Keis, you opened Pandora's box here & I expect there to be some heated debate! :-) This topic can degenerate into a "religious" battle.
Anyway to answer a few points:
Jameswei - nice cut & paste from your Aleph owner's manual. Show that there are so many diff ways to say the same thing & that there doesn't seem to be any consistency in the audio industry. They use whatever terminology that sells the most equipment!
Search thru that manual (I've read the X series manuals on-line & I know that it exists in there FOR SURE) & see if you can find this quote from Nelson Pass: "I ask anyone to show me an electronic circuit & I will show them the feedback path".
Which leads me, nicely, into my next point:
Keis: you are DEAD wrong when you say that Ayre circuits have zero feedback! Sorry to be so blunt but this topic has been dealt w/ here on Audiogon N number of times. Just search the archives! Member Aball nailed it quite well - "The marketing gurus have done a great job of convincing audiophiles that zero feedback is the way to go but no functional amplifier design actually exists without any feedback".
Precisely correct!
This is also corroborated by member Gregm post about stabilizing.
Ar_t asks "stabilizing what?" Stabilizing the transistor bias point. This is of utmost importance - the bias point. Shift the bias point puts a transistor in Class-A or class-AB or Class-B or Class-C or Class-D, etc. When designing any electronic circuit, be it vacuum tube or s.s. - the designer spends 70% or more of their time ensuring that the bias point of the device(s) is(are) correct for the intended application & that the device will remain in class-A/AB/B/C/D, etc over the entire signal excursion. If the bias point is not stabilized, the device drifts with input signal (called signal level distortion, which is a really bad thing) & you can hear it.
No electronic circuit on Planet Earth exists w/o negative feedback! however small the negative feedback, it's always there. To that effect, the Ayre circuits probably use local feedback either around an individual device or around a small cluster of devices. I don't know which but it's got to be one of these if they are claiming "zero feedback".
Whoever came up w/ IM distortion tests for audio amps must have done it to get good test-bench performance numbers 'cuz it's generally know that THD numbers & sonic performance have little or no correlation today. large amounts of negative feedback gave excellent test-bench performance but the amp sounded like sh$$ in a home stereo system. Amps having negative feedback usually lack macro-dynamics & they usually have a flat (2-D) soundstage. That's because the negative feedback loop allows only so much excursion of the transistor bias point before limiting it. The bias point is never allowed to leave a certain region (a very tight region if feedback factor is very high & a looser region if the feedback factor is low[er]) thus the transistor doesn't distort much & THD measures very well!
The reason for inserting negative feedback is that there was a very cool device that was invented in 1948 called the BJT! After several years of development & use in other areas, it was "ripe" for use in audio in the late 1960s & early 1970s. It was supposed to be the next revolution & was supposed to replace the venerable vacuum tube. Like sh$$!!!
The vacuum tube is a device whose operation can be described by oridinary physics (the stuff you learnt in high school) & from these equations you see that gain is a linear function of current & other physical parameters of the tube.
OTOH, the semiconductor transistor is a square-law (MOS) or an exponential law (BJT) device! Thus, the distortions produced by s.s. devices is totally diff from that produced by a linear amplification device (vacuum tube). There is much more odd order harmonic content in s.s. amplified music than there is in vacuum tube amplified music. Plus, owing to the non-linear relationship between gain & physical parameters of the s.s. devices, this distortion rises more rapidly than it does for a tube circuit. So, how to curtail this distortion?? You guessed it - negative feedback!
This negative feedback is our best friend when we are designing electronics for most all other applications EXCEPT audio because we want to do signal processing that is usually some means to an end other than critical listening.
However, for critical listening, negative feedback is our worst enemy. Makes the amp test & measure excellent but makes it sound like cr**!
It's taken us over 30 years to come to grips w/ this - slowly admitting it w/ each new generation of s.s. gear until we come to year 2004-2005 when manuf are openly advertising zero global feedback. What if there was some real intelligent designer back in the 1970s who realized the "hazardous" effects of negative feedback after a few releases of audio gear that had it & he spoke up against using it, I think that his business would have long gone under even tho he would have been correct!! It's taken us this long.
