Bombaywalla, perhaps the point was missed by me. As far as realization, others may indeed come to the same conclusion and others may not. How ever I think it important to realize that our systems are complex and some attributes of sound that may be particularly pertinent to an individual may be better served due to technical or budgetery needs to use certain components that that may be better served by alternate means of amplification. Just so we undertand each other, I don't take any personal offense by your opinion. I just thought that another perspective was in order. Best Regards. |
I don't know of any designer that looks to feedback as a solution to bias stabilisation in a SS amp. Lowering noise, distortion and output impedance, yes. Increasing bandwidth and input impedance, ditto.
Maybe the guys who design tube gear do, but I don't hang around with any.
But since this was supposed to be about a "historical look at amps", and feedback...........let me ramble on some about the stuff that I have done.
Mid 80s.......overall feedback, but much less than usual. Maybe 10-20 dB.
Late 80s.....no loop feedback in the voltage gain stage, loop feedback for the outputs. (Similar to Stasis.)
Mid 90s.......no loop feedback anywhere.
Early 00s........"digital" amps. Tons of loop feedback.
Looks like we have come full circle. |
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? |
Bombaywalla, that solid state most often uses feedback, may not be the perfect solution to a problem. On some level it works. On the other hand these problems are not the only ones that exist in audio amplification as we know it. All designs have problems that have less than perfect solutions. Yet, on some level they work. If a particular solution doesn't work satisfactorly for you, well, then you have options. Some may have measurable issues, that sound good enough for you. Some may have currently immeasurable issues that sound sound poor to you. Fine, but, "sh$$", "sh$$!!!" and "cr**!" is purely subjective and is also fine. To extrapolate that to mean that those who have made different choices than you, is due to ignorance is well, eh, ignorant. BTW, I plead guilty to some level of ignorance, but, I don't think I've been brain washed by the twists & turn of marketing dudes any more than I have been by this most recent post. |
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.
|
How do you figure it has to do with stabilising?? Stabilising what?
And which way do you use for marketing, and which for precision? By precision, do you mean THD numbers of 0.00001%? Seems to me that is done for marketing, as the amps in the mid-70s using that approach sounded rotten.
Some of us design it so that it sounds best to us. Whether others agree or like it is another matter. ("Some" being most every amp designer that I know.) |
SS uses feedback to stabilise the circuit. In the more exotic designs this f/back is local. OTOH tube circuits can be stabilised w/out any type of feedback (even there it's not plain sailing -- it requires effort).
Objective measurements: let me insist that measurements are useful and can be reliable. Depends what you're measuring FOR, i.e. it's not always for sound reproduction.
If for marketing, then you go one way. If for guessing the precision of amplified signal, you go another way (i.e. checking into amp-speaker interface matters, et alia).
A Norwegian called R. Lian had looked extensively into this matter. To a certain extend so have we all: if I'm to check for amp performance, WHAT (and how) should I measure?? Cheers |
Keis, I believe it is impossible to build a practical amp using discrete fets that doesn't use some type of feedback. I'm going to need more proof than the fact their web site uses the term "zero feedback." I sent an email to Ayre asking them to clarify. I'll let you know if they respond.
Aball, my functional amp uses zero feedback. That is the beauty of the triode vacuum tube. They are linear enough to use without any feedback. |
The Ayre amps use local feedback, sometimes called degeneration, and no loop feedback.
The Stasis amps had no feedback from the speaker output to the signal input, but instead had feedback loops applied to "building blocks".
The new generation of so-called "digital" amps (the self-oscillating varieties) do have overall feedback loops from the output to the modulator. Some sense current, some voltage, some both. |
Aball, the Ayre products have no local or global feedback. That was my point. They now do exist and I don' believe Ayre will be alone for long. If you don't believe me go to Ayre.com (no I'm not an Ayre rep but I do own a K1xe.)
Ayre has zero feedback designs in their video circuits also. |
Be careful not to confuse global and local feedback. The comment:
"Feedback circuits have been with us since the 1920s and we are now just elliminating this basic design feature in modern amps and preamps"
is quite incorrect. There are designs with no global (aka "overall") feedback which does have some benefits - and thus some disadvantages too - but they definately still use feedback in the circuit. It is just applied around the transistors instead of around the circuit.
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. |
I still find the following quote from Nelson Pass most amusing. It comes from the owner's manual for my Pass Aleph 3, written early in the 1990s:
"When I started designing amplifiers 25 years ago, solid state amplifiers had just achieved a firm grasp on the market. Power and harmonic distortion numbers were king, and the largest audio magazine said that amplifiers with the same specs sounded the same.
"We have heard Triodes, Pentodes, Bipolar, VFET, Mosfet, TFET valves, IGBT, Hybrids, THD distortion, IM distortion, TIM distortion, phase distortion, quantization, feedback, nested feedback, no feedback, feed forward, Stasis, harmonic time alignment, high slew, Class AB, Class A, Pure Class A, Class AA, Class A/AB, Class D, Class H, Constant bias, dynamic bias, optical bias, Real Life Bias, Sustained Plateau Bias, big supplies, smart supplies, regulated supplies, separate supplies, switching supplies, dynamic headroom, high current, balanced inputs and balanced outputs.
"Apart from digitally recorded source material, things have not changed very much in twenty five years. Solid state amplifiers still dominate the market, the largest audio magazine still doesn't hear the difference, and many audiophiles are still hanging on to their tubes. Leaving
aside the examples of marketing hype, we have a large number of attempts to improve the sound of amplifiers, each attempting to address a hypothesized flaw in the performance. Audiophiles have voted on the various designs with their pocketbooks, and products go down in history as classics or are forgotten. The used market speaks eloquently: Marantz 9's command a high price, while Dyna 120's are largely unwanted." |
Actually, the leader of that movement was/is Nelson Pass, whose patented STASIS amplifier designs of the 1980s and 1990s were revolutionary in their time. They featured Zero Global Feedback and were available in both Pure Class A, such as the SA-3, SA-2, and legendary SA-1; and high bias A/AB, such as the S-300, S-500, and S-1000. |
