# SSF project prototype preview



## SturmMD (Dec 30, 2008)

I'd like to share some info about an exciting SSF project I have been working on for many months. I now have some prototype hardware.

In summary it will be an adjustable SSF filter with 0.1Hz resolution, different gain settings, and preset filter types.









As seen in the photo above, it has a 3 digit LCD, AC power supply, RCA input, and a 1/4" phono balanced output.

A microcontroller controls the LCD, frequency, and filter type on the analog board. I haven't written the microcontroller
code yet, but I have assembled and am testing the analog board shown below.









There are 8 selectable gain settings from 0dB to 13dB (plus an additional 6dB for a balanced output).
The following graph shows the response of the input stage before the HPF.









The filter frequency is controlled by a clock signal generated by the microcontroller. In this testing I used an external signal.
The following graph shows the consistency of the response as the frequency is varied.









The last graph shows the frequency response in the following modes: 2nd order butterworth, and 2nd order peaking filters with 
peaking of 3.0dB, 2.25dB, 1.5dB, and 0.7dB (Q=1.3066, 1.172, 1.044, 0.904).
There will also be a 4th order butterworth mode. I found that the SSF frequency was very accurate.










Please hold off on the extensive questioning - let me save my precious free time for working on the project.
Don't hold off on the encouragement though


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## ironglen (Mar 4, 2009)

SWEET!!!:yourock:


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## Mike P. (Apr 6, 2007)

Looking Good!


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## collo (Dec 23, 2006)

Those boards look fantastic!! :T


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## onebadmonte (Jan 7, 2009)

Very nice. Cant wait to get a hold of one.


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## Mike P. (Apr 6, 2007)

onebadmonte said:


> Very nice. Cant wait to get a hold of one.


+2 on that!


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## tonyvdb (Sep 5, 2007)

Your on to something here SturmMD, Looking forward to the finished product:T


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## jagman (Jul 8, 2006)

1+

Looks primising. I'm sure you already know this but you have a couple competitors priced just under a $100. I really like the accuracy of your filters and the adjustable output gain (especially WRT using this with prosound amps). Keep up the good work!


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## ironglen (Mar 4, 2009)

jagman said:


> 1+
> 
> Looks primising. I'm sure you already know this but you have a couple competitors priced just under a $100. I really like the accuracy of your filters and the adjustable output gain (especially WRT using this with prosound amps). Keep up the good work!


Yeah, but 'they' don't have the easy and accurate dial-in freq-- 'they' are more like a guess of the freq. ED told me to get my voltmeter out and play test tones through their device while checking output voltage...c'mon! As if I'm going to do that each time I change my input device, or better yet, maybe I should scribble the actual numerical freq's on the faceplate:rant:


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## jagman (Jul 8, 2006)

Hmm. I have tried contacting Bob @ CSS a couple times with questions about the Reckhorn B2 but I keep getting pointed to this forum (with most of the questions going unanswered). He admitted that the 10Hz SSF is really about 12Hz. If you're saying the top dog suggested you do your own testing on your device to be sure it's doing what you want it to (and he's suggesting it may not), that brings to mind serious issues with the reliablity of the product. If SturmMD's new mule does what the B2 does but more reliably then sign me up  (especially if the cost is similar).


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## Blaser (Aug 28, 2006)

Hi Sturm,

Can you give more details about the project? Is this intended to be produced so that anybody can buy? Do you expect a release date soon?


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## ironglen (Mar 4, 2009)

jagman said:


> Hmm. I have tried contacting Bob @ CSS a couple times with questions about the Reckhorn B2 but I keep getting pointed to this forum (with most of the questions going unanswered). He admitted that the 10Hz SSF is really about 12Hz. If you're saying the top dog suggested you do your own testing on your device to be sure it's doing what you want it to (and he's suggesting it may not), that brings to mind serious issues with the reliablity of the product. If SturmMD's new mule does what the B2 does but more reliably then sign me up  (especially if the cost is similar).


Oh, the ED ssf works, it's just that the freq adjust is not precise, meaning that you don't KNOW that it is set at a particular freq, but rather an estimate. So, given you have a tuned sub (which is why you need a ssf, right???) you'll typically want to set your ssf freq a bit lower, but with these devices you don't KNOW the freq you've set the ssf. 