Today s.s. gear is making some real strides in sounding "real"......................very much like vacuum tube gear always did before it!!! LOL!
The transistor was to replace the vacuum tube - like hell it did! Just like CDs were supposed to replace vinyl!
Vacuum tube circuits still sound the best & any top-notch system has tubes in it. It'll always sound the best to a human ear as long as it remains a linear amplification device.
And, vinyl is more coveted today than it was during it hey-days.
What peeves me the most is that the users in the audio community are not as informed about electronics as they should be esp. if they are pre-disposed to spending large amounts of money on gear. The marketing dudes still seem to rule the roost & they still seem to twsit & turn details to enhance a sale & even brain-wash a user into thinking what THEY want the user to think.
Even more pissing off is when you try to spend some time to teach them, they come back & bite you! This move suggests to me that they want to remain ignorant. Well, ignorance is not a bliss......................it's ignorance!!
FWIW. IMHO.
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Unsound,
Read your post.
My post WAS NOT an attack on any person or persons. LET ME MAKE THAT CLEAR if it was not.
2ndly, I NEVER meant to say that my choices are better than yours (Unsound) or anybody elses. Let me MAKE THAT CLEAR as well, if it wasn't. (I should know better than that!)
You, Unsound, might not have been brain-washed by the marketing dudes, which is probably good for you. My statement was a general one. Most of the people fall for the marketing ploy & you often read posts supporting the marketing propaganda. Of course, there are people who have learnt & know better than to succumb to it.
Indeed what sounds "correct" to me might be/will be different from what sounds correct to someone else. I do find, however, that the longer a person stays in this audio hobby & gets many chances to hear diff gear & make direct comparisons of s.s. vs. tube gear in the same system, the better are his/her chances of realizing the positive attributes of linear amplification devices over non-linear ampl devices. That person might still not want to own tube gear - fine! - but the realization does set in.
This point might have been missed by you? |
Keis,
I was not shouting! I used capital letters to emphasize a word/words. In an all-lower case post, it seemed reasonable to use capital letters to stand out. Sorry that you thought I was shouting.
I re-read you orig. post - certainly appears that you succumbed to Ayre's marketing hype. There was no way to know otherwise. Your latest post suggests otherwise. Wish that you had written the words in your latest post in your original one!
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Re. feedback in bias ckts - yes, I agree that you do not need to used feedback to design a bias circuit. That's *not* what I meant to say! I re-read my orig post & I cut & paste from there:
"That's because the negative feedback loop allows only so much excursion of the transistor bias point before limiting it".
what I was trying to say was the transistor's bias point's movement along the load line. That's the excursion I'm talking about. Not the static/DC bias point, which is what I think Ar_t & Herman are talking about.
Negative feedback curtails the effective load line movement keeping the transistor in it's linear region of operation. Just trying to clarify my point.
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True, today's class-D power amps of tons of negative feedback. I'm wondering (without actually having studied a class-D amp in any details) if today's negative feedback is diff from yester year's negative feedback OR if feedback is feedback is feedback?
In yester year's feedback, as Keis pointed out, the output signal is fedback to some point in along the forward gain path. So, this feedback is directly messing w/ the wanted output signal & botching up the sound.
Today's class-D power amps seem to be using a high-speed switching power supply that makes an attempt to follow the envelope of the music signal while the transistors are merely turned on/off at that same rate. By making the switching speed 64X or 128X or 256X 20KHz, the power supply is able to quantize the music envelope quite well.
There must be an error signal generated to ensure that the power supply is correctly tracking the envelope. It *appears* that the fedback signal is not directly in the music signal path (maybe I'm wrong & it is?!)? Maybe that's why these digital amps (despite the tons of feedback) sound much better than amps of the prev many generations? (I have a friend who seems to love his Rowland 201 mono blocks more than his prev Rowland Model 2). |
Hi Ar_t
reading your post suggests very clearly that you are a manufacturer of audio equipment. However, you did not make this disclaimer. We love to hear from you audio industry insiders but we also would appreciate if you would be open about it & let us know that you are directly involved.
