Designing Crossover for Two-way Speaker System

Nowadays, filter design has become relatively straightforward. If you have access to the frequency response data for your drivers, testing can be carried out using software. In the past, a certain level of knowledge was required to calculate the necessary cutoff frequencies for each component. However, with modern software, understanding how each component impacts the frequency response is sufficient, allowing you to input values and assess whether the curve aligns with your desired specifications. While there is no strict rule for the arrangement of components, numerous common configurations can be found online. A good starting point is to implement a low-pass filter for the mid-range driver and a high-pass filter for the tweeter, and then proceed to develop the rest of the circuit by strategically placing components.

This project began with my acquisition of some inherited speakers. Subsequently, I purchased used drivers and set aside the old ones. Upon inspecting the speakers, I noticed a lack of filtering. The connections were made with screw terminals, and a basic capacitor served as the low-pass filter for the mid-range driver. Additionally, there was insufficient attenuation within the speaker cabinet. It was clear that a renovation had to be done.

Supplies

For designing the filter, you will need:

• Frequency response and impedance curve data for each driver: Typically, these files can be obtained from the supplier or manufacturer's specification sheet. If you're working with older drivers, finding this data might be a bit more challenging. In my case, I couldn't find an exact match for the model number, but I did discover something that close to the model series. In such situations, Google can be helpful.
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• Simulation software: I use Xsim for this task, but there are other excellent alternative programs available. It's important to note that Xsim is compatible only with Windows.

You might also require:

• File-making software: I utilized FP-Graph Tracer for this purpose. This software is capable of converting curve drawings into files for frequency response and impedance. FP-Graph Tracer is compatible with both Mac and Windows.

Step 1: Tweeter

For my tweeter, which is a 25 TFF H515 6-ohm model from Seas, I could only locate a handmade drawing within a PDF document. Unfortunately, the drawing was not particularly user-friendly for my specific purpose. Nevertheless, this document did contain all the necessary details I required, even though it took me some time to comprehend everything.

The drawing within the PDF actually comprised two images in one. In order to prepare these drawings for use with the FP-Graph Tracer program, I did as follows:

1. Made a copy of the pdf file. Then flipped one of them upside down, and saved it as an impedance graph.
2. Zoomed in and followed the curves with a pencil to make them curve thicker and clearer. Additionally, I used different colors to enhance their distinction from the decade lines.
3. After these adjustments, I saved them as two separate files and subsequently opened them in FP-Graph Tracer for further processing.

Step 2: Mid-range

The mid-range driver, also sourced from Seas, is a P21 REX H402 model. Once more, I opted for the closest match I could locate on the internet. I followed the same procedure as I did with the tweeter's drawings, using different colors to delineate and distinguish the various elements within the drawings.

Step 3: FP-graph Tracer

FP-Graph Tracer is relatively straightforward to use. To begin, position the cursor in line with the outer ends of the chart. Fill in the lower value and higher value as required, and select logarithmic scaling. Then, proceed to click on the graph until the entire graph is marked with a new color.

This chart includes three graphs in one. These slightly different curves likely represent variations in the microphone's angle relative to the speaker during testing. However, for the purpose of this filter design, the central position holds the most significance. Thus, I focused on the curve associated with the highest magnitude, marked with 0 degrees.

In the impedance curve, the chart exhibits a slightly different format. The Y-axis follows a linear scale, while the X-axis shares the same axis as the frequency response, just in the opposite direction.

Once the correct graph is marked, and all the necessary ranges are set, the document can be saved in the desired file type.

Step 4: Designing Filter

The simulation software plays a crucial role in helping us determine the most suitable circuit for the filter. Our primary goal is to ensure that the different drivers cross over at the right frequency bandwidth, resulting in a total frequency spectrum that is as flat as possible.

Within the frequency response figure, we observe three distinct graphs: one for speaker 1, one for speaker 2, and one for the entire system.

