Replacement Diaphragm for P.Audio BM12CX38?

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danvprod

Replacement Diaphragm for P.Audio BM12CX38?
« on: 22 Jul 2017, 11:46 pm »
Hi folks. I am still trying to trace down a mismatch between my left and right super V HF speaker. Does anyone know where replacement diaphragms for the HF driver can be purchased? I have about a 10 dB difference between the left and right speaker centered at 1.8 kHz that follows the driver. My last shot here is to replace the diaphragm. I don't believe it to be the amp, room or crossover.

If someone has the spec sheet of the P.Audio BM12CX38 please PM me. I'd appreciate having that for my records.

Jonathon Janusz

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Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #1 on: 23 Jul 2017, 12:09 am »
http://reconingspeakers.com/product/p-audio-bmd750-bmd740-diaphragm/ - one source I found with a quick google search.

The HF driver in that coax was (if I remember correctly) BMD750 series ONE (they revised all their drivers in that series to a "series two" that doesn't share compatible replacement parts). 

There are a few vendors on ebay selling replacement parts, but if it were me and I was going that route considering you are trying to match them, I might order a pair (or two) of them to measure and make sure I get as close to a matched set as I could, as some of what is offered there are aftermarket parts.

T/S parameters from US Speaker:

http://www.usspeaker.com/paudio%20bm12cx38-1.htm

If you are picking up spares, one tweak I had done that squeaked out a little more performance out of the driver was having the diaphragms cryo'ed; according to Dave at P.I. Audio the cryo smoothed the tweeters out a bit more than the same driver broken in and stock.

You might also ask Danny if he could call up the distributor he got the drivers from in the first place, as they may likely have spares still easily available?

Hope that helps!

danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #2 on: 23 Jul 2017, 12:14 am »
Thank you, that does help very much. I was confused about the series ONE vs. TWO. I think replacing both diaphragms at the same time is a good idea.

Interesting thought on the cryo treatment. I'll give Dave a call.

danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #3 on: 23 Jul 2017, 12:32 am »
FYI, this is what I am measuring.



I've tried reversing the amp channels (left and right speaker cable swap to see if it is a tube or amp channel). I've tried moving the HF driver from the left P.Audio driver to the right -- the dip follows the driver.

And I've windowed out the room (I'm only using a 5 ms window with the mic inches from the front of the speaker). It's not a polarity problem in the XO (would have not followed the HF driver). I definitely think that this 10 dB gap centered at 1.8 kHz is pulling my phantom center to the left. 

Jonathon Janusz

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Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #4 on: 23 Jul 2017, 01:54 am »
When I was cruising ebay for you, I saw this listing:

http://www.ebay.com/itm/PAIR-of-P-Audio-BM-D750-Bolt-On-High-Frequency-2-Compression-Drivers-TESTED-/192219681748?hash=item2cc12f93d4:g:-28AAOSwgv5ZRHP1

This a a $180 OBO listing for a pair of complete tweeters.  If you could get them for a low enough price, it might not be a bad idea just on the off chance that it isn't the diaphragms but something more in the tweeter that is causing your issue.  $180 is a pretty good price as-is, but I'm betting you could probably offer your way to a better deal.  In any case, having a complete set of spares isn't a bad idea regardless.

The only thing you would need to do to is swap the machined out back plates on your drivers for the solid ones on the donor drivers; four allen bolts and about five minutes time, tops.

Just another thought.

danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #5 on: 23 Jul 2017, 10:29 am »
Missed out on those CDs, unfortunately. They were bolt on, though, so best case I would have been able to harvest the diaphragms, but not much else. I'll keep an eye out for a backup pair, wouldn't be bad to have on hand. In fact, I'd love to have a backup pair of the whole BM12CX-38 unit.

In the short term, I ended up ordering replacement diaphragms from Loudspeakers plus, which I beleive to be genuine P.Audio not 3rd party made to P.Audio specs.
https://www.loudspeakersplus.com/ProductDetails.asp?ProductCode=P%2DAUDIO%2DBMD750%2E8RD&CartID=1

Hopefully this does the trick.




danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #6 on: 23 Jul 2017, 06:29 pm »
For grins I took out the MiniDSP to try and correct the difference. Applied a + 9 dB boost @ 1.9 kHz Q 2.6 to try and balance out the two channels. You can see the deleterious effects of doing that in the harmonic distortion response. Imaging is better but worse all around and adding that much EQ is killing my max SPL, as you might expect.

