Mark Audio CHR-70 Application Thread

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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
 
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.
 
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
 
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.
 
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...

http://www.diyaudio.com/forums/markaudio/153091-new-chr-70-a.html#post1944788

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...
 
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

Sorry, the Translation is from Google :D
 
Hello,
I plan to build a MMAR 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
 
frugal-phile™
Joined 2001
Paid Member
Where you will be placing the loudspeaker you probably do not need a filter. Simulated FR is attached. Efficiency is the same as specification for the CHR-70 at 84.5 dB. You can looslry fill the box with polyester wool or line the walls with 1/2" wool or cotton felt.

dave
 

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@planet10,
is the Alpair 7 better then the chr-70 :confused:
Fits the Alpair 7in the same mMars as the chr-70.


none of us @planet10 have yet to hear the Alpair 7, but even so the question of "better than" will probably be best answered as: "maybe - that can depend on your criteria" (i.e. is it a better match/value for you)

for example, I happen to prefer the paper coned (CSS branded) EL70 over the CHR70 in normalized versions of 2 quite different enclosures with which I've had opportunity to spend much time ( i.e the milli-size series, and the microtower), but as always, YMMV
 
frugal-phile™
Joined 2001
Paid Member
The milliSize sealed enclosures are 5.5 litres and about as small as i'd go.

I see lots of FRs from Germany (i guess mostly from Timmermans) that have filters with more parts than some of my amplifiers, so i'm guessing that is where your question comes from.

I avoid filters as much as possible, and that is where i'd start if i were you. I know the German diy magazines have covered both of these drivers, if you find after you have lived with them for awhile that you feel the need to play, then i'd use those as a starting point.

dave
 
Not exactly to the milli plans, but they are about 5.5 liters and the backs of the drivers are braced.

Rather than fuss with getting the brace exactly the right height, I left it about 1/16 of an inch short and stuck some duct sealer to the back of the driver before I installed it.

2xd7E.jpg
 
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