Marchand bassis for changing Q - diyAudio
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Old 31st May 2012, 10:05 AM   #1
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Default Marchand bassis for changing Q

I have a bassis that I used for years on my home system before I had powered subs that could stand on their own.

I was wondering if anyone ever used one in a car. I know Phil Marchand had a mobile isolated power supply for it back 20years ago when I built mine.

Anyway, although ported speakers really cant take advantage of its bass extension function, you can still use it to adjust the Q of the system. Phil describes its use on ported systems in this fasion can make the system respond as if it were a transmission line. You have two knobs for Q, one being to set where your speaker is at and the other to raise or lower the Q to the desired quality.

I'm just wondering if anyone used it for this at home or a car because it sounds like it can really add to the performance of any system. I will dig mine up tomorrow. I can temporarily put it in my truck using AC until I get a mobile power supply and see what it does to the Transient response of my ported TC LMSr 12".

Last edited by SpinMonster; 31st May 2012 at 10:12 AM.
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Old 31st May 2012, 09:20 PM   #2
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The bassis works on the same pricile as feedback in a servo sub in that it alters the input signal to make up for what the sub isnt doing right.

In this case you can dampen the input signal so the output reflects the final system Q you want. So if you have a ported box that rings on with a high Q, you select a system Q of .5 to critically damp the input signal so the sub's resulting sound has the Q you want. the transient response of a transmission line can be duplicated on a ported sub. Its sudden and decays fast.

I'm trying it within the hour.

Marchand replied to my e-mail. He does have a mobile power supply for the unit. 4 week lead time since its not a common request. Im in.
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Old 1st June 2012, 11:36 PM   #3
NEO Dan is offline NEO Dan  United States
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The BASSIS is not going to improve the transient response of your subwoofer, if anything it would be worse. This is due to the nature of analog EQ filters, if the Q is high they can ring or smear the response in the time domain.

Just about any quality subwoofer will net an extended bass response in a small "room" like that provided by a vehicle. The "cabin gain" or transfer function is what allows "rubbish" car audio drivers to produce acceptable bass extension, it is sometimes necessary to put a "better quality" driver in a very small enclosure to prevent it from sounding fat or slow because it already is capable of the extended bass response with out the gain provided by the vehicle.
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Old 2nd June 2012, 04:32 AM   #4
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Quote:
Originally Posted by NEO Dan View Post
The BASSIS is not going to improve the transient response of your subwoofer, if anything it would be worse. This is due to the nature of analog EQ filters, if the Q is high they can ring or smear the response in the time domain.

Just about any quality subwoofer will net an extended bass response in a small "room" like that provided by a vehicle. The "cabin gain" or transfer function is what allows "rubbish" car audio drivers to produce acceptable bass extension, it is sometimes necessary to put a "better quality" driver in a very small enclosure to prevent it from sounding fat or slow because it already is capable of the extended bass response with out the gain provided by the vehicle.
Did you ever use one? I own it for 20+ years.

It wasnt going to be used for extension.

Having used it in my home on ported speakers, I know it doesnt make things worse.

My car sub is a LMSr 12 TC sounds in a 1.4 cubic foot box ported to 27 with a group delay of 24ms. I would charactorize it as punchy and detailed after trying it in 7 alignments from 2.05, 1.7, and 1.4 cubic ft boxes ported varying from 25 to 27.5hz. I also tried two sealed boxes at 1.05 and .85 cubic feet. I was trying to lower the Q of the input signal to result in the same Q change of the sub. You say the unit doesnt do this? What was your personal experience with it?
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Old 2nd June 2012, 06:41 AM   #5
NEO Dan is offline NEO Dan  United States
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I built/own one for 5+ years, I'm not currently using it, I don't need it in my small system ATM.

The device is a nice combination of filters, and there are benefits to be had. But it is not as transformative as you appear to believe it to be. If you put energy in that excites a resonant system - guess what - it resonates, the BASSIS can't fix that. It'll allow you to put less energy in where it resonates if you adjust it correctly, but it will not "fix the problem" with the box.

