RTA 12B General Questions

Sitka

OK, so I'm in the midst of upgrading a pair of 1982 RTA 12B speakers. Because of the unique "tweeter above the cabinet" layout, I've got a few questions.

I'd like to install hurricane nuts, but the two MW's reside so close to the top, there's a giant wad of sealer glue behind 4 of the screw holes. Is it OK to take my multi-tool and hog out the glue enough for the hurricane nuts?

I also want to upgrade the internal wiring with a more substantial wire gauge. Simple enough until you get to the 3 pin connector up top. I saw somewhere here that somebody replaced theirs with an XLR connector and jack. Would that work?

Thanks, pics to come!

xschop

Hog away.

Jazzhead

I have original RTA-12's. I have a top connector with an intermittent connection problem and this was what @westmassguy (DHS Speaker Service) recommends. I'm upgrading the connectors when I get free time:

https://www.parts-express.com/Neutrik-NL4MP-Speakon-4-Pole-Panel-Mount-092-052?quantity=1

https://www.parts-express.com/Neutrik-NL4FX-Speakon-SPX-Cable-Connector-4P-092-190?quantity=1

https://www.parts-express.com/Neutrik-SCDP-0-Sealing-Gasket-for-D-size-Connectors-Black-092-094?quantity=1

https://www.amazon.com/dp/B09T9626D3?tag=bravesoftwa04-20&linkCode=osi&th=1&language=en_US

I had to chisel hot glue away from the screw holes as well. Hurricane nuts and better internal wire are audible upgrades. Some folks here recommend Supra Rondo wire. I used Neotech OCC wire. You might consider upgrading the binding posts while you're at it (another audible improvement). Cardas is a good choice.

Sitka

Thanks Jazzhead, will look into all you suggested. Did you find an audible improvement with Cardas vs. cheap Chinese copy? As much as I'd like to use Neotech OCC, I'll use 14 gauge copper wire. Trying to keep the budget under $1K while maintaining real quality improvements.

pitdogg2

Cheap Chinese binding post copies are no better than stock. Save your money.

Jazzhead

I hear you on the budget business. I was shocked at how bad the stock posts sounded when I replaced them (decades of oxidation?) I haven't heard the Chinese copies, but I'd go with something of decent quality. Good connectors make a difference. Suggestions folks? Also, there must be some "bang for buck" internal wire options out there, perhaps some folks on the forum can suggest?

F1nut

I like the WBT-0708Cu Nextgen.

Sitka

I see your point on the Chinese knockoff argument since brass is no match to pure copper or silver. However, I did find pure copper insulated binding posts on ebay (shipped direct from Shenzhen, China) for $20 a set of 4. I'm sure they're no match for real Cardas or Nexgen, but maybe a little closer?

BTW, it was the glowing reviews by Jazzhead and others of the RTA 12 that convinced me to pick these up and give em the treatment. They're getting the treatment, but with an eagle eye on the budget. So it's Clarity CSA throughout, Mills resistors, plus what we're discussing here, Armacell and Dynamat Extreme. I'm with F1nut on the foam lined inner baskets - no on that. The nice thing about it all is I can always upgrade later if warranted.

xschop

What will you do to mitigate the lobing of the side-by-side drivers?

Gardenstater

I would be suspicious of those Chinese binding posts that say they are Pure Copper. Unless they say solid copper they are probably copper plated. Even the Cardas binding posts are Tellurium Copper, which is a high copper alloy. Copper is not easy to machine because of the softness.

joebass3

xschop wrote: »

What will you do to mitigate the lobing of the side-by-side drivers?

If he hasn't already, he should check out @Jazzhead thread that addresses it. Definitely should be on his list of upgrades! https://forum.polkaudio.com/discussion/173834/westmassguys-idea-employing-damping-to-mitigate-comb-filtering-and-lobing/p1

Gardenstater

I think the effects of combing and lobing are much diminished in the 12B, which is when they introduced the mirror imaging of the speakers and where both drivers share 500Hz and below but only one driver gets 500 Hz to 2kHz.

Jazzhead

Yes, the 12b got the crossover that rolls off one driver:

https://www.hifi-classic.net/review/polk-audio-rta-12b-339.html

So, no need for the westmassguy lobing mod.

Jazzhead

@Sitka , are you installing Black Hole 5 or Sonic Barrier behind the drivers? The standard treatment is a 4in x 4in or so piece behind each to clear up the midrange and control any "shouty" characteristic at higher volumes. Some RTA-12 (b and c) owners have mentioned that this "shouty" nature persisted to a degree even after the standard treatment. I'd be curious to learn what you hear when finished, and think it might be interesting to experiment afterward with a greater amount of Black Hole 5/Sonic Barrier behind the outer divers (the ones that are rolled off) to see if that improves the presentation (further accentuating what the Polk engineers intended). It would be a very easily reversed experiment.

Sitka

They are listed as "100% pure copper". We'll see in 2-3 weeks - report to come. Neutrik connectors also on order.
As far as comb filtering, I'm willing to experiment. On my M 10B's I played around with 4" vertical cardboard "separators" which produced a big audible improvement to imaging. To my surprise, it's the tweeters that create soundstage and imaging. If their out put blends into the output of one MW, no problem. But the vortex created by 2 MW swallows up the lower half of the tweeter signal - mostly in the first 4" past the baffle. I would imagine the RTA 12 layout with the tweeters physically separated, directed by several plates and stepped back goes a long way to improving the situation.