Sturm's device, at least to me, would be worth more than the off-the-shelf devices currently available, even if you have to build it yourself from a parts list. (I better start saving!)


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## brucek (Apr 11, 2006)

> but with these devices you don't KNOW the freq you've set the ssf.


Why not just test the device with REW and make some marks on the dial? I wouldn't trust a device even if it had marks on the dial - I'd want to test it myself and look at the curve. It takes only a few minutes to do...

brucek


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## ironglen (Mar 4, 2009)

Old computer, no rew...my opinion is unless sturm's design is either faulty or quite costly, manufacturers should be making a better mousetrap rather than expecting the consumer to (unnecessarily) have additional (costly) equipment to use their device accurately and follow up my making marks on the face. 

Could you imagine after purchasing your automobile having to buy a radar gun and test your vehicle throughout the speedometer's range, and then write the actual speeds on the dash with a sharpie???


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## jagman (Jul 8, 2006)

Not only that, but testing the accuracy of your SSF in room is pretty difficult given all the interactions with room boundries. One would need to take your box outside and lift it up with a fork lift a la Kevin Haskins. Hard to do with an IB sub.


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## brucek (Apr 11, 2006)

> manufacturers should be making a better mousetrap


There's a cost involved with that. We're discussing ~$100 devices here. I don't expect much from these cheap boxes.



> testing the accuracy of your SSF in room is pretty difficult given all the interactions


The suggestion was to test the device with REW, not test the sub. It's a simple electronic sweep of the device itself to obtain a response graph - no room, no sub.

brucek


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## ironglen (Mar 4, 2009)

I'm going to have to get into REW sometime, probably after I move to a permanent place, so I can do the treatment thing, etc for the best sound.


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## jagman (Jul 8, 2006)

brucek said:


> There's a cost involved with that. We're discussing ~$100 devices here. I don't expect much from these cheap boxes.
> 
> The suggestion was to test the device with REW, not test the sub. It's a simple electronic sweep of the device itself to obtain a response graph - no room, no sub.
> 
> brucek


That makes more sense :duh:... and would be pretty easy to do. The only risk is it may be like luck of the draw on accuracy. The 10Hz SSF may be 12Hz but it could be 14Hz. Same with the boost at 20Hz. I suppose if a device tests inaccurate enough one could return it... but then shipping costs get involved. Do you take a chance on a second device that may be better or possibly even worse? I suppose it depends on the whether the manufacturer pays for shipping.


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## jagman (Jul 8, 2006)

SturmMD. Sorry for going OT. Your device looks cool and I can't wait to see how it turns out. If your device is more accurate than your comps then you will have a real place in the market. Good luck.


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## jagman (Jul 8, 2006)

Any progress?


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## SturmMD (Dec 30, 2008)

jagman said:


> Any progress?


I've been trying to find time to start working on the microcontroller code. I have a deadline at work next week that I have been working on recently. After that I hope to get to it.


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## jagman (Jul 8, 2006)

Thanks for the quick reply . Hopefully you'll have something functional for the masses sometime during Q1 next year.


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## jagman (Jul 8, 2006)

Have you had any time to work on this? It has every feature I want... a highly adjustable and accurate SSF, one band of highly adjustable parametric EQ, an RCA input and a fully balanced XLR output with adjustible gain and the requisite +6dB gain. That would be perfect for an infinite baffle sub using a pro amp. 

One specific question... do you know if a time delay is induced when the SSF and/or parametric EQ are used? I ask because my receiver has a very sophisticated auto EQ that measure and accounts for time delay to the msec. The problem is it autocorrects to a flat response. I would like to have the SSF implemented before the auto EQ is run but I would implement the boost at 20Hz afterwards to create a house curve. The drawback is if there is too much of a delay it could make things worse. Please keep this in mind as many receivers and pre/pros have very sophisticated auto EQ programs and a lot of people use them.


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## SturmMD (Dec 30, 2008)

jagman said:


> Have you had any time to work on this? It has every feature I want... a highly adjustable and accurate SSF, one band of highly adjustable parametric EQ, an RCA input and a fully balanced XLR output with adjustible gain and the requisite +6dB gain. That would be perfect for an infinite baffle sub using a pro amp.


