Mains and sub integration.

[Pancakes]

I'm nearly done designing the box etc for my home sub and am not totally inexperienced with box design and how it wil sound in a room etc but haven't tried to integrate a vented sub before. The design I want to go with is 2 x 12's (isobarik) in a 85L box, with two 100mm x 870mm vents.

There isn't much in the way of range for sound DIY in NZ for anything like a good price and not many cheaper amps. I was looking at one from Jaycar which seems to be a very similar if not same unit as is available thru dealers in the States also. I will defeat the 6Db boost at 35Hz because I would rather have less volume at this frequency and not have the lower notes overpowered, this isn't going to be a crazy big HT setup but hopefully some good tight fill in a small box for not much coin.

My mains amp has an "EQ" function with line out/in I had assumed (thats where things go wrong eh!) that the line in would be the full signal and the line out would be the balance left, still at line level from the built in X/O. It seems from digging further but maybe not far enough (as I don't have the answer yet) that the output is still just the same signal as the input. As the subs seem to be less than ideal for what I'm doing (have done mainly vented mains with no sub or sealed mains and sub before so it hasn't been so much of an issue) they have what seems to be a large delay as they get to the tuned frequency.

Is this delay too big and will this cause phasing and imaging issues with the two sets ending up out of phase? I have attached the WinISD group delay screen so you can see what I'm seeing.

I know this is probably the bare bones of the info needed to give an answer but it is pretty long as it is so please ask for more info if it will help!!

Thanks, Brian.

[Mike P.]

What subs are you using? If you could post the T/S parameters or a link to them, it would help.

[Pancakes]

These are the only subs I have found in NZ that are in the budget and go low enough in the small space I have (that I have found so far). They are discontinued now so I have ordered the last two.

Specs,




Gain plot from WinISD,

[Mike P.]

I modelled the sub in WinISD and the results are not very good. That is to be expected since the Xmax is only 5mm. In a isobaric configuration in a 85 liter box tuned to 23hz, input power is 60 watts before maximun excursion is reached. A subsonic filter will be needed ar 21hz to protect the drivers from over excursion.

[Pancakes]

Ha ha! I didn't notice that typo before. I use the info of the hard copy spec sheet I have for these. Have used them before but for small sealed boxes and the X-Max is 12mm in the spec sheet which is correct from what I have seen the drivers do. 5mm X-Max, you must have been trying hard to word that nicely!

I know they aren't ideal but as I have set a strict budget these are about it I recon. Sorry for not picking that up! I have check the other specs and they are all as per the hard copy sheet.

[Mike P.]

It also shows 2 entries for Qms. One of them should be Qes. Once we get the parameters right I'll try modelling it again.

[Pancakes]

The mechanical is Qms, Electrical is Qes.

Nom inpedance ................... 4 Ohm
Power Rating ...................... 180W
Sensitivity at 1W@1m .......... 87.85dB
Voice coil Resistance (Re) ..... 3.5 Ohm
Resonant Frequency (Fs) ...... 31.27Hz
Mechanical Q Factor (Qms) ... 7.698
Electrical Q Factor (Qes) ....... 0.566
Total Q Factor (Qts) ............. 0.520
Equivalent volume (Vas) ........ 69.49Lt
Voice coil diameter ............... 65.5mm
X max ................................ 12mm
Magnet Diamter ................... 156mm
Magnet height ..................... 40mm

Thats all the specs I have.

[Pancakes]

I want to take the weight off my mains but don't want to lose any directional sound, my thoughts at the moment are do I make a two way line level X/O and leave the sub amps built in X/O as high as it'll go? Not sure where I want to cross over exactly till I hear it too but it's music only not HT and the mains sound nice and rich but roll off making the lows way under the music level. I could set the sub amp X/O around it's lowest setting, 50Hz then on the return signal back to the main amp make a line level high-pass at around 30 - 35Hz so that load is taken off the main setup. I don't know whats supposed to be the "right" answer but I feel like directional hearing still functions to an extent quite low so as I write the last idea seems to be sitting in my brain right.

Thanks for taking the time to de-cypher my thoughts and help out! I only have one mate who is into DIY sound so its great to have a group to bounce idea's off.

Right now listening to Athlete - Tourist. Wicked album. Needs a sub tho to warm up the bottom end!! haha hah

[Pancakes]

just thought, how do you measure amp impedance (the main amp in this case?) to calculate the components correctly? If I guess out too far I'll be in the wrong fequency altogether!

Mmmmmmm, is this the right this to be doing while I'm off work with a head injury? Need to turn the thinking tap on!

[Mike P.]

85 liters tuned to 23.5 with 2 - 100 mm ports 914mm long is what I came up with. 380 watts takes the subs to maximum excursion at 32 and 21hz. You will need a subsonic filter at 21hz to protect the drivers. If it were me I'd go with 120 liters tuned to 20hz. You would be giving up a few dbs in the low end and gaining more low end extension, which is what you want with a HT sub. Just my 2 cents.

[Pancakes]

It will be a music sub only so super low would be nice but the F3 at 20 - 21Hz range is ok. Good thought on the filter especially as it will roll off so fast the audible difference will be jack all. I really wanted to keep the whole unit under 100L and I'm looking at 120L exterior volume as it is to get the net 85 - 90L ports and braces etc. Any good places I should look at to make a subsonic filter or are units available at a reasonable price on the net or mail order etc?

Tested my EQ outputs just before and they are affected by the volume control which isn't what I expected so the plan looks like I can take that line level out, split off a signal for the sub and route a line back in but maybe with a 24dB/octave filter @ 35Hz? Would have to play with the sound once it's set up.

How do you measure amp impedance? I have done speakers a bit to make the crossovers right as the impedance changes the frequency for the same components (but I'm sure you know that) but is the resistance in the amp static or does it change with volume etc? What range of resistance would I expect to see from my amp and do I measure from the line in RCA's or what?

More questions I'm sorry but I am getting somewhere!!

[Mike P.]

Oops, I understood this was for HT. If it will be a music sub only, then the 85 liters net volume tuned to 23.5hz will work just fine. There won't be any real need for a subsonic filter. You haven't said what amplifier you will be using for the sub, but most plate amps have a built in subsonic filter.

I'm not sure why you need to measure the amplifier impedance. The specs will tell you whether it's 8 ohmns stable or 4 ohms stable or both.

[Pancakes]

I was wondering about the amp impedance cos I could make a line level XO for the mains when the sub is active. My amp has what is labeled EQ line out and in, these are before the amp but are volume controlled so I can play the line out to a splitter at will give signal to the sub and also signal back to the mains. I would like the sub to play from 40 - 50Hz (may change once I hear it in the room) but want to highpass the mains at maybe 30Hz @ 24dB/octave while the sub is active so they aren't working hard to do something the sub will be doing already. The amp has a button to turn the EQ out function on and off so the setup could be mnains only or mains + sub at the touch of a button.

Sorry for not mamking clear I was meaning a music sub especially in an HT site!

Will use one of these amps, they are the same as far as I can tell but the exchange rate might mean I'm better off getting one in from the states.

http://apexjr.com/Apexsenior.htm

NZ one;

http://jaycar.co.nz/productView.asp?...=&SUBCATID=443