PMC OB1 mid range problem

ozgur_turel · « **on:** 4 Mar 2009, 09:42 pm »

Hello,

I have had some issues with my OB1's and I would like to share them with you. Maybe some of you can provide some feedback about the possible reasons of my troubles.

I bought my OB1's about three years ago and after a year I realized that one of the mid range drivers was producing a bizarre rattling kind of sound. Not always though. Only when I am listening to piano music and on some certain notes. And when the volume is beyond a threshold - not too high though, about 1/3rd of the volume dial. When that special note is hit on the piano, the sound is as if you left a bottle on the top of the speaker and it is shaking or rattling.

At the time I contacted my sales person and he sent me a replacement driver which solved the problem. He told me that the reason why this happened is because I was driving my speakers with an amplifier that did not have enough power, so the sound signals were turning into square waves. This was causing problems with the speaker because there were not designed to function under square waves.( I was using an arcam solo at the time - was saving money to buy Bryston).

Anyway, after he told me that the source of the problem is my amplifier, I unplugged my speakers and ordered my 4BSST. Since the day my new amplifier arrived, I have been using it to drive the OB1s.

Two years fast forward, and I am having the same problem. I am not sure if it is the same speaker. But the problem is identical.

So, my question is this: Am I doing something wrong and ruining my speakers? Or, is there a design issue that someone out there is having similar problems?

Regards,
Ozgur

PS: By the way, I am a proud owner of all Bryston/PMC system. I completed the system last year with BCD1 and BP26DA. I cannot explain how much this system adds to my life. I typically work long hours and there is nothing like coming home, opening a beer, and hunting through my CDs for long forgotton favorite tracks. Thanks Bryston. Keep up the good work.

AnthonyH · « **Reply #1 on:** 4 Mar 2009, 10:14 pm »

Hi

I've got a very similar setup to you and have never had a problem like this with my OB1's. I listen to a wide variety of music at all volumes and no rattle other than objects in the room from time to time.

Hopefully Ian (mvwhiting_83) from PMC will be able to help though.

Levi · « **Reply #2 on:** 4 Mar 2009, 10:15 pm »

Hi Ozgur,

+1 with Anthony.

I don't notice distortion you described with my OB1 even with insane volumes.

Levi

b5pt9 · « **Reply #3 on:** 4 Mar 2009, 10:17 pm »

Check this out, might be relevant: http://www.audiocircle.com/index.php?topic=58853.0

ozgur_turel · « **Reply #4 on:** 4 Mar 2009, 10:18 pm »

Actually, at high volumes, the distortion is quite masked.

By the way, as far as I can see, the problem shows itself only with pianos. Never human voice, a symphony or a chamber piece caused a problem. But maybe the problem is there but it is masked. I don't know.

jethro · « **Reply #5 on:** 4 Mar 2009, 10:27 pm »

Welcome ozgur,

Do you have a test tone CD ? Perhaps you can disconnect one speaker and
see if a single tone causes the noise that you are hearing. This might help
you track down the possible causes.

ozgur_turel · « **Reply #6 on:** 4 Mar 2009, 10:30 pm »

I don't have a test tone cd. Is it something I can download and burn?

MOD: Looks like it is. I will test it this evening and report the results.

mvwhiting_83 · « **Reply #7 on:** 5 Mar 2009, 12:06 am »

Hi Ozgur,

I'm sorry to hear about the trouble you are experiencing, surely I know the great joy you get from a cold brew and beautiful music... So for anything to hinder that would most annoying.

I would recommend for you to get a test disc of some sort. I have the Ayre Acoustics - Irrational, but Efficacious disc, it has a sweep on there that helps with frequency specific problems. I know there are a few others out there, I got this one from Music direct. I would ask that you remove the jumpers on your binding posts and connect directly to the midrange in question. Play it back at the levels at which you experienced the problem. Check and see that it is the driver and not a cabinet resonance of some sort. Is it the same recording that you are hearing this buzzing? Or is it with all piano music? Worse comes to worse, you can give me a call at the office and I will be more than happy to get you a new midrange.

Bassmann · « **Reply #8 on:** 9 Mar 2009, 04:58 am »

Hi ozgur_turel

I own a pair of OB1's, hooked up to a 4BSST.

Their has been an occasion or two in about 2 years where on certain music / frequency / volume when i thought i have heard a very slight resonance or imperfection emanating from the midrange unit. I wondered personally if it could be due to not having a rubber gasket to dampen vibration in the plastic housing? but also maybe that the OB1 is not hardcore enough for me (maybe IB2 or MB2 better for me

)

If its not the driver at fault, I'd try rolling a thin layer of blu-tack and pressing it into the outside edge of the driver cut-out so that when you screw the driver in place, there is constant pressure all the way around the outside edge of the driver housing.

Would be nice to see rubber gaskets on all the drivers really, just to be sure.

One thing I did in the past was change my speaker cable to a tri-wire QED XT-400, and WOWSERS, that opened up the sound way too much - and definitely made what you describe more likely to happen, especially I imagine with piano music. I reverted back to my previous cable, bi-wire (vdh cs122) which smoothes the sound slightly to my ears.

Hope this helps

ozgur_turel · « **Reply #9 on:** 10 Mar 2009, 12:41 pm »

Sorry to respond so late. I never got a chance to try a test disc. However, I confirmed that the problem was with the mid range, not the cabinet. (I just swapped the mid range of the buzzing speaker with the non-buzzing one. ) Moreover, the problem is more audible once the mid-range is hanging out from its place.

I'll contact my salesperson and see if my warranty still works. Thanks for all the feedback.

Ozgur.

Levi · « **Reply #10 on:** 10 Mar 2009, 01:07 pm »

This is good to know Ozgur. Definitely an isolated incident.

Levi

Moon Doggy · « **Reply #11 on:** 10 Mar 2009, 07:02 pm »

Does it only happen on one particular piano CD? I had a rattle in my speaker on a piano recording as well. I was worried that I had cracked the driver. Then I realized it was a flaw in the recording where the mike was picking up a rattling from the piano itself.

mvwhiting_83 · « **Reply #12 on:** 10 Mar 2009, 07:38 pm »

Could your OB1 be too revealing? I actually had a customer with and TB2 swear that there was a cabinet resonance problem, he sent them to me... It was in fact an artifact in his recording.

ozgur_turel · « **Reply #13 on:** 10 Mar 2009, 07:59 pm »

I am having the problem with multiple cd's.

I don't think the reason of the buzzing is OB1 being too revealing either, because if that were the case, when I swap the mid-range, the problem would persist in the same channel (I have two OB1s, remember?).

Another explanation of the experiment is that, maybe, the speaker that I thought was OK is actually not revealing enough, though I am not buying that explanation. The CDs that are showing the problem were OK just a few weeks ago. And one of the CDs is my favorite and I used to listen to one of the tracks once every two weeks at least. So one of the the speakers became more revealing all of a sudden after two years, and nothing happened to the other? I am not buying that.

I will download a test CD and see if I can isolate the frequency.

mvwhiting_83 · « **Reply #14 on:** 10 Mar 2009, 08:34 pm »

This singular midrange problem is entirely odd. Let me know what you need, I will get it done.

audiokiep · « **Reply #15 on:** 10 Mar 2009, 09:43 pm »

A test tone CD may not reveal the fault frequency. You need to do a sweep and have a way to determine the actual frequency when the problem occurs.
I would suggest using some computer software to measure and/or generate signal. There is lots of freeware out there.