Dual sub integration using the MiniDSP

This very comprehensive application note was written by community member Neutro on AVSForum. We're very fortunate to see dedicated members taking the time to write a great step by step like this one.
To those of you who have ways/ideas on a little tutorial, make sure to send it to us. And once again a big "Merci" to François!


 

The goal of this thread is to discuss a handful of topics regarding the integration of one or more subwoofer, using a MiniDSP 2x4, to your average home theater setup. While the MiniDSP is a great platform to perform subwoofer equalization, this won't be discussed here in detail. Rather, I want to focus on how to:

  • Perform the actual connexions and setup of the MiniDSP and subs in the system
  • Use the MiniDSP as a splitter to simplify the setup for multiple subwoofers
  • Easily level-match multiple subs using the MiniDSP
  • Tweak the sub integration with the mains using the MiniDSP
  • Add a house curve on top of whatever EQing is performed by the AVR

 

 

Gear

The required gear for this tutorial is simply:

  • An AVR
  • Up to four subwoofers
  • An appropriate number of subwoofer cables
  • A MiniDSP 2x4
  • A MiniDSP plugin

The basic MiniDSP 2x4 is sufficient for the job but of course, other models can also be used, which would allow for integrating even more subwoofers. Many MiniDSP plugins can also do the job; basically the following plugins have all the required blocks and can also be used to EQ the subs to boot:
  • 2-way Advanced Crossover
  • 2.1-way Advanced Crossover
  • 3 or 4-way Advanced Crossover
  • 2x8, 4x10 and 10x10 Crossover

Note that the sub channel in the 2.1-way Advanced Crossover plugin would not be used. This is because the MiniDSP platform can be used as a digital crossover for mains, hence why it has two inputs and four outputs. We won't use the MiniDSP that way, and thus simply require one output per sub. This means that a single MiniDSP 2x4 can be used to level-match and delay-adjust up to four subs, provided a splitter is used to feed the subwoofer channel to the two inputs. In the rest of this tutorial, I will use the 2-way Advanced Crossover. Note that upon buying a plugin, MiniDSP will send a download link via email: the plugin is not factory-installed on the MiniDSP itself.

Setup

The physical setup depends on the AVR capabilities and, of course, the number of subwoofers. In all case though, connections are fairly simple.

Powering the MiniDSP

The MiniDSP can be powered via USB or using the DC Phoenix-type block connector on the back. Once included in the setup, it has to be powered for the subs to get any signal. USB power can come from a computer or a standard phone charger. DC power can come from just about any wall-wart type power supply capable of 4.5 to 24 V. If used, the terminal from the DC power supply should be cut and bare wires inserted in the Phoenix connector using the correct polarity. Failure to use the correct polarity will probably damage the MiniDSP. Note that in my experience, connecting and disconnecting the USB cable (thus powering on and off the MiniDSP if the back connectors are unused) does not lead to any loud pops and is thus safe to do with the subs turned on.

Connecting a single subwoofer

Connecting a single subwoofer to the MiniDSP is as simple as it gets. The MiniDSP sits between the AVR and subwoofer as in the figure below, with the AVR's sub pre out being fed to Input 1 on the MiniDSP, and Output 1 of the MiniDSP linked to the line level input of the subwoofer:

Connections with miniDSP - nanoAVR - Subwooferx1

Connecting two subwoofers

Assuming the AVR in the setup has a single sub pre out, connecting a second subwoofer simply involves plugging it into Output 2 of the MiniDSP as in the figure below. The MiniDSP thus acts as a splitter.

Connections with miniDSP - nanoAVR - Subwooferx2

If the AVR in the setup has two sub pre outs, but cannot distinguish between individual subs (i.e. the two outputs are just part of an internal splitter), then the easiest way to integrate the MiniDSP is still to only use one of the AVR's two subwoofer outputs and let the MiniDSP assume the role of splitter.