If you were to try and fill in a narrow cancellation node with a high EQ boost you might create a filter that exhibited some ring to it, but other than that I doubt you'd notice.

If you want a TL just build one. If you want purity of sound in the vehicle try an aperiodic IB. Better yet BUILD a portable HIFI / PA type system and go outdoors where there are far fewer obstacles to achieving sound quality.

What were the results of your test?
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Old 2nd June 2012, 06:59 AM   #6
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Quote:
Originally Posted by NEO Dan View Post
I built/own one for 5+ years, I'm not currently using it, I don't need it in my small system ATM.

The device is a nice combination of filters, and there are benefits to be had. But it is not as transformative as you appear to believe it to be. If you put energy in that excites a resonant system - guess what - it resonates, the BASSIS can't fix that. It'll allow you to put less energy in where it resonates if you adjust it correctly, but it will not "fix the problem" with the box.

If you were to try and fill in a narrow cancellation node with a high EQ boost you might create a filter that exhibited some ring to it, but other than that I doubt you'd notice.

If you want a TL just build one. If you want purity of sound in the vehicle try an aperiodic IB. Better yet BUILD a portable HIFI / PA type system and go outdoors where there are far fewer obstacles to achieving sound quality.

What were the results of your test?
OK, I understand what youre trying to say. For starters, I dont have an underdamped crappy sub. Its got as low a group delay as I could build in a ported system and isnt muddy or boomy. When I listen to good recordings I'm in heaven. What I was looking to do was enhance poor recordings that seem to be over EQ'ed. So since the speaker is properly built and capable of great performance, the recording is the culprit and since its a signal, it can be altered to be 'less worse'.


I find my TC sounds 12 now to be one of the most detailed systems I've built. I am going to put the bassis in to see the effect first hand. I wasnt using a $50 auto parts store crap speaker in a poorly tuned box. I agree it cant fix that.

Last edited by SpinMonster; 2nd June 2012 at 07:05 AM.
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Old 2nd June 2012, 07:13 AM   #7
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Hey, if you can design a transmission line for a TC sounds LMSr 15, I will build it. My theater is 500sq feet with 9.5' ceilings and a 16 foot screen. I have 4 of these 15's in two ported enclosures that are nearly 400lbs. If its feasible and technically sound, I will build it. If you look for the thread for my car sub, you will know the lengths I will go to to get what I want for performance. I just dont know anyone with good theory on how to actually design a TL. Just a lot of people who talk a good bit.

Any ideas?
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Old 2nd June 2012, 07:17 AM   #8
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What would you do differently from what you have now, do you need louder/lower???
What are the details for your existing boxes?
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Old 2nd June 2012, 07:51 AM   #9
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Originally Posted by NEO Dan View Post
What would you do differently from what you have now, do you need louder/lower???
What are the details for your existing boxes?
Each TC 15 has 4.2 cubic feet X 2 drivers in each box(8.4 cubic feet net per sub). It is tuned to 25hz. The group delay was staggering in 6.1 cubic feet with a 20hz tuning so this alignment was far better sounding.

It lost 3hz extension in exchange for the much lower group delay, detail, and punch.

The vents are 5X20".

The subs are about 400lbs 1.5" thick MDF all panels. Its 52x24x23.

Each is powered by Peavey CS4080hz's at 8ohms delivering 4000 watts each. The subs never bottom out as the amps near soft clipping. These amps are heavy with huge transformers and caps.

The subs are in the coners of my 500sq foot room with 9.5' ceilings. The room opens to the next part of the house which is 4000sq feet.

The subs and screen for size requirements....the screen is 16ft diagonal.

Click the image to open in full size.Click the image to open in full size.

Im looking for better transient resonse (though this sounds great now)...attack and efficiency. Size can be 8 feet tall and I can fit it but 2 feet x 2 feet are the H/W limits.
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Old 2nd June 2012, 08:07 AM   #10
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You could go a lot lower with the tuning and push the group delay down an octave and out of the area where you seem to be picking up on it.

Methinks I've been to this movie before...
Click the image to open in full size.
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