Successful Conversion of RM30Ms to RM30Cs

RGordonpf · « **on:** 3 Dec 2007, 04:20 am »

Hi All,

It seems like almost everyone is tweaking or modding their VMPS speakers. I had to join in, so this weekend was spent making a simple, inexpensive mod that yielded major sonic improvements. I have a Larger sub with the VSS driver upgrade. I also have a pair of RM30Ms that are from the first production run. I had been using deHavilland 845-G mono block SET (Single Ended Triode) amps to drive the RM30Ms full range. The upgraded Larger, which has a dual voice coil, was driven by a pair of Daytona Loudspeaker 500 watt plate amps. These plate amps are the smaller version of the 1000 watt plate amps that Big B uses in his Powered Bass Systems.

This system was very musical with all of the usual audiophile superlatives regarding imaging, focus, soundstaging, etc. What the system lacked, however, was bass slam. The problem was that the RM30Ms go down to 37Hz. This is so low that the Larger could not do its thing without either overloading the room or adding a big bass hump to the frequency response which was perceived as warm, muddy bass.

The mechanical solution was twofold. First, a capacitor was placed in front of the inputs to the deHavillands. This rolled off the bass at 3db at 50Hz. Secondly, the 10" side firing woofers were pulled from the RM30s and replaced with 8 ohm, 10 watt non-inducting resistors. This was the easy part. The hard part was repositioning the RM30s so that the room frequency response was as smooth as possible. Thus, I spent most of today with my Phonic PAA2 spectrum analyzer moving the now RM30Cs around the room. After many tries I found the spot that measured the best and also sounded the best. It was about two feet further into the room than the best spot for the RM30Ms which had been close to the front wall. I then fired up the Larger and with the PAA2 set the gain, crossover point, and single band equalizer. Those Daytona plate amps are really fast and easy to set up. Thanks Big B.

The benefits of these changes are twofold. The SET amps are not being driven as hard because the low bass is being rolled off. This resulted in a much clearer midrange. The cap at the input does add a sonic signature, but that loss is more than made up for by the clearer midrange. The second benefit is that the sub is now crossed over at 60Hz instead of 40Hz which allows the sub to do more of its magic. Also, one low bass source is clearer than three bass sources. Thus, I have cleaner, clearer bass with more dynamics and much more slam. Big B has said that the RM30Cs with a sub are better than RM30Ms with a sub. He is absolutely correct.

Two weekends ago I ran my RM30Ms with a pair of Herron mono block amps. The Herrons are excellent sounding solid state amps with an incredible bottom end on the RM30s. Unfortunately, they don't have that SET magic on top. My new RM30C setup retains that SET magic. The bass is not as authoritative as it was with the Herrons, but it is close enough that I won't worry about it any more. I am a very happy camper.

Thanks for making such great speakers Big B. I should listen to you more often.

Brian Cheney · « **Reply #1 on:** 3 Dec 2007, 04:31 am »

If you would like to complete the upgrade to the C version and eliminate the 8 Ohm 10W resistor, I can send instructions for wiring the two front 6 1/2" in parallel (currently in series, then in parallel with your resistor) and the crossover mod which compensates for the change in impedance.

I think you'll like this version even better. Plus, our new 6 1/2" Megawoofers with the metal phase plug, 40oz magnets and 4 layer voicecoils are available at a nominal charge of less than $300 for all four.

Russtafarian · « **Reply #2 on:** 3 Dec 2007, 06:49 pm »

I would be interested in this M to C conversion mod as well. My RM30Ms are also from the first production run and have not been modded or upgraded except for the CDWGs which I don't use. The CDWGs added a peakiness in the midrange that made the speakers unlistenable.

Where can I get lambswool to replace the fiberglas behind the panels? Aside from the new megawoofers, what other mods or upgrades should I consider?

Russ

John Casler · « **Reply #3 on:** 5 Dec 2007, 04:01 pm »

Quote from: Russtafarian on 3 Dec 2007, 06:49 pm

I would be interested in this M to C conversion mod as well. My RM30Ms are also from the first production run and have not been modded or upgraded except for the CDWGs which I don't use. The CDWGs added a peakiness in the midrange that made the speakers unlistenable.

Where can I get lambswool to replace the fiberglas behind the panels? Aside from the new megawoofers, what other mods or upgrades should I consider?

Russ

Hi Russ,

Knowing you, the conversion to Direct Drive and OXO or D-OXO would likely be up your alley.

You could also try the SR-71 internal cabinet damping.

Brian Cheney · « **Reply #4 on:** 5 Dec 2007, 04:14 pm »

The new Megawoofers are slightly larger diameter than the old graphite woofers. You would need to route out the front baffle cutouts about 3/16". If you can do that the upgrade is highly recommended.

Russtafarian · « **Reply #5 on:** 5 Dec 2007, 06:47 pm »

Thanks John & Brian.

John will you be at the LAOCAS party nest week? We can talk more then.

Also, where can I get lambswool for restuffing the midrange chambers?

Russ

RGordonpf · « **Reply #6 on:** 5 Dec 2007, 07:09 pm »

Hi Brian,

I found a local cabinet maker who can route out the cut-out. So that problem is solved. As long as I have the drivers out, should I replace the fiber glass filler? Orca Design, the makers of Black Hole 5 now have Black Hole Stuff. I just got the following e-mail from Orca chief engineer Alan Hulsebus:

" Black Hole Stuff is 100 percent cotton. Cotton fibers are much smaller diameter than other fibers so they have greater aerodynamic drag. Individual fibers also have very good mechanical damping and a rough surface for good friction damping.

I did a comparison test of all the normal materials, polyfiber, acoustistuff, wool, fiberglass, acoustic foam and Black Hole stuff.

I used a sealed test box and measured the impedance curve. Black hole stuff had the lowest resonate frequency, lowest peak height and no secondary resonances. In a 1 cubic foot box with 100 percent fill, the apparent box volume increased 40 percent - this is very useful for car stereo subwoofers where box size is the biggest problem. This is also
extremely helpful for midrange speakers to lower the resonance frequency and remove all acoustic resonance.

In frequency response measurements Black Hole stuff was the smoothest. More of the small peaks and valleys were removed.

If you have a vented box, maximum fill is 25 percent so that you do not damp the port resonance frequency."

Black Hole Stuff costs $10 per cubic foot. However, it is only sold in cases of 12 cubic feet. I think that is a bit much for stuffing the woofer and midrange cavities of the two RM30Cs.

Any thoughts?

matix · « **Reply #7 on:** 20 Feb 2010, 07:21 am »

Quote from: RGordonpf on 3 Dec 2007, 04:20 am

Secondly, the 10" side firing woofers were pulled from the RM30s and replaced with 8 ohm, 10 watt non-inducting resistors.

I have just moved my RM30M into a dedicated listening room measuring only 5m by 3m. As you can imagine, the speaker is way too big for this room and I have bass problems aplenty. I have the same idea to pull out the side firing woofer. But being a non DIY guy, I am wondering how to put up this non-inducting resistor. Is soldering required? I am also thinking of disabling the passive radiators below as I can feel one of the cone is already coming apart, and probably emitting distorted sounds. Please excuse the naive post, as I don't really play a lot with the equipments and only just enjoying the music...