Also, THX amps are all like 6 watts per channel. As I just said in the other thread, give me a 100-150$ 1 watt AAA headphone amp!
You essentially already have one in the Atom, AFAIK. IIRC the Atom and Heresy and THX tech are all feed-forward topologies. The engineers on here can comment further, but my basic understanding of their respective designs is they all execute basically the same amp cicruit idea in slightly different ways.
Feed back - Circuit/Mechanism adjusts itself depending on load.
In other words: Feed back designs do not make assumptions about the load side, they are built to adjust themself to it.
Feed forward - Circuit/Mechanism reacts to the input in a predefined manner.
In other words: Feed forward circuits expect the load to be a certain way, loss of control signal or the load requires cut-offs/safeties.
The Schiit Heresy is feed forward. The Op-Amps in the output stage do not care for anything near them. They do what the input Op-Amp feeds them.
The JDS Labs Atom could be feed back. Black PCBs suck when you want to follow the traces… TI uses a feedback resistor in their reference circuit.
The SMSL SP200 THX Amp is another black PCB hides interesting bits. In the datasheet for the Op-Amps used, audio application is more of a side note. As with the Atom, TI has a feedback resistor in their reference circuit.
TL;DR
Schiit Heresey → Feed Forward
JDS Labs Atom → Most likely feedback.
SMSL SP200 → Impossible to say without taking a multimeter to it
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Opamps are usually designed around around inverting feedback configurations. I wouldn’t call them feedback a the signal ratio between the input resistor and the feedback resistor is how one sets the application state. As long as the op amps used aren’t past their slew rate limit they just work. To reduce the need and worry about being slew rate limited, multiple gain stages are cascaded (hence how the Schiit Fulla 3 -> Hel -> Heresy scale up in power output).
The THX amps do something different but the op amp designs are literally virtually all the same minus manufacturing difference with the op amps and how well the matching can be done by the time they’re manufactured (it’s so small that quality components with op amps should not need matching).
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Might be a language barrier, I do not understand what you try to tell me.
Negative feedback circuit in its simplest form:
This is the “typical application” for op-amps.
In the JDS Labs Atom, it looks like so:
Path from output of the TI LME49600 is red. At the Via, it R5 is negative feed back to the negative pin of the JRC NJM2068. No idea what the NJM2068 feeds without taking a multimeter to it.
In case of the SMSL SP200: No idea what is going on.
Would need this area as high-res as possible.
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Excellent! Yeah, the non inverting feedback is what I was getting at and something I wouldn’t call “feed forward”. Generally, feed forward is used in FM transmitters and is a pain because they’re moderately unstable and have a frequency multiplier like function. That screen shot, perfect and showing exactly what I’m talking about and shows literally how simple op amp designs can be.
It is not feed forward, it is feed back 
The controll circuits for feed forward (when done properly in discrete hardware) are rather complicated. And to be honest, I do not see any point in using them for audio.
FM gear is a beast I am not educated nor qualified to talk about.
To quote Texas Instruments:
Proper board layout is crucial to getting the maximum performance
As in: A bazillion capacitors and resistors in places where short traces should be is key to a good circuit. No Capacitors for marketing wank sake and so on. Everything should have rhyme and reason.
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I think we’re oversimplifying things here a bit.
Feedback as far as my limited understanding goes accomplishes basically 2 things for an amp. Noise reduction, since you feeding back some part of the noise introduced by the amplifier back, and reduction in the output impedance.
The reduction in noise is possibly the most interesting part, because it removes 2nd harmonic noise introduced by the circuit, and that’s often considered to make an amplifier more pleasing to listen to.
Feedback in most audio amplifiers is often local (around a single amp section) AND global (around several sections).
There seems to be a fair amount of debate in the audio amp design community, as to how much feedback and of what sort is the right way to go. That seems to range from feedback good to amps should not have any feedback.
Interestingly the no feedback camp seem to be at the higher end of the spectrum, where they can reduce the noise by spending more on component quality, and The PSU.
I saw an interview with Nelson Pass who had a somewhat more pragmatic view point than most I’ve seen, his view was to use it to tweak an already good circuit rather than trying to fix a bad one with it.
The THX AAA is supposed to be a feed-forwards design, but that’s not unique it’s been used in audio amps (albeit less frequently) since at least the 80’s.
It helps, but I need it to extend further to the right.
Apart from that, very good photo.
Not realy. The first photo you posted has the section in question in very crisp detail. The global shot is as good as the one I have myself (and is clear as mud when you zoom in).
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Local feedback is standard. Not just because everyone just does it, but because it helps the circuit correct itself. Even then, you need to keep traces for feedback as short as possible and resistors as close to the input pins as possible. Else you have stray capacitance (causing oscillation) or pick up interference.
Global feedback is an entirely different beast and IMO not adviseable for audio applications due to the immense frequency spectrum.
Negative feedback resistors
The way I learned it, Vout = Vin * (R2 / R1) with R1 being input resistance and R2 being feedback resistance. R1 and R2 should be as close as possible to the Op-Amps Vin- pin.
Vin is line level, Vout is what the headphone or following stage gets.
“No feedback” only works when either the user sets a range (which will be fairly limited) or the control circuit can aquire some info about the load (making it a feed back IMO).
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There are a lot of “Zero” feedback amp designs in the multi thousand dollar amps. You’ll commonly see even SS amps with 0.1% distortion or worse, and output impedances in the 1 Ohm range.
They’re all discrete designs of course.
Half the first watt stuff is 0 feedback.
Seems to be a philosophical point for some audio amp designers.
I guess it’s similar to the push/pull vs SE thing, push/pull eliminates any noise introduced by a PSU, but it also eliminates 2nd harmonic distortion, which is sometimes considered a bad thing.
Yeah, that’s another… audiophile problem, lol.
(Really interesting, though!)
(He says, while listening to music going through a tube buffer.)
In control theory we call that proportional controllers. Normally the Class A - D designs off hand all do not use feedback in the designs. With op amps, you MUST use feedback loops. It’s just inherent in their design and operation (for analog signals).
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In reply to the question this thread raised:
Drop + THX AAA ONE Linear Amplifier
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Yup.
As long as they don’t just shoot themselves in the foot by selling it 200$ lol.
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This didn’t age well
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149$ for the preorder at least. Overpriced for some, endgame for others. 
Sure thing is, here, the consensus appears to be “don’t buy THX if you have 500$+ headphones, you’ll just waste your money”.