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In Reply to: RE: Hey, why not ?!? Attractive pre's for reasonable cost posted by Feanor on June 04, 2022 at 05:38:50
And bucks the usual trend employing a fully discrete, low feedback class A output stage!
Follow Ups:
I don't recall anything one way or the other about discrete gain stage. But however achieved, distortion is extremely low, (per Denafrips).
Dmitri Shostakovich
I don't recall anything one way or the other about discrete gain stage.
... If Denafrips gets the low distortion they claim using limited feedback, and I don't think this is achieved simply by avoiding op amps.
Dmitri Shostakovich
Use inherently linear devices that don't require boatloads of *correction*!
Companies have been doing that for decades. ;)
Ah! you mean TRIODES!
the voltage-in to current-out transfer curves all deviate from a straight line in some way or another. But, if you can get the voltage gain you want with only small current pertubations then distortion can be low. This works for a preamp but not for a power amp!
FETs & tubes have square-law transconductance, BJTs are exponential - none are linear. In general, if you arrange things to allow the signal current excursion to be along only a small part of the transconductance curve that might be linear enough (i.e. very strong class A biasing). Or, add some degeneration (local feedback) - that will straighten things out.
Whenever audio designers say 'no feedback' they mean no global feedback around multiple gain stages. Any designer who eschews local degeneration for audiophile sensibilities is not really a designer! Except in the case of tubes - if you degenerated the circuit you would not be able to hear the differences in non-linearity between tube types so clearly. That is a win-win, poor circuit design* gets passed-off as voicing/tuneability and everyone is happy:)
*I am joking about poor design. If it is what customers want then it is the right thing to do.
Edits: 06/06/22
According to Boyk and Sussmann, the triode actually follows a 3/2 law and not a quadratic law like FETs. This makes them somewhat more linear.
I'm not technical enough to understand the mathematical nuances but I understand the gist of it: if a little feedback can be bad, a lot of it is great. Yes, I know, your device must have enough "open loop gain" to sustain a lot of feedback.Follow the link below to find it ...
Dmitri Shostakovich
Edits: 06/07/22
That is a good article but a bit esoteric. It is not applicable to our discussion here about the inherent linearity/non-linearity of active devices but it is about global feedback around a whole amplifier and how to add more. To keep stability, usually open-loop gain is rolled-off from a very low frequency so there is a lot of 'excess gain' at low frequencies to correct distortion but at higher frequencies less correction can be made. Mr Putzey presents a method to increase the gain at higher frequencies and still keep the closed loop system stable.
I believe the idea goes back to Bob Cordell for audio amplifiers but, I suspect, the concept itself is a lot older in the general field of system design and servo control.
Unfortunately my lack of technical and math savvy means most of the article is mumbo-jumbo to me.
Putzeys's final conclusions, viz. that is essentially that there is no such thing as too much feedback, and that includes multi-stage feedback loops as well as local strike me as valid based my listening to my Purifi amp. ("Degenerative" is term I hadn't hear before reading this thread).
E-Stat got on about the supposed inherent linearity of amplifier components which you, Duke, addressed. However E-Stat's fundamental agenda is anti-feedback in general; this is misguided, IMHO, based on Putzeys and other commentators of the last couple of decades. E-Stat's archaic views on feedback are the basis for his categorical hatred of typical, integrated op amps which is, according, also misguided.
Dmitri Shostakovich
My categorical hatred is based on hearing literally dozens of high feedback amps.
I have heard a number of Putzey's designs where of course he is putting his theories into practice. The sound? Not musical.
E-Stat is not misguided, no-feedback designs often (but not always) sound closer to real music. High feedback designs have yet to produce a single example I could point to where I say "yes, it can be done this way"
Listening to audiophile preference for over 20 years I understand there are many who prefer the "musicality" of tube amps (and some S/S amps). Descriptions often include sound that's full, smoother, and (sometimes) having layered or "holographic" imaging. I have heard these qualities myself from certain equipment and understand what those folks are talking about. I agree that people are entitled to like these attributes.
Your explanation is that negative feedback robs the sound of these qualities because it causes higher order harmonic distortion.
My hypotheses is what what mainly contributes to these attributes isn't exclusively the absence of high order harmonics but, rather, the presence of 2nd and/or 3rd order harmonics.
I believe that lots of feedback reduces all harmonics and that includes high level harmonics. But it also greatly decreases 2nd & 3rd order harmonics and I believe that is why many do not like ultra-low distortion amps such as Purifi or Benchmark.
Dmitri Shostakovich
Lots of feedback creates more and more harmonics, even if very low in level, that acts like an artificial noise floor, which is signal modulated...because it's not really noise.
Reminds me of what we jokingly called "accu jack" back in the engineering department in my early career. It was a pneumatic analog servo controlled throttle actuator for a car gas pedal. If the PID control wasn't set just right accu jack's shaft would cycle in and out rapidly and uncontrollably.
