[Bob Wilcox]

After eyeing the RMX pictures I have been toying with the idea of raising the ribbon panels slightly forward of the speaker cabinet - nothing radical say 1/2 to 3/4 inch so the grills will still fit .

I would do this by either using nylon spacers and longer screws or by building an MDF frame to fit over the current cut-out in the cabinets. While this may lessen reflections off the cabinet, it would place the ribbons forward of the woofer and tweeter mountings.

Has anyone tried this or have any thoughts as to any downside?

Thanks

Bob

[JoshK]

I've thought of this too....  Your not alone

[audiochef]

Bob,
I'd be a little leary of putting forth the already speedy mids.
For the purpose diffraction ,try experimenting with foam or felt lappels.

[ekovalsky]

Be careful how you attach foam, if you pursue that option.  One AudioCircle member had significant damage to the piano black finish when the foam was removed, prior to sale of the speakers.

[Bob Wilcox]

Thanks for the feedback.

I once used some custom speakers with 180 degree tweeter dispersion and a "acoustically deformable" rear ducted port. With the right orchestral program material, the illusion was striking at times. Some instruments appeared to emanate from beyond the room boundaries. This was in a pretty large room - a loft apartment actually.

My present room is around 14 by 16. So, there are limits to how much I can effect things by speaker and listeming chair placement. The RM-40s are pretty close to the side walls.  My racks are on the back wall and cannot be relocated.  I have already done the woofer swap and that helps in this particular room. If I had a larger room I would want to try it both ways.

I would like to achieve some additional image width and a bit more center fill.

I have already experimented with felt and foam. Never found much of an effect with the felt. I attached it with that semi-sticky translucent Scotch tape and it came off easily. I could not find a satisfactory set up. I would end up adjusting the driver levels and end up where I started.

With foam, I ended up with 1 to 2 inch strips right at the sides of the array (except by the tweeters). I hold this on with adhesive backed velcro pads . These pads come off periodically and seem to be doing no harm to the finish. I find this a slight improvement. The foam  prevents a little bounce harshness that was there regardless of driver level setting. It is probably less absorbtive than the felt.

As far as driver offset, does anyone know if the RM-40 supposed to be a time-aligned driver design? I am thinking of adding less than an inch of projection.

I am also curious about the need to extend the side walls with rigid material. Does the ribbon box provide any acoustic benefit to the mids or is it simplly to protect the mids from being vibrated by the other drivers?

Any further thoughts are appreciated.

Bob

[John Casler]

With foam, I ended up with 1 to 2 inch strips right at the sides of the array (except by the tweeters). I hold this on with adhesive backed velcro pads . These pads come off periodically and seem to be doing no harm to the finish. I find this a slight improvement. The foam prevents a little bounce harshness that was there regardless of driver level setting. It is probably less absorbtive than the felt.

Hi Bob,

While most people don't know this "Veritone MINIMUM PHASE Speakers" are not time aligned but the phase coherency is addressed by Brian in the electronics.  (part of his "MAGIC")

So if you made an adjustement in speaker alignment, it could affect the "phase" slightly.

As far as adding acoustic foam "lapels" it is imperative that you use at least 3" foam.  The smaller foam will not be sufficient to absorb as needed.

You might even find that felt backed foam might provide an even better absorbsion.

I used 3" wedge foam strips and when I do it again, I will use 4", with felt backing.  I will also make a suport mechanism that allows the foam to be mounted using the holes already in place from the speakers grill pegs.

With the acoustic foam strips in the 3-4" range you also slightly reduce "side reflections" to a greater degree, than smaller 1-2" strips.

[Ric Schultz]

Bob,
If you move the midranges forward they would be less time aligned....not a good thing.  Generally, when the voice coils are aligned it is close.  The point that the sound starts at in a woofer is near the inside by the dustcap, this is where you would want the midranges to line up to....at least a starting point.  I have my midrange set back about one inch on my highly modded 626s and it would be even more with a 10 inch woofer. Makes big difference to have drivers time aligned.  So, if you want to time align then you want to move the woofers forward, not the midranges.  

If you want to make a big difference in sound for cheap and easy, then change the fiberglass insulation behind the midranges to long hair wool.  I have suggested this to several people here and they all find it very worthwhile.  It sounds way more open and electrostatic with wool behind the midranges compared to fiberglass.  I hope Brian reads this and starts doing this, it should be standard and done at the factory!

