October 04, 2017

The Case for Active DSP Crossovers

Active DSP Crossovers Are The Future!

Nearly all speakers require crossovers.  Historically crossovers have been passive. From the lowliest 2-way budget box to those 10 foot tall high-end monster cabinets you see in shows and magazines that cost as much as an average house – passive crossovers.

Despite the ubiquitous nature of passive crossovers, active crossovers are in fact better than passive crossovers in almost every way except for their cost and complexity. Thankfully, the cost and complexity of active crossovers are being mitigated by the rapid evolution of digital signal processing (DSP).

DSP technology is getting more powerful and easier to use and more affordable than ever. DSP use is on the rise and that is nothing but good news!

What is a Crossover and Why Do I Care?

The speaker is arguably the single most important component in your audio system. Sure, other components matter, some more than others, but it’s the speaker that ultimately turns all that voltage and current into the actual sound we love to listen to.

Most speakers use 2 or more audio drivers to cover the full 20-20,000 hertz audible range. Why not just use a single full range driver?  For the simple reason that it’s really really hard to produce the full audible range from a single driver and do it really well including full bass.

With multiple drivers you need to divide the full audible spectrum into frequency sub-ranges and feed each sub-range to a specific driver designed to handle that frequency range. Most speakers are 2-way, fewer are 3-way, and fewer yet are 4-way or higher. “Ways” refers to the number of drivers in each speaker. Low and high pass filters are both used as well as other more specialized filters. Combined, these filters are referred to as the speaker’s “crossover”.

“Passive” means the crossover is driven by the energy (voltage and current) in the audio signal coming from the amplifier. You could say the amplifier does all the work and the filter components “passively” respond.

Typical passive crossover components include resistors, capacitors and inductors as shown in the graphic above. These passive crossover components are mounted on a board and that board is mounted inside the speaker cabinet. Simple, out of sight, and largely out of mind. As with all things in audio, the cost of individual passive crossover components run the gamut from dirt cheap to you’ve-got-to-be-kidding-me nose bleed expensive.

The Downside of Passive Crossovers

The advantages and disadvantages of passive crossovers are fairly well understood and are summarized in the table below.

Add them all up and the disadvantages of passive crossovers are not trivial. Passive crossovers WILL degrade the overall sound quality of your speakers.

ADVANTAGES DISADVANTAGES
Simple (plug and play) Crossover interference (back EMF) with amplifier signal
Single speaker cable (per speaker) Loss of speaker damping and amp's direct control of driver especially near the crossover frequency
Single stereo amp Higher loading on amplifier with greater losses requiring higher wattage
Usually less expensive Uneven phase shifting between drivers with different impedances
Variable and nonlinear responses with changes in power and temperature
Higher THD and intermodulation distortion, interference patterns, amplitude irregularities, driver resonances, cone breakup, and degraded off-axis response
Greater probability of amplifier clipping and driver damage due to higher power and complex impedance loads.

 

Why then are passive crossovers so prevalent if they’re so flawed?  Passive crossovers are used for obvious reasons; they are simple to build (although not that simple to design), relatively low cost, and yes they do work.

If you are a fan of the “if-it-ain’t-broke-don’t-fix-it” philosophy then by all means enjoy your speakers with their passive crossovers. But if you’re looking for ways to materially improve the performance of your audio system then I suggest it’s time to leave passive crossovers behind and make the move to active crossovers.

Why Active Crossovers?

Because active crossovers sound better! In fact, switching over to active crossovers was one of the best moves I ever made with my personal gear (aside from adopting an LDR preamp).

Active crossover sound better than passive crossovers because they eliminate all of the disadvantages of passive crossovers. The advantages of active crossovers are listed in the table below.

ADVANTAGES DISADVANTAGES
Direct control of each driver by each amp channel Inherently more complex
Simple and easier impedance loading on amp Potentially increased noise 
No parasitic power losses At least twice the number of cables
No loss of damping (driver control) Requires multiples amps or amps with multiple channels
Less likelihood of clipping with clipping limited to single driver Usually more expensive
Consistent crossover behavior regardless of power level or signal content
Less loading on each amp with load divided among multiple amps
Less distortion
Highly flexible and adaptable especially with DSP technology

Do Active Crossovers Cost More?

If you’re adapting your existing speakers to active crossovers you will definitely spend some more money. On the other hand if you’re building your own speakers you can avoid pricey passive crossover components altogether.

Cost more than what exactly?  What I’m suggesting is that before you spend another nickel on some exotic tweak, pricier cables, wooden blocks, or even a new set of speakers, consider making the move to active crossovers first. Keep your existing speakers, adapt them to an active crossover,  and you will likely be pleasantly surprised at the improvement in their performance.

The primary additional costs for a DSP crossover is the DSP crossover itself plus a second 2-channel amplifier. Of course there are also numerous additional interconnect cables required. Like so many things in high performance audio you can spend a little or a whole lot. I submit that DSP need not cost an arm and a leg. For example a $200 DSP from miniDSP does an outstanding job.

How Do Active DSP Crossovers Work?

As mentioned above, a typical active DSP crossover requires the DSP unit itself plus at least one additional 2-channel amplifier as shown below. Let’s stay with a 2-way system for now. If your speakers are 3-ways you’ll need two additional 2-channel amps.

Active crossovers do the same thing passive crossovers do – they split the audio signal into different frequency range buckets. Whereas passive crossovers operate downstream of the amplifier where there is high voltage and current levels, active crossovers do their work upstream before the amplifier using either a digital input or line level analog inputs.

The figure above shows a typical active DSP crossover for a 2-way speaker. Here we have a conventional 2-channel preamp feeding into a DSP crossover. The crossover splits the left and right channel audio signals into separate “hi” and “lo” frequency signals for both the left and the right channels resulting in  4 analog signals each of which needs its own amplification.

With active crossovers the only thing each amplifier “sees” is a single driver with a relative simple impedance characteristic. All things being equal, your audio system WILL sound better with an active crossover in a one-amp-per-driver configuration.

How Many DAC Channels did You Say?

Like the name says, DSPs are in fact “digital”. They perform the crossover process in the digital domain using specialized high-speed processors and lots of cool math. This means the incoming 2 channel audio signals must first be converted to digital via analog-to-digital converters (ADCs), then processed computationally into 4 channels, and finally converted back into 4 analog channels via 4 digital-to-analog converters (DACs).  

Note that if your source is mostly digital to begin with you probably already have one 2-channel DAC in your system. This means you now have not only a 2-channel ADC in your system, you actually have a total of 6 (SIX!)  DAC channels as well.

DSP Plus 4 Channel Analog Preamp

An alternative approach is shown in the diagrams below. Here we’ve moved the DSP upstream into the digital domain where it takes its input directly from your digital source. The DSP also accepts an analog line input such as from a Phono Preamp.

The first diagram uses the 4 DAC channels embedded in the DSP to generate the 4 analog outputs. In the second diagram an external pair of dedicated 2-channel external DACs are used instead and the DSP operates entirely in the digital domain.

With this approach we’ve eliminated the ADC altogether and we’ve gone from 6 DAC channels to the minimum 4 required.

The challenge with this simplified system is now you need a 4 channel analog preamp. 4 channel preamps are are not common.

Tortuga Audio is considering offering a 4 channel LDR preamp for this type of DSP crossover application. This would be an ideal application for our latest V25 Preamp Controller.


The figure below is the same as above except 2 dedicated external DACs are used instead of using the 4 DAC channels of the DSP.





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