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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Wikipedia says "sacrilege" is the violation or injurious treatment of a sacred object. Well, I committed sacrilege today.
But let me back up.
Some of you may remember I have two pairs of Beovox M70s. One pair (without frets) had been relegated to the garage system (Beosystem 1900), while the other pair is in my main two channel system. You know that pair - check my avatar.
Go ahead. Look at my avatar. I'll wait for you.
Earlier this year I built a pair of REDACTED which replaced the S40s in the spare bedroom system (Beosystem 2400). The S40s went out to the garage and replaced M70 pair 2. And then it was on to a new project: making a center channel speaker from the spare M70s.
Over the past few weekends I've played around with mockups and crossover configurations (by reusing parts on hand).
The project goals:
1) make a center channel that sounds good in a 5.1 system where M70s are the front left and front right speakers
2) make a center channel that is a visual match to the M70s
My first attempt in meeting Goal 2 will be to repurpose one of the M70 boxes. Thus the need for sacrilege.
A few smart raps with a hammer on the inside of the baffle resulted in a crack.
Then it was just a matter of repeated blows to the front of the baffle until bits started breaking off. (The baffle is a somewhat brittle plastic.)
To my surprise, I rather quickly got to this point. Here you just rap down and the piece pops out.
And now I have four sides of a box.
The next steps will be to (1) downsize the repurposed box to center channel size, and (2) cut the new baffle. I've got the wood for that, and Mark from Meniscus has new crossover components heading my way.
I take it that you are re-using the small enclosure for the mid-range units?
Peter
Peter: I take it that you are re-using the small enclosure for the mid-range units?
That is the plan.
Mark at Meniscus filled my order same day, so I've already got the crossover ready to solder. Cut the baffle this evening, also. Will try to get a hole or two cut for the drivers tomorrow night. Should be doing some listening tests by Sunday evening.
How are you modifying the original xover? Just copying it and changing values for different impedances due to two drivers won't be enough, for one thing the tweeter level will need to be at least 3 dB higher to compensate for there being only one of them. I don't know how much padding they had on it in the original xover, I hope the output of a single tweeter will be enough. Usually they are a fair amount more efficient so you should have enough output but I don't know how efficient the midranges are. Also, the change in driver positions relative to the baffle dimensions will have an effect, though it may be relatively minor and noT important.
An interesting project though.
Jeff
I'm afraid I'm recovering from the BeoVirus.
Jeff: How are you modifying the original xover? Just copying it and changing values for different impedances due to two drivers won't be enough, for one thing the tweeter level will need to be at least 3 dB higher to compensate for there being only one of them. I don't know how much padding they had on it in the original xover, I hope the output of a single tweeter will be enough. Usually they are a fair amount more efficient so you should have enough output but I don't know how efficient the midranges are. Also, the change in driver positions relative to the baffle dimensions will have an effect, though it may be relatively minor and noT important.
The M70 uses four drivers: 10" woofer, 5" "phase link," 2" midrange dome, 1" tweeter. The published crossover frequencies are 500 Hz and 4500 Hz.
Yes, that's correct. 4 drivers, yet just 2 published crossover frequencies.
The M70 crossover has 9 capacitors, 4 inductors, and several resistors, I don't remember off the top of my head. Something like 5 or 6.
So - heck no, I am not copying the original crossover, as I'm omitting the 10" woofers. The first experimental crossover will be a simple first order with crossover frequencies of 500 Hz and 4500 Hz.
I had it in my head all along that I would be cutting the M70 box down to size. Think of turning it on its side, then taking 3 or 4 inches off the bottom. But then Jeff's question got me thinking - why not try a mockup with both tweeters?
But this uses the whole box. Why not just stick with a center channel that's just the original M70? Can't get a better sonic or visual duplicate that an exact copy.
So anyway, I cut a different baffle today, and was able to get all the drivers mounted. The baffle has not been fixed to the old box as yet. I need to remove the drivers and paint the baffle black first.
