B&W's not so bad...

madmax

In September of 05 I was excited about finding some vintage B&W's from 1972 in great shape. They were DM4's. I posted this thread. http://www.polkaudio.com/forums/showthread.php?t=32455&highlight=antique+shop
The data sheet said "suitable for amps between 10 and 30 watts" so I hooked them up to a 50wpc amp. I was very disappointed. They worked but sounded like crap. They got unhooked and thrown in the basement. They even followed me to Colorado when I moved because I had not thrown them away yet. Last weekend I gathered up a bunch of stuff to fill my trash can and these were among the stuff in line. The can filled up and there was not enough room left for the speakers. I thought "I'll dump them in next weeks trash".

Well anyway, since they were just sitting there on the garage floor, I decided to take them back in the house and hook them up to my big rig for the hell of it. I turned on the manley amps in tetrode mode which is 100wpc and turned the dial up part way. There it was, a very poor sound. In the big 20' x 30' room I knew they had no chance anyway. I placed them on two big pieces of PVC pipe as stands to see if that got me anywhere. OK, so they were not so bad as far as soundstage but they were not nearly loud enough to enjoy.

I turned them up to use the full 100 watt peaks and they started to sound nice and full. Still, they just didn't have much to offer. I was surprised they didn't blow a tweeter or something since they were meant for a 10 to 30 watt amp. I knew I was throwing them away anyway so I set the amps up for 200wpc by flipping a switch which puts them in pentode mode. As I got into the 150 to 200 watt peaks these things came alive. I turned on the bass transducers to give them a little bit of bass below 60 hz. The soundstage was big and open and the volume level was much closer to my normal speakers. I tried song after song and found myself looking at the two pairs of speakers in front of me and thinking that it could be either set. Wow, what a surprise! I listened for about 5 hours before going to bed. I was just waiting until some drivers started to fail but they kept on.

The next day I ran through a bunch more music just to see if I had the same impression. Sometimes I'm just in the mood or whatever and everything sounds good whether it does or not. I had the same good time with them as I did the night before. After another 4 hours I shut them down. What a surprise this is! I guess I'll use a nice 200W amp and maybe put these somewhere, maybe in the media room where I keep all the records, tapes, cd's dvd's etc. Or, maybe in the garage if they don't like a small room.

So here is my question: why would anyone rate a speaker which obviously wants between 100W and 200W at 10W to 30W? I'm thinking back in 72 they wanted to sell them to more people and most people had something under 60 watts.

beardog03

They probably thought they would only be hooked up to a avr type reciever

I like your horns though !!

madmax

I don't think they had avr receivers in 72. Thanks for the complement. Most of the time I keep them covered up with a black cloth because they are too distracting.

beardog03

I bet the tubes sound real smooth through those bad boys !!

Face

How do you get away with only 100wpc powering those horns?

What happened to your woofers?

mantis

I never heard a B&W speaker I didn't like something about it. THe old ones are very nice. I run into them all the time. It seems old Rotel or Adcom goes hand in hand.

madmax

Face wrote: »

What happened to your woofers?

As good as the Duos are there is something wrong with the bass. It is generally reported as a dip around 160 hz. The reason for this dip is the designers tried to do too much in the single bass modules. They gave up some of the upper bass to allow the bass modules to also act as a subwoofer. I'm in the process of trying different drivers/passive radiators to replace the originals. It will no longer be a subwoofer when I'm done, only the bass section down to maybe 40 or 50 hz. I will then leave the sub bass up to a seperate subwoofer. Some people just remove a wire from one of the drivers and allow it to act as a passive. This is almost good enough but I think I can do better.

Most speakers give up one thing or another to achieve something else no matter what the price tag.

Face

What is the enclosure's displacement?

Do you have a certain load and sensitivity in mind?

How will you be crossing them to the horns, passive or active?

And are you sure the dip at 160hz isn't floor bounce? An easy test, lay a bass trap on the floor between the woofer and mic.

madmax

The 160hz dip has been measured by others and this problem has been frequently discussed. I actually read about it before getting the speakers and noticed it first hand while auditioning. The bigger brother, the trio, does not have the problem because of a bigger mid horn with a lower crossover.

I'm not sure where I'm going yet. The midrange and treble horns have their own passive crossovers. First thought was to use a passive radiator and one of the original speakers. Each bass module has its own active module (SS, ewe) and I planned on using it so efficiency isn't really an issue. The second thought was to use two higher efficiency drivers and a dedicated external tube amp. I know I can't reach 104db eff with this size and two 12" drivers. Third thought was to build a real bass speaker, 104db efficiency, whatever size it must be, and use with whatever amplifier I'm driving the horns with. The current box is about 2 or 3 cubic feet. I have not measured internally yet.

While thinking through another speaker system I'm playing with I found suggestions to make sure the bass cabinent is at least 36" wide for some reason. Something about half room or full room something or other. Need to look into that a little more. I actually have room now to do whatever I choose but I hate to get too big.

Oh, and I'm not measuring them yet, just playing around. I like to get things to sound fairly close and then have my friend bring his equipment over to see how close I am. I guess I should buy a setup for myself but I have not.

Face

36" wide?! F-that.

So the woofer/mid crossover point is 160hz? That would mean you still would need a 28" baffle so baffle step loss isn't an issue. Or, you can just build a 12-16" baffle and add a BSC circuit: http://diyaudioprojects.com/Technical/Baffle-Step-Correction-Circuit-Calculator/
The only issue then is the loss of some efficiency. So either look into a pair of high efficiency pro audio drivers or pick up a pair of hi-fi drivers with a beefy SS amp.

madmax

So how did you calculate I need a 28" baffle?

madmax

So now I'm wondering. The current bass speakers are only 14" wide. That means the sounds below a given frequency are traveling around the speaker causing a 3db rolloff below that given frequency. Does that mean I could put a wider baffle (26") on the present cabinent and make it roll off at a higher frequency? Am I getting the baffle step idea right?

Face

That's a possibility.

Two things. The baffle would have to be reinforced so it doesn't vibrate.

You can experiment with cardboard or plywood to find the correct width and to ensure you're not creating peaks elsewhere. But you would need a measuring setup(less than $200).