However, if the AVR can distinguish between individual subs, as is the case with AVRs equipped with Audyssey SubEQ HT, then for it to be able to treat the subwoofers independently, it is required that each AVR sub pre out be connected to one input of the MiniDSP, as in the figure below. Using the 2-Way Advanced Crossover plugin, Input 1 of the MiniDSP will be linked to Outputs 1 and 2, while Input 2 will be linked to Outputs 3 and 4. It is thus necessary to plug the second subwoofer to Output 3.

Connections with miniDSP - nanoAVR - Subwooferx2 - Audyssey

Connecting up to four subwoofers

Since the two inputs of the MiniDSP 2x4 are totally independent (for example, using the 2-Way Advanced Crossover plugin, there is no way to send signals from Input 1 to Outputs 3 and 4), it is necessary to use an external splitter before the MiniDSP (and thus using its two inputs) to connect more than two subwoofers on the MiniDSP, as illustrated below:

Connections with miniDSP - nanoAVR - Subwooferx4

Connecting the MiniDSP to a computer

The MiniDSP can operate without being connected to a computer. However, configuration of the plugin must be done by plugging the MiniDSP to a computer via USB. During configuration, unless the MiniDSP is connected to a DC source, it will then be powered by the computer's USB port.

Plugin exploration

The plugin application can be started even if the MiniDSP is not connected to the computer, and can be safely explored this way. Even when the MiniDSP is connected to the computer via USB, no firmware will actually be loaded on the MiniDSP unless the green sync button on the top right is clicked. When started, the application should look like the screenshot below:

miniDSP 2 way crossover plugin

This tutorial is not meant as a complete tour of the 2-Way Advanced CrossoerPlugin and of MiniDSP applications, and thus, we will concentrate on the plugin's block diagram below the four tabs. Each yellow box is a clickable module in which parameters can be changed.

The plugin's architecture is quite clearly mapped in this block diagram. Both inputs share the Input Gain block, meaning they are affected by it simultaneously. Downstream from the Input Gain block, the two branches of the plugin don't cross again. In that regard, as mentioned before, the output pairs (1 and 2 vs 3 and 4) are completely independent with the 2-Way Advanced Crossover plugin. Using one or two subs, the bottom branch of the diagram will be completely unused, unless the AVR is SubEQ HT-equipped, in which case the two branches' independence becomes handy.

Downstream from the Input Gain block, there is a first Parametric EQ block in each branch. On the top branch, this block is common to Output 1 and Output 2; on the bottom branch, it is common to Output 3 and Output 4. Thus, any filter programmed in this block will simultaneously affect the connected outputs.

This is in contrast with the Parametric EQ blocks that are downstream from the Crossover block. These further PEQ blocks are specific each output. Finally, the Delay/Gain/RMS block is a single block, but can be used to adjust the gain, delay and polarity of each output independently. It can also be used to mute each sub independently using a single click.

Configuring the Crossover block

The first step to configuring the plugin is to click on the green Sync button and wait until the pluggin application is synced to the MiniDSP. When synced, any change to the plugin will be loaded and effective immediately on the MiniDSP.

Our first task will be to configure the Crossover blocks so that they act as a perfect splitters instead of the default crossover. This is essential to all subwoofer configuration since by default, unneeded low or high pass filters are applied by the Crossover blocks. Note that if Outputs 1 and 2 only are used, the second Crossover block need not be adjusted.

The internal structure of the 2-Way Crossover block is illustrated in the diagram below:

Internal structure of the 2-way crossover

The block is thus a splitter, followed in each branch by a low-pass and a high-pass filter. By default, the upper branch has the high-pass filter enabled and the lower branch has the low-pass filter enabled.

Clicking on a Crossover block brings us to the 2-Way Crossover block menu. The lower part of the application, below the plugin's block diagram, is now split in two. On the left are the parameters for the four filters; on the right is a frequency response diagram for the block (the top and bottom outputs are in different colors).

Our goal is to bypass all filters in order to simply have a perfect splitter. First click on the "CH1 BandPass filter OUT1" button, and click on the two BYPASS buttons. Do the same after selecting the "CH1 BandPass fitler OUT2" button. Now the Crossover block should have no effect short of just splitting the signal, as illustrated in the screenshot below:

miniDSP 2-way crossover plugin

At this point, the signal from the AVR's sub pre out should be able to reach all subs without being affected by any filter in the path.

Level-matching multiple subs

(Note: AVRs equipped with SubEQ HT automatically level-match two subs; this step can thus be ignored in such setups. It can however be used in conjunction with SubEQ HT to level-match pairs of subs in setups with more than two subwoofers.)

When using multiple subwoofers, one of technique is to ensure that all units play test tones at the same level at the listening position. Typically this operation involve lots of fiddling with the sub's gains, which can be tiresome when controls are hard to reach or when subs are far apart. Ensuring that the combined output of multiple subs reach the right level can be equally equally tiresome. The MiniDSP and 2-Way Advanced Crossover plugin can be used to elegantly level-match all subs without having to fiddle with the subs' gains nor with any setting in the AVR.

The first step is to set the subwoofer gains (on the sub's amps) at a relatively high level -- the MiniDSP can only attenuate the signal it gets from the AVR. In my own case, this allows me to set the gain on the subwoofers to the maximum setting, as recommended by the manufacturer. All subs can be set to the same gain.

The second step is to click on the Delay/Gain/RMS block corresponding to the connected subs in the plugin application. The lower part of the application now displays level meters for the two outputs for that block, along with a few other controls. Note the very handy Mute buttons, in particular.

To level-match the subs, first mute all subs but one and start playing a calibration tone. For example, when starting Audyssey, it typically asks the user to set the subwoofer gain in order to get 75 dB, with the measured level displayed on the TV. While only one sub is playing, adjust the volume to 75 dB using the slider along the meter for the sub that is playing. Now repeat the operation for all sub, muting all others and adjusting the sliders so that they all produce 75 dB at the listening position.

miniDSP 2-way crossover plugin

Now all subs, individually, are set at the proper volume. Unmute all subs and click on the Input Gain block. The calibration tone should be much louder with all subwoofers playing. Note that if two subs playing together don't sound louder than a sub playing alone, there could be a driver polarity issue, with the subwoofers actually playing out of phase. Interestingly, the 2-Way Advanced Crossover plugin can be used to reverse the polarity of each output signal, using the "Invert" button in the Delay/Gain/RMS blocks.

The objective is now to lower the overall volume until the 75 dB target is achieved while keeping the same balance between the subs. This is easily done by clicking on the Input Gain block and lowering the gain slider until the target level is reached. Thus, each sub is individually set to 75 dB using the output gains, and the common input gain is used to adjust their combined level to 75 dB.

Tweaking delays

(Note: AVRs equipped with SubEQ HT automatically adjust the delays of up to two subs; this step can thus be ignored in such setups. It can however be used in conjunction with SubEQ HT to adjust delays of pairs of subs in setups with more than two subwoofers.)

Many AVRs will be able to adjust the delay of each channel, including the subwoofer, so that all sound waves arrive in synchrony at the main listening position. In particular, failing to correctly adjust the delay of the subwoofer channel may introduce cancellation between the mains and the subwoofer in the crossover frequency range. Verifying this require either a good hearing and lots of experienc, or more reliably, a frequency response measurement. Frequency response measurements are not covered in this tutorial, but can be performed rather easily using an Omnimic system, or using the free Room EQ Wizard (REW) software from HomeTheaterShack and one of the many available microphones or SPL meters fit for the job. MiniDSP's own UMIK-1 microphone is particularly well-suited for the job.

