====== Advanced Nested Architecture: what it is, what it isn't, and whether it's important ======
TL;DR: It's not important.
This is our first installment of "From the Engineer's Desk." If you like it, please let us know. (Tweak this text depending on the platform: "Like" for FB, etc.)
Now that our NP1 power amplifier and CA1 control amplifier are shipping, we thought it might be a good time to discuss some of the technical highlights in these products, starting with Advanced Nested Architecture.
Advanced Nested Architecture (ANA) is our name for the nested feedback technique we are using in the NP1 and CA1. It may show up in future designs as well.
===== Nested architectures in general =====
In a classic non-nested class-AB audio amplifier design, there are a number of voltage and/or current amplifier stages connected in series with a single global correction loop that wraps around all stages.
In a classic nested architecture, there are two or more subsystems, each with a correction loop of its own, that are then present inside the global "wrapper" correction loop. Thus, a typical nested audio power amplifier may consist of a small-signal input stage with some feedback around it driving an output stage that is a conventional power amplifier with some feedback around it, with a final correction loop from the output of the output stage to the input of the input stage.
Pedantic digression: Any non-trivial control system is technically made up of several feedback-controlled subsystems, and if there is a loop around them they are therefore technically a nested architecture. Your regular power amp has an input stage that likely has some emitter or source degeneration. That's feedback. The driver stage probably does as well. That's feedback. Etc.
The distinction with what I and others call nested architectures is that the subsystems can be lumped together into two or more major subsystems that are each controlled by correction loops of their own. A final global correction loop around all stages completes the design.
It's not spectacularly challenging to make nested audio architectures work. However, the same can't be said regarding making them work //well//. There is a lot of devil in the details.
===== ANA =====
ANA, then, is our specific approach to designing nested architectures that aims to optimize performance for dynamic music signals. In a nested architecture, there are different ways you can distribute stage gains and the frequency shaping (a.k.a, compensation) required to keep things stable and clean. ANA simply identifies our approach to resolving this juggling act. [Elaborate]
Regarding specifically the NP1 and CA1, the output stage is a conventional class-AB amplifier that operates at very close to the desired final system gain. The input stage is a high-performance operational amplifier that operates at close to open loop at low frequencies and is contoured at high frequencies to yield well-behaved output over a range of output levels. The global correction loop involves some additional high-frequency compensation to improve transient characteristics. The result is a very linear and robust system that were are ecstatic to see is making people very happy.
===== Q & A =====
==== Q: Isn't this just an opamp in front of a power amp? ====
No. The crucial distinction here is that the power amp is in the opamp's correction loop. This yields at least two advantages. First, especially at low frequencies, the amount of loop gain increases dramatically, which yields significant gains in correction for the power amp's residual non-linearity. Second, the overall system performance is dominated by the opamp's input stage, and recent opamps have gotten very, very good. You essentially get small-signal linearity with large signal outputs.
==== Q: Isn't this just a classic composite or hybrid design? ====
In a way yes, but there is a crucial difference between a classic composite or hybrid design and a nested architecture. In a classic composite design, there is essentially only one feedback loop -- around the entire circuit. Anything else is there just to make things work. In a nested architecture, there are effectively three (or more) important loops: the one around the input stage, the one around the output stage, and the one that encases both. Does this matter? Not especially, at least to a first approximation. In both cases, the effective total correction is the same; it's just distributed differently. In practice, a nested architecture may make the input stage work less vigorously than a classic composite equivalent because a bulk of the output stage's correction has already been performed by the output stage's correction loop.
==== Q: Is this new technology? ====
A: Not really. Nested architectures have been around since about the time feedback theory was first formalized, and they can be found in many fields. However, their use in commercial power amps is rare to nonexistent, so there's that.
==== Q: So what makes ANA special? ====
The effort required to balance all the factors to get optimal performance out of real-world nested implementations is immense. It's relatively easy to make nested architectures work. It's an entirely different matter to get them to work in the real world, with real world devices and real world loads. Design is always about tradeoffs. We spent a good amount of time working to make sure the tradeoffs were all the right ones for amplifying music.
==== Q: Does an amplifier now need to use ANA to be good? ====
No. ANA is just another approach to designing amplifiers. In the case of the NP1 and CA1, it was the right choice given the design goals and constraints. It may not be in applications with different goals and constraints. That said, we are having a lot of fun spinning different designs with it and are consistently impressed with the results.
==== Q: Will you provide me with a schematic of the amp so I can see exactly what you are doing? ====
No.
==== Q: Your answers seem a bit vague and possibly even evasive at times. Is this deliberate? ====
It might be.