David - Just For Fun

[Bill Fitzmaurice]

Once again; Thanks, Bill. How does one break in the Midbass drivers? I know how to break in the sub driver. I will add the caps to the mid crossover.
Here are pics of the build thus far.

The same as with a woofer, but at higher frequency so you don't have over-excursion. 10-20% below Fs is appropriate.

[Steve Regier]

New coils arrived today... off to the shop for install! I'll post results.

[chrapladm]

[Steve Regier]

OK, Bill was right. There was enough room to work inside the driver chamber. It was a bit like building a ship in a bottle. However, my results are a bit confusing but here 'goes.
I replaced the coil in the low pass section. With the low pass to spec the sub still seemed weak and sterile. I then bypassed the low pass completely and the sub boomed to life. It was a bit hooty I'm sure due to the super efficiency of the horn but the low end was pounding just the same. I went to the Erseaudio.com site and calculated a 3rd order low pass unit. I had parts on the shelf and built a low pass using the specs from Erse. The 18mH coil in the plans was replaced with a 10mH. The 220mF cap was replaced with a 300mF unit. And the 10mF inductor was replaced with a 3mH unit. With this new low pass the sub section started sounding right...tight and crisp and loud.
Now for the mid section. I followed the plans and checked polarity in the mids and added the extra 16.8 mF poly cap that Bill recommended. The mid section performance still seemed unchanged. I noticed that with only one mid driver running the mid section performed as expected; Full, loud and clear. I connected the second driver and the out put dropped 5dB. Now, I thought it might be polarity but I double and tripple checked and the drivers are in polarity with each other. I even did the battery test and they move the same direction. Now for even more oddities. The single driver sounded best with the second one removed and just a hole in the baffle where it belongs. I then connected the second driver without installing it and got a 5dB drop in output. I then noticed that the drop was not instant it happened over the period of a second or so. It was like something electronic was saturating. I went back to several crossover calculators for a second order band pass and found that in every case the 3mH inductor specified in the plans was a 0.3mH inductor in the calculator. I made the switch and the mids came to life. Could this be a misprint in the plans? One other issue...I noticed that when I mounted the second mid driver output in the mid section was reduced by 1.5 dB but the range and clarity of the mid section remained high. Could this be due to the small chamber for the mid drivers? Could they use a larger chamber to be all they can be?
Checking the high pass with the calculators it remained unchanged. The drivers had about 6 hours of breakin at the time. I put the cab to the test. The bottom is full and rich but loses energy below about 35Hz. The top end matches the bottom quite well and has plently of punch and clarity. This all matches the expected performance that Bill shows on his graph. I will have more pics and testing tomorrow ...but overall "little cabinet with a big sound, David truely lives up to it's name".

[chrapladm]

WOW......I hope it all comes to your specs in the end.

I think I maybe sold on getting a pair of Davids for my L/R.

Can the Davids play volume wise louder than your SLA's?

I was just wondering because I just wish my SLA's could play louder and seeing I might be building these Davids I needed to ask. Oh and how many full sheets of plywood does the David require?

Sorry just noticed you have a few different SLA's. I dont know what your extreme SLA is but I have a SLA w/ nine JVC 3x5's and eighteen Onkyo tweeters per speaker. SO thats what I would be comparing them to.

[chaywood]

.
Now for the mid section. I followed the plans and checked polarity in the mids and added the extra 16.8 mF poly cap that Bill recommended.

Hello baronvonsteve,

Did you notice in the plans that the mids and tweeter are wired in reverse polarity ?

CH

[Bill Fitzmaurice]

The 18mH coil in the plans was replaced with a 10mH. The 220mF cap was replaced with a 300mF unit. And the 10mF inductor was replaced with a 3mH unit. With this new low pass the sub section started sounding right...tight and crisp and loud.

By raising the coil value and lowering the cap value you haven't actually changed the crossover frequency, what you've done is to change the filter Q, from about .5 to about .7. That could result in boomy response depending on room placement. But if it works for your placement that's OK.

I made the switch and the mids came to life. Could this be a misprint in the plans? .

It is, it should be 0.3mH, not 3mH. Broadband output will drop with two drivers compared to one, but that's because with one driver the crossover function isn't correct.
As for the polarity issue that's covered in the plans.

[Steve Regier]

The 18mH coil in the plans was replaced with a 10mH. The 220mF cap was replaced with a 300mF unit. And the 10mF inductor was replaced with a 3mH unit. With this new low pass the sub section started sounding right...tight and crisp and loud.

By raising the coil value and lowering the cap value you haven't actually changed the crossover frequency, what you've done is to change the filter Q, from about .5 to about .7. That could result in boomy response depending on room placement. But if it works for your placement that's OK.

OK, thanks Bill. However, I lowered the coil values and raised the cap value. Does this mean that the sub would be less boomy? The sub does sound more energetic this way.

[Steve Regier]

.
Now for the mid section. I followed the plans and checked polarity in the mids and added the extra 16.8 mF poly cap that Bill recommended.

Hello baronvonsteve,

Did you notice in the plans that the mids and tweeter are wired in reverse polarity ?

CH

Yes I did notice, thanks. I performed the polarity test as described in the plans.

[Steve Regier]

WOW......I hope it all comes to your specs in the end.

I think I maybe sold on getting a pair of Davids for my L/R.

Can the Davids play volume wise louder than your SLA's?

I was just wondering because I just wish my SLA's could play louder and seeing I might be building these Davids I needed to ask. Oh and how many full sheets of plywood does the David require?

