In this application note, we will show you how to design an active 2-way speaker with a 4-output miniDSP processor.
- 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. Align the drivers
- 7a. Add the crossover (conventional)
- 7b. Add the crossover (linear phase)
- Wrapping up
1. What you will need[Top]
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A 4-output miniDSP processor, such as:
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miniDSP Flex (with or without Dirac Live)
Whichever one you choose, be sure to refer to its User Manual and make sure you have all the software installed and working before starting this app note!
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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 or UMIK-2.
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Four channels of amplification. Two stereo amplifiers can be used or a single multichannel amplifier.
Below is the block diagram that applies for processors without Dirac Live. We will 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 to tame room issues. The 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.
Next is the block diagram that applies to processors with Dirac Live. In this case, we will use the output channel PEQ (parametric EQ) to correct for the response of the individual drivers, and allow Dirac Live to perform overall response shaping in addition to room correction. The Crossover block will be used to implement a conventional (Linkwitz-Riley or Butterworth) 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 speaker, a 5" or 6.5" woofer and a 1" dome tweeter are common choices. Search online forums to see what others are using and 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 a typical connection, using the 2x4 HD as an example:
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 miniDSP Device Console allows any input to be routed or mixed to any output. This a key element of its flexibility. To implement a two-way crossover, rename input and output channels and set up the routing as shown in this screenshot:
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 speaker driver, see our app note Loudspeaker measurement with UMIK-1 and REW.
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. You can also do this with the aid of the AutoEQ feature of Room EQ Wizard.
Ideally, flatten the response to an octave above or below the crossover frequency (for woofer and tweeter respectively).
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 our example, be careful not to correct for peaks and notches caused by the room – you will do room correction later when the speaker is in its actual location in 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. Align the drivers [Top]
You will probably need to adjust the level of one of the drivers to match the other. Usually, this is best done by using the output channel gain adjustment control to reduce the level of the driver that measures highest. Since you have flattened the driver responses, this should not be difficult – just measure both of them (one at a time) and make sure that the responses overlap near the crossover frequency.
In most cases, you will need to time-align the drivers in order to get good phase-matching at the crossover. See our app note Time-aligning speaker drivers with UMIK-1 and UMIK-2.
7a. Add the crossover (conventional) [Top]
To implement the crossover, there are two options: IIR (conventional) and FIR (linear phase). In this section we will describe the IIR version.
Click on the Crossover button to access the settings. Set a low pass filter on the woofer and a high pass filter on the tweeter. We suggest that you use Linkwitz-Riley (LR) 24 dB/octave filters, but you can use slopes from 6 up to 48 dB/octave. Here is an example low pass filter for the woofer:
Here is an example high pass filter for the tweeter:
Now measure the response of the complete speaker! You may need to fine-tune the crossover settings to get the smoothest response around the crossover frequency:
- Adjust the delay between the drivers slightly.
- Adjust the equalization of one driver or the other near the crossover point.
- Use an asymmetrical crossover – for example, BW 18 dB/octave lowpass on the woofer and LR 24 dB/octave high pass on the tweeter.
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:
7b. Add the crossover (linear phase) [Top]
(This section applies to the 2x4 HD and Flex without Dirac Live only. It does not apply to the DDRC-24, SHD Series, Flex with Dirac Live, Flex Eight or Flex HT.)
Instead of a conventional IIR crossover, the FIR filter blocks on the output channels can be used to implement a linear phase crossover.
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 HD and Flex without Dirac Live, 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:
Wrapping up[Top]
Once you're done, put the speakers in their designated locations in your listening room! Then perform room correction and tailor the final sound:
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If you are using a 2x4 HD or Flex without Dirac Live, use the input PEQ blocks. For example, use a low-shelf filter to add a little bass boost. You can also use peak filters to compensate for room modes. Do this carefully and mostly use cuts rather than boosts.
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If you are using a DDRC-24, Flex with Dirac Live, or SHD Series processor, run a Dirac Live calibration. Use the target curve to tailor the overall sound signature.
That's it for this app note! Have fun, and let us know about your active speaker experience in our forum.
