 Audio Research VT-150SE mono amplifiers I’ve been using for the past twenty-five years on SoundLab speakers. I've been comparing them directly.

The VT-150SEs are rated at 130W pc and they use eight (four per channel/amplifier) of the 6550 power tubes (GE), eight 12BH7 drivers (four per amplifier) and four (two per amplifier) 6922 input tubes. It also uses tubes (two 6550s and a 12AX7) in the power supply regulation. Power supply energy storage: 420 joules for each channel/amplifier.

When comparing the VT-150SEs on SoundLab A3PX full-range ELS, the Reference 150SE tonal balance is slightly leaner (taut) upper bass and slightly more forward upper mid-range, using the 8 ohm taps on the amplifiers. Speakers are set flat (0) with treble at 12 o’clock position.

If you are convinced that best amps can be measured, you most likely hit a goldmine of technical wisdom, although the term "smooth" might be not universal enough to define it. However, in my opinion, that is not even possible, especially because amps do not make sound on their own: the sound you hear exists in a relation to all component in the signal path.

I define 'smooth' as a lack of audible higher ordered harmonic distortion as per my prior posts to this thread. The idea that amplifiers do not make a sound of their own is false; we've been hearing these 'sounds' for the last 6 decades. Some amps are smooth and might be a bit soft in the bass, others do bass alright but are harsh in the highs. That this is so is well-known and not controversial.

In the end, considering price, the real question is personal. I am not sure moving over makes much sense. Moving up yes. So if you have a piece of gear you are happy with change for change’s sake doesn’t make sense to me especially in Audio.

The most important factor in a good audio system are the synergies among amplifier, speakers and room.

Finally. 
We audiophiles tend to get buried in the technology weeds. The key numbers/measurements for amplifiers are those that indicate how they will drive your speakers.(power, current, impedance etc etc etc).

The technology or design philosophy that achieves those numbers is at best secondary at worst a red herring. And really what counts is how it all sounds to you.

 

Wise post.... Thanks....

There are certainly areas where significant improvements have been made over time. Anything in the digital realm, streaming, music storage etc.

Way back when I purchased (on sale) a Krell KST100 amplifier. It anchored many changes (upgrades attempted and upgrades achieved) upstream. After twenty plus years, some issues became apparent and I was faced with the dilemma of purchasing a new amp or servicing my Krell.

Looking at the market place, reading reviews and posts on websites (this one mainly) I called Krell. Had a brief discussion with a tech there who went over the ins and outs of bringing my KST to spec and resolving any problems they found.

The current marketplace for amplifiers that would deliver the the same or better attributes of the Krell would require quite a bit of money. At least what I paid for the Krell and quite a bit more than the cost of “refurbishing” it.

Luckily, especially given the original packaging was long gone, Krell is located less than an hour’s drive from me. I drove the amp up to Connecticut, the tech met me outside with a wheeled cart! The amp weighs around 45 pounds with sharp heat dissipation fins.

For around $1500 and a few weeks I had a like “new” 100 watt (measures 130) clean across the spectrum, the amp handles just about any speakers well, superb bass control (good slam factor).

The service at Krell is superb. The techs are enthusiastic helpful and a pleasure to deal with. I got good reasonable advice. 

I think the main advances in power amplifiers these days are size related with class D. Integration with pre amps and/or all sort of one box combinations. Was the Krell the last word in amplification? No. Even when new Krell made “bigger better” amps. The KST was actually their “dip” into the more “affordable” domaine…we are talking some $2750 back twenty five years ago! In today’s money?

In the end, considering price, the real question is personal. I am not sure moving over makes much sense. Moving up yes. So if you have a piece of gear you are happy with change for change’s sake doesn’t make sense to me especially in Audio.

The most important factor in a good audio system are the synergies among amplifier, speakers and room.

Finally. 
We audiophiles tend to get buried in the technology weeds. The key numbers/measurements for amplifiers are those that indicate how they will drive your speakers.(power, current, impedance etc etc etc).

The technology or design philosophy that achieves those numbers is at best secondary at worst a red herring. And really what counts is how it all sounds to you.

