In this application note, we will show you how to design an active 2-way loudspeaker with the miniDSP 2x4 HD. Using the 2x4 HD, you can either create a conventional (Linkwitz-Riley or Butterworth) crossover, or a linear phase crossover.
- 1. What you will need
- 2. Select the speaker drivers and design the enclosure
- 3. Getting connected
- 4. Set up routing
- 5. Measure and equalize the drivers
- 6. Add the crossover (conventional)
- 7. Add the crossover (linear phase)
- Wrapping up
1. What you will need[Top]
- A miniDSP 2x4 HD (boxed or kit) together with the matching 2x4 HD1 plugin.
- Ability to run acoustic measurements. You will need a measurement program such as the freeware Room EQ Wizard (REW), and measurement hardware for which we recommend the UMIK-1.
- Four channels of amplification. Two stereo amplifiers can be used or a single multichannel amplifier.
This is the block diagram of the 2x4 HD1 plugin. A good approach is to use the output channel PEQ (parametric EQ) to correct for the response of the individual drivers, and the input channel PEQ for overall response shaping and taming room issues. The Xover (Crossover) block can be used to implement a conventional (Linkwitz-Riley or Butterworth) crossover, or the FIR block used to implement a linear phase crossover.
2. Select the speaker drivers and design the enclosure [Top]
If you are starting from scratch, you will need to select the drivers for your speakers. There are literally hundreds of drivers available for DIY use at all price levels, so it's impossible to give specific recommendations here. For a small two-way loudspeaker, a 5" or 6.5" woofer and a 1" dome tweeter are common choices. Peruse the online forums to see what others are using and to ask for recommendations for your particular project.
If you are building your own box, you will need to design it. The most important factor is the internal volume, and if it's a ported box, the size and length of the port. Fortunately, there are a number of free programs that do the complex math for this based on the Thiele-Small parameters of the woofer. For example, a popular Excel-based program is Unibox.
If you are modifying an existing speaker from passive to active, then you have the enclosure and the drivers already. In this case, you will most likely need to remove the internal crossover and add a second pair of binding posts.
3. Getting connected [Top]
This diagram shows how to connect everything up. (Please follow the procedure in the User Manual on initializing the miniDSP and familiarizing yourself with the 2x4 HD1 plugin before doing this.)
It's recommended to put a capacitor in series with each tweeter as shown. This will help to protect the tweeter from any turn-on and turn-off surges from the amplifier, or if you accidentally send low frequency test signals to it.
4. Configure routing [Top]
The 2x4 HD1 plugin allows any input to be routed or mixed to any output. This a key element of its extraordinary flexibility. To implement a two-way crossover, set up the routing as shown in this screenshot. (See the User Manual for how to rename the input and output channels.)
5. Measure and equalize the drivers [Top]
Once you have built the box and mounted the drivers, you will need to measure the drivers one at a time. (You only need to do this for one speaker.) For more information on how to measure a loudspeaker driver, please see our app note Measuring a loudspeaker with the UMIK-1.
Use the PEQ blocks on each output channel to shape the response of each driver so that it is flat over its operating range. Use "Peak" type filters to flatten peaks (with negative gain so they create a notch) and "High-Shelf" and "Low-Shelf" type filters to straighten out the overall response.
Below is an example measurement for a woofer. The various features of the measurement and the areas to correct are marked on the graph, along with the corrected response in light blue. If using an indoor measurement (as in the example), be careful not to correct for peaks and notches caused by the room.
Here is a graph of the tweeter, measured before and after correcting its response. When performing a tweeter measurement, start the sweep at a frequency so as not to strain the tweeter (for example, start at 1 kHz, not 20 Hz).
6. Add the crossover (conventional)
There are two options for implementing the crossover: IIR (conventional) and FIR (linear phase). In this section we will describe the IIR version.
To open the crossover settings screen, click on the Xover button on the Outputs screen. Set a low pass filter on the woofer and a high pass filter on the tweeter. As a starting point, try using Linkwitz-Riley (LR) 24 dB/octave filters. You can then experiment with lower or higher slopes, from 6 dB/octave up to 48 dB/octave.
Then you can measure the response of the complete speaker! Use the output level controls to match the signal levels from the woofer and tweeter. If you have a notch at the crossover frequency, you probably need to invert the polarity of one driver. You will most likely need to fine-tune the crossover settings to get the smoothest response around the crossover frequency:
- Time align the drivers
- Move the filter corner frequency of one driver up or down a little
- Use an asymmetrical crossover, for example BW 18 dB/octave lowpass on the woofer and LR 24 dB/octave high pass on the tweeter
- Adjust the equalization of one driver or the other near the crossover point
This REW plot shows the response of the woofer and tweeter in our example speaker with crossover filters in place, and the combined response after crossover fine-tuning:
7. Add the crossover (linear phase)
Instead of a conventional crossover, the 2x4 HD can be used to implement a linear phase crossover. Each output channel has 1024 taps accessible by clicking on the FIR button on each output channel.
Use the excellent freeware program rephase. See "Example 2: a linear-phase crossover" in the app note The rePhase FIR tool for a tutorial example. Note that for the 2x4 HD1 plugin, you will need to set the taps parameter to 1024 and the rate parameter to 96000. (Instead of 2048 and 48000 as shown in the app note example.)
Here is an example two-way crossover from that app note:
That's it for this app note! Have fun, and please let us know about your active loudspeaker experience in our forum.