Also, the dome midrange has something of a wave guide built into the driver. I need to do some sanding/routering on the baffle to extend the wave guide into the baffle. This 5/8" MDF is about 3/16" thicker than the plastic baffle of the original speaker.
But I'm done for the day, I think. It's still pushing 90F here in Florida, and I need a break. Plus Michigan football (I think we're playing Jaff's Boilermakers) and Detroit Tiger baseball will be on in a bit.
Rich:Plus Michigan football (I think we're playing Jaff's Boilermakers)
Stick with your project ... It will be less painful than watching the forthcoming Weinerine smackdown.
Well, I'll say you get the prize for a design that will maximize comb filtering at all frequencies! I was going to suggest that you either use one tweeter or mount them one on top of the other rather than side by side and mount the mids as close to each other as possible.
The mid domes are the phase link drivers, they cover a very small range of frequencies in the dip between the cone mids and tweeters. The purpose was to address a dip in the power response into the room that was a result of the crossover between the mids and tweets, this dip typically did not show up in the on axis frequency response but it affected imaging and dispersion into the whole room. If you're using them like a real mid and making this a three way you will have a really hard to near impossible time matching the timbre of the M70s, especially using first order xovers. The xover of the M70 being a phase link setup is pretty complex and it's going to be hard I think to timbre match even with the same drivers.
Not trying to dissuade you from what looks like a fun project, just want to point out some issues you will need to address. Do you have measurement equipment? I have the Parts Express TrueRTA setup and a calibrated mic, unfortunately it's packed as I'm like two weeks away from moving or I'd love to have helped you with measurements and such since we're in the same town.
Wrong. The 5" cone is the phase link.
I always thought it was the mid dome. Learn something new every day. In that case, I suspect your problems will be even worse in trying to match the mains if you're now using those mids as midwoofers. Any reason you dropped the large woofers completely other than space?
If I gave the impression that I've settled on a design, I'm sorry. This is an experiment. I'm going to try at least a half a dozen configurations.
Oh, and it's something of a goof. Wasn't that apparent as well?
I got the impression you were more focused on matching the M70s, a more serious project. I can certainly think either approach, serious or crazy experiment, is fun and worthwhile! I didn't mean to imply otherwise, just was trying to point out areas of potential difficulties if you were more serious. Gods know I've decomposed and repurposed/redesigned plenty of speakers in my time, I've got a set of drivers from the old 70's, and highly underrated, Sansui Linear Motion LM 110 and 220 speakers waiting for me to turn them into a pair of bipoles. and I don't even have the original xovers for a guide.
How low do you think the cone mids will go? Down to 150 hz common in center channels or higher? I've never measured a set of mids out of a Beo speaker.
Dom
2x BeoSystem 3, BeoSystem 5000, BeoSystem 6500, 2x BeoMaster 7000, 2 pair of BeoLab Penta mk2, AV 7000, Beolab 4000, BeoSound 4000, Playmaker, BeoLab 2500, S-45, S-45.2, RL-140, CX-50, C-75, 3x CX-100, 3x MCL2 link rooms, 3x Beolab 2000, M3, P2, Earset, A8 earphones, A3, 2x 4001 relay, H3, H3 ANC, H6, 2014 Audi S5 with B&O sound, and ambio
Jeff: How low do you think the cone mids will go? Down to 150 hz common in center channels or higher? I've never measured a set of mids out of a Beo speaker.
The cone mids, mounted in a paper box, with a beach towel inside for damping, go down past 70 Hz easily. A bit surprising.
DMacri:What if you used all the drivers and crossovers of a single speaker and just rebuild the baffle for horizontal orientation? Certainly less ambitious, but you would end with a much better sonic and visual match to you fronts.
The 10" woofers are being held in reserve for the next "goof experiment": powered LFE speaker.
Wow! 70 hz, that is surprising. Truly a mid woofer rather that a true midrange, but probably chosen for its dispersion over the phase link range more than bass. Should work admirably for a bass limited center channel.