@fleschler 

Please correct me if you think amps should have both zero global and zero internal negative feedback

As wth nearly all things concerning audio it just depends. NFB will decrease the amplifier output impedance and increase damping factor.  For some speakers this will be beneficial.  There are speakers designed with the intention of being driven by  zero or very low NFB amplifiers. It will definitely depend on specific amp-speaker characteristics and appropriate matching compatibility. 

BTW I bet you modified amplifiers sound fantastic!

Charles 

My primary amp is a pair of Altec 1569A theater amps. WARNING-Using these unmodified in the home is dangerous as the 700 volt operating current puts out not insignificant amounts of radiation. My amps were gutted but for the case and transformers. They run at about 425 volts and the transformers are always barely warm to the touch. Six tubes replace the four and the two input tubes are 6GU7s with NOS RCA 6SN7s cathode follower. No one uses my 6BG6 output tubes. The amps are fully voltage regulated including the bias (very stable). It has four levels of adjustable feedback starting about 6db which I prefer. The internals have oil filled caps and there are 2 huge 3X7 electrolytic storage caps on top. They kill my EAR 890 amp. My second system has the best Dynaco 70 ever with tremendous bass control and dynamics. Again, moded to the central tap on the transformers with voltage regulated design including the tube bias and superior large electrolytic storage caps. Mr. Record says it’s the only good Dynaco 70.

I do not understand the "negative feedback" posts without describing what type of negative feedback. There is global negative feedback (my EAR 890 has none) and internal negative feedback which should be employed or else risk distortion. I think current VAC amps have 7 db of negative feedback and internal feedback. Another friend has a personally repaired RM9 ultralinear amp that sounds very nice. It’s in the design that counts, not the "modern" and "new parts design" that makes a great amp. True, my amps are radically different than most. Audio Research makes crude/primative versions of voltage regulated amps today which is inferior in the design in my amps sophisticated voltage regulation.

Please correct me if you think amps should have both zero global and zero internal negative feedback.

Transient response never goes out of style

VTL  250 mono blocks ,  I own for 20years , heavy yes , upkeep can be a pain , but can always be fixed , by a descent tech. I purchased them used for 3000  Used ,New today out of reach , I guess when I bias them , I appreciate them even more.

that's a good question. when you say "Can an amp be 'timeless' and compete with today's amps". you have to then ask what is it for? What is its purpose or function? If its to amplify a signal to drive a transducer then yes Any amp that functions as intended can compete with any other amplifying device.  Now you need to be specific and set up parameters to compare the two amps and decide upon listening decide which meets your expectations and which doesn't This is not including instances where a load may be 2 ohms impedance and for whatever design considerations are employed the amp is not stable to drive the load and thus your not able to judge the amp simply because its not functioning. Hi-Fi is an art and subject to any and all considerations of ugliness or beauty.

The key of the audio system is to release the emotion while you are listenning If you feel it and you love it,you definitely do not need change as you are not in audio business.

You need change if you feel boring, you feel fatigue ,you can not listen for long time,then it is demonstrate that you need do some change.

The 1912 DeForest single ended triode is an example of a classic that has been resurrected. One manufacturer described the change to class B as making compromises to improve efficiency and other tricks to enhance measurement of distortion already below what can be heard. This was done for marketing purposes. A TAS review of a McIntosh amplifier described it as sounding like a single ended triode amplifier. Don't automatically newer will always be more desirable than classic vintage gear. "The best" will not be the same for everybody's preferred sound.

theaudioatticvinylsundays.com

Thanks for the link to your site  it seems very interesting...

Plinius! For sure. 

My equipment is 15 to 66 years old. 
Most of my guests think it sounds spectacular. 

theaudioatticvinylsundays.com

I happen to have lots of vintage equipment and 10 integrated SS amps and 4 tube amps.I don't buy anything new.I like to find vintage equipment. I can't afford the new stuff.

OP, what exactly would be the problem if it WERE a hotrod job?  If you use "upgrade parts" [a relative term, I acknowledge], maybe you end up w/a BETTER sound that what it was originally...I say this because I am using an OLD Sansui integrated, slightly hotrodded [by someone else], driving some old Pioneers that are majorly hotrodded [by me].   WAY better sound; I sit and shake my head in wonder every time I listen to it.

But go w/whatever you like, obviously ~ just saying that ruling out upgrading something [like caps] might be your sonic loss>>>>>

Apologies if I duplicate other’s comments.

I think the simplest designs with solid components, whatever their strengths or weaknesses, have longevity, whatever your judgements are of their fundamental architecture or their particular performance. More complicated ones compete with successors’ evolutionary tweaks.

The other issue is that everything ages. There can be a multitude of performance changes that result from that, more or less amenable to intelligent maintenance.

I second @lancelock with the vintage theater amplifiers. I have the little brother of the 1570B, the Ampex 6516 monitor amplifier, that was used to master the soundtracks of movies. Of course, it needed heavy bringing up to date to be used as a stereo amplifier, but all that is largely cosmetic as we are talking about adding modern connectors, and also about taking out the first gain stage (as the input was tape level.)

Mine had seen extra heavy duty in a movie theatre for decades, so a nice tune-up was already in demand. Not because of the heavy use (which it can easily take, as it was designed to be operated 10h a day, every day, for decades), but simply because of the age.

Compared to the 1950s we have drastically better capacitor technology, and a LOT can be done to optimize the internal wiring.

Few other mods done, which are modifications and not just "oil change": power tubes running in triode mode, separate filament transformer, the C input supply changed to CLC input, feedback loop drastically reduced (and then fully removed).

For those measurement savvy, I measured it with the minimal feedback (less than 1dB), at full power output (20W) the low extension was -1dB at 10Hz. I have not re-measured after taking ou the NFB completely, but I can report that the sound is way more dynamic, live, and bass much more 3D and grabbing than with it.

Now, back to the sound quality....

It's simply astonishing. It is as emotional, pure and human as the best examples of SET amplifiers are. They have produced the MOST LIFELIKE violin reproduction I ever heard from a stereo. Well, they clock the only instance a violin felt real from recording, in any system ever. (That is was not "close" or "almost", but the real deal.) That says a lot, as even most ultra high end systems reproduce violins in cat major or flat mouse E sharp, or plastic being shredded by sharp steel A keys...

So, it's a rare form from any amplifier of any age to do the most basic form of justice to violins. The venerable Ampex excels at it. Yet, not just that... I can listen to Karyyn - Jaina or Billie Eilish When I was older, and feel as if I'm transported to another reality... these songs contain high levels of subsonic information, and these babies can deliver it like nothing I ever heard. (Well, that might show my narrow-experence with HE systems boasting sub-20Hz provess... to tell the truth, we, audiophiles never listen to such material in our audio get togethers, so who knows really.)

So yes, I think "vintage" amplifiers are well and truly deserving of our attention, whether you are an UBER-fidelity fan (for life-like violin reproduction), or a hard-core rave party addict who wants to feel the marrow moving inside his bones (wants more than just a kick-in-the-chest...) 

And then, of course, there exists the prospect of overthinking things until.....

My old Phase Linear 400B amp is almost 50 yo. I doubt that I could tell the difference between it and my newer amps in a blind AB test. Maybe others could, but as others here have said, amps haven't changed much over the years. Transistors and resisters are quieter and capacitors have improved greatly but does that end up with a better sound?

What this means is its possible to build a solid state amp that is every bit as smooth in the mids and highs as the best tube amps and leave nothing on the table in terms of detail, sound stage and the like, in such a way that vintage amps simply cannot compete (not that they sound bad, just they don’t sound as good). This is one of those things that is easy to hear and easy to measure.

If you are convinced that best amps can be measured, you most likely hit a goldmine of technical wisdom, although the term "smooth" might be not universal enough to define it. However, in my opinion, that is not even possible, especially because amps do not make sound on their own: the sound you hear exists in a relation to all component in the signal path. Insisting on individual component’s quality, as promised by manufacturers marketing materials, rather than emphasizing the signal path synergy is not the optimal formula to enjoy audio but, of course, it can be easily the most expensive one...

yes

Benchmark moved the game a head with their current technology that's in the AHB2.

My bet is if you plugged your tube pre into this and kept the rest of your system as is you'd be more than pleased with the results! And if not, you can return. Worth a shot!

Let’s simplify negative feedback = taking something ( the AC audio signal aka music ) that has ALREADY happened, flip it around out of phase ( that’s the negative part ) and feed it back into the input where something new and unequal is happening ( unless you think sine ways are music )… then apply some critical thinking….

I feel like Pavlov’s dog waiting for the bell to ring…
(like expecting every impulse transient will have a ringing exponential decay like a Cadillac with no shock absorbers going down the road.)

A better approach these days might be to estimate what the error is going be a priori and just apply that as (or along-with) the input. So more of a feed-forward approach, or pre-emphasis in control theory I think?

Then there is no feedback, and the input is driven towards a lower error. But that sort of begs a fully digital system IME.

i have not been keeping with amp design, other than the Bruno work, which Ralph mentioned. So maybe a lot of stuff is happening I am unaware of?

When it comes to amps, there seem to be a plethora of folks who like a specific sound. Some will call that sound ’distortion’, others will call it ’neutrality’ and still others will call it ’transparency’. Tube amps have a specific ’sound’, that sound is generally more pleasant to the human ear than some of the typical ss Class D sound that most folks associate with the class. OTOH, there are folks who prefer this very sound that others decry. At the end of the day, I believe it is the ’flavor’ of the amp sound that we choose...and the appeal to not only our pocket book, but also our aesthetics. Most amp manufacturer’s know this...and design accordingly.

Another question is: has the basic circuit of the tube or ss amp design changed that significantly over the years to call it a total re-design? That is a question for the more technically advanced here.

Thanks...

You are very clear...

C’mon folks, apply a little critical reading and thinking skills. Read and see what is clearly between the lines. While I had always admired Ralph and thanked him for his participation here (though disagreeing with his continued insistence that higher cartridge loading is always best unless RFI interferes and that correspondingly, RFI is the chief factor upon which cartridge loading should be chosen), there is clearly a theme here towards promotion of his Class D amps. I have to wonder how a customer of Atma-Sphere who just paid 20K+ for a pair of the MA-1’s a month ago must feel when it’s principal proclaims his own product to be by nature inferior to his new solid state alternative.

And I am also surprised and even angry at myself to only now notice that Ralph is so reliant upon that which can be measured.

:) Critical thinking. I like that: far too little of that going on these days; people have taught themselves to disable their critical thinking. Dietrich Bonhoeffer wrote an interesting paper (which later got him killed) regarding this topic.

Some loudspeakers are not meant to be driven by an amplifier that behaves as a voltage source. ESLs are a good example. For those, tube amps remain the more viable alternative (and just for the record, the last set of MA-1s, which did ship about a month ago, are in use with a set of Sound Labs; something our class D simply can’t make the power to drive because of the impedance. So to my understanding, he feels pretty good.)

Regarding the cartridge RFI thing, that’s easy enough to demonstrate:

http://www.hagtech.com/loading.html

If you have a one meter phono cable, 20pf/foot is typical. find the inductance value of your LOMC cartridge and drop the values into the calculator. You’ll find that the resonant peak is in the RF range. This calculator is based on simple math that comes from electronics 101 in the first week.

Regarding our Class D amplifier, you brought that up, not me. Again, I was addressing the original question, which, again, the answer is that there are semiconductor devices now that didn’t exist 25 years ago, allowing designers to build amps that are actually successful at getting rid of audible distortion (harshness and brightness being an example that might measure quite low on the bench), whereas prior to that time, getting rid of harshness in solid state amps was done with a simple technique called ’lying’.

That lying is why for the last 60 years audiophiles have had to take equipment home to find out if it sounded OK in their system. Its going to take a while to overcome that learned behavior! the first and hardest thing will be to convince people this is all real. As audiophiles we’ve been lied to for so long we take it as rote.

Regarding measurements: One of the problems that many audiophiles have is that the spec sheet seems counterintuitive; amps with lower distortion often sound worse than those with significantly higher distortion. This is because the ear is sensitive to higher ordered harmonics and the harmonics are not ’weighted’ on spec sheets (and the amps with higher distortion are often tube amps with a lot of 2nd and 3rd harmonic). Amps with low distortion figures often have unmasked higher ordered harmonic content at low levels, but the ear is keenly sensitive to these harmonics as it uses them to sense sound pressure, so has a 130dB range to support!

But we can measure what’s important! And reliably predict what the amp will sound like based on those measurements. However most of these measurements rarely if ever show up on spec sheets. Here they are:

1) distortion vs frequency. If rising at all past 1KHz, expect brightness; I explained why earlier.

2) distortion spectra. To mask distortion you have two avenues, either get the distortion product well below -100dB or mask the higher orders with a more prodigious 2nd or 3rd harmonic. SETs use the 2nd harmonic, our OTLs use the 3rd. The distortion spectra should remain consistent at all frequencies, not just 100Hz or 1KHz! FWIW zero feedback tube amps can do this last bit quite well.

3) distortion spectra at -6dB of full power (you never see this one). It should be consistent with the 1 Watt spectra. SETs often fail this test; higher ordered harmonics appear in their output at this level (usually on transients) causing the amplifier to sound ’dynamic’. I’ve found that when audiophiles discuss ’dynamics’, for at least 80% of the time, the word ’distortion’ can be substituted for ’dynamics’ without changing the meaning of the conversation.

If the amplifier is well behaved in these regards it will sound smooth and musical; the lower the distortion the more transparent the amp will be because distortion obscures detail. This is why I don’t like SETs because they often have several orders of magnitude more distortion, causing them to lack detail. Detail does not have to be associated with brightness although it often is because typical solid state amps that are low in distortion are often bright and harsh, although they are at the same time more detailed. You know you’re making progress when the sound is smoother and more detailed both at the same time!

Measurements are really really helpful if the designer understands the how the ear perceives distortion. IOW you can design the amp to work with the human ear rather than to look good on paper; if you really know what you’re doing you can achieve both!

@fsonicsmith

Thank you for bringing to my attention the Ampsandsound gear, a brand I do not know, until now.  I looked at their website and their design and build look really interesting.  I love amps built using turret boards.  The layout of their wiring is extraordinarily neat and uncluttered and it appears to be extremely easy to service--nothing has to be disassembled to get to a component.  I like the way that all wires that have to pass through the chassis or internal walls goes through a high quality passthrough grommet.  The only giveaway that it is not a truly old school build is the use of plastic ties to hold the wires together.   

@larryi 

Great post. I wish you could hear my Ampsandsound Nautilus which is a modern recreation of the HK Citation II. It puts out 8 watts of SE triode. The amp is overbuilt to the extreme. It is perhaps half the size of my ARC Ref 80S and yet weighs the same. I wish you could hear it not because I have any reason to believe it is any better or even as good as your low powered amps but because your impression would be interesting given your library of knowledge and amps. 

When @teo_audio writes positive feedback, does he really mean feed forward?

Technically (as in: labeled for least potential in confusion), feed forward, yes.

The significant sort of modern advance is Class D amplification.  The better designs are pretty much indistinguishable, to me anyway, from other high end solid state designs.  But, where one does not need a whole lot of power, my preference is for low-powered tube amps.  I find that the better low-powered tube amps (roughly 40 watts or less)  to be more engaging and lively sounding at lower volume levels than solid state amps (you have to crank the volume up just a bit more with solid state to feel the music come alive).  I also think that such tube amps have a more natural sounding attack to the note (less of an artificial sounding "edge"). 

The amps I own include a parallel single-ended 2a3 amp (Audio Note Kageki), and a pushpull 349 amp built in Italy by Aldo D'Urso (sort of a replica of a Western Electric 133 amp using original Western Electric transformers and other vintage parts).  The parallel 2a3 amp puts out something like 6.5 watts, the 349 amp something around 5.5 watts.

My personal preference is for low-powered triode tubes (e.g., 2a3, 45) in either single-ended operation or pushpull, and certain pentode/tetrode tubes (KT-66, 6L6, 350B, 349) in pushpull operation.  I tend to dislike high powered tube amps that use multiple higher output tube types--the sound becomes hard and has a "glare" that I find a bit annoying.

I really like certain OTL amps as well.  They have an "immediacy" and sound extremely exciting and dynamic compared to most other amp types.  Some are operationally fussy (not the Atmasphere amps), and are a bit scary to me because I have speakers that are close to irreplaceable.  One of the best amps I've ever heard is a custom-made OTL amp.  

