Phase linear subtractive crossover for subwoofer / speakers

[Yevhen]

Or you can go for a fully software-based DSP solution just like I did with my 2-way DIY system:

DIY Audio Nirvana 12" + Dayton Reference FAST project

I am sharing with you yet another DIY project we have done with my neighbor (who is a carpenter...:)) much earlier this year It is a FAST (Fullrange Assisted Subwoofer Technology) system consisting of 12" Audio Nirvana Classic fullrange speakers and Dayton Audio RSS315HF-4 subs I could not find...

www.audiosciencereview.com

I am using Jriver with various VST plugins, for crossover I am using Acon Equalize 2 in mixed phase mode. I have tried tons of EQ plugins for the crossover and this is by far the best I managed to find
You can see on the below how the wavelet diagram (=impulse response by frequency) looks like when using mixed phase mode crossover vs. traditional minimum phase crossover (that equals to hardware based crossovers)

Mixed phase mode:
View attachment 155491

Min phase mode:

View attachment 155492

In both cases crossover was set to 80Hz, 24dB/octave slope

Or you can go for a fully software-based DSP solution just like I did with my 2-way DIY system:

DIY Audio Nirvana 12" + Dayton Reference FAST project

I am sharing with you yet another DIY project we have done with my neighbor (who is a carpenter...:)) much earlier this year It is a FAST (Fullrange Assisted Subwoofer Technology) system consisting of 12" Audio Nirvana Classic fullrange speakers and Dayton Audio RSS315HF-4 subs I could not find...

www.audiosciencereview.com

I am using Jriver with various VST plugins, for crossover I am using Acon Equalize 2 in mixed phase mode. I have tried tons of EQ plugins for the crossover and this is by far the best I managed to find
You can see on the below how the wavelet diagram (=impulse response by frequency) looks like when using mixed phase mode crossover vs. traditional minimum phase crossover (that equals to hardware based crossovers)

Mixed phase mode:
View attachment 155491

Min phase mode:

View attachment 155492

In both cases crossover was set to 80Hz, 24dB/octave slope

Thanks! Nice graphs! 30ms That's quite a lot, do you hear the difference during the attacks? Could you please try to describe it?

 

[Yevhen]

Then I’d say: good luck soldering your analogue EQ room correction every time you decide to move the sub. If you really go the analogue route, at least make sure you have the usual sub controls like phase and crossover pots.

Yeah, that might be a problem. Luckily I have only one nasty mode at 40Hz in the office. So I planned to add EQ with 1-2 parametric channels (variable Q, freq and depth).

In the living room, I have 15, 30, 40Hz + strong harmonics. There I just give up and plug the ports of the speakers (of course with old socks)

 

[Yevhen]

25l seems small? It is closed?

Yes, closed. It also seems small to me. But very attractive. I see it gets an extra 1-1.5dB of sensitivity at 20Hz in the 30-35l box compared to recommended 25l.

 

[Yevhen]

What do you guys think about this crazy idea?

If I place sub with internal electrical delay in LPF just next to me, would it compensate for the acoustic delay from the speakers? That's very attractive, because sub in that position should not trigger 40Hz room mode

 

[voodooless]

In general it’s a good idea to place the sub halfway a wall.

But have you actually looked in detail at those filters? Only the 1st order filter actually sums properly, but for a sub, it’s rather useless. 4th order would be much better. But check out the summation and response:

It’s basically unusable. 4th order is only applied to the high pass, not the sub. It’s just 20 dB down at 20 kHz. That’s not very practical.

Conclusion of the article should speak volumes:

So, if anyone was ever mildly curious, now you know why I have not (and will not) publish a project based on what I consider to be a seriously flawed design.

 

[Yevhen]

Did you already build the sub? I am running my main speakers full range and using an SVS subwoofer to extend and smooth the low end. The DSP done by SVS has a lot of flexibility (separate control of LP filter cutoff and slope, polarity, phase, and PEQ). I only apply this at the very low end so it doesn't seem like too much of a religious violation!

Yes, I also considered that solution but thought that DSP might create an extra delay compared to the clean path in the main speakers. That's why started considering these phase linear crossovers. But actually might be interesting to try a hybrid solution: phase linear analog crossover + DSP in sub for fine-tuning. The analog part will do the main job and might offload the DSP (let it process faster). Would that work? I considered DSP plate amps from MiniDSP or Hypex, but they are quite expensive. And cheaper alternatives are probably not as fast, not sure.

 

[sarumbear]

Does it make sense to use a subtractive filter to align the delays in sub and speakers?

Subtractive Crossover Networks

Derived (or Subtractive) Crossovers - are they any good? Why aren't they used everywhere?

sound-au.com

I quote from the conclusion section of the link you posted as my comment.

For the sake of a few more parts (or fewer parts if a phase shift network is included), you can have a 24dB/ octave filter that works properly.

So, if anyone was ever mildly curious, now you know why I have not (and will not) publish a project based on what I consider to be a seriously flawed design.

 

[richard12511]

I've always been pushed away by the learning curve, do you have any sources i can read about subwoofer design (Sorry OP for hijacking your thread).

Not really, as I’ve never done it, but I was seriously going that way at one point. I was constantly reading the DIY forum over on AVS, which I found very helpful. I’ve never read this forum’s DIY section, but my guess is it’s even better. I was super close to trying to build Danley DS10 kit at one point 4 years ago(ish) but a 40% off one day Black Friday sale for JTR RS2s happened to pop up that I couldn’t resist.

Making a great loudspeaker is much more difficult, as the NFS tests here have shown. Subs are much simpler, though, especially true for sealed designs.

