RTA 12B General Questions

F1nut

FYI, copper is hard to beat for conductivity - even better than gold!

Duh!

Gardenstater

Even though the resistivity of Rhodium is 2.53x the resitivity of Copper, a 1 micron layer of Rhodium that has, say, 10 sq. mm. of contact area would only represent an added resistance of 0.000000043 Ohms.

If it doesn't sound good it isn't because of the resistance IMO.

Sitka

And it turns out lead and tin are terrible conductors! It's all here in this handy list:

https://en.wikipedia.org/wiki/Electrical_resistivities_of_the_elements_(data_page)

F1nut

Cardas Quad Eutectic Silver Solder

Gardenstater

Even a typical solder with Tin/Silver/Copper has about 13% IACS or about 7.6x the resistivity of Copper.

F1nut

Cardas

​"No other solder flows as well, solidifies as uniformly, sounds as good or provides as durable and long lasting solder joints. There's a reason Quad Eutectic is considered the best in audio, and is widely used outside of our industry as well."

Sitka

OK, the pure copper binding posts arrived today. As promised, I filed one of them and can now confirm they are indeed 100% pure copper. They are pretty hefty beasts, plus they are dual connect for banana plugs or spade lugs. The nuts are copper plated brass. Not too bad for $18 shipped - I would buy these again.

Gardenstater

Pure copper determined by visual appearance/color I presume? If you ground the end, now try scratching a pre 1982 penny (95% copper). If it scratches it they aren't 100% copper. If it doesn't scratch that try scratching a piece of copper water pipe. They could be copper plated red brass vs yellow brass. Red brass has 85% copper. I think this is why they justify selling connectors and such as "red copper", which isn't even a proper metallurgical name.

Maybe they are 100% pure copper in which case you got a great deal. When I was looking into it I saw some other chinese made ones for the same price which said copper plated.

I think pure copper, in addition to being harder to machine because of the softness, can have issues with the threads stripping out on a binding post if too much torque is used.

Sitka

Well . . . I was unable to scratch either a 1973 penny or a copper plumbing pipe. They appear to be 100% copper!

My theory is that some factory in Shenzhen was contracted by a company like Marantz to make 50K binding posts (plated in gold). The factory then made an additional 250K units (plated and unplated), and that's what we see on ebay. If I'm right, these will eventually sell out and that will be it for these. However, buyer beware as I did see some listed as "copper plated".

Pure copper is impossible to machine, but being so pliable it's easy to take a casting and press it into a binding post or bolt, complete with threads. The nuts are brass because the threads were machined.

xschop

Maybe you got ahold of TeCu😎

Gardenstater

I admit I've never tried threading soft copper with a die or cutting the threads on a lathe but it is said to be very likely to smear rather than form proper chips and it is picky as to the cutting fluid used. Cardas may be making their binding posts out of pure copper now where before they were TeCu. It is so hard to be sure because they are all pretty vague about materials.

Molding exterior threads doesn't seem feasible to me however because of the issue of draft angle at the parting line.

I think rolling or forming the external threads may be the way to go with copper.

Sitka

Not very likely TeCu, just a chunk of plain Cu. Make no mistake, these are second rate compared to Cardas and their peers. But they are a step up from bottom tier junk and they fit my budget. All I want to do is get as much as I can of the high current from my amp delivered to the drivers. These will help!

Sitka

I'll defer to you Gardenstater - you are a pro at these things. Just ordered some copper ring connectors for the back ends of these binding posts. Previous speakers I used standard brass crimp ring connectors.

Gardenstater

Copper ring connectors is a good way to go. I soldered the return wires on my Cardas but then switched tacks to the gold plated ring connectors. Probably a higher conductivity connection. I remember complaining to them that they only supplied one nut per post and it was gold plated brass. I believe they are now gold plated copper and according to an email they supply 2 per post.

No expert but thanks. Just learning and sharing.

Sitka

Switching subjects to the 4 MW drivers that came with this set of speakers. None of them had labels attached, but I found 4 labels inside the enclosures - 3 MW 6600x and one MW 6501. Two of the drivers looked the same and ohmed out at 7.4 each. I think these might be MW 6600x. The other two did not look the same, as one has a felt dust cover and one has a woven fabric dust cover. They are pictured below.





One of them came in at 7.6 ohms (might be a 6501?) and the other came in at 6.6 ohms (maybe a 6500?).

Any suggestions as to what I actually might have? I'm just guessing, but it looks like I might want to get a 6501 or two 6600x drivers.

Gardenstater

I never heard of a 6500 with anything other than a felt dust cap. Assuming all the 7.x drivers are either 6600X or 6501. The 6500 would probably have a smaller magnet dia. than the others (not sure). 6.50 Ohms would be the DC resistance which is pretty close to what you got. The only other drivers that would have resistances close to that would be 6503 and 6510.