As for solid state amps, the ones I've heard that I particularly liked have also been low-powered, specifically the First Watt J-2 (borrowed from a friend for two weeks) and their SIT amp (heard in a friend's system).  These have a slight bit of an artificial edge to the attack of a note, but, they don't sound as dead and unengaging as most other solid state amps.  I also like some Ayre amps because they sound relatively relaxed without sounding dead; if you need more power, they are a good choice. 

Well this is the only modern amp that I like better than what I have. 

https://dandagostino.com/products/relentless-amplifier

A listener would have to compare them in a direct A/B setting.

i think so also...

At the end acoustic/psycho-acoustic experience in a room decide what will be our choices, because nevermind the theoretical level of improvement, no ears and no room are exactly the same ...

The law of hearing are the same but the ears differ...The needs of people differ too...

And if technoloy improve always with time, there is a plateau of maturity where new technological improvement rival other different technological  advantages improvement with different trade-off...

Well stated Charles.

@fsonicsmith

And I am also surprised and even angry at myself to only now notice that Ralph is so reliant upon that which can be measured

Ralph is an accomplished electrical engineer and also a successful audio electronics manufacturer. I certainly can appreciate his reliance and support of formal measurements as it simply reflects his training and background.

In addition I find his explanation of NFB as utilized in the manner he describes proper application. It is logical and coherent. My primary objection is the assumption that this approach is inherently going to result in a better sounding amplifier than the OP’s VAC Renaissance amplifier. A false assumption in my opinion. A listener would have to compare them in a direct A/B setting.

I just don’t buy the notion that new technology is without debate always the better performing/sounding choice. It has to be confirmed under real world listening/audition  conditions not just in theory.

Charles

You are right...

When people dont understand something they search for simplistic  explanation and they mob together...

I don't think accusing people of audiophile heresy is productive.

 

Burn him at the stake!!!!   I am really getting a kick out of this place.

I was curious, so I tried a search. "Class D" and then found I could filter by user, so I filtered for atmasphere. It appears that atmasphere has been communicating about their class D amplifier for almost 5 years. It could be longer but I got bored of searching.

I have worked in semiconductors and batteries. Everything new we release is better and better rendering even last years products effectively obsolete. I find this a strange thing to complain about. When I read atmasphere's other posts in this thread, I would be happy he does rely on measurements. He seems to have a handle on how those measurements will make his product sound. How can that not be a good thing?  This is audio. I don't think accusing people of audiophile heresy is productive.

What this means is its possible to build a solid state amp that is every bit as smooth in the mids and highs as the best tube amps and leave nothing on the table in terms of detail, sound stage and the like, in such a way that vintage amps simply cannot compete (not that they sound bad, just they don’t sound as good). This is one of those things that is easy to hear and easy to measure.

Bruno Putzeys wrote a wonderful article on feedback.

For example in a class D amp that is self-oscillating,

C'mon folks, apply a little critical reading and thinking skills. Read and see what is clearly between the lines. While I had always admired Ralph and thanked him for his participation here (though disagreeing with his continued insistence that higher cartridge loading is always best unless RFI interferes and that correspondingly, RFI is the chief factor upon which cartridge loading should be chosen), there is clearly a theme here towards promotion of his Class D amps. I have to wonder how a customer of Atma-Sphere who just paid 20K+ for a pair of the MA-1's a month ago must feel when it's principal proclaims his own product to be by nature inferior to his new solid state alternative. 

And I am also surprised and even angry at myself to only now notice that Ralph is so reliant upon that which can be measured. 

No I meant in a much more simple fashion, i.e. a portion of the output is subtracted from the input. Literally negative. It would technically be out of phase but that is by definition. 

Yes- we use a resistive divider network to do that in our smaller OTLs (we run 2dB of feedback in the M-60). You either do it that way or thru an active device, such as a tube's cathode while the input signal is applied to the grid.

But the feedback network itself can be simple or complex, depending on (...how well its designed...) the expectations for it, such as keeping unwanted frequencies out of the loop. For example in a class D amp that is self-oscillating, the feedback network defines the oscillation frequency (known as the 'oscillation criteria') since so much feedback is applied that the amp goes into oscillation as soon as its turned on; the amp then uses the oscillation as the switching frequency. That network can be a 4th or 5th order loop with 6 or 7 variables; for that you'll need a computer to sort it out.

