TL;DR: It's not important.

Now that our NP1 and CA1 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 power amplifier and CA1 control amp. It may show up in future designs as well.

This post tries to clarify what nested architectures are and what makes ANA special.

Nested architectures in general

In a classic non-nested class-AB audio amplifier design, you typically have a few voltage and current amplifier stages connected in series with a single global control loop that wraps around all stages.

In contrast, in a nested architecture, there are two or more series connected subsystems, each with a control loop of their own, that are then present inside an outer "wrapper" control loop. So, a typical nested audio power amplifier may consist of a small-signal input amplifier with some feedback around it driving a power stage that is a conventional power amplifier with some feedback around it, with a final feedback loop from the output of the power stage to the input of the input stage.¹

It's not all that challenging to make nested audio architectures work. However, it's quite challenging to make them work well. There is a lot of devil in the details.

Advanced Nested Architecture (ANA)

Advanced Nested Architecture (ANA) 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 (i.e., compensation) required to keep things clean and stable over all operating conditions. ANA simply identifies our approach to resolving this juggling act.

In the case of the NP1 and CA1, the output stage is a conventional class-AB amplifier with a gain that's very close to the desired overall system gain. The input stage is a very carefully selected, high-performance operational amplifier that runs at high gain at low frequencies and is contoured at higher frequencies to yield well-behaved output over a range of output levels when the global control loop is closed. The global 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.

That's it.

But you have questions ...

Q & A

Q: Isn't this just an opamp in front of a power amp?

No. The crucial distinction here is that there is a feedback loop that wraps both the opamp input stage and the power amp circuit. This yields at least two advantages. First, especially at low frequencies, the amount of loop gain increases dramatically, which yields significant correction gains for the power amp's residual non-linearity. Second, the system performance is dominated by the opamp's input stage, including interactions between the input and the source driving it. And some recent opamps have gotten very, very good.

Q: Isn't this just a classic composite design?

There is a crucial difference between what people typically call composite designs and what we consider to be a nested architecture. In a classic composite design, there is essentially only one feedback loop -- around the entire circuit. Everything else is there just to make things work. So, you might have an opamp input stage that's running open loop driving a power amp stage that's also running open loop, with one big loop around everything and some compensation parts sprinkled throughout to keep things stable.

In a typical nested architecture, there are typically three important loops: one around the input stage, one around the power stage, and one that encapsulates both. In addition, with a nested architecture the power stage typically will have a gain not horrendously different from the overall system gain.

Does this matter? Not especially, at least to a first approximation. In both cases, the effective correction can be about the same; it's just distributed differently. In practice, a nested architecture may make the input stage work less vigorously because a bulk of the power stage's correction has already been performed in the power stage.

Q: Are nested loops a new technology?

Not really. Nested architectures have been around since about the time feedback theory was first formalized, and they have been used in many fields. However, their use in commercial consumer power amps is very rare to say the least.

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.

Q: Will you provide me with a schematic of the amp so I can see exactly what you are doing?

No.

Q: Does an amplifier now need to use ANA to be good?

The marketing answer is, "Yes, definitely. It obsoletes everything that came before it."

The engineering answer is, "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. In an application with different goals and constraints, it may not be. That said, we are having a lot of fun spinning different designs with it and are consistently impressed with the results."

Now, pick the answer the makes you the happiest.

---