Mark Audio CHR-70 Application Thread - Page 33 - diyAudio
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Old 17th January 2010, 01:07 PM   #321
Henkjan is offline Henkjan  Netherlands
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Quote:
Originally Posted by planet10 View Post
Henkjan,

What is the material thickness on those?

dave
16mm
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Old 23rd January 2010, 01:49 PM   #322
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Mark Pointed me to this thread as I just got a pair of CHR-70s that I intend to use in a "mini-console" project. This will be table top semi-portable system designed for iPod/MP3 players and other line level sources incorporating the 70s as the mains driven by a custom built tube amplifier and a built in subwoofer (around 8") driven by solid state amplification.

I have gotten the drivers broken in by playing them at low and then moderate volume for about 3 days straight. I am going to knock up some test boxes to give them a listen and make sure that I am on the right track before going on with the project. Judging from the testimonials I have read I suspect these will do just fine.

I will have to try some different amplifier topologies to see what is needed but I am hoping that a 2 or 3W SE will suffice (cross over at 80Hz) but can go PP if necessary.

I modeled the drivers in a sealed enclosure and maximally flat gives a cut off point of around 80Hz which is coincidentally about right where I wanted to cross. I could make the corner frequency of the coupling capacitor between the VAS and power stage of the full range amp 80Hz also which would give a 3rd order total roll off to mate nicely with a standard subwoofer amp crossover. Simple and very forgiving I think.

However, another thought occurred to me that might have some advantage in further unloading the full range amp from dealing with low frequency energy. By ditzing with the size and tuning of a vented enclosure I was able to get a peak of 5 or 6dB (simulated) right around 80 or 90 Hz. As I recall it resulted in a 0.25 cf enclosure with a 2" port of two or three inches in length. By moving the coupling cap filter up to around 160 to 180Hz the response ended up being very similar to that of the sealed design. By doing this I am able to reduce the drive to the full range amplifier by and additional 6dB at 80Hz thus giving that much more headroom in the pass band.

What do you think of this second approach? Do the phase contortions pose a problem? I know the cabinet tuning is more difficult due to being a ported design of course. Another possible solution is to do a more normal ported design which extends the response well below the 80Hz mark and then use a more complex high pass filter (2nd or 3rd order) to mate to the subwoofer. This no doubt has advantages and disadvantages. The most obvious disadvantages are the more complex filter may require additional active devices and could have a negative impact on imaging properties. The overall unloading of low frequency energy from the amp and the overall phase response, however, might be better.

Any thoughts you might offer are appreciated. Also if there is interest I could start a build thread as I get things going.

mike
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Old 23rd January 2010, 01:57 PM   #323
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i think i read somewhere that a sealed driver have better transients than a BR ported one... also i'm pretty shure you'll gain much unloading the driver from LF content. advantages in unloading the amp i don't know and can't comment on, would you care to explaing more on this?
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Old 23rd January 2010, 02:48 PM   #324
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Sure. The amp that I intend to use will have limited power capability. Since the low frequencies consume the greatest amount of power any of the lows that we can filter out before the power amp stage will preserve the limited power capability for the frequencies we are interested in amplifying allowing us to play louder than we otherwise would be able to.
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Old 23rd January 2010, 03:19 PM   #325
Henkjan is offline Henkjan  Netherlands
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so you'll make a BR and then apply a series cap? this could very well work (will need some real life testing). I've seen something similar in the german magazine Hobby Hifi a couple of years ago. the result will be a very low cone excursion which helps keep distortion low, so should give a cleaner sound. intersting, keep us posted if you actually try this
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Old 23rd January 2010, 03:39 PM   #326
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Yes but the series cap is in the amplifier not the speaker. This way the power section of the amplifier is relieved of some heavy lifting.

In the amplifier (unless DC coupled) there is a capacitor between the plate of the voltage amplification tube and the grid of the power tube. This capacitor in combination with the grid leak resistor (resistor from power tube grid to ground which establishes the 0V DC reference voltage) forms a high pass filter. Normally this cap is made large enough to pass all audio frequencies without significant phase shift. In our case we just make it smaller so that Zc=Rgl at the desired frequency. Presto, instant 1st order low pass filter with no additional parts.
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Old 23rd January 2010, 04:28 PM   #327
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Just stick with sealed...

dave
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Old 29th January 2010, 02:45 PM   #328
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Question (possibly obvious and noobish) for CHR-70 (MK1) owners out there - what size/type screws did you use to mount?

I know the new MK2s come with M4 wood screws, and that others have posted that this is life-changing...

New CHR-70

But for those still building with MK1s, are M3s required? Not too keen on drilling out anything to #6 if I can help it!

Thanks as ever...
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Old 29th January 2010, 06:07 PM   #329
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I use brass screws (robertsen from Home Despot, careful you have a proper pilot hole and don't tighten too much or you'll twist the head off). I think they are #6 (smallest size available). I use the same srews for the newer drivers too.

dave
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Old 4th February 2010, 08:07 AM   #330
snooze1 is offline snooze1  Germany
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Hello planet10,
I plan to build a mMarS for Markaudio CHR-70.
The box is indeed closed. I need a blocking circuit to adjust the Lausprecher or does it not?
Have summaryplot frequency response and efficiency of the box? What must be insulated? The box stands on the desk near the wall.

Michael

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