AudioAsylum forces this in a certain way by putting "(M)" after the moniker (sure one could lie but it wouldn't go over well when one is caught). We don't have that system here on Audiogon & so we rely on the honour system. So, if could please honour the honour system, we'd appreciate it. Thanks! |
Ar_t, I indeed see that you are a commercial user but that's only when I do a "member lookup". How about 1 of these suggestions:- * put a disclaimer in each post you make saying that you belong to such-&-such audio outfit. (this is tedious, I agree) * you can sign off each post with you name & audio outfit like, say, Roy Johnson does in his posts: http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1062780030&openusid&zzRoyj&4&5#Royj* Another example: Ralph Karsten of Atmasphere uses his company's name as his moniker "atmasphere". The name is well-known & it's easy to see from who the post is from. |
Ar_t,
I see that you are being extremely difficult on a simple matter like this!
I note an extreme resistance on your part to openly cite your affiliation w/ your company thinking that somehow by revealing it we are going to jump on you.
For such a simple matter as this you are making excuses.
Whether you are low profile or not is of no concern to me. What I/we want to know from your posts is that you are an audio industry person & weight your comments accordingly.
And, I see that you are trying to avoid this at all costs.
Something does not seem correct to me! No wonder AudioAsylum designates their dealer sub-forum as "Shady Lane"!
Forget about the other people who "I know of some people (on other forums) that make countless useless posts, only so that they can get their name in front of as many people as possible. Again."
You post sensibly + make very clear your association with the audio industry & you will *not* end up looking those other people & neither will you create any confusion for us. You haven't been long enough on these Audiogon forums - we can handle quite a bit of confusion before we cry "uncle"!
You are *not* coming across clean, Ar_t - something is not right here! |
Keis,
Thanks for posting the response from Charlie Hansen. His mostly detailed post does clarify his intentions & it is exactly what I envisioned his circuits to be - use mostly or only local negative feedback.
He is correct in writing that there is no standardized terminology for describing circuits. Often a circuit topology will acquire a certain name if it is used often enough & over a period of time. If one is in the business of creating electronic circuits, then, it is generally understood that 'feedback' pertains to 'negative feedback' unless otherwise stated & also that it pertains to some degree of 'global feedback' - either around the entire ckt or part of it. It is tacitly understood, once you graduate from EE school, that no device works w/o local feedback. so, if you are in the biz of making ckts, why even bother talking about it 'cuz it's a for-gone concl that it's omni-present.
HOWEVER, the audio market does not consist of merely EEs - it's got all walks of life. So, is it OK to assume that these people, non EEs, will know (& even care) about "generally accepted terminology as used by major semiconductor manufacturers and experienced designers around the world"? Under my breath I say "bullshit! they won't".
So........here you are, a well-known audio brand, selling seemingly good sounding equipment to many, many non-EEs BUT.........using "generally accepted terminology as used by major semiconductor manufacturers and experienced designers around the world"!!!
IMHO, the manuf (& I'm *not* singling out any 1 in particular 'cuz I feel that they are all guilty of this) is doing the minimum possible to enlighten the public about his/her product on his/her website. Hot issues within the audio community (& global negative feedback is one of them) are high-lighted in the marketing text almost the same way it would have been written in a technical data sheet.
Hell, the techincal data sheet is read by trained eyes (in the art of circuit design) while the marketing text is read by mostly untrained eyes!
It boils down to what is convenient to the manuf in the sale of the product. If you end up being less educated about, it's probably better for the manuf.
You fell for his "zero feedback" marketing stuff & really believed that he had no feedback in his circuits. If you had not asked for a more detailed explanation from the manuf, none would have been offered!
THAT IS my issue & is what I tried to highlight in my original post - Don't just eat what they give you hook, line & sinker. Ask for a more detailed explanation. They do not need to reveal any trade secrets or proprietary info for you to better understand their product & its benefits to you.
So, I'd like to ask - in this audio market, who is serving who? Is the manuf serving us the consumer or is the consumer serving the manuf?
