THT low profile build.

[Rick Lee]

Whoa!!!! YOU GOT A HARLEY 5 STAR!!!!! Congradulations!!!!

[But it Hertz]

I will treasure this, always

[But it Hertz]

If you put the braces in first you can't adjust their position to remove any warp.

Yep, can't argue with that. Also, I wasn't actually looking for warp at this point. I totally got distracted by the alignment. I learned so much. Can't wait to start the next build!

An excellent build otherwise.

Thanks!

[But it Hertz]

Wait, I was under the impression pocket screw joinery was not very useful in 1/2 ply. Did you have any trouble?

Well, I knew it was a bit of a gamble but not necessarily because it was 1/2 inch ply.

I was more concerned about removing material thinning the wood. Not sure if this is a valid concern but I decided to fill the pockets with PL when I was done just to be safe.

Every screw tightened up perfectly. The only difficulty I had was for panels 3 and 5. I had to set the jig to 3/4 inch thickness to make it work with the angles.

---------

If anyone is interested this is the jig I purchased. Its one of those rare items on Amazon that has overwhelming positive reviews. There's a neat video as well.

I also got the face clamp... turned out to be worth the extra cost IMO.

Lowes has them in stock a dollar or so cheaper than Amazon prices.

[But it Hertz]

So here is the chamber complete and ready for the cover.



Did some clean up and made good the edges. Haven’t rounded anything over yet but sanded all over so it is nice and smooth.

So glad I did the pocket screws…. There was no patching up to do. Just clean up! Took me about 2 hours.





Next up was the binding posts. I screwed some scrap inside the chamber to hold the pin for the circle jig. The nuts you see are just used as spacers. I couldn’t find any shorter screws!



The completed binding post. I was a annoyed that I got some tear out there on the right. My trial hole went perfectly! Can’t explain it.



Put some speaker gasket around the hole. On my previous projects I used stuff from home depot but this stuff is soooo much better.



At last!!!



Checking for leaks. None around the speaker. I had to refit the binding post with speaker gasket after I put on the cover.



Next, into the office to the test rig!

[But it Hertz]

At first I was disappointed. The low frequency response was no better than the T18. I had been seduced by all those wonderful SPL charts that disappear off below 20Hz at 80 or so dB.

I had nothing like that and you could totally hear it.

So put it in a corner firing upwards and did some sweeps with REW.

The red line is the response of the THTLP with no EQ and the green is after I wrestled it into giving me some low frequency response with my miniDSP.



You can see I really had to boost the low end to get it going. I put in 7dB (according to the plug-in) at 28 Hz and took a load out over 45 Hz to get the green line.

I’m very pleased with the results. Listening to various film trailers really brings this sub alive and some of my music torture tracks really sound better than ever.

Question: Some of you will remember my torturing of the Infinity 860W in my T18 after the MCM 55-2421 came unglued. As a result I have been treating this driver with kid gloves and have been really sympathetic… especially as it’s much more expensive.

Having learned what a speaker in a horn sounds like under duress I was disappointed to find that “those sounds” start to happen way lower than the advertised RMS power level. You can really hear it when using test tones, say 25 to 30 Hz.

So the specification says this driver is 500w RMS and 1000w peak. So by my calculations I should be good for just over 44v war volume, peaking over 60v correct?

This driver starts making odd noises after only 32v which by my calculations is only about half that, 250w.

Have I messed up with my calculations? Have I got a dud driver? Am I not understanding something?

Don't get me wrong, this thing sounds FANTASTIC but I paid the premium for a 500w RMS driver. If the spec was 250w RMS I would be delighted!!

Any ideas?

[DJPhatman]

So the specification says this driver is 500w RMS and 1000w peak. So by my calculations I should be good for just over 44v war volume, peaking over 60v correct?

This driver starts making odd noises after only 32v which by my calculations is only about half that, 250w.

Have I messed up with my calculations? Have I got a dud driver? Am I not understanding something?

