finished Krutke's MTM

scone · 18th January 2005, 02:50 AM

I finished John Krutke's Vifa XG18 / Seas 27TDFC MTM. Kudos to John for a very awesome design!

I'll post a few pics to this thread. The cabinets are made from 3/4 in. Baltic birch plywood.

I think I'm having trouble with the cabinet stuffing. The speakers sound a bit shrill on things like vocals and violins. The interior walls (except for the baffle) are lined with a 3/4 thick layer of light foam insulation which one of my friends bought from madisound a while back. I'm not sure what it's called. On top of that there is a light layer of polyfill with a slightly thicker layer near the bottom where the crossover lives and a thicker layer along the back to reduce reflections.

The crossovers were built as suggested out of bennic polys and two eagle 5ohms in parallel per channel. The resistance in the crossover measure at 2.6 ohms.

I used both screws and glue to construct the cabinets, so they are quite sturdy but not airtight by any means. The rear of the cabinet is only attached with screws at the moment, but I put a 1/8 in. thick layer of weather stripping to seal the joint, so it's not exactly leaking air.

Here's what I observed after listening and tweaking the speaker for a few hours. The bass response is improved and the speakers seem to sound less harsh when I do not seal the back with weather stripping. In fact if I loosen the screws holding the back on slightly, the harsh effect seems to go away. I'm not sure what that is doing to the frequency response... I guess the speakers aren't broken in yet, but I'm not sure how much that is going to tame the harshness of the tweeter. Anyone have any idea what the problem could be?

scone · 18th January 2005, 02:55 AM

here's a picture of the inner cabinet construction.

wigginjs · 18th January 2005, 02:55 AM

Nice build quality, these speakers have a very unique look. I'm not sure what might be causing the problem, but if breaking the seal on the back panel seems to help you could always use WinISD (or some other such program) to calculate a port and turn it into a vented enclosure. That would give the same preasure equalization as having the seal broken, but without the unpredictable effect on the frequency response. Prehaps a more experienced forum member can actually help you figure out your proble, this is more of a work-around.

scone · 18th January 2005, 02:59 AM

here's a pic of the foam stuffing

Zaph · 18th January 2005, 12:17 PM

Hello,

Those look pretty nice so far. I have a few suggestions to improve the sound.

1) Countersink the tweeter. (doesn't look like this has been done) This has a major effect on the frequency response.

2) Round over the front corners of the enclosure. This also has an effect on the frequency response. 1/2" is good, 3/4" is better. Also, a 3/4" 45 degree chamfer can be done easily with a table saw.

3) Add more damping material. What you have now is not enough. At a minimum, fill the boxes 2/3 of the way full with Acousti-Stuff brand stuffing. Place most of near the back of the enclosure and leave about 4" clear from the back of the driver. If you can't get that easily, Dacron pillow stuffing will do.

4) Break them in more. If needed, hook them up in series to keep a lighter thermal load on your amp. Hook both speakers up out of phase and then facing each other at medium to high levels for about 8 hours. When they face each other about 4" out of phase, it will hardly be heard at all while they break in.

Then of course, check the obvious stuff, like driver polarity (all drivers are in phase) and crossover construction. If any of the shunt connections are out, that will majorly mess up the response. Of course if the series components are out, you get no sound. So you know where to look.

Also, I'd like to ask - Are the insides of the woofer hole rounded over? For ideas on rounding over the edges, have a look at what Merlinx76 did with his pair. (just saw these last week, great work) Looks like solid roundover edges.

Good luck, and let us know how it goes.

John

Zaph · 18th January 2005, 01:07 PM

Quote:

Originally posted by Zaph
Hello,

Those look pretty nice so far. I have a few suggestions to improve the sound.

Just a couple extra things I thought of:

5) Add a bracing crossmember from side to side in the middle. There could be some cabinet wall vibration.

6) If possible, glue the back on rather than bolting it. This will add a little stiffening to the enclosure.

Sorry, I'm a stickler for details.

Jim85IROC · 18th January 2005, 05:52 PM

I would also suggest using something in the corners to ensure an airtight seal between panels. Silicone is cheap and effective, just make sure not to reinstall the drivers until the silicone has cured properly.

merlinx76 · 18th January 2005, 08:13 PM

When I first put the XO together I had a 2.1 ohm resistor. I really found they sounded great...but a tad bright (I guess I like a little bright) I went to 2.4 ohms and they lost all their charm... So back to 2.1 and that's where I am at right now.
I still gotta take everything apart to stain more, varnish, and add a brace which I neglected in my haste.