I am currently writing the code for the microcontroller to control the circuit and display.



jagman said:


> One specific question... do you know if a time delay is induced when the SSF and/or parametric EQ are used? I ask because my receiver has a very sophisticated auto EQ that measure and accounts for time delay to the msec. The problem is it autocorrects to a flat response. I would like to have the SSF implemented before the auto EQ is run but I would implement the boost at 20Hz afterwards to create a house curve. The drawback is if there is too much of a delay it could make things worse.


All analog filters introduce delay that varies with frequency. Also, I believe that millisecond-accurate delay compensation has decreasing benefit and requirements relax as frequency decreases into the very low bass range. I understand the desire to minimize it though for the best infinite baffle sound.

One point I'd like to clarify is that the "parametric EQ" (the gain peaking) is linked to the highpass frequency since I am just changing the Q of the filter. There is not a peaking filter separate from the subsonic filter so you cannot set the subsonic filter and then dial in gain at as specific frequency. You can use this program written by Collo to quickly graph the peaking filter choices of this circuit. Select "peaking" to see how the circuits response reacts to the different gain presets. Right now the circuit has 0.7, 1.5, 2.25, and 3dB peaking settings.

Similarly, you can use WinISD to plot the group delay of these filters. Use type: highpass, SOS user-specified, 2nd order, and Q values of 0.904, 1.037, 1.172, or 1.307.
The following pictures are an example of this

One important thing to consider is to also include all the highpass poles of the signal chain in the simulation. I have measured mine as (1st order poles) Receiver 2.5Hz, BFD 3.7Hz, EP2500 4.1Hz, and SSF 1.95Hz.

My signal chain alone creates the following group delay (in milliseconds); which is unavoidable besides reducing the amount of equipment in the signal chain.









The next figures uses my PR sub design as I compare the different filter options:
(blue) 4th order butterworth
(white) 2nd order butterworth
(yellow) 2nd order, 0.7dB (Q=0.904)
(orange) 2nd order, 1.5dB (Q=1.037)
(red) 2nd order peaking, 2.25dB (Q=1.172)
(green) 2nd order peaking, 3.0dB (Q=1.307)

I adjusted their frequencies so that they had equal height peaks (below and above tuning frequency) in the cone excursion graph.








When comparing two filter choices, any advantages (higher excursion at a lower frequency) in this graph translate to small, but meaningful (~1dB) SPL output advantages.

For the peaking filters I also reduced their gain by the equivalent amount so that the comparisons would be fair. The final filter transfer functions (with signal chain) are then shown below.









and the additional group delay (ms) caused just by the filter type choice.









So you can see that you just have to compare the simulations and make tradeoffs between group delay and the low frequency extension.


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## jagman (Jul 8, 2006)

Wow... thanks for the thorough explanation!


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## Chester (Feb 19, 2007)

May I ask what SSF actually stands for?  sorry... lol


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## Mike P. (Apr 6, 2007)

SSF = subsonic filter. Same thing as a rumble filter or HPF which is a high pass filter.


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## jagman (Jul 8, 2006)

You probably already know this but a subsonic filter is used to protect your subwoofer from bottoming out from the really low frequencies it wasn't designed to reproduce. It also frees up your sub amp to use it's reserves only with the frequencies your sub was actually designed reproduce.


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## Chester (Feb 19, 2007)

Yeah, I understood what the filter was doing from the response graphs  just was not sure what the abbreviation was


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## dhnjp1 (Nov 18, 2008)

Awesome project with a great set of features! :T

One thing I hope you don't have a problem with is clock noise in the passband. You don't often see a switched-cap pure highpass for that reason (usually a bandpass SC or a HP with analog low pass below the clock freq. on the output).

--Dan


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## dynamowhum (Oct 3, 2006)

The new year is here and would like to know how the project is going. How are you planning on marketing this?


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## SturmMD (Dec 30, 2008)

Yes, I'm still working on it. Disappointingly, I haven't been able to put enough time into it - though the time consuming parts are complete. Everything is built and some initial verification is done. I'm am just at the point where I am writing the software for the microcontroller that manages everything and runs the display.
And yes, the goal is to market this somehow with/by htshack


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## dynamowhum (Oct 3, 2006)

Very nice indeed Dr. Sturm. Do you plan on offering it as a kit or a complete unit? Also do you have a ball park price point in mind that you could share?


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