The best way to resolve dips in frequency response due to cancellation between subwoofers and mains is to use real-time analysis (RTA), which is available in REW. Once engaged while playing pink noise, the dip caused by cancellation can be lessened or even totally removed by tweaking the delay of each output in the plugin's Delay/Gain/RMS output blocks. Note that due to the nature of subwoofer frequencies' interaction with the room, substantial delay can be necessary to improve cancellation. The equivalent distance (obtained by dividing the speed of sound by the delay introduced, and displayed in the plugin controls) can be quite difference from the physical distance between the subwoofers and the listening position.

Introducing a house curve

AVRs equipped with subwoofer equalization capabilities such as Audyssey MultEQ XT and XT32 most often will only EQ the frequency response flat. While other technologies such as Audyssey DynamicEQ and THX Loudness Plus can alter the flatness of the equalization to introduce more bass at lower listening levels, sometimes user still prefer an additional oomph.

Most often, the subwoofer(s) will be ran "hot", meaning that either the gain of the subwoofer(s), or the AVR's subwoofer trim level, will be inched a few dBs higher to reflect that preference. While very simple to implement, the caveat is that the boost will affect equally all frequencies played by the subwoofer(s). The MiniDSP can be used to easily tweak the balance of those frequencies using the Parametric EQ blocks of its plugins.

Note that the Parametric EQ blocks are independent, and if more that two outputs of the MiniDSP are used, it may be required to copy the parameters from one block to another. This can be done manually, or in some cases using the Copy buttons in the Parametric EQ blocks. The Parametric EQ blocks upstream from the Crossover blocks can thus be made to quickly share the same parameters. The Parametric EQ blocks downstream from the Crossover block can also be copied in pairs: Output 1 with Output 3 and Output 2 with Output 4.

Each Parametric EQ block can introduce up to six biquadratic ("biquad") parametric filters. The frequency response of each block is conveniently displayed while adjusting the parameters, which can be entered either via the user interface, of by importing a text file (which itself can be generated by REW). The parameters can also be introduced by hand using the same syntax found in this text file using the "Advanced" interface, with the help of the Biquad Calculator tool provided in the plugin application under the tab of the same name.

Using Parametric EQ filters (PEQs) to flatten the frequency response can be tedious and is better left to software such as REW. However, tweaking a flat response to introduce a house curve can be done by adding a single PEQ to the setup, as done in the figure below:

miniDSP 2-way crossover plugin

In that example, dual subs are connected to Outputs 1 and 2. Thus, the Parametric EQ block upstream of the Crossover block is used to introduce the house curve, in order to simultaneously affect the two outputs. Filter EQ1 is selected -- this is the only filter having a non-zero gain. All filters having a zero gain are effectively bypassed, but each filter can be explicitly bypassed using the "BYPASS" button without changing their parameters.

The parameters are set as to provide a gently-rising house curve that gradually boosts the frequency response to reach +6 dB at 20 Hz. Note that this provide substantial reinforcement of the deep rumbles produced by sub-audible frequencies. To achieve this, a low-shelf filter type has been selected, with a 40 Hz corner frequency, a gain of 8 dB and a Q-factor of 0.5. Those parameters can be adjusted on-the-fly in any situation (including when watching a movie or music), and the effect of the filter on the frequency response can be seen instantaneously. It is thus easy to adjust the shape of the house curve even with limited knowledge of what each parameter will do. Moreover, it is very useful to develop an intuition about what the parameters actually mean.

Another type of house curve that is easy to introduce is a peak filter type. Introducing peaks around 60 or 80 Hz, with substantial gain, one can significantly enhance the "chest thump" or "slam" that a subwoofer will produce. Experimenting with the frequency, gain and Q-factor of single peak filters can help understand where such effects come from. Such filters can easily be bypassed later if a flatter response is preferred.

Saving

While all changes will be loaded immediately on the MiniDSP while the plugin is in sync with the device, it can be useful to save a configuration for later retrieval. This can be done in the System Settings tab. Configurations are saved on your computer as a standalone file that can be loaded again at anytime.