This is just one of many tweaks I offer my customers who buy VMPS speakers from me.

[zybar]

I can vouch for the positive impact changing the fiberglass for wool had on my RM 40's.

It will cost you about $20 from Madisound and say an hour or so in labor.

Just be very careful when taking out the fiberglass and putting in the wool.  There are wires that you wouldn't want to rip out.  

George

[Bob Wilcox]

Thanks for the suggestions guys.

John

The foam I am using I paid way too much for 20 years ago. It is 3/4 inch egg shell. I use it in 1 inch wide strips right at the edge of the ribbons. For child reasons, the grills must stay on and the 3/4 inch height allows this.

Ric

The idea of moving the mids back is interesting. They would be recessed inside the cabinet - would this not limit dispersion to the sides unless some cabinet surgery was involved? As I mentioned, I would like to increase the image center fill and this seems counterintuitive.

Zybar

The wool mod sounds cheap enough and I will probably try that.

Does anyone have an opinion concerning the need to have a solid side wall piece if the drivers are offset outward or inward? They could just be suspended on spacers.

Whatever I do, I will get very familar with some recordings that do some heavy phase tricks so I can judge the effects before and after. When moving drivers, too much time elapses to judge subtle changes.  Increase or decrease in image size is fairly noticable and changes in the behavior of the phase effects on discs such as Amused to Death and Bowies's Reality should be quite noticable.

Bob

[audiochef]

I've  also been experimenting with insulation for the mids. I first tried a little acoustastuff that I had left over with positive results. I then went super economy  and just got some cheap polyester fill for stuffed animals  at the craft store.
Since the stuff is so light and fluffy, I packed it in, abut a 3/4 pound each mid enclosure.
I noticed an imediate  difference just like Rick schulz describes, more open ,and to me less boxy and less beamy.
Bob, if your searching for more width and center fill, this tweak will definately help.

[Ric Schultz]

Bob,
No, move the woofers out.  You definitely don't want to move the mids inside the cabinet....on my modded 626s I turn the cabinet upside down and place the midrange/tweet on an open baffle that can be moved back on top of the bass cabinet.....no can do with RM-40s.  You can try some kind of spacer on the woofers to move them forward about 1.5 inches.  Pretty crazy, I know.  I am sure you won't try it.  Don't blame you.  Enjoy.

[BrunoB]

Bob,
No, move the woofers out.  You definitely don't want to move the mids inside the cabinet....on my modded 626s I turn the cabinet upside down and place the midrange/tweet on an open baffle that can be moved back on top of the bass cabinet.....no can do with RM-40s.  You can try some kind of spacer on the woofers to move them forward about 1.5 inches.  Pretty crazy, I know.  I am sure you won't try it.  Don't blame you.  Enjoy.

I would be interested to see pictures of your modded 626s!

Bruno

[Bob Wilcox]

Ric wrote

You can try some kind of spacer on the woofers to move them forward about 1.5 inches. Pretty crazy, I know. I am sure you won't try it. Don't blame you. Enjoy.

Ric

If I had the wood working skills I might. The woofers are supposed to be sealed to the cabinet to work with the passive radiator so individual spacers on each screw would not cut it. I would still need to seal the circumference.  I would need to make 4 big circular spacers instead with the same size opening as the cabinet holes. If I could do that I could probably make the new grills to fit as well.

I have ordered the lambs wool. This weekend I may mount the ribbons on 1/2 inch spacers and see what happens. Time alignment would be nice but imaging is a bigger priority at this point. The issue is mainly with SACD/CD where the images seem shrunken compared  with vinyl or my Scott tuner.

Bob

[John Casler]

Big B can correct me if I am in error, but again, moving VMPS drivers out of their present plane will move them "out of phase" as it is my understanding that Brian "adjusts" for "phase" via crossover electronics.

He has been doing this for years, in fact the name VMPS is based on this practice.

As far as I know, Minimum Phase, and Time Aligned are one in the same thing as far as result, except this works better.

[Ric Schultz]

Brian does not do anything in the crossover to account for time difference between the drivers.  He uses first order crossovers that are in themselves fairly phase coherent.  Actually his high pass crossovers use a cap and a coil but the coil value is lower than usual for the crossover point so basicially the crossover is 6db per octave near the crossover point and switches to 12 db per octave lower in frequency.