I think, it's a bit of both - it has to be a closed loop (i.e. there has to be negative feedback between manuf & consumer. I'm almost laughing as I write this but I'm quite serious) system in that the manuf makes product to sell to the consumer so his/her tastes must be taken into account + the consumer should be able to feedback his/her preferences to the manuf & have the products evolve. However, in audio, where electronics is used to perform the signal processing, the consumer has to learn the appropriate language to have a meaningful conversation w/ manuf. To that effect, the manuf has a vested interest to teach the consumer some aspects of electronics. If that doesn't happen, the loop opens & you begin to get cr** electronics entering the market & marketing types taking ownership telling you what you should have & what you shouldn't have.
From a EE perspective I agree w/ Charlie but from a strictly consumer point of view I do not.
I have no ax to grind as I'm strictly a consumer & I'm free to choose which ever brand I like.
I might have less experience than the Burr-Brown engineers but the repetoire is fairly diversified.
(BTW, I've heard Ayre designs on several occasions & I have always liked Charles Hansen's designs tho' I have never owned one myself. I have no quarrels w/ Ayre or Charlie Hansen - just for the record. Ayre appears repeatedly in this post simply because you posted something about their latest product offering & for no other reason).
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Just one comment re. Ar_t's prev post:
"You say that you don't want feedback there.......ok......here are your options:
Take the resistor out. Connect the emitter to ground. Great, now you have something that won't bias on in a linear manner. You have what is called a Class C amp. Great for RF, useless for audio."
even when that resistor in the emitter leg is removed there IS local feedback in that device. It's the intrinsic emitter resistance called "little re" + any package lead resistance. The issue is that it's really very small & of no practical consequence i.e. not large enough to bias the transistor in its linear region.
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IMHO, there are several reasons that amps s.s. amps (or sand amps as I like to call them) sound better:
* firstly, the transistor devices themselves have gotten radically better than they were 5, 10, 20 years ago. Ar_t did cite this as well. The distortions from these newer devices is much lower than what it used to be so amps using them can play louder for longer. Tighter manuf tolerances also makes it easier to match them. Many manuf like Pass, Rowland, Symphonic Line & a whole host of others use several of them in parallel for high current outputs.
* 2ndly, the s.s. amp designers themselves have grown in their skill set to design these amps. I bet that Ar_t can testify to this! :-) It takes a while & several generations of products to understand how the semiconductor devices *really* works & how to coax the best from it.
You can see this in the realm of CD music too. When was the CD 1st introduced? If anyone of you has a CD, say, from 1985 or so & you compare it to a recent re-issue of the same music, you can tell that the people re-mastering the CD have a vastly better understanding of the whole process.
The growth of the individual amp designers to skillfully use the semiconducting device to its inherent strengths is what I consider a more important reason for better sounding amps. You can have the best BJT or MOS but if the implementation is poor, you'll still get bad sound.
* 3rdly, *more* s.s. amp designers understand & believe within themselves that there is no or very little correlation between THD, TIM measurements & sonic character of the amp. Thus, applying large amounts of negative feedback to ensure that the amp measured superbly in some reviewer's lab is not a top priority any more. There have always been s.s. amp designers thru the yester years that believed in less global feedback & we consumers have voted w/ our money by owning these products.
* 4thly, there is a lot of admission from the s.s. amp design camp that the vacuum tube, tho old & to some unreliable, was & remains a really fantastic amplification device for audio where lack of harmonic & inter-mod distortion is king. You'll often find people seeking a "tubey sounding s.s. amp" - the forums are littered w/ such posts. Why are these people seeking such an amp?
Also, note that some of the best s.s. amps sound like a good tube amp. Anytime I have ever tried to find an adjective to describe a good sounding s.s. amp I've mostly come up with "sounds like a tube amp!". Fancy that!!! I wrote this in my orig post (which Unsound picked up on). The vacuum tube might be a real old fart but it remains the most linear amplification device that we can work w/ practically. Any world class system will have it somewhere in the chain. Real good s.s. amps only approach that quality of sound.
FWIW. IMHO. YMMV. |
i, too, thank Ar_t for his candid & informative posts. Such info sharing has been a long time coming (we got a lot of this in the speaker forum from Roy Johnson when one member 'innocently' asked what the diff between time & phase coherence was & which one was more important. Many of you might remember that valuable thread) & much appreciated by us. The glimpse of the real inside scoop.
