15 ohm driver & TQWP for small room?

[waam68]

Hi
A while back I built a 6B4G 3.5 W SET amp based on the diytube "GetSetgo" board. I run this into a rebuilt 1950's set of (3 way) Wharfedale W3's & they sound really good together. That said I am about to start a 2A3 Loftin white build & thinking ahead I would like to try & build a really simple pair of TQWP's to "see what FR is all about". My listening room is really small at about 12ft by 12ft & the floorspace I have left is only about a foot & a half square at each corner although height isn't a problem. All my diy amps have 15ohm output transformers. As my tools & skills are pretty limited I was thinking some kind of voigt pipe & have found some 15ohm FR drivers for reasonable money with these specs

DCR=6.0+/-0.3ohm

ACR=14.98 ohm @100hz, 15.4ohm @1.0khz

Efficiency>= 92dB/W/M

45hz - 18.5khz +/-8dB , fo 62hz

L= 0.31mH, Q= 0.12 @1.0khz

As I know nothing about speaker design does anyone know if these drivers would be suitable and/or can anyone recommend a plan? bearing in mind my space limitations.

Thanks
Mike

[AstroSonic]

Sounds like you have some nice projects going. Just curious: which drivers are these? We will need some T/S parameters in addition to those you posted: at least Qts and Vas.

You will find with full range drivers that ultimate satisfaction will not be related to bandwidth (bass and HF extension), but to response irregularities in the upper mids and low to mid HF. You need to choose a driver you can live with.

The 15 ohm impedance is pretty limiting these days. 15 ohm Lowthers come to mind, but there are also a few vintage 15 ohm drivers. Perhaps 2 nominal 8 ohm drivers could be wired in series and used in a dipole with some form of pipe loading (MLTL?).

Good luck,

Bob

[waam68]

Hi
The 8" 15 ohm FR drivers were from Taiwan & that's all the info posted in the ad. I do have a 3 matching vintage 15 ohm "Grampian reproducers" FR drivers in storage but all are 12" which is too big for the space I have left...WAF factor is an issue! Lowthers would be far too expensive for this project. I guess my options would be:

-two 8 ohms in series ...any suggestions on cab/driver combo's that might work? or maybe the simplest route

- swap out my output TX & build Bob Brines style voigts...any recommendations for cheapish drivers that would work in this kind of cab?
Mike

[AstroSonic]

Mike,

You will have many more choices by getting iron with multiple secondary taps (4, 8 and 16 ohm, or even just 4 and 8 ohm). Then if you want to try a TQWP, choose a proven design (and driver) by searching on this forum. After some research you will likely find that other enclosure designs may meet both your space and performance requirements.

There are several fine, low cost full range drivers currently available. As with most things, the better drivers generally cost more. You can always try full range at lower cost. If you like it then you can upgrade.

The 2A3 will require relatively efficient speakers if you are to have any headroom, even in your 12x12 room. So, choose a compact design that uses drivers that have adequate efficiency. Watch the placement requirements: some designs require placement well away from walls, while others may require corner placement.

Regards,

Bob

[mashaffer]

I find the listed Q a little bit hard to believe. 1.0 I would believe, but 0.1? It must be a very powerful magnet. If you don't have the means to measure the parameters there are a couple of possibilities. You can try open baffle quite easily but if room size is a problem that may not be practical.

The other possibility is to do some cut and try transmission lines. I would start with a line about 50" long or so with the cross sectional area of the closed end about 3 times the surface area of the cone (not the whole driver just the cone) and the open end about equal to the area of the cone. Make it in the form of a folded line of three segments with the driver mounted at the apex of the first bend from the closed end (1/3 of the line before the driver 2/3 after the driver). lightly stuff the volume before the driver to just after the driver with polyfil.

See what that gives you and adjust from there. That should get you in the ball park if it has fairly normal parameters I think. That should tune it down near 40 Hz or so so you will need to be sure not to dump a lot of sub 40Hz power into it.

Lengthening the line will tune it lower as will increasing the taper up to a point. I would be careful about tuning it much lower than 40Hz given the fs of your driver because you start to reach a point where you get ragged response on the bottom end and excursion problems. More stuffing will damp high frequency resonances more but reduce bass output some also. Increasing the area of the two ends of the line will usually increase bass output but can make it peaky while decreasing it will do the opposite (kind of like more damping).

Before all the calculations were worked out this kind of cut and try was the only way to do it. I see no reason why you couldn't do your initial mockups using OSB to keep the costs down and just account for the fact that you are probably going to have more resonances from the cabinet. When you are sure of the design you want to use then make it up in plywood or MDF as you prefer. Just one option.

[bear]

If ur taking about Martin King's stuff - it seems to work tremendously well in a small room... follow his recipe and you should be "good to go".

_-_-bear

Post some jpegs of the mystery drivers??

[waam68]

OK Thanks for the pointer. I don't suppose you have a link to an example TL?

I do need somehing with a small footprint but tall is no problem.

Here are some pics:

Mike

[bear]

search for: Martin J King and transmission line or TQWP... should bring up his site.

_-_-bear

[brsanko]

Looking at those pictures I can tell you with absolute certainty that those speakers do not have a Qof 1.2. The magnets are ceramic ferrite and very small I would venture a guess they are more likely 1.2 which is pretty weak. But they look like they have pretty light paper cones so the midrange should be decent and the phase plugs indicate decent high frequency but not necissarily extended. It would be hard to design a horn when the T/S #s are so uncertain. Open baffel Xed over >100hz should sound good and the advice about experimenting with a TL sounds pretty feasable. I would avoid BR with those small magnets but experiment away that's the funnest part of this hobby.

[bear]

The other thing is that Qt would not be measured at 1.0kHz - it would be measured at
or around Fs... you would have to actually measure the T/S parameters yourself to use these drivers...

_-_-