Audio Note AX-Two Crossover Project

Two coils, two caps, and a resistor, arranged across two drivers with the purpose of dividing the energy between them evenly. This is what I came up with after several months of experimentation. I tried almost everything, but it was the series crossover network that brought resolution and peace to my weary soul.

We don’t often see series dividing networks in commercial or DIY applications. This breed is even more obscure than tube amplification and analog playback put together. But here we go — the sidewalks along the beaten track are sometimes more desirable.

Setting off like Alice in Wonderland, my initial excitement caused by bringing the crossovers out of the enclosures gradually dried up, giving way to the sober realization that the possibilities for improving the performance of the original crossover network were rather limited. When I say this, I mean that I really tried a variety of things — not just different parts, but also tweaking values up and down, experimenting with different filter orders, attenuation slopes, and whatnot. In the end, I reached the point where I had to admit that the original parallel network was simply spot on, and I wouldn’t touch it except to use higher-quality parts.

Despite the confirmation that the original crossover, developed by Andy Whittle at Audio Note UK, was optimal, I was still unsatisfied with the performance and was scratching my head as to what to do next. Out of curiosity and desperation, I eventually decided to give the series crossover a try. After some research, I came up with what should have been a series approximation of the original parallel dividing network. During my research, I noticed that there is a lot of controversy surrounding the topic of parallel versus series crossovers, but I got the impression that, due to their subtractive nature, series crossovers are harder to design and model than their parallel counterparts.

Even though it was far from optimal, the initial prototype showed quite a lot of promise. I was struck by the impression that I was listening to a single point source — the tweeter and woofer seemed to work as one unit. Suddenly, things became more focused, dense, and saturated. It was a new and completely overwhelming experience. As it turned out, this promising start was the beginning of a journey that took me several months to complete. I am glad I went through it, and I would like to share some tips on crossover development that I learned along the way.

A Few Tips for Beginners

1. Never give up on the series crossover network as an alternative to the parallel one before you have even tried it.

2. Do not start with complex, high-order arrangements. Begin with simple first-order filters and try to get the best out of the imposed limitations before continuing with increased complexity.

3. Don’t bother spending time on impedance linearization networks or voltage dividers before you have fully explored the basic design. In my experience, every element or group of elements connected directly across the driver’s terminals has mostly adverse effects.

4. Don’t take for granted the superiority of certain components based on their price tag and number of likes. In my experience, common wire-wound coils outperform both backed-varnished and copper-foil types, which also cost more.

5. Pay attention to how the total DC resistance in the woofer’s signal path affects the fine balance between bass extension and midrange clarity. Since the DC resistance influences the total Q of the system and is mainly contributed by the coils in series with the woofer, one can fine-tune a circuit by choosing coils wound with wire of a specific diameter. For this project, I chose a 0.47mH coil made with 1.2mm wire by Monacor and another 0.68mH coil made with 1.4mm wire by Mundorf. This combination gives a balanced blend of qualities that I really enjoy.

6. Use resistors of good quality. They are usually not expensive compared to the other parts, but they contribute greatly. I find Ohmite wire-wound non-magnetic resistors superior to the common MOX types.

7. Use a breadboard for rapid prototyping, fitted with connecting terminals, so that you can easily swap parts on a whim.

8. During the evaluation process, use program material of a wide variety. A good dividing network is able to draw attention to the performance itself, not to the sonic artifacts. Even lesser records become thoroughly enjoyable, which is one of the most desirable features.

9. Start a project diary. As the project progresses, a stage will inevitably be reached where you might think, “That’s it!” Days or even weeks will pass before you decide to change anything. It is important, before making any changes, to draw a schematic with component values and everything in order to have a snapshot of the current state. Also add a short description of the improvements you hear compared with the previous version. Keeping track of the milestones is important, as it will serve as a version control system that allows you to revert to an earlier iteration if something goes wrong. It will also give you an overview of the actual progress.

No measurements of any kind were performed during the development process; the network was tuned completely by ear. Relying entirely on my senses to assess the reproduction qualities allowed me to concentrate fully on the perception of the playback, but sometimes longer pauses were needed, including visits to live performances as well as references back to the original crossovers for comparison. During the process, there were a few intermediary stages when I was already quite satisfied with the results, and it was only curiosity and the urge to experiment further that pushed me forward.

The proposed second-order series crossover network shares a common tonal balance with the original parallel one used in Audio Note AX-Two stand-mounted loudspeakers. The parallel crossover impresses with exemplary transparency and layering, to the point that it feels somehow immaterial and holographic. As a consequence, it could direct the listener’s attention toward sonic artifacts and provoke analytical reactions. The series network, on the contrary, delivers a more focused, dense, and saturated presentation that manages to convey the atmosphere of a real performance better.

Although I find the series crossover vastly superior, it could also be a matter of individual preference. Designer preference, along with the fact that series networks are harder to model and measure, might give us some explanation as to why this type of network has not received wider acceptance. As far as I know, series filters are used by very few people in the industry. One is Tony Gee, whose capacitor test page is quite popular among audio hobbyists. The other is Misho Mironov, known for his artisan products and patent on a series dividing network based on a transformer, whose primary winding performs the low-pass filter function while the secondary serves as an HF attenuator.

This piece might have been named “In Praise of the Series Dividing Network” as well, because my intention was to tell you how good this breed of network sounds. I would be delighted if someone built that network to try it with existing Audio Note AX-Two speakers or simply used it as a starting point for a future project.

Update — July 2021

I came full circle and eventually returned to the original parallel crossovers. These are simply unbeatable. They strike such a fine balance that I simply could not match it with the series version, so I gave up on it.

Update — January 2022

Finally, the AX-Two crossover project was completed. As mentioned earlier, the original parallel crossover network was chosen as the basis for this build.

The fact is that I was disappointed. I didn’t like their sound at all — it was quite congested, mechanical, and heavy. My assumption was that the solid-core copper wire was coloring the sound, so I rewired the entire circuit, with the exception of the ground bus, using Duelund silver foil in silk wire. And voilà — what a nice surprise! The sound was transformed completely. It opened up nicely, revealing plenty of detail without being analytical in any way. I have been using them for more than a month now, and I can’t deny that I am very satisfied.

Thank you for stopping by, and as always, happy listening!