Design of an active 2-way loudspeaker with DSP and active crossover

Design of a 2-way active loudspeaker with miniDSP

In this section, we'll show you how to design a 2-way active loudspeaker with a 4-output miniDSP processor. This app note is applicable to:

• 2x4 HD (miniDSP-2x4-HD plug-in)
• DDRC-24 (DDRC-24 plugin)
• miniDSP Flex (with miniDSP-2x4-HD or DDRC-24 plug-ins)
• SHD Series (SHD Plugin)

If you use the miniDSP-2x4-HD plug-in, you can implement a conventional crossover (Linkwitz-Riley or Butterworth) or a linear phase crossover. If you use the DDRC-24 or SHD plug-in, you can implement a conventional crossover (Linkwitz-Riley or Butterworth); you will be able to use Dirac Live to correct the impulse response, perform room correction and customize the final sound.

Contents

• What will you need
• Select speaker drivers and design the enclosure
• Connect
• Set the path
• Measure and equalize drivers
• Align the drivers
• Add crossover (conventional)
• Add crossover (linear phase)

• What will you need

A 4-output miniDSP processor. See the list described above. Refer to its User Manual and make sure you have the DSP software up and running before starting this read.
Ability to perform acoustic measurements. You will need a measurement program such as the free Room EQ Wizard (REW) software and measurement hardware for which we recommend the UMIK-1 or UMIK-2.
Four amplification channels, i.e. 2 stereo power amplifiers or 4 mono power amplifiers.

Below is the block diagram of the MiniDSP-2x4-HD plug-in. We'll use output channel PEQ (parametric EQ) to correct individual driver response and input channel PEQ to shape overall response and to tame room issues. The Xover (Crossover) block can be used to implement a conventional crossover (Linkwitz-Riley or Butterworth), or the FIR block used to implement a linear phase crossover.

Annotated miniDSP HD block diagram

Next is the DDRC-24 plugin block diagram. The SHD plugin is very similar. In this case, we will use the PEQ (Parametric EQ) output channel to correct the response of individual drivers and allow Dirac Live to do overall response shaping as well as room correction. The Xover (Crossover) block will be used to implement a conventional crossover (Linkwitz-Riley or Butterworth).

Annotated DDRC-24 block diagram

• Select speaker drivers and design the speaker

If you're starting from scratch, you'll need to select 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 advice here. For a small two-way speaker, a 5" or 6.5" woofer and 1" dome tweeter are common choices.

If you are building your own loudspeaker, you will need to complete the design first according to your tastes and needs. The most important factor is the internal volume and, if it is a speaker with reflex ports, the size and length of the port. Fortunately, there are a number of free programs that do the complex calculations for this based on the woofer's Thiele-Small parameters. For example, a popular Excel-based program is Unibox.

If you are changing an existing speaker from passive to active then you already have the enclosure and drivers. In this case, you will most likely need to remove the internal crossover and add a second pair of binding posts.

• Connect

This diagram shows a typical connection, using the 2x4 HD as an example:

active two-way speaker miniDSP active speaker

We recommend placing a capacitor in series with each tweeter as shown. This will help protect the tweeter from being switched on and off by the amplifier or accidentally being sent low frequency test signals.

Generally with super tweeters capacitors from 1.0 to 1.5mF are used.

ATTENTION: When carrying out important imports or changes, it is always advisable to switch off the final amplifiers to avoid damaging the drivers.

• Set the input/output paths

Each plug-in allows you to route or mix any input to any output. This is a key element of its flexibility. To implement a two-way crossover, set up the routing as shown in this screenshot.

miniDSP two-way active speaker routing

• Measure and equalize drivers

Once you've built the speaker and mounted the drivers, you'll need to measure the drivers one at a time. (Do this for one loudspeaker only.) For more information on how to measure a loudspeaker driver, see "Speaker Measurement with UMIK-1 and REW".

Use PEQ blocks on each output channel to shape each driver's response to be flat across the operating range. Use "Peak" type filters to flatten peaks (with negative gain to create a notch) and "High-Shelf" and "Low-Shelf" type filters to straighten the overall response. You can also do this with the help of the Room EQ Wizard's AutoEQ feature.

Ideally, flatten the response to an octave above or below the crossover frequency (for woofers and tweeters, respectively).

miniDSP active two-way speaker Example of parametric equalizer

Below is a measurement example for a woofer. The various characteristics of the measure and the areas to be corrected are marked on the graph, together with the correct answer in blue. If you use an internal measurement as in our example, be careful not to correct the peaks and notches caused by the room: you will perform the room correction later, when the loudspeaker is in its actual position in the room.

example woofer miniDSP two-way active speaker

Here is a graph of the tweeter, measured before and after correcting its response. When taking a tweeter measurement, start the sweep at a frequency that does not stress the tweeter (for example, start at 1 kHz, not 20 Hz).

Example of miniDSP two-way active speaker tweeter

• Align the drivers

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's gain trim control to reduce the level of the driver that measures higher. Since you've flattened the driver responses, this shouldn't be difficult - just measure both (one at a time) and make sure the responses overlap near the crossover frequency.

In most cases, it will be necessary to align the drivers over time to achieve a good phase match at the crossover. See our section "Time Aligned Speaker Drivers with UMIK-1 and UMIK-2".

• Add crossover (conventional way)

To implement crossover, there are two options: IIR (conventional) and FIR (linear phase). In this section we will describe the IIR version.

Click the Xover button on the Output tab. Set a low pass filter on the woofer and a high pass filter on the tweeter. We suggest using 24 dB/octave Linkwitz-Riley (LR) filters, but slopes from 6 up to 48 dB/octave can be used. Here is an example of a low pass filter for the woofer:

miniDSP low-pass filter active two-way speaker on woofer

Here is an example of a high pass filter for the tweeter:

miniDSP high-pass filter two-way active speaker on 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:

• Slightly adjust the delay between drivers.
• Adjust the EQ of one driver or the other near the crossover point.
• Use an asymmetrical crossover, such as BW 18 dB/octave low pass on the woofer and LR 24 dB/octave high pass on the tweeter.

This REW graph shows the response of the woofer and tweeter in our example speaker with crossover filters in place, and the combined response after the crossover is tuned:

Combined example of miniDSP two-way active loudspeaker

• Add crossover (linear phase)

Instead of a conventional IIR crossover, the MiniDSP-2x4-HD plug-in can be used to implement a linear phase crossover. Each output channel has 1024 taps accessible by clicking the FIR button on each output channel.

Use the excellent freeware program rephase. See "Example 2: Linear Phase Crossover" in the "The rePhase FIR Tool" section for a tutorial example. Note that for the MiniDSP-2x4-HD plug-in, 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 example from the section).

Here's an example of a two-way crossover from that app note:

example of linear phase of a miniDSP two-way active loudspeaker

• In the end...

Once you're done, place the speakers in their designated positions in your listening room, then perform room correction and customize the final sound:

If you are using the MiniDSP-2x4-HD plug-in, use 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 ambient modes. Do this carefully and mostly use cuts rather than boosts.

Good music!