The system graph represents the ultimate outcome as we construct the circuit. Our ultimate objective is to achieve a flat curve, enabling the speaker to reproduce the entire bandwidth effectively. However, achieving this with frequency-dependent components can be challenging. It is natural to expect a slight decrease below 100 Hz, primarily because the mid-range driver is not designed for lower-frequency reproduction. In fact, for modern music, it can be desirable to have a minor positive boost in the lower bass frequencies, typically within the range of 100 Hz to 500 Hz. Fine-tuning adjustments like these are often better accomplished through the amplifier's EQ section. Nevertheless, I found it necessary to bump up around 100Hz to keep more of the lower spectrum less attenuated.

Several considerations should be kept in mind when designing the circuit for audio filters:

1. LC Filtering in Series: It is preferable to use LC filtering, involving components such as inductors and capacitors, in series with the speaker. This choice is made because resistors tend to introduce more noise. However, it is perfectly acceptable to use resistors in parallel with the speaker since they are involved in filtering out specific frequencies.
2. Resistors for Adjusting Magnitude: Resistors can be valuable for lowering the magnitude of the frequency response. For instance, the tweeter is typically louder than the mid-range driver due to the physics of the drivers. The tweeter's smaller membrane requires less energy.
3. Power Consumption and Component Tolerance: It's important to consider how the system responds to changes in amplifier power. Paying attention to the power consumed by the filter components, especially resistors, which may draw a significant amount of power. I am aiming for low values on these resistors and ensure they have a reasonable tolerance for power consumption. When dealing with amplifier outputs, I am looking for the RMS power rating, which can often be found in the spec sheet. Inputting this value into the simulation software allows you to assess the power consumption of each component in the chart. If a resistor requires a high power tolerance that is not readily available, it is possible to employ two resistors in parallel to distribute the current flow, thus allowing the use of smaller tolerance resistors.
4. Crossover Frequency: The crossover section between the tweeter and mid-range driver should typically be situated around 2 - 3 kHz. However, in my case, it's slightly higher, at around 3.8 kHz. This gave the system a flatter curve than it would otherwise. This adjustment allows the mid-range driver to handle a broader range of frequencies that the tweeter might otherwise have been responsible for.
5. Polarity Reversal: Experimenting with the polarity of one of the speakers can significantly impact the system's overall response. In my case, inverting the connection of the tweeter made a substantial difference around the crossover point, leading to an improved curve.

Step 5: Building Filters

Now that the filter schematic is complete, the next step is to source all the necessary components. During my component search, I discovered a local supplier specializing in components designed for audio systems. While it's possible to use standard components from general electronic suppliers, there can be potential issues such as resonance frequency, EMI (Electromagnetic Interference), microphonics, and others. To avoid these problems, I preferred to select components that are specifically suited for the audio system's requirements.

For constructing the circuit, I had some PCB prototype boards on hand. However, these inductors are quite heavy, and the prototype board alone is not sturdy enough to support their weight. To address this, I 3D-printed some supports and attached them to the prototype board. The prototype board is secured in place with screws in each corner and a few more around the heavier components. Additionally, to ensure the inductor coils remain firmly in place and to reduce tension on the solder joints, I used a bit of double-sided tape.

For added convenience and serviceability, I connected cables to terminals to make these filters more user-friendly and easier to maintain.

Step 6: Making Speaker Grill

The speaker grills for these speakers had become old and worn, prompting me to seek out a new retro-style grill that could be easily attached with magnets. In my search for a retro-style dust cloth, I came across a linen-based cloth on Aliexpress. To create the frame for this cloth, I designed it using Fusion360 and then printed it in multiple parts. In each corner of the frame, there's space for a magnet, which is attached with double-sided tape. The cloth is wrapped around the frame and secured with screws on the interior. Additionally, there's a structural feature on each side of the frame that fastens the cloth to a track on the interior.

Throughout this project, I drew inspiration and received valuable guidance from various YouTube sources. I'd like to give a special shoutout to Kirby Meets Audio, Impulse audio and Toids DIY Audio.