I was very happy in the first place but they are still improving with time. Thanks for sharing your designs with everyone Zaph. Nice work!

scone · 18th January 2005, 09:07 PM

thanks for all of the suggestions everyone. I added more stuffing to the inside and it helps to curb the high frequencies a little bit. The bass got a little deeper as well, but now borders on boomy. Unfortunately, I won't be able to do any woodwork until I go back home from college for an extended weekend... all of my tools are in my parents' garage

I'll play with the stuffing a little bit more this weekend to see if i can get that right at least.

btw- John, you are correct, I did not countersink the tweeter. My circle guide didn't make arcs that small and I did not want to attempt to do it freehand. Do you think it would really make a difference to the high frequencies? If so I'll give it a shot.

merlinx76 · 18th January 2005, 09:08 PM

I never did round the inside edges behind the woofer. I don't have a router and the guy I hired to do the rest of it was poorly equipped, cost a lot and did a pretty bad job (albeit entirely due to being poorly equipped). Hopefully its not that big of a deal on this size driver. With the woofer countersunk the baffle's only 1/2" thick.

18th January 2005, 02:55 AM	#3
wigginjs diyAudio Member Join Date: Dec 2003 Location: Columbia, SC	Nice build quality, these speakers have a very unique look. I'm not sure what might be causing the problem, but if breaking the seal on the back panel seems to help you could always use WinISD (or some other such program) to calculate a port and turn it into a vented enclosure. That would give the same preasure equalization as having the seal broken, but without the unpredictable effect on the frequency response. Prehaps a more experienced forum member can actually help you figure out your proble, this is more of a work-around. __________________ The Four Chairs DIY Home Theater

18th January 2005, 12:17 PM	#5
Zaph diyAudio Member Join Date: Sep 2003 Location: Wisconsin	Hello, Those look pretty nice so far. I have a few suggestions to improve the sound. 1) Countersink the tweeter. (doesn't look like this has been done) This has a major effect on the frequency response. 2) Round over the front corners of the enclosure. This also has an effect on the frequency response. 1/2" is good, 3/4" is better. Also, a 3/4" 45 degree chamfer can be done easily with a table saw. 3) Add more damping material. What you have now is not enough. At a minimum, fill the boxes 2/3 of the way full with Acousti-Stuff brand stuffing. Place most of near the back of the enclosure and leave about 4" clear from the back of the driver. If you can't get that easily, Dacron pillow stuffing will do. 4) Break them in more. If needed, hook them up in series to keep a lighter thermal load on your amp. Hook both speakers up out of phase and then facing each other at medium to high levels for about 8 hours. When they face each other about 4" out of phase, it will hardly be heard at all while they break in. Then of course, check the obvious stuff, like driver polarity (all drivers are in phase) and crossover construction. If any of the shunt connections are out, that will majorly mess up the response. Of course if the series components are out, you get no sound. So you know where to look. Also, I'd like to ask - Are the insides of the woofer hole rounded over? For ideas on rounding over the edges, have a look at what Merlinx76 did with his pair. (just saw these last week, great work) Looks like solid roundover edges. Good luck, and let us know how it goes. John __________________ -Zaph\|Audio-

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18th January 2005, 05:52 PM	#7
Jim85IROC diyAudio Member Join Date: Dec 2003 Location: Stamford VT	I would also suggest using something in the corners to ensure an airtight seal between panels. Silicone is cheap and effective, just make sure not to reinstall the drivers until the silicone has cured properly.

18th January 2005, 08:13 PM	#8
merlinx76 diyAudio Member Join Date: Nov 2004 Location: The Sticks, NS	When I first put the XO together I had a 2.1 ohm resistor. I really found they sounded great...but a tad bright (I guess I like a little bright) I went to 2.4 ohms and they lost all their charm... So back to 2.1 and that's where I am at right now. I still gotta take everything apart to stain more, varnish, and add a brace which I neglected in my haste. I was very happy in the first place but they are still improving with time. Thanks for sharing your designs with everyone Zaph. Nice work!

18th January 2005, 09:07 PM	#9
scone diyAudio Member Join Date: Nov 2004 Location: Austin, TX	thanks for all of the suggestions everyone. I added more stuffing to the inside and it helps to curb the high frequencies a little bit. The bass got a little deeper as well, but now borders on boomy. Unfortunately, I won't be able to do any woodwork until I go back home from college for an extended weekend... all of my tools are in my parents' garage I'll play with the stuffing a little bit more this weekend to see if i can get that right at least. btw- John, you are correct, I did not countersink the tweeter. My circle guide didn't make arcs that small and I did not want to attempt to do it freehand. Do you think it would really make a difference to the high frequencies? If so I'll give it a shot.

18th January 2005, 09:08 PM	#10
merlinx76 diyAudio Member Join Date: Nov 2004 Location: The Sticks, NS	I never did round the inside edges behind the woofer. I don't have a router and the guy I hired to do the rest of it was poorly equipped, cost a lot and did a pretty bad job (albeit entirely due to being poorly equipped). Hopefully its not that big of a deal on this size driver. With the woofer countersunk the baffle's only 1/2" thick.