 

[Yevhen]

But have you actually looked in detail at those filters?

Like it is shown in the picture, parasitic delay in sub might be even useful, so phase linear filter is not necessary, I could use classic 4th order LPF.

But if want to place the sub just next to the speakers (which seems more practical), there I indeed need to cut the highs of the sub.
I just thought that the last schematic from that article can bring higher suppression than just the 1st order. If not, that will be disaster, because the closed box sub would need additional 6dB oct compensation so 1st order will only make it flat

Figure 14 - Phase Corrected Subtractive Crossover

Need to check it in simulation, I'm not getting what it actually does.

 

[pjug]

Yes, I also considered that solution but thought that DSP might create an extra delay compared to the clean path in the main speakers. That's why started considering these phase linear crossovers. But actually might be interesting to try a hybrid solution: phase linear analog crossover + DSP in sub for fine-tuning. The analog part will do the main job and might offload the DSP (let it process faster). Would that work? I considered DSP plate amps from MiniDSP or Hypex, but they are quite expensive. And cheaper alternatives are probably not as fast, not sure.

Two subs, one with analog filtering and the other with DSP seems reasonable. But for the analog I wouldn't build your own fixed filter when you can buy a plate amp with the filter and other adjustments built-in, which should make it easier to integrate. It seems the whole point of your OP is to have time alignment. I only care about phase as it relates to getting the best frequency response I can do (smooth crossover between mains and subs).

 

[Yevhen]

Two subs, one with analog filtering and the other with DSP seems reasonable. But for the analog I wouldn't build your own fixed filter when you can buy a plate amp with the filter and other adjustments built-in, which should make it easier to integrate. It seems the whole point of your OP is to have time alignment. mean 2 subs. only care about phase as it relates to getting the best frequency response I can do (smooth crossover between mains and subs).

No, no, I didn´t mean 2 subs. Just one sub with DSP and analog crossover in front of it. I thought the analog part might do all the dirty (time-consuming) job and let the DSP do fine-tuning. But as voodooless mentioned, that subtractive crossover is only doing the 1st order LPF, so not so much help for DSP.

Personally, I could live with uneven freq response in the bass region, but when the sub is playing with a long delay (due to time misalignment and/or long tail @ room mode freq) it´s no go for me. I have tried 4-5 different subs in my living room, but couldn´t really enjoy the music. I had exactly the same feeling with old guitar processors - you hit the string, hear it live but then the sound comes from the speaker with a 5-10ms delay and the brain cannot process it as one single action, feels like somebody else playing it.

 

[voodooless]

Note that that analogue thing is not phase linear either! You cannot make a phase linear filter in the analogue domain (well, you could emulate a fir filter with analogue parts, but that would be madnesses ) the 1st order variant just has the same phase response for both low and high passes, but 90 deg appart.

Why not invest in something that can DSP both mains and sub. That should solve any issues.

 

[Yevhen]

Note that that analogue thing is not phase linear either! You cannot make a phase linear filter in the analogue domain (well, you could emulate a fir filter with analogue parts, but that would be madnesses ) the 1st order variant just has the same phase response for both low and high passes, but 90 deg appart.
View attachment 155660

Why not invest in something that can DSP both mains and sub. That should solve any issues.

Yeah, I agree, investing in DSP would be a solution but I still want to keep it simple, like click on the button and enjoy some music for 5min during the break from work. Could you please recommend some DSP box with 3-4 outputs? Preferably with >115 dB SINAD . And what's the price level? Analog part on opa1612 would cost me 10-20€ as I already have plenty of components from the old guitar pedal projects.

 

[voodooless]

MiniDSP SHD would come to mind, or the studio variant if the SINAD is not good enough (pick your own DAC’s). It’s not very cheap though.

 

[Yevhen]

Thanks, SHD seems to be interesting, around 1k + DACs. Need to dive into it a bit.

Did you try to estimate the effectiveness compared to acoustic room treatment? I mean what if I upgrade my diy speakers somehow to make them dig deeper and the rest spend on acoustic panels and bass catchers?

 

[voodooless]

Did you try to estimate the effectiveness compared to acoustic room treatment? I mean what if I upgrade my diy speakers somehow to make them dig deeper and the rest spend on acoustic panels and bass catchers?

I have no idea.. have the same question basically. It’s not just cost though, usually it ales end up looking quite hideous with a lot of treatment. I guess usually a mix would be recommended, which makes it not really cheaper. And some things you just cannot properly fix with DSP.

 

[pjug]

Looks like plate amplifiers are available with DSP, not too expensive:

Home

Dayton Audio SPA500DSP 500W Subwoofer Plate Amplifier with DSPDayton Audio SPA500DSP Subwoofer Plate AmplifierElite Control with DSPDayton Audio's SPA500DSP true 500 watt digital switching plate amplifier features advanced signal processing control that every audio enthusiast wants. With Class D...

www.parts-express.com

edit: how do you stop the link from showing as "shopping"?

 

[Yevhen]

I checked those Dayton amps quickly but didn't find any info related to DAC quality. Most probably, it will be a downgrade for me.

Current plan:
1. Build the sub with conventional analog LPF and play with the box volume & port tuning of the floorstanders.
2. If not satisfied, try to cut out room mode at 40Hz with an analog filter.
3. If not satisfied, try to find a MiniDSP SHD for 1-2 weeks to understand if that´s my thing.
4. If not satisfied, try an acoustic treatment of the room and boost the bass response of the floorstanders.
5. If not satisfied...

 

[Yevhen]

Thank you all for the comments, seems like that subtractive crossover is not the best idea because of the poor filtering in the low-freq path.