The DDRC-88BM plugin includes miniDSP's flexible and powerful signal processing functionality on each output channel, along with full signal routing/mixing. This can be used to implement active speakers.
Note: configuring the DDRC-88BM plugin for active speakers should be done before performing your Dirac Live calibration.
The example project [Top]
Figure 1 illustrates a multichannel active system in which four channels from an A/V receiver or processor are input into the DDRC-88A: left front, right front, center, and LFE/subwoofer. The front and center speakers are active two-ways. A great many other combinations are also possible – with a single DDRC-88A you can implement four two-way active speakers, two four-ways, or two three-ways and a two-way. You can even add on multiple subwoofers and optimize them as described in Optimizing multiple subwoofers with the DDRC-88BM and Multi-Sub Optimizer. For more complex systems, multiple DDRC-88A units can be used.
Figure 1. Connection of an example active multichannel system
Note: when connecting power amplifiers directly to speaker drivers, a protection capacitor in series with each tweeter is strongly recommended.
1. 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. 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.
2. Set up the Mixer tab [Top]
The Mixer tab is the key to implementing an active speaker system. This is how to set it up for the example system:
The output from Dirac Live channel 1 is sent to output channels 1 and 2. These will be used for the woofer and tweeter of the left front speaker, as indicated by the labels at the top. (The names are changed on the Outputs tab.) The right and center speakers are set up in the same way. The two remaining output channels are used to drive two subwoofers.
The Routing tab doesn't have to be set up specially for this application. However, since only four input channels are used, this can be indicated like this:
3. Measure and equalize the drivers [Top]
You will now need to measure the drivers one at a time. (You only need to do this for one speaker.) For information on how to measure a loudspeaker driver, please see the app note Loudspeaker measurement with UMIK-1 and REW.
Then 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 (e.g. start at 1 kHz, not 20 Hz).
You will generally want to use the linking feature on the PEQ blocks to make the same settings for corresponding left and right speakers. With the example system, link channels 1 and 3, and 2 and 4.
4. Add the crossover
Open the crossover settings screen by clicking on the Xover button on the Outputs tab. For a two-way speaker, set a low pass filter on the woofer and a high pass filter on the tweeter. For a three-way speaker, the midrange driver will need both high pass and low pass filters. 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.
Now measure the response of both drivers together. Use the output level controls to match the signal levels from the woofer and tweeter. You may need to fine-tune 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 with crossover filters in place, and the combined response after crossover fine-tuning:
Wrapping up [Top]
Once that's all done, you can position the speakers in their proper positions, play music and listen to the results! You may wish to continue to experiment – for example, by trying a different crossover frequency or slopes. It is easy to do A-B auditions by setting up different configurations and switching between them with a remote control.
If you have spare output channels, you may also want to consider using them for additional subwoofers and optimizing as described in our app note Optimizing multiple subwoofers with the DDRC-88BM and Multi-Sub Optimizer.
Once that's all done perform perform your Dirac Live calibration as described in the User Manual (pdf). Then set up your bass management as described in the app note Bass management with the DDRC-88BM. Have fun, and let us know about your experiences in our forum.