@atmasphere 

Your participation and patience on this Forum is something for which I am truly grateful. Comments from you are always illuminating and helpful.

Thank you.

No I meant in a much more simple fashion, i.e. a portion of the output is subtracted from the input. Literally negative. It would technically be out of phase but that is by definition. 

When @teo_audio writes positive feedback, does he really mean feed forward?

In addition to what you write, is not the negative in negative feedback, simple because it is subtracted?

If I get your meaning, probably not. In most amplifiers the feedback is applied to a node that has non-linearities, so often feedback signal itself contains distortion not present at the output! For example in a typical solid state amp, the input circuit is a differential amplifier. One of its inputs (differential amplifiers have two, one inverted and the other non-inverted) takes the signal. The other takes the feedback. But the device (transistor) isn't perfectly linear... Of course, if you applied enough feedback, you could overcome this issue...

Thank you @atmasphere , skimmed the linearaudio link, will delve in later. Looks like it will not be that hard to follow.

In addition to what you write, is not the negative in negative feedback, simple because it is subtracted?

Let’s simplify negative feedback = taking something ( the AC audio signal aka music ) that has ALREADY happened, flip it around out of phase ( that’s the negative part ) and feed it back into the input where something new and unequal is happening ( unless you think sine ways are music )… then apply some critical thinking….

This statement is false. Its based on the idea that there is a time delay between input and output. What actually happens is there are frequency poles in the amplifier that cause phase shift; on an oscilloscope this appears as a time deviation if you are measuring in the range of the phase shift.

Filter theory tells us that there will be phase shift at some high frequency (in an amplifier, the high frequency roll off will be on a 6dB per octave slop initially; this will impose phase shift to 1/10th the cutoff frequency). As you increase frequency, eventually the phase shift is so acute that the feedback becomes negative instead of positive, and the amp goes into oscillation. Norman Crowhurst was writing about this 65 years ago; none of this is controversial. When the amp goes into oscillation, it can be said that its phase margin has been exceeded.

http://www.tubebooks.org/Books/crowhurst_basic_3.pdf

(link above for those interested in how this really works)

This explains what is really happening and is not some sort of pseudo neo-science.

So if one has been paying attention, one should see what the problem is with feedback in traditional amplifiers (which includes all tube amps). You have an output transformer in most cases, and it causes a fair amount of phase shift, often inside the audio passband! As a result, it poses a limit on how much feedback can be used, and is a guarantee that the feedback will cause harshness and brightness as a result (I already explained why).

Futterman applied 60dB of feedback in his OTLs, but the problem he was up against was not a lot different: his design not only had significant poles in the circuit design, but he also had overall insufficient Gain Bandwidth Product. This means that feedback was decreasing at higher frequencies and so harshness was the result. Because of the frequency poles oscillation was an ever-present danger as well.

Bruno Putzeys wrote a wonderful article on feedback.

https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf

It explains much of what I’ve been talking about here in greater detail. Don’t worry if you can’t follow the math 😁 You’ll note that he mentions Peter Baxandall’s writings of how feedback imposes distortion of its own, reflecting Norman Crowhurst’s observations (and explanations) of that from 20 years earlier. Those individuals never got the chance to play with an amp with sufficient bandwidth and loop gain! Its precisely that (sufficient bandwidth and loop gain) which is why an older amp might not keep up with a state of the art design (although it might easily keep up with new amps that are rehashed circuits from earlier decades).

I did not cover the aspect of trailing layers of various odd ordered harmonic distortion patterns, as I try to take care of that when I'm messing with an amplifier execution, at the fundamental level. It is a huge part of the coloration, as Ralph says. But so is the other aspect that I speak of. Have to be careful, though, or we'll be delving into some thing about big boxes on cables and an infinity of poles of articulation.😉

It has become an interesting thread. Two people are talking about negative feedback. One clearly understands the topic and appears to be able to talk about it in great length. The other appears to talk at great length. It is up to the reader to decide who is correct. I enjoy both, but for different reasons.