Sorry just noticed you have a few different SLA's. I dont know what your extreme SLA is but I have a SLA w/ nine JVC 3x5's and eighteen Onkyo tweeters per speaker. SO thats what I would be comparing them to.

That David watt for watt stays right up with my big SLA Etreme which uses 6 5" woofers and 12 Goldwood tweeters. The SLA is a bit more articulate and a tad less harsh. The David seems a bit hot in the 2-3 KHz range.
The build used just short of 2 60" X 60" sheets of 1/2" BB.

[Bill Fitzmaurice]

OK, thanks Bill. However, I lowered the coil values and raised the cap value. Does this mean that the sub would be less boomy? The sub does sound more energetic this way.

My bad, that was pre-coffee dyslexia. Lowering the coil value and raising the cap does raise the filter Q, which gives a sharper cutoff but with higher output at the corner frequency, and that might lead to boom. The lower Q of the original filter gives less output at the crossover frequency with a more gradual roll off slope, which compensates for the rising response of the sub and allows it to run to 200Hz without being too loud compared to the lower end of its bandwidth. But placement also affects the woofer response, and some placements will favor the higher Q filter.

[Steve Regier]

OK, so I was listening to David sitting down when I posted previously. It sounds good but a bit harsh. Louder than my big SLA but not as articulate still pretty good, right? Ewetho came by yesterday to help with next build and audition David...we never made it to the next build.
He plays the Binks CD and walks and listens and crouches and listens getting more frustrated as he goes. (Ewetho is a mucian and the mastering engineer at our studio. We call him "Goldern Ears". If a mouse farts off pitch 1/2 mile away he'll hear it.) He has me crouch down to the optimum listening level even with the tweeter as I had been doing yesterday. The David sounds great. "Now stand up and listen", he commands. I listen. At a listening distance of 8' as I stand the highs dissapear after only 12" of evelation. "That's one hell of a vertical node", he states. "You can sit and listen in the sweet spot but try standing and moving about. The sound picture changes and it is nowhere near as articlulate as the SLA. I really wanted to like this cab", he says,"But I can't. It has too many dispersion issues, it's shouty and the fine detail heard in the SLA is missing!" I was devistated. He's right. What to do - We stared at eachother and thought about drinks but we must soldier on. Bill's stuff is usually exceptional. What are we missing? We look at MTM specs and see that nodes are an issue but there would be issues with other arrangements as well. We considered replacing the MTM with an SLA at the top but that meant a lot of chopping and reworking. For comic relief I removed the GR 130's and installed some coaxial Pyle car door speakers. We fired up David and the nodes were attenuated to a very acceptable level. Hmmm...We then bypassed all the electronis and powerd the mid section directly with only a 5.1uF Poly cap on the tweet with the 130's reinstalled. This was even better. We were hearing detail in the music and the blend between tweet and mid was smooth and the nodes while still present were only a Db or 2. OK, before we chop this thing up I'll pull all the electronics and we'll start over. We decided to continue using the revised Low Pass and created a High Pass for the Mids and Tweet after reviewing the driver specs from GR-Research. The 130's can play much higher. We are going to let them. They also have a cone designed to smooth HF rolloff. Let's take advantage of that as well. Ewetho got the parts together while I gutted the driver chamber. To Bill's credit I WAS able to remove everything though the access cover. We installed the new electronics and had a listen. (Schematic posted below) and WOW What a difference! The cab was alive and vibrant with excellent detail. The mid harshness was gone and the nodes were minor in intensity to the point of almost unnoticable. "Now we can build the other one", Ewetho declares. I agree but first we'll finish this one with a nice cherry veneer and some old school grille cloth.
Pics to follow.

Thanks to Bill for another great design and an another adventure.

[ewetho]

Well, we were gonna get started early but the hour drive home at midnight after all day yesterday was a bit rough.

Well well well!!! The David lives to see another day. Once we opened the crossovers up it sounds almost as good as an SLA up top. We are nitpicking here but VERY VERY close now. Would not hesitate to build them this way. It definitely is better than anything in Best Buy or Fry's even in the soecial rooms!!!

For example we can tell in Scorpions Were Still Loving You, we can now tell the drummer is using brushes on the Hat very distinctly even listening to it's shimmer and shine which you could barely hear before and telling it was the brushes was impossible. This is one area the SLA wins in that you can also tell the articulation of the brushes on the Hat with the wipes and so forth not just the strikes and subsequent shimmer of the hat and brushes.

As for the bottome end, can't wait for some corner loading as they sound very good and very promising for placing in a corner. AWESOME!!!

Killer speaker Bill!!!

[chaywood]

Hey Guys !

Good thing you watched all the episodes of McGyver and know how to build crossovers using the parts off a junk microwave oven !

Looking good so far... and it must sound perfect to make it through quality control !

Hope to see some cherry wood soon !!!!

CH

[Bill Fitzmaurice]

I've never experienced any of your dispersion issues with mine, though of course I have different drivers. Still, things shouldn't be all that different. But an MTM is intended for listening on the vertical axis, so that may be all you're experiencing.
I can't try mine with your crossover as it's in my workshop, current temperature 15 degrees, but once the weather warms I'll fool with it.
As to your crossover, a couple of things jump out. First is the 1st order high pass on the tweeter, which doesn't offer much protection. Its corner is up around 5kHz, though, so that helps. But I'd make it 2nd order at least. Second is the lack of a low pass on the mids, which will cause bandwidth overlap with the tweeter. I'd be rolling them off at 5kHz. That's a bit higher than usual with fives, but the GRs have good off-axis that high, so it's not problematic. Also be sure to try swapping the tweeter polarity, in an MTM it's critical, especially with respect to vertical dispersion.