Hi everyone, reading this thread made me think that some of you might not know how to properly make a crossover so here it is.
In making a RC (resistor-cap) crossover circuit you should know first how every components work. Every positive and negative connection or complete loop to a driver is called a circuit. Connecting a resistor parallel with the driver, decreases the resistance of the circuit. While connecting it in series increases the resistance. Meanwhile, the capacitor determines whether the crossover will be a low-pass (only allows frequency below the cut-off frequency) or a high-pass filter (only allows frequency above the cut-off frequency). Frequency cut-off (fc) is where the filter starts to drastically curve down the frequency sent to the driver. You should connect the capacitor in series with the driver if you want it to be a high-pass filter, or connect it in parallel if you want it to be a low-pass filter.
Now in making a crossover circuit, you should determine which frequency should you cut each of the driver in your iem. This should be the same or close to the cut-off frequency (fc) of the next driver. For example, a low-pass @220Hz for the subwoofer and a high-pass @200Hz for the midrange driver. It's better for those two fc to overlap than to have a gap. That's because of the phase shift that's happening the more you get far from the fc. I won't dwelve to much with that but that's just the way it is.
Now, this frequency cut-off for an RC filter has a formula, Fc=1/(2πRC). R being the overall resistance in a loop/circuit including the impedance of the driver at the cut-off frequency you wanted (see the graph in datasheet, you should also consider if there is too much fluctuation at your desired frequency cause this will affect the frequency roll-off of the circuit), and C as the capacitance of the capacitor you need to use. You need to keep your resistor lower if you want your iem to be sensitive rather than to make it a power hungry beast. That means you should mostly adjust the capacitance than to resort on using higher resistor.
The formula in finding the right value of your capacitor is C=1/(2π x fc xR). The numbers x10-⁶ or 10^(-6) in the result means you need a capacitor in microfarad (uf) and x10-⁷ or 10^(7) if you need it in hundreds of nanofarad(nf). But most of the time it would be in microfarads. Just use the nearest available capacitor in your kit or just add the capacitance of different capacitors by connecting them parallel to each other (not in series). Just make sure that the result would be close enough.
Now, the resistors in a crossover circuit are primarily used to formulate your desired cut-off frequency. If you want to decrease the volume of the driver, either you use a higher resistor (connected to the driver in series) or you use an l-pad.
L-pad is used to decrease the volume of the driver while maintaining the resistance R across the circuit. It uses both a parallel and series resistor. Here are the formulas and an example from chatgpt:
Remember, the resistance R in the formula fc=1/(2πRC) is the overall impedance. With a series resistor, the R is the sum of impedance plus the resistor.
For using a parallel resistor, the formula is R=(Id x Rr)/(Id + Rr). Id being the impedance of the driver @ the desired cut-off frequency while Rr is the resistance of the resistor. With a resistor in parallel, you cannot just add the resistance together like in a series connection. When you add a resistor in parallel, you add another path for the current to go. It's like opening another gate. Thus, it decreases the overall resistance and also lowers the power received by the driver making it sound a little quieter.
Though resistor in parallel is commonly used in high-pass filter and has the benefit to stabilize the impedance in the circuit, it's not always necessary. You can just use it to lower the overall resistance R if the impedance of the driver is high enough for you to achieve your desired cut-off frequency and use a resistor in series if the resistance is too low. Just add it after the capacitor.
But still resistors are not specifically used to decrease the volume of the driver. It will only work like that if you don't have a capacitor connected, thus it is not a crossover circuit but merely an attenuator. But of course, the general rule of thumb is that using higher resistors will still lower the volume of any driver may it be in a crossover circuit or not. That's why you can see some manufactured iems use a large tantalum capacitors in their iems. They're keeping the resistance lower and just using higher capacitors to compensate.
"But how about if the impedance of the driver itself is already good enough for the equation?"
Good question, if the impedance (Ω) and volume of the driver itself is good enough, then resistors are not necessary anymore. Meaning, you can have both high-pass and low-pass filters without any resistors. Resistors are just used to alter the computation. So, you would only need a capacitor to be connected either in parallel or in series with the driver. Except of course if you want to lower the volume of the driver. Then you would use an l-pad to keep everything in place.
Here are some examples of my own crossover circuits:
3.3 Ω resistor in parallel and a 10uf in series for a high-pass filter to be used in a tweeter. The impedance of the driver at that frequency is 7Ω. The cut-off frequency is close enough to 7kHz. I just did trial and error from my available resistors (2.2Ω and 1.1Ω in series to produce 3.3Ω).
10Ω resistor in series with the driver with a impedance of 27 at 200Hz. I used a 20uf capacitor to be used as a low-pass filter for the subwoofer.
4.7Ω resistor in parallel with a midrange driver with a 15Ω impedance at 200Hz. I used a 220uf tantalum capacitor to be used as high-pass filter. I used a large capacitor to retain a low resistance across the circuit. But here, as an example, we can just use a 50uf capacitor without any resistor connected to still produce a cut-off frequency of around 200Hz.
Hope this helps, if there were anything wrong on what I said just feel free to correct me. For those who haven't still learn how to use crossover circuits, use this technique together with other techniques like tube length and diameter to see further improvements in your frequency graphs. Lastly, if you want to make a dip in your frequency, just search the formula in getting the tube length and diameter of the tubes to be used. I know there is, I have read that. Just use some AI like chatgpt to search for it. I know crossover circuits would take up space, but still if you can fit it, it would be better. Hopes for your improvement. That's all thank you!