Going to put everything back the way is was and cross my fingers that the replacement diaphragms do the trick, with no DSP or other processing needed.



drewm

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #7 on: 23 Jul 2017, 08:04 pm »
I'd be surprised if the driver is the issue seeing the very normal frequency response all the way up top. Have you tried reversing polarity on the tweeter just to see if that dip flattens out? It almost seems like one of those drivers is wired up with reverse polarity, as that dip is right around what I would guess is the crossover frequency.

danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #8 on: 23 Jul 2017, 09:50 pm »
Thanks for the thoughts on the polarity, @drewm. IIRC, the XO point of the V is 1 kHz. My measurements would seem to indicate that the XO is well behaved and drops considerably when the polarity is reversed from how they should be wired up (white cable to red terminal; red cable to black terminal).



poseidonsvoice

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Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #9 on: 23 Jul 2017, 11:29 pm »
 :scratch:

You purchased the speakers used yes?

Did the suspected driver spontaneously acquire that dip at 1.8khz after it was QC checked? It is unfortunate this happened in the field and I guess it can happen! It also brings up the question on if others with Super V's should do as you have done to make sure their speakers are to spec as it may or may not be obvious.

Nice graphs illustrating the issue.

Best,
Anand.

drewm

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #10 on: 24 Jul 2017, 12:51 am »
Interesting.

The other thing I was thinking, the rear area of the tweeter has been milled out right? It would be interesting if someone replaced the entire coax driver with a factory one and didn't do the milling to open baffle the tweeter. Without that milling work I would guess the FS of the tweeter would be higher and it wouldn't play as low. It's just quite a mystery as frequencies above the area in question behave just fine.

danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #11 on: 24 Jul 2017, 01:16 am »
I did purchase these used, although this is not a new issue. It's been a "feature" of the right speaker for as long as I had them, I think. I'm just finally digging in to addressing it.

At first when I purchased them, I set them up in my room and listened for a while, not taking any measurements (because they sounded so good and I was just enjoying the music). This was also before adding any treatments to my room.

I did always feel like the image pulled a bit to the left, but had figured it had something to do with my room.

It could be that there is just that big of a difference in the response between the two drivers, perhaps. i.e. there is nothing wrong with the driver, there is just some variation between the two. We will certainly know more when I swap in a new diaphragm later this week. Either way, it is a good idea to have an extra pair of replacement diaphragms on hand.

So that all being said, I don't think this a spontaneously acquired dip, but I will figure out what is going on. I'd love if some of the other Super V owners who have the gear to make measurements would see how close their left and right P.Audio drivers are in terms of FR. I'd be curious.

Danny Richie

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Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #12 on: 24 Jul 2017, 04:00 pm »
It is likely a driver issue. You can confirm by unscrewing the tweeters and swapping them.

I also personally built that pair out for a show in Denver, and I have measured and tested that pair. They matched perfectly at one time.

danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #13 on: 24 Jul 2017, 04:06 pm »
Thanks, Danny. I'm sure were perfectly matched at one time and I hope to get them back there! I'll update this thread when I get the new diaphragms and swap them out.

AKLegal

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #14 on: 24 Jul 2017, 05:46 pm »
I have a pair of BM12CX38s.  I was keeping my Super Vs around because I didn't know if I would ultimately prefer the NX-Ottica mtms over them (its a very close contest by the way).  I don't have the original boxes and packaging though.  They saw about two years of use.  PM me if interested.

danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #15 on: 24 Jul 2017, 11:40 pm »
Sent you a PM @AKLegal.

danvprod

Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #16 on: 26 Jul 2017, 11:24 pm »
New diaphragms came today from Loudspeakers Plus. Ordered on Sunday and they are here on Wednesday. I love them. Pretty easy job to replace the (although slightly stressful).

Quick measurements are much more promising.  :D



I am very happy about this. The red trace is the right speaker, which had the 10 dB dip at 1.8 kHz. That is gone. I'm happy with the tweeters being within a dB between 1kHz and 10kHz.

Thanks everyone for the help.
 

Danny Richie

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Re: Replacement Diaphragm for P.Audio BM12CX38?
« Reply #17 on: 26 Jul 2017, 11:33 pm »
 :thumb: