Mark Audio CHR-70 Application Thread

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Nanook

Nanook

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Joined 2004
or ya could always set up a threaded insert on the back (from the inside), and use a suitable length bolt to couple the back of the driver to the back of the box. Just don't over-tighten
preiter: nice looking boxes (and not a bad table either).
 
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M

MJK

Disabled Account
Joined 2002
quarter-wave.com
Nanook said:
or ya could always set up a threaded insert on the back (from the inside), and use a suitable length bolt to couple the back of the driver to the back of the box. Just don't over-tighten.
Click to expand...

I had somebody come to my house once with a speaker that had a nice brass bolt through the rear wall pressing on the back of the driver's magnet. It was a really solid heavy duty cabinet with an 8" full range driver. Unfortunately when I ran some sweeps to measure the electrical impedance, it became clear that the bolt and the back of the driver were "rattling". You could not get enough linear compression between all of the stiff parts to keep everything in contact. Any small motion of the driver's magnet unloaded the bolt and caused a loss of contact. The impedance plot was very hashy, back off on the bolt so it was not touching and it cleared up. I am not convinced that this is a good idea.
 
markaudio

markaudio

Member
Joined 2008
markaudio.com
Nanook said:
or ya could always set up a threaded insert on the back (from the inside), and use a suitable length bolt to couple the back of the driver to the back of the box. Just don't over-tighten
preiter: nice looking boxes (and not a bad table either).
Click to expand...

Hi Guys,
This particular idea has just been brought to my attention. Please note the following:

NONE of the Markaudio drivers are designed to take a positive stress load on the rear of the chassis, in particular the motor assembly (back-plate).

This practice needs to be VERY carefully controlled to avoid the risk of distorting the driver. This practice could result in the driver's power-train being pushed out of alignment if you get it wrong.

Cheers

Mark.
 
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planet10

planet10

frugal-phile™
Joined 2001
planet10-hifi.com
Paid Member
markaudio said:
This practice needs to be VERY carefully controlled to avoid the risk of distorting the driver. This practice could result in the driver's power-train being pushed out of alignment if you get it wrong
Click to expand...

Actuallty this is also true, althou perhaps not as dramatic, with pressed steel baskets & even cast baskets. You do not want to stress the basket.

A bolt also concentates the force, Andrews suggestion of a force spreader is a good one. I'll stick to holey driver braces thou.

dave
 
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Ian444

Member
Joined 2008
I cut up thin cardboard shims from a manilla folder and glue them against the brace with PVA, each shim is around ten thou (0.25mm), usually I need 1 or 2 or 3. Then mount the speaker while the glue is wet, making sure there is no glue between the back of the speaker and the last shim. Seems to work well. With 10 thou at a time, you can get it just right.

Ian.
 
P

Peter Menting

Member
Joined 2003
When I built my Alpair 10 OB's, I Glued a 1" thick Oak "Puck' to the rear of each driver, with a threaded insert at its center and used this to "Magnet Mount" the drivers. It would be easy to bring a threaded rod out of the puck with a locknut, two fender washers and a spring and "load' it against a brace such as the ones that dave uses!

Just a thought.
Pete
 
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mashaffer

Member
Joined 2004
tubiespad.angelfire.com/index.html
Made a test box

Back in Post 322 I mentioned that I was building a portable mini-console for my granddaughter and had gotten a set of 70s for that purpose. Well I finally got around to knocking up something for testing.

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I took the easy way out. I had some old Onkyo mini-speakers on hand so I took the guts out of one and knocked up a quick and dirty particle board baffle. It is a pretty sturdy box with lots of fiberglass stuffing. I just did one to start.

I didn't have a mono setup (or one with a mono switch) at my finger tips so I just used one channel of my HT and turned the balance all the way over and threw in a Phil Driscoll CD. I was quite surprised by how good the mid-bass was (down to about 60Hz I estimate was quite good). Plenty of SPL and what seemed to be very accurate midrange. Very clear and dynamic it was and the highs were right there (as high up as my old ears can hear anyway) but not exaggerated.

So far so good. I will make up the other one so I can do some more detailed testing and then I will be ready to try different amplifier topologies.

As to the built in subwoofer I am thinking of using a long throw 8" and trying out that trick where you use a box made intentionally too small and operate it entirely below the box roll off with a compensating 2nd order low pass filter. This would be in order to keep size down without sacrificing too much extension.
 
planet10

planet10

frugal-phile™
Joined 2001
planet10-hifi.com
Paid Member
The LS3/5A are in the midst of restoration. A friend bought them at the local hifi emporium (he always wanted a pair -- he still has his BC-1 i sold him in the late 70s), and when the milliFonken destroyed them, we opened them up to find that someone had taken the carcass of a dead set of LS3/5A and faked it. Wrong B110, dimpled T27, totally different XO.

Parts are mostly in the UK at the moment, awaiting one last bit before wending their way here.

dave
 
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