Sitka

Yeah it's a head scratcher for sure. The 6.6 ohm unit has the woven fabric dust cover, and the magnet diameter is about .125" smaller compared to the 7.6 ohm unit. One option I have is to use the four known 6501 MW's from my M10's. There's currently a pair of 6600x drivers on ebay for $110 which I may go for.

F1nut

The one on the left is a made in Mexico driver from the early '90's.

pitdogg2

I have spares of both of those drivers. Whether mw6600 or mw6501 both IIRC had felt dust caps.
Again IIRC both were very very close in ohms. If you look at the SDA 1, SDA 1 Signature and SDA 1a the only changes occurred to the cabinet. (inside enclosure for upper MW and ports disappeared) Also if you can find the directions to take the SDA 1 to SDA 1a you can see the removal of the inside enclosure and IIRC an inductor.
If you like at some point this weekend (if I get time) I can ohm out both and take pictures. I did at one time have some Mexican MW6501.
I always thought the 6501 was just a renamed driver. At that time frame it seemed Polk just went up in driver name by 100. MW6500 and MW6600 seemed to be the only two drivers in use, then it went to MW6500, MW6501, MW6502 etc. etc.
I'd seen old advertisement stating Polks NEW driver and new speakers RTA 12 and the MW6600 was the new driver. The 6501/6600 are the only two drivers that went from standard speakers (RTA) to the SDA series, then it all changed once again.

Sitka

Wow, you guys are good! Here's the loose 6501 label I found inside the cabinet.



Pitdogg, I'd be interested in your 6501 or 6600x if it ohms out in the mid 7's and if you're willing to sell. My theory is that at some point around 1993 one of the 6600x drivers seized and was replaced by Polk with a 6501. Later another 6600x seized and was replaced with a random MW 65xx driver (the one with the 6.6 ohm reading).

Sitka

I'm going to go ahead and purchase some MW 6600x drivers that are on ebay. Might as well have the proper drivers after all the work going into this project.

Here's some pics as we go into the home stretch.

Faustin

Looking really, really good. Some days I miss my 12's. When the cross overs where completed on mine, it took about 200 hours for them to open up. Some people pooh pooh caps burning in, but mine changed drastically. Had several friends notice the difference. Hope you enjoy.

Sitka

Thanks Faustin! I used Clarity caps on my M10 crossovers and they for sure took 200 hours to burn in. Actually they varied in sound quality for the first 200 hours, sometimes good sometimes not so good. I kept the volume down during that time. I'm looking forward to plugging these 12's in, and hopefully no problems - there's a lot of wires and components in these.

For anyone following along, the above photo of the binding post cup, I nearly didn't get it to fit in the hole. The side by side configuration for the caps is not recommended because even exceeding the edge of the PC board by 1/4" is a problem. My other binding post cup is under way and for that one I stacked and strapped the caps. Photos to come.

Sitka

Well these speakers are back together and all plugged in and they sound absolutely incredible even without hardly any burn in time. The sound stage and imaging is just jaw dropping. I want to thank everyone who helped out - this forum is great!

To follow up, I ordered 2 MW 6600x drivers, and they both measured 7.4 ohms resistance, matching two of my mystery drivers. I'm pretty confident I now have 4 6600x drivers.

Here are the upgrades I made:
Clarity CSA caps
Mills resistors
Copper binding posts with copper ring connectors
Internal wiring upgraded to 16 AWG (did require carefully drilling larger holes in the boards)
Neutrik connectors
Hurricane nuts
Dynamat Extreme
Armacell gaskets
Sonic Barrier (4" x 4" squares behind each MW)

Here's the cabinets before I installed the drivers and PR



And the binding post cup with stacked caps



Next up is to veneer these as time allows.

F1nut

I'm glad you like the results.

Faustin

Wait until you get some time on them. You will Really be blown away. The 12's were really hard for me to part with. I veneered mine as well. Looking forward to seeing yours completed.

Sitka

Wow Faustin, great job on those. Curious what adhesive did you go with? A wood worker friend suggested I go with iron on adhesive. It's water base, and it rolls on with a rubber roller, let it dry then iron the veneer on.

I can't wait to log some time on these, after a year of running a pair of M10B's (which sounded equally great but almost zero imaging), I'm ready to "see" the music again.

Faustin

I used PSA veneer. Adhesive backed. Worked well for me.

Sitka

Thanks Faustin, I'll take a look around, PSA sounds a lot easier than glue.
OK, so after some critical listening - these speakers have better sound and more detail than the M10's, and on well mastered recordings the sound stage is jaw dropping. Hi def tracks even more so. Even my cat is looking around, like man what did you do to those speakers? All that and they're being driven by a Yamaha A-S2200 with rated power of 150W at 4 ohms (dynamic power 190W). Probably explains why the sub bass is a little understated. Needless to say I'm now on the lookout for a decent amp with more wattage.
Overall, these speakers do not disappoint!

F1nut

You should have plenty of bass. Do the air leak test.