If you stagger the drivers than you get true time alignment.  This is what Vandersteen, Dunlavy, Theil and others do (they use both 6db per octave/first order crossovers and they stagger the drivers).

The only way to know what staggering/aligning the drivers can do is to try it and listen.  Once you have done this then you have true knowledge (knowledge based on direct experience).

[John Casler]

Since I would do a poor job of explaining this (obviously) the quote below is from Brian, explaining "minimum phase" and how it is acheived in VMPS speakers.

Minimum phase response means that the envelope of frequencies arrives at the ear at the same time and in phase.
 
To do this I use first-octave crossovers (phase linear) and wire all drivers electrically in phase.  Phase plugs in the woofers move their acoustic center forward to the blunt tip of the phase plug, and into the same plane as the mid and treble ribbons.  Since these are planar they do not need to be moved back relative to the woofer's acoustic center.  You can experiment with this by tilting the 626's back a few degrees at a time.  You will hear the sound deteriorate.   Optimum listening height for the 626's is about a 8" window between the upper third of the mid panel and the top of the tweeter diaphragm.
 
There are "time aligned" speakers like Thiel and Vandersteen that wire midranges electrically of inverted polarity relative to the woofer and tweeter, which results in measurable better amplitude response.  Of course it also destroys the integrity of the music signal outside the crossover region.  Phase changes 180 degrees as the signal exits crossover and into the passband, a highly deleterious and audible effect.
 
Push pull motors, in phase electrical wiring, first order networks and very wide range transducers are some of the keys to my designs, but hardly all the considerations necessary for good sound.

[Bob Wilcox]

Thanks for the info John.

I did try my experiment of moving the ribbons 1/2 off the box. I used 1/2 inch nylon spacers. The midrange does seem a bit quicker and I needed to increase the tweeter level ever so slightly. For those disks that do phase tricks, the placement in the sound field of various effects is slightly altered.

With more typical CDs, the center fill is increased. In my first post, I mentioned some of the constaints in my listening room. I will receive the lambs wool late next week and try that. I will probably remove the spacers Tuesday as I will have lived with this set-up for a while and need some time back with the normal mounting to be able to compare the lambs wool mod to stock.

So combining Brian's explanation with Ric's, if the ribbons are brought forward, the tweeter should be as well and longer phase plugs used on the woofers?

Bob

[Ric Schultz]

All I know is what I hear (direct knowledge).  And when I moved the midrange back one inch relative to the 626 woofer it sounds much better.  I don't really think the phase plug in the woofers moves the acoustical center to the front of the woofer....but, I could be wrong.  And the phase plugs on the RM-40s woofers do not extend all the way to the front of the speaker. As far as tilting the speakers back, well, you are now not on axis with the midrange or tweeters or woofer and since the mid and tweets are so directional, it would make sense that you would screw up the sound.  Again, the only way to know if it will be better is to try it.  Someones thoughts about it are really just that....thoughts.

Bob,
Of course, the midranges would seem faster when you move them forward.  This is because the mid/lower highs are hitting your ear even earlier than the lower frequencies.  If you like this, fine, but it is going away from time aligning.  

The wool mod is what you want.....you cannot make a particular source image better than it does.  If your turntable and tuner image a certain way and your other sources do not then you need to change the other sources.....it cannot be the speaker as it does the "good" imaging on the other sources.  Of course, felting and foaming will help limit dispersion/defraction and help imaging and clarity.  I have always done this on every speaker I have ever had since 1980.

Yes, of course, you would have to make circular extension rings/spacers for the woofers if you were to move them forward.  It would be a lot of work but one could do it with the right tools and some, say, one and a half inch MDF or maple would be better (and way more expensive).

Enjoy

[Ric Schultz]

BTW,
There are electical tests that are helpful in time aligning.  I think you can use a square wave through the speakers and look at the results from a good mic on a scope and also remember people putting the mid and woof in separate rooms and using square waves similtaneously and using one scope for both in summing mode and moving one of the drivers back and forth till a good square wave is had and then using those measurements in the final speaker.  Maybe Brian has done this type of test and can tell us his findings or his opinion about this type of test.

[Ric Schultz]

Just remembered that stereophile magazine uses a pulse step test where one can see how the different drivers affect the step response.  A time aligned speaker would have a pefect step response (straight up and then like 45 degrees down).