Designing a new top for Polk RTA 12 speakers

Michael8it

I have a pair of Polk RTA 12 speakers that I am refinishing and will be designing a new top to replace the black 'hats' that came with these. I want to design a new top out of wood (my choice for this project is cherry) to enclose the tweeter and crossover.

As designed the tweeter sits 3 1/4" back from the front edge (and other drivers). The tweeter has the phase reversed to compensate for the time shift. I am thinking about moving the entire array forward so the tweeter is aligned with the other drivers, but I don't want to distort the image. If I move it forward, it would be mounted similar to a Monitor 7 or 10. I am pretty sure I need to reverse the phase from how it is now. Is that my only concern, or would moving it distort / change the image?

I want this top to be 'plug and play' with the least modification possible done to the speakers. If I move it forward, I can use the two metal posts as guides for mounting this, along with the screws that currently hold the crossover in place.

Thanks,

pitdogg2

can you say "time Alignment" that is why the tweeter sits back. just like speakers that slope towards the wall.
others will chime in but moving would be a mistake IMHO. But I'm digging what you are planning on with the hat. Are you planning on the hat being wood on 3 sides?

Nightfall

I wouldn't move the tweeters. IMHO.

Michael8it

I know the time alignment is important, and IIRC the crossover plays a part in this as well. The only reason I was thinking of moving it forward was to avoid removing the two metal posts that secured the hats.

The new 'hat' will have about a 6" face (same and the mount for 12B's, then angle back on each side by 30 - 45 degrees, and extend about 6" until they clear the crossover on each side, then straight back to the back of the Cabinet. There will be a top and bottom of wood as well (bottom is just 1/8" plywood to aid in mounting) and will fit right over the screws coming out for the crossover.

The whole cabinet will be re-laminated in Cherry, and then I will take 1/2" x 1 1/2" Cherry and frame the sides and front with it to give the cabinet dimension. The trim will extend down to about 3" below the cabinet and will form the legs for it. I will put removable spikes in the legs. The whole cabinet will have a mission/shaker feel to it and match some of our other furniture. A bit hard to describe but I know where I am going with it and will post pictures when its done. It sounds a lot harder than it will be. The face will also be laminated in cherry and will have no grill, so the drivers are exposed.

Nightfall

What condition are the stock top hats in?

westmassguy

There's a reason the tweeter and crossover assembly are exactly where they are. I would not change it. Find a way to incorporate it's location into your design. If it has the little anti diffraction ramp in front of the tweeter, leave that as well

DaveHo

+1

Moving the tweeter, or boxing it in on three side with wood will destroy what they are capable of.

TennMan

If you are going to move the tweeter in line with the front baffle you might consider finding a pair of SDA 2B or CRS+ crossovers to install to get the SDA effect. They will be called FrankenPolks anyway so if you are going to modify them you might as well go all the way. I added SDA CRS+ crossovers to a pair of monitor 10Bs and they sounded great. I preferred them over to the CRS+.

Michael8it

DaveHo, that is a bit of a concern, but the Tweeter is already boxed in by the frame. Only the RTA-12 has a true free standing tweeter. The only opening in the back is space for the magnet. I don't see how this would affect it. If it was on a completely flat surface, like it's mounted on a Monitor 10, I could see that impact the image the speakers produce. But in this design, in the projection path of the tweeter, it will be the same as it is now.

There is a ramp on the front, which I would design into this.

Dskip, thanks, but I don't want new speakers. These are cosmetic changes that will not affect the integrity of the speakers.

Nightfall, the stock hats are in very good condition - and already sold...

Nightfall

Michael8it wrote: »

Nightfall, the stock hats are in very good condition - and already sold...

Bummer.

There is also another ramp built into the top hats.

zingo

My buddy did testing adding and removing a hard top above a tweeter, and it did make a big different. The RTA12s are supposed to be open on top, and I wouldn't put anything on top of them that isn't acoustically transparent on all sides.

drumminman

The hats on my RTA 12C's have a solid back panel, and fabric sides and top. This is important for the speakers to image like they do. I would not move the tweeter or you'll likely ruin part of what makes these sound so good.

It'd also be easier to work on the hat design than move the tweeter, reverse phase, etc. What about a new hat constructed out of solid hardwood to match your veneer, with open panels on the sides and top with fabric inserts?

Michael8it

drumminman wrote: »

What about a new hat constructed out of solid hardwood to match your veneer, with open panels on the sides and top with fabric inserts?

Now were talking! Make a new frame out of hardwood and put the fabric panels inside..... Basically the same as the existing hats only 'inside out'.

I curious though as to why it makes such a big difference to have the tweeter mounted open like it is. Can anyone help explain that?

drumminman

Michael8it wrote: »

Now were talking! Make a new frame out of hardwood and put the fabric panels inside..... Basically the same as the existing hats only 'inside out'.

Yep, that's what I'm thinking.

Michael8it wrote: »

I curious though as to why it makes such a big difference to have the tweeter mounted open like it is. Can anyone help explain that?

My best guess is that they have wide horizontal and vertical dispersion, the latter being helped by the angled deflector attached to and just below the tweeter.

I had to replace one of my hats which had the frame break at the point it was welded or soldered to the back. When I looked at it to see if I could repair it I noticed that the front corner posts are made of plastic with vents all through them, I suppose to allow the tweeter output to pass through. You might want to incorporate this design feature into the ones you build.

The corner posts could be two pieces joined at the top and bottom by wooden connectors, with the middle open and covered with with mesh fabric to match what you're planning to use on the rest of it.