Edits: 06/08/22
the reproduction of music is what matters most. :)
Enjoyment/engagement are what I prize most and I'm not too worried if it means my system is coloring/supplementing the source. I suspect many feel that what they prefer must be objectively better too and that might not be the case. Audio is broad church that allows many philosophies and it is not like we're talking about aircraft landing radar where it either works or people die! So, some things in audio that don't work so well still have a place and our world would be duller without them.
nt
Dmitri Shostakovich
Same .002 THD+N @ 1kHz (using zero feedback) per Stereophile review :
To be sure, the SP20 adds a phono preamp, etc.I'm not sure what Reina means, though, when he says, "The result is the SP20 ($9000), a fully balanced, pure class-A preamplifier that uses no overall loop negative feedback."
Is "no overall" feedback the same as Zero feedback?
Meanwhile the Athena claims THD+noise an order of magnitude lower than the SP20, i.e. 0.00012% vs. 0.002%. Per 6Moons they claim "Low", not zero, feedback. But I'll wait for Amir to the measurements to confirm the Denafrips' claims, (I might have a long wait).
Anyway, I don't suffer from 'feedbackphobia' to the extent you do.
Dmitri Shostakovich
Edits: 06/06/22 06/06/22 06/06/22 06/06/22 06/06/22
I use measured specs [from ASR, Stereophile, etc] mainly as a starting point to weed out obviously defective products. I don't suffer from op-amp or feedbackphobia but I do look at the overall design to see where the designer's head is, but in the end its the sound that counts most to me.All three ARC "tube" preamps that I once owned suffered from solid-state-itis. To be expected I guess from what were essentially JFET solid-state preamps with a tube buffer bolted on the output. Additionally these all used $2.00 IC chip volume controls. You have to move into the large five figure price range before ARC gives you a better volume control. I'd much prefer a good ALPS pot or better yet, discrete resistor volume control. Many preamp manufacturers do too, including all of the 'high-end' established brands I listed below. As they say, the volume control makes or breaks the preamp.
If these ARC "tube" preamps sounded like tubes to me, I wouldn't be so critical of them. But they didn't. To me a REAL tube preamp doesn't just bolt a couple tube buffers onto the output stage as ARC does. BAT doesn't do this. CJ doesn't do this. Aesthetix doesn't do this. Manley doesn't do this. VTL doesn't do this. VAC doesn't do this. Cary doesn't do this. Etc. Again, you have to move into the large five figure price range before ARC gives you a REAL tube preamp with hopefully a better volume control. And I would hope that these five figure ARC tube preamps sound like tubes.
Edits: 06/06/22
That is Sonic Frontiers Line 1 SE+ which has no s/s stages. But, OPPS !! , it does have PGA2311 volume control chips that have op amps, and very robust S/S power supply.
It's very transparent and very neutral but not very tube-like. I'm sure morricab would hate it.
Dmitri Shostakovich
BAT makes a very good preamp. Aesthetix too. Not as big a fan of CJ or VAC pres.
Check these out though for something different:
https://aries-cerat.com/incito-product-page/
https://aries-cerat.com/incito-s-product-page/
Very low distortion, tube rectified, transformer coupled output triode preamps.
Incito has resistor ladder volume control Incito S has transformer volume control
Most preamps have multiple stages with a unity gain output buffer being the final one. The objective is lowering output impedance and the 6H30 is well suited to that function. Different companies choose tube or SS for either gain or buffer.VTL and PS Audio take the opposite approach of the SP20 by *bolting* a MOSFET buffer to the 12AX7 or 12AU7 based gain stage for the 7.5 or BHK Signature .
And I would hope that these five figure ARC tube preamps sound like tubes.
My goal is for components to sound neither *like* tubes nor SS.
Edits: 06/06/22
(at 4X the cost)Completely hand assembled in Minnesota vs automated assembly in China.
Is "no overall" feedback the same as Zero feedback?
Local, aka "degeneration" vs loop feedback between multiple stages. Explained by Nelson Pass
Anyway, I don't suffer from 'feedbackphobia' to the extent you do.
Avoidable with small signal circuits and useful in low values (15 db) with power amps for overall stability and lowered output Z.
Edits: 06/06/22
No integrated circuits in the gain stage.I guess the trend you are referring to is the use of op amps.
Dmitri Shostakovich
Edits: 06/04/22 06/04/22
But lets not ignore the fact that those Burson "discrete" op-amps that some folks love all use huge amounts of negative feedback just like IC op-amps.
Let's be aware that the Burson discrete op amps are designed, (in varying degrees depending on the model), to mimic tubes by causing highish amounts of 2nd & 3rd order distortion. For instance see this distortion spectrum for the Burson V3 (as reported by Sparkos labs, see link) ...
Contrast that with the result for the Sparkos SS3602 which happens to be what I prefer in my VTV Purifi amp with VTV buffer ...
I tried Burson V6 Vivids, (supposedly less tube-like), in the VTV but found detail and dynamics to be compromised vs. the Sparkso.
BTW, unlike E-Stat I'm not obsessed about the evils of feedback nor of op amps, (be they integrated or discrete). Old fashioned thinking, (see E-Stat & Morricab), holds that negative feedback is bad because it causes high-order distortion -- and so it does!!. But while small to moderate amounts of feedback cause high-order HD, lots of feedback reduce all orders of distortion, high as well as low.
Dmitri Shostakovich
Edits: 06/06/22 06/06/22 06/06/22 06/06/22
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