DSP Active Crossover

I'm considering trying a DSP-based active crossover in my system. I did a search to see how much this has been discussed, and most of the posts are pretty old or about active speakers. DSP technology has changed a fair amount in the last 15-20 years.

My system is digital only, and my speakers are 3-way, so it's not particularly complicated. I've been looking at the Danville Signal dspNexux 2/8 which has two channel in (with digital inputs) and eight balanced analog outputs. This appears to be available with AKM AK4499 DACs which are fairly well regarded sigma-delta DACs (although I don't know how good their implementation is).

This product has a fairly rich DSP software environment for programming filters, time delays, etc., so it should be fairly straightforward to set it up to replace my passive crossovers. 

My biggest reservations are 1) giving up my Denafrips Terminator+ DAC and nice-quality DIY preamp, and 2) using the DAC's digital volume adjustments. 

This unit is about $3K (maybe a bit more with the AK4499 DACs), so isn't terribly expensive. From the limited research I've done, this unit appears to be higher sound quality than the miniDSP or DEQX boxes, but I could be wrong. All my amps have balanced inputs, so I'd prefer to use a unit with balanced outputs. 

So, what I'm wondering is if the benefits of active crossovers and dsp equalization will outweigh the lesser DAC quality (assuming this is the case) and lack of analog volume control (currently using a relay switched attenuator). I'm also wondering if there are other dsp audio processors that I should consider (digital inputs, at least six channels out, ideally with balanced outputs).



Given how good your DAC, preamp and passive crossover are......I can see moving to a stock electronic xover as being just a side ways or lowering quality move.  The analog input and output stages are just op amps in the Danville (nothing like the analog stages in your preamp!!!).....and how good are the power supplies and routing and wires and jacks?  It is not made anything like the preamp you made or the DAC you have.  You are removing an excellent passive simple crossover for a way more complex and not as well done thang.  A better A/B would be to use the Danville as the DAC and preamp, as well.  Therefore you are not as much adding something but just changing.  You would then have no external DAC, no external preamp, and no analog cables between them.  You just run the digital signal directly into the Danville and then through their DAC and output stages and then right to your amps.  This is how Rich Hollis is using the Danville.  He uses no preamp or external DAC....he gets some pretty darn good sound.  And if you modded the output stages of the Danville you would then get even better sound.

This is why I am such a fan of digital amps.  You have no DAC, no preamp, no analog cables and you also have no analog stages on the output of the crossover or normal analog amps with feedback (or no feedback).  The digital signal from your source goes directly to the chip in the digital amp and then the PCM is converted directly to PWM and then drives the GaN output stages that drive your speakers.  By using two stereo digital amps you can bi-amp using the Minidsp Flex of SHD and you can build a two way speaker that is out of this world transparent.....because it has no analog crossover and has these low distortion digital amps (sonically low distortion)......all drivers mounted on open baffle and driven directly without connectors from the amps.....this is unheard of simplicity.  Of course, you have a 3 way speaker system with servo amps so going this way would make it way more complicated.....so this would not work for you.  I am sure you have seen my webpage that describes what I am talking about.  This is world class amp/speaker system for $6K.  If you wanted to go bigger you could run 6 12 inch woofers per side on their own baffle and run 8 planars in a long line source on another baffle fo another $2K.  Imagine the impact and transparency of such a monster.......and still only $8K or so.  We are talking 6 12 inch woofers and 8...eight inch planars per side.....open baffle, fully equalized and time aligned.....would blow you away...........just like your current system.....but look at the cost difference!  The only thing you would not get with such a system is super low bass like with the servos.  

Here are a few suggestions to make you current analog xover better.

1. Change the foil coil on the input of the midrange to a 12 gauge version (looks like 14 gauge? is what you are using?  Also, you need to go into the inside of the coil (all coils you use) and out the outside....or the outside to ground if used as a shunt.  I like the Jantzen 12 gauge wax paper coils.  Mundorf has some newer foil coils that are damped as well.