Don't get me wrong, this thing sounds FANTASTIC but I paid the premium for a 500w RMS driver. If the spec was 250w RMS I would be delighted!!

Any ideas?

Thermal limits (manufactures Pe) is meaningless. Because of the throat pressures and excursion limitations exerted with horns, there are excursion limits. Bill will have to fill in this limit, but 32VRMS should have been making your building shake like it was in an earthquake!

Recheck for leaks, run some OUTDOOR RTA sweeps if you can, and post the results. 1/3 octave RTA is all that is necessary.

[Bill Fitzmaurice]

Thermal limits (manufactures Pe) is meaningless.

+1.

32VRMS should have been making your building shake like it was in an earthquake!

+2.

Recheck for leaks, run some OUTDOOR RTA sweeps if you can, and post the results. 1/3 octave RTA is all that is necessary.

+3. You can use higher resolution than 1/3 octave if your EQ has higher resolution than 1/3 octave, but if it doesn't, what's the point? And getting the most out of a sub is just like buying real estate: what matters the most is location, location, location. The worlds best sub simply won't work if you put it in the wrong place.

[shawn_g]

What are your room dimensions? Room modes might be playing havoc with the lower frequencies.

[Radian]

+7 dB??

Looks to me more like the filters are AFU.

What's you're x-over point and where's a picture of how the subwoofer amp is hooked up?

For a primer, see: http://www.billfitzmaurice.info/forum/v ... =5&t=18545

[But it Hertz]

OK, I’ll do some RTA analysis in my drive way.

What’s the best signal to use? Sweeps , pink noise or what?

Is it OK to sit on the little tables you see above or would on the floor be better?

In answer to the questions / comments in no particular order:

Re 32v and the volume. Yes it was loud!

Re Max voltage… I thought I heard people talking about where to set the limiter based on max RMS all the time? I must have misunderstood that.

Re leaks… I’m totally confident there are no leaks but I will check again.

Re picture of my amp set up? What? I can’t hear you… you are breaking up….

The test room is small to be fair 10x11 feet with a ceiling sloping from 8 to 11 feet.... (my office where things get sorted before being unleashed on the wife)

Re cross-over point… I have been experimenting. Right now it is 95 Hz, Butterworth 48db crossed to my SLA’s. But I continue to experiment.

Re the filters being AFU. I’ll start a fresh again today just to be sure.

Thanks for the support!

[But it Hertz]

OK.

Derp derp derp!!

Turns out for whatever reason (operator error) REW had defaulted back to my internal laptop microphone instead of the external soundcard and hence, SPL meter!! It was a total derp and probably alcohol related.

I feel such an idiot!! Apologies for wasting peoples time on this!!

So here we have 3 lines with no smoothing:

Green is SPL meter unplugged for a sanity check!!

Red is the THTLP with no Eq. Sat in the corner of the room firing towards the ceiling and measured from the listening position in my office – which is pretty much the opposite corner behind me. I’m much happier with the look of this curve!! It matches Bills charts!

The Mustard colored line is with the Eq applied from last night accidentally using the internal laptop microphone!! You can see how crazy I got with boosting the low end.



So I'm going to spend a couple of days with it and get some sensible EQ going!

Take two.

[Bruce Weldy]

I feel such an idiot!! Apologies for wasting peoples time on this!!

No need.

That's the great thing about this forum - when something looks out of whack, there are enough people here with experience to point you in a few directions to find the fix.

[shawn_g]

I've had the same issue with REW from time to time, so no worries. The new chart looks MUCH better.

[Radian]

The test room is small to be fair 10x11 feet with a ceiling sloping from 8 to 11 feet.... (my office where things get sorted before being unleashed on the wife)

Re cross-over point… I have been experimenting. Right now it is 95 Hz, Butterworth 48db crossed to my SLA’s. But I continue to experiment

Bummer that the room is not going to play well with the sub at 30Hz for the moment.

It'll be good to see a raw chart when it's finally in place.

The limit for the THT is 28V.