CHR-70 bass reflex - diyAudio
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Old 21st February 2012, 12:25 PM   #1
Andy17 is offline Andy17  Germany
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Default CHR-70 bass reflex

Since I got some useful information from this forum, I also would like to contribute something here ... only a small speaker .
I come from Germany and so my english is not perfect!
First some background.
I was not so happy with the center speaker of my home cinema system, a Canton CD50 II.
The main problem ist not the Canton itself, but in my eyes it did not harmonize with my front speakers ("old - DIY" Dynaudio Sirius).
A center with the same drivers is not possible as Dynaudio do not sell drivers anymore. Furthermore I want a "small" center.
First I thought about a construction like the Canton, but in simulations it is clear that it is not a good idea to put so many drivers in a horizontal line - for a center!
So I thoght about a fullrange driver - ok these drivers have also some limitations - but I always wanted to build a speaker with a fullrange, why not a center.
So I searched about fullrange speakers and came across MarkAudio an this forum.
From the datasheet the CHR-70 (gen.3) seems to do quite well in a small enclosure and it is quite cheap so it is not fatal if I won't get it work like I want .
I put the driver data in BoxSim an simulate a bit. I end with a 7l / 59Hz bass-reflex enclosure.
These would fit in my shelves also, so maybe I will build 2 more as front speakers!? As I want a thin box I use "only" 12mm birch plywood and put some bracings in. This should enough for such a small enclosure. A part of the enclosure is a sandwich with sloped inside against standing waves. I am not a cabinet maker and with this method the outside is an "easy" rectangle and inside no one would see "the flub"
The outside is not finished but a first hear impression is really good. I put 0,39mH || 3,9Ohm in as it sounds very bright. But it should play same time before a final decision.
Here are 3 early pictures and planing piktures with Sketchup.
Attached Images
File Type: jpg BOX1.JPG (209.0 KB, 1067 views)
File Type: jpg BOX2.JPG (180.2 KB, 1002 views)
File Type: jpg BOX3.JPG (171.6 KB, 1002 views)
File Type: jpg BOXs1.jpg (58.8 KB, 987 views)
File Type: jpg BOXs2.jpg (98.7 KB, 985 views)
File Type: jpg BOXs3.jpg (68.3 KB, 354 views)
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Old 21st February 2012, 08:17 PM   #2
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Hi Andy, (Guys)
Many thanks for posting your project. Its very interesting and looks very nice. Could you post the build dimensions? If you'd like to send me a dimensional drawing, I'm happy to put your plan on the new Markaudio site:

Plans | Markaudio

Looking at the pics and your text, this box design has an internal volume of around 7 litres. As a guide, box volumes from 8 to 10 litres offer closer optimisation for the Gen.3 CHR (subject to power-handling needs). There's a plate mounted from the front baffle, close to the underside of the driver (see arrow on pic). Markaudio drivers generate a large backwave (relative to cone size), so its sometimes better to avoid having this internal box feature so close to the driver. These 2 design elements could increase internal compression on the rear of the driver's power-train. Some losses in the bass output are likely. This is normal for smaller BR and vented boxes. Using an inductor in series may help to reduce the "bright" sound.

Adding bracing on box projects using under 18-mm thick walls is a good idea.

You could also experiment by adding some loose low density damping material (see pic).

Cheers
Mark.
Attached Images
File Type: jpg BOX2.JPG (281.7 KB, 317 views)
File Type: jpg 26-09-2003018s.jpg (90.5 KB, 271 views)

Last edited by markaudio; 21st February 2012 at 08:27 PM.
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Old 21st February 2012, 08:55 PM   #3
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Quote:
Originally Posted by markaudio View Post
Adding bracing on box projects using under 18-mm thick walls is a good idea.
I'm a big bracing advocate, but one does need to consider the build material... 12mm BB is approx as stiff as 20mm MDF so they would require similar bracing on that basis, higher mass with MDF means even more bracing required to push panel resonances up to where they become innocuous,

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Old 22nd February 2012, 06:58 AM   #4
Andy17 is offline Andy17  Germany
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Quote:
Originally Posted by markaudio View Post
Hi Andy, (Guys)
Many thanks for posting your project. Its very interesting and looks very nice. Could you post the build dimensions? If you'd like to send me a dimensional drawing, I'm happy to put your plan on the new Markaudio site:

Plans | Markaudio
Many Thanks!
If I find some time I will create a (good) dimensional drawing and send it to you.
First a quick dimensional Sketchup export. The hollow space of the sandwich part is filled with quarz sand.