Herman, well put in your last post. I join you in asking your question.
"The issue here seems to be concern that Ayre is the one playing fast and loose with buzzwords, created just for marketing measures. I disagree. I believe that is others who are guilty of it, and perhaps Ayre is being cast in with them."
True, "others" have coined these terms. however, these "others" were EE & maybe even the IEEE! They were *not* engineers/designers making audio equipment to the general consumers. Amongst the EE, who are schooled in the art of ckt design, it is fine to use these terms. One is amongst peers of similar knowledge bases. But.......the consumer?
what i was insinuating in my last posts re. Ayre & other manuf loosely using the feedback terminology is that Ayre has done nothing to educate their clients on negative feedback, done nothing over time to dispel the doubts on this topic, done nothing to educate them on the types of feedback, done nothing to show them how their products are any diff from the others in a way that the layman comprehends it. All of this while they seemingly have been the longest term practising proponent of "zero feedback". good companies are responsible for not only good products but also for educating their customer base. Great companies are even reformers.
With audio hobby in a decline, the companies surviving in this realm should think of this (honestly educating the customers) as a self-preservation act.
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Muralman1,
i'm afraid that I'm unable to make any concrete comments on class-D amps. i've not heard one yet. |
Hey Ar_t,
I cut & paste an exerpt from your earlier post:-
"The crux: "current-feedback" amps have a bandwidth that remains constant, regardless of gain. Traditional feedback amps do not: as the gain increases, their bandwidth goes down."
This does *not* read correct to me! AFAIK, the gain-bandwidth product of a voltage-mode amp is fixed for a certain topology implemented by the designer & with a fixed set of devices. What I have seen tho is that as one increases the gain, the dominant pole frequency goes down & when the gain decreases, the dominant pole frequency increases. (you've got to have compensation somewhere in there to counter-act the phase shift of the music signal as it propagates thru the amp). However, the unity gain frequency, often used as the bandwidth of the amp, remains fixed. The unity gain freq can be increased by reducing the in-circuit parasitic poles (better devices), cascoding & by increasing the bias current.
Maybe I understood incorrectly. Perhaps you can clarify?
BTW, there is a company called Comlinear that makes current-mode opamps & other electronics. I don't think that it's audio grade stuff (but then it might be!) A lot of their circuits are based on the current conveyor (CC II) concept. I've never used them personally but their architectures sure look different than traditional voltage-mode amps. They also seem to be perhaps the lone star player in that field. |
agree, Herman, time to exit this thread.
Ar_t has been very helpful in revealing the dark secrets of amp design. I don't know if this "So why do you feel threatened when I tell the dark secrets of the world of amp design." was directed at me of Muralman1? IFFF it was directed at me, I'll say that I am not threatened in the very least. Au contraire, I stand to gain a lot more knowing the info shared in the thread. Your having 20 years of product design doesn't diminish my capabilities one bit. In fact, we can talk at the same level, which might be enjoyable.
Anyway, what should have been learnt from this thread is that manuf will do the bare minimum to disclose aspects of their design. Even good/very good manuf held in high(er) esteem by consumers. There is no standards committee in this audio consumer market that'll keep them honest. The only people who can do this is, us, the consumer. This will happen ONLY if we consumers are educated enough to ask them pointed questions that'll give us an idea of what the manuf might have done inside the product we are about to spend a ton of cash on. Like Ar_t said, if someone is putting down $3K on audio gear, expect some serious questions to be asked! Not unreasonable.
Also, don't simply eat hook, line & sinker what the manuf has to say about his product. Ask some serious questions. Try to understand their implementation.
By keeping manuf-consumer loop closed, both parties will benefit - we will supply feedback about their products, keep them honest & let them know where their performance is sub-par. Hopefully this will not fall on deaf ears & they'll make products that we are seeking using the best possible methods to implement them. You want audio to be around, we'd better help each other. No other way out. (you do this at your place of work with your suppliers, correct? why is audio any different?) |