Gardenstater

Gardenstater wrote: »

I think the effects of combing and lobing are much diminished in the 12B, which is when they introduced the mirror imaging of the speakers and where both drivers share 500Hz and below but only one driver gets 500 Hz to 2kHz.

Doh! 600 Hz !

Call me suspicious of Chinese products but I got some binding posts that were supposed to be "solid copper conductor" and I could scratch a copper penny with them. Turns out there was a small rod of solid copper inside, presumably.

I wouldn't put it past them to say 100% pure copper when they are 100% pure copper plated.

Sitka

Your suspicion may well be true - or not, but for $20 I had to find out. When I get them, out comes the file, and a few strokes will tell the story. If they're 100% pure copper plated gun metal, they're going back. Another work around is to use copper nuts and bolts, also from China on ebay, listed as solid pure copper. As I said, I'm sure Cardas is superior, but their price is really high.

Jazzhead, I do plan to include Sonic Barrier or possibly Blackhole 5 (which appears to include some mass damping).

Jazzhead

If you strike out with the Chinese posts, the WBT Nextgen posts F1 suggested above are 20% off on the VH Audio site currently. If they didn't sound good F1 wouldn't recommend them and Chris VenHaus wouldn't sell them.

Gardenstater

Another alternative is these tube connectors for $59 per set of 4, which I have not personally tried so I am not endorsing just putting it out there for sake of interest.

https://gr-research.com/product/electra-cable-tube-connectors/

https://www.youtube.com/watch?v=gInqJGvMT_8&t=738s

https://www.youtube.com/watch?v=uevd5RFXwCU&t=627s

Sitka

Thanks Gardenstater - very interesting stuff on that video. It did convince me to head back to ebay and purchase 8 rhodium plated copper spade connectors for $14 (made in USA and free shipping). Those will replace my banana plugs. He also praised the Neutrik connector and then downplayed it because the nuts are steel. Will keep you posted as these parts roll in.

Sitka

Those Neutrik connectors arrived and I've been getting them ready for install. The gentleman from GR Research was right, the male connector wires attach with a steel screw. But they say soldering is possible, so I'll give it a go tomorrow. Very cool connectors though.

To attach the base to the top of the speaker cabinet, the existing hole has to be enlarged to 7/8", and some of the rubber mat has to be removed to make way for the Neutrik gasket. Of course, I'll **** on some caulk sealer as well. The base handles 3/16" Faston connectors, but I opted to solder instead.



Also a pic of the hogged out wax and hurricane nut. The multitool works great for wax removal.

F1nut

What model Neutrik socket did you use?

This is the one to use, https://www.neutrik.com/en/product/nlt4mpxx-bag

It's heavy duty, air tight with 1/4" faston tabs. Use gold plated fastons with it. The 4 screw holes are designed for 3m screws. I use stainless steel screws, lock washers and nuts.

Mate with this male connector, https://www.neutrik.com/en/product/nlt4fxx-bag

Sitka

Thanks F1nut - excellent suggestions! I went with the Neutrik connectors Westmassguy suggested to Jazzhead. The base is not as heavy duty as the one you suggest, but I think it'll do. You both went with the same male connector (with the steel screws). Neutrik says soldering is possible and my concern is whether solder will adhere well enough because the machined wire insert is a hefty piece of metal.



A possible option would be to get a long M4 brass screw and cut it into pieces, and file a slot for a screw driver. Did you solder yours?

F1nut

Hmmm...no screws on the ones I've used, but they are the older version of that model. Yes, I soldered the wires without any issues...does take some heat though.

Dug up a pic.

westmassguy

The screws may be steel, but the little sleeves that come in direct contact with the wire are not as I recall.

pitdogg2

On my Dreadnought connectors I tossed the little sleeves and the screws worked just fine to hold the wire very well. The sleeves actually worked against me as my wire was large.

Sitka

Thanks guys, really great we're getting our Neutrik info into one thread. It took some sleuthing to figure out that the NL4-FX has the screws, and the NL4-FXX Bag (the ones in F1nut's photo) is solder only. Parts Express doesn't carry the NL4-FXX Bag, but B&H Photo does. So it sounds like I can just go with the steel screws for now and maybe order the NL4-FXX Bag units later.

Sitka

The 8 rhodium plated copper spade connectors for $14 on ebay arrived, and they are indeed 100% pure copper plated in rhodium. Very nice quality, fit well on my 10 AWG speaker wires, and most importantly there is an audible improvement in the sound. They're hooked up to my M10B's, with binding posts that are banana plug only (brass). Still waiting on the copper binding posts, but I think they too are banana plug only. I have to say I'm tempted to try the GR Research tube connectors.

F1nut

Hate to break this to you, but rhodium plating is not a good choice as it is significantly less conductive than gold, less than half as much.

Sitka

Quite true, however even with the micron thick layer of rhodium, I still got an audible improvement in output. FYI, copper is hard to beat for conductivity - even better than gold!

pitdogg2

Sitka wrote: »

Quite true, however even with the micron thick layer of rhodium, I still got an audible improvement in output. FYI, copper is hard to beat for conductivity - even better than gold!

Correct and Silver is the best at conductivity even tarnished. Gold is very good at not tarnishing like copper and that is where it beats copper in hostile enviroments. They use rhodium on stuff that gets hard use to protect the softer metal. They often flash silver with rhodium as well. I have found the flashed silver to have a much harsher sound signature than the plain silver stuff I've used. (binding post, IEC and outlet plugs and banana plugs) I attributed that to the rhodium coating.