2. Change the Erse coil to a copper foil coil.  Even in a compensation circuit it still makes the same difference.  Erse coils are veiled....big time.

3. Change your speaker input connectors to WBT copper nextgens and cut off most of the solder tab in the back.  Then remove those massive speaker banana posts from your speaker wire and form the end of your speaker wire into a loop and tin it and wrap it around the WBT jack or make your own tinned hook out of 14 gauge VH audio cryoed ofc wire that you use as a spade.  This is much more transparent than what you have.  Even better would be to hardwire your speaker cable directly to the wires in the crossover....or use my plastic clamp system as shown on my Peachtree GaN 1 page.  I have not used any speaker connectors on any speaker or amp in over 40 years.

You could also try my Ground Enhancers and Music Purifiers on the input of your xover....mucho better sound for practically nada moolah.

Whatever you do....please....have fun.

BTW......I am pretty sure you would get much cleaner bass if all the servo woofs were all facing forward.  Does not make sense to have two facing backwards.  Even Rich Hollis is doing it the way I suggest....so did GT Audioworks (who now uses other non servo woofs....but all facing forward)


@ricevs - I have been using the dspNexus as the DAC/preamp, so I'm feeding it USB audio directly (no conversion from analog). I don't think it makes sense to use my Terminator + and preamp ahead of the dspNexus. 

I agree that the dspNexus could be improved. I think the biggest weakness is the power supply - the simple SMPS that is currently used could be improved a lot with a good linear supply with shunt regulation. I think if I decide to go this route, I will probably implement my own power supply. 

The biggest advantage I see to use a DSP crossover is to be able to implement steep filters with no added noise. If I implement analog active crossovers, the steepest I'd probably be able to get away with is 24db/octave. This is still better than the 12db/octave passive crossovers I have now though. 

I agree there is a bit of room for improvement in my current crossovers, but this is clearly diminishing returns. Still, I may consider it at some point. I've just got too many projects going on in parallel right now, and I'm having trouble actually finishing any of them 😀.

As far as facing all the drivers in the woofer towers forward, I have not been able to discern a difference. I had them facing forward in my triple towers and I think the quad towers are just as tight and clean sounding - probably a bit more so - than the triples. There isn't an easy way to turn them around anyway. All the holes for the wires are only drilled on one side. 

With such a tall tower, I'd be a bit nervous about having all the drivers facing forward since the weight distribution would be heavily toward the rear. Even the triples were slightly tippy. 

Hi Everyone,

I would like to discuss the choice of the switching power supply in the dspNexus. Commercial switching power supplies solve a lot of practical problems when converting AC mains to low voltage supplies in a commercial product. They are not ideal.

I evaluated 4 different switchers during the development process of the dspNexus. Most suffer by having insufficient current capability with the negative supply. This was clearly audible with the headphone amplifier even with 100 watt units. The one I chose was good in this regard. It is the only board in the dspNexus that I did not design.

That said, a large linear mains supply is not a panacea. There are three linear supplies between the DACs, ADC and Clocks. These are not all LDOs since high frequency power supply rejection is also important.  The goal is to have a low noise, low impedance supply located near each target device.

Designing good audio products is a process of making many small decisions. Collectively, the choices you make determine the outcome. I started making high end consumer audio products 45 years ago as an analog engineer. I have used DSPs for 30 years, so I have a bit of experience.  I welcome discussing design philosophies and I hope many of you will consider the dspNexus as a good solution for your systems. I will be at CAF and I hope to meet many of you there.

Al Clark
Danville Signal


@dsp - Al, I didn't mean to imply that the dspNexus isn't a well engineered product.  I would expect that replacing the SMPS with a linear supply that was good enough to provide a noticeable improvement to the sound quality would more than double the cost and significantly increase the size of the enclosure (also increasing cost).

As is, the dspNexus provides a lot of value for it's price point and is very nicely constructed, allowing easy upgradeability to the DSP and DAC modules. 

I look forward to meeting you at CAF.