Michael8it

Even with a wide horizontal and vertical dispersion pattern, I don't understand how sound being projecting forward from the tweeter would be impacted if the space behind the tweeter is enclosed. I agree that everything in front of the tweeter should be open, i understand that part. But why the space behind the tweeter needs to be open is what I don't get.

drumminman

IDK, maybe sort of like an open baffle design. My best guess is enclosing it will kill alot of the imaging.

zingo

Installing new capacitors in the crossovers is something Polk never could do due to budget constraints. Enclosing the top hats is fundamentally changing their original design. Have you asked Polk about your new design idea?

Michael8it

I already recapped them which made a Hugh difference. My hats are from RTA 12's - I actually converted these to 12b's. the back of my hats are not solid, which would indicate that Polk changed this over time.

ZLTFUL

Bear in mind that when a speaker, in this case, the tweeter, produces sound, it does create a wave of sound to the rear. See the picture below for reference...
The common theme in people saying don't modify the design is because of imaging. I can definitely see putting a baffle near or around the back side of the tweeter effecting how the total sum of the sound would radiate from the driver.

As an experiment, make a 3 sided box out of 1-1.5 inch foam insulation approximately the size of your tophat and place it on your speakers and give it a listen. I am willing to bet that you lose a ton of depth and width to your soundstage.
The crossover tends to be the key in these open baffle designs. The crossover can effect your phase, transient response, etc...all things that combined with physical time alignment placement of the drivers make the speakers what they are.

Michael8it

Wow. THANK YOU. This is very valuable information indeed.

Faustin

I have a set of 10a's and 12b's both recapped, dynamatted etc. Both sets are close to being burned in. I have them set up next to each other and can switch back and forth to compare sound. Love the 10's but the 12's have more depth and "surround sound" and I would say it is the way they were engineered and designed with the tweeter mounted open instead of in an enclosure. I think it would be a mistake to enclose them.

Nightfall

Faustin wrote: »

I have a set of 10a's and 12b's both recapped, dynamatted etc. Both sets are close to being burned in. I have them set up next to each other and can switch back and forth to compare sound. Love the 10's but the 12's have more depth and "surround sound" and I would say it is the way they were engineered and designed with the tweeter mounted open instead of in an enclosure. I think it would be a mistake to enclose them.

Definitely no comparison. RTA-12's, IMO, are by far and away the best non SDA vintage Polks. They give the smaller SDA's a run for their money and would bet that most would prefer them to even early SDA's that have side by side S1000's.

Michael8it

I appreciate everyone's feedback, and this is exactly why I made the post on the forum here. I would have spent time and effort and money building something that would have killed the sonics of these speakers, which is exactly why I like them!

I am still going to refinish them but designing new hats is proving to be complicated to say the least. I may go back to an earlier thought of moving the crossover inside the cab and leaving the tweeter all by itself on the top. I'm undecided on that part. Could just take a 6" piece of 3/4" cherry and shape it just like the existing tweeter mount. Would give it a fresh look and still leave it exposed, and would be much easier to make than a whole new hat.

westmassguy

Michael8it wrote: »

I appreciate everyone's feedback, and this is exactly why I made the post on the forum here. I would have spent time and effort and money building something that would have killed the sonics of these speakers, which is exactly why I like them!

I am still going to refinish them but designing new hats is proving to be complicated to say the least. I may go back to an earlier thought of moving the crossover inside the cab and leaving the tweeter all by itself on the top. I'm undecided on that part. Could just take a 6" piece of 3/4" cherry and shape it just like the existing tweeter mount. Would give it a fresh look and still leave it exposed, and would be much easier to make than a whole new hat.

That sounds like a very interesting design. You could make the tweeter mount and the little diffraction ramp out of cherry. Just keep small fingers away from them

Michael8it

DSkip - the vinyl covering the wood on one side of the cabinet has peeled off. Hence the refinish. Plus, they look a bit dated, and the 'cleaner design' is also to keep the fiancee' happy....... A happy wife = a happy life

joelll

I have a fairly early pair of the RTA12's (serial numbers in the 12-0032n range for the speakers, and around 22-0041n for the tweeter assemblies that fit on top; I'd love to know if the serial numbers started at 001, 101, 201, or 301). The "hats" that I have are wire frame on an MDF base, with no ramp, and open (though grille fabric) on all four sides and the top.

Pictures at: http://world.peace.net/~joelll/polk/m12-photos-large/

While I like the idea of moving the crossover inside and putting the tweeter all by itself on a mounting, I think you'll still need just a little dust protection for it. Ideally, I'd want to find a spare set of crossovers (probably harder to find than hen's teeth...) so I could go back to non-disassembled originals.

There's another speaker with similar design with the tweeter free-air mounted on top under a "hat", the Epicure 3.0
Some info (with photos) about the 3.0's can be found here: http://www.humanspeakers.com/e/epicure-3.0.htm

Michael8it

My speakers were identical when I bought them, but I was able to get 12B crossovers and drivers for a very fair price.

I have an extra set of 12B crossovers, but not 12's. And yes. they are for sale! You could convert your current pair to 12B's and store the crossovers you have.
I looked at your photos and it looks like you have original crossovers - upgrading your caps would make a world of difference.

I am still trying to figure out how to design a fabric grille into this open tweeter design. Don't have that one figured out.

As far as the serial numbers go Geppy1 is an aficionado on these speakers, he may be able to answer that question.

Michael8it

Four years later... I am finally getting around to this...

http://forum.polkaudio.com/discussion/176596/polk-rta-12-b-overhaul