Quote:
Originally Posted by markaudio View Post
...There's a plate mounted from the front baffle, close to the underside of the driver (see arrow on pic). Markaudio drivers generate a large backwave (relative to cone size), so its sometimes better to avoid having this internal box feature so close to the driver.
I put this plate at the same distance than the top and side walls so that the "pressure" to the driver should be consistent!?
I know the driver is (to?) close to the walls but I want a thin Box to put it in my TV Rack. The front wall around the driver is beveled.
Quote:
Originally Posted by markaudio View Post
Using an inductor in series may help to reduce the "bright" sound.
With the 0,39mH || 0,39Ohm in series it seems quite good, the "bright" sound was without any part in it.
As I cover the Box with wood stain and wax the next listinig has to wait a bit.

Quote:
Originally Posted by markaudio View Post
You could also experiment by adding some loose low density damping material (see pic).

Cheers
Mark.
Of course I put some damping material (Sonofil) inside

Andreas
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Old 22nd February 2012, 09:48 AM   #5
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Quote:
Originally Posted by Andy17 View Post
Many Thanks!
If I find some time I will create a (good) dimensional drawing and send it to you.

Andreas
Hello Andreas,
Great, put your name on it and email me as a PDF if possible: support@markaudio.com
Cheers
Mark.
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Old 27th February 2012, 07:39 AM   #6
Andy17 is offline Andy17  Germany
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As there is some discussion about power handling
Alpair's aren't woofers
some notes to my box. Even if this is a small Box, it is a bass-reflex!
So frequencies around fs and < fb are critical.
I attach a simulation from BoxSim.
With damping (red line vs black) the fs can be controlled.
To be safe, frequencies under fb shoud be controlled by a filter.
With a simple 1 order passive filters you may not get the desired effect.
Either the filter frequency is to high or the effect is to low. (50Hz - blue line).
As I use it as a center (or front) with an AV receiver (aktiv filter) a 3. order even at 40Hz should be ok (green line).
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File Type: jpg ShoeBox_W_V.jpg (88.9 KB, 264 views)

Last edited by markaudio; 28th February 2012 at 10:51 PM.
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Old 28th February 2012, 10:56 PM   #7
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Quote:
Originally Posted by Andy17 View Post
As there is some discussion about power handling
Alpair's aren't woofers
some notes to my box. Even if this is a small Box, it is a bass-reflex!
So frequencies around fs and < fb are critical.
I attach a simulation from BoxSim.
With damping (red line vs black) the fs can be controlled.
To be safe, frequencies under fb shoud be controlled by a filter.
With a simple 1 order passive filters you may not get the desired effect.
Either the filter frequency is to high or the effect is to low. (50Hz - blue line).
As I use it as a center (or front) with an AV receiver (aktiv filter) a 3. order even at 40Hz should be ok (green line).
Hello Andreas, Guys,
If you have a filter design, would be useful to post your idea.

Re filters in general. Much depends on each individual Diyer, his/her needs, preferences and system operation. Personally, I err on the side of purist so rarely apply filtering, taking care when running drivers. All said, I appreciate that for others, protective filtering might a sensible practical option.

Cheers
Mark.

Last edited by markaudio; 29th February 2012 at 02:54 AM.
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Old 29th February 2012, 07:45 AM   #8
Andy17 is offline Andy17  Germany
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For Hifi(Stereo) Music I personally don't like filters either.
My stereo amplifier has only a volume control.

As center or front at the home cinema system the AV-receiver calibrate
the room and uses DSP so here I have no problem to use the digital filter
of the receiver. BluRays uses heavy dynamics with very low frequecies so
here I like to be save for the speaker.

I have no expert knowledge in filter design!!!
If someone really want a passiv filter I tried a simulation with BoxSim.
As the filter is seeing the "whole box" (to my knowledge) a impedance correction is needed.
To use "standard" devices I get a 2. order filter at ~43Hz.
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Old 29th February 2012, 01:07 PM   #9
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Are these inductor values really 33mH and 10mH? This would be a staggeringly expensive filter. I haven't even found an inductor large than 20mH.
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Old 29th February 2012, 01:44 PM   #10
Andy17 is offline Andy17  Germany
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Yes, for such a low frequency (40Hz) you need these values.
Of course no air-coils. I do not know US shops and parts.
Here are e.g. Mundorf coils quite commen.
http://www.mundorf.com/english%201.1/spulen-kern.htm
FERRIT-core coils > 10mH are about 10.
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