RTA 12C's upgraded for forum member, w/ pics

mlhm5

Late to the party, however never seen anything like it. Absolutely the envy of every reader on this forum.

pickoid

Yes, this thread was the inspiration for the restoration work I did on my 11's. If I've viewed this thread once, I've viewed it a hundred times. Ideas I blatantly stole include the use of T-nuts and stainless steel cap-head machine screws, blue/black/white heat shrink on the internal wiring, damping material on the MW and passive radiator baskets, and just the general motivation to go for it and re-cap my crossovers. I realize that a lot of these ideas may not have originated from this thread, but nothing else lays it out and documents everything so well in one place. My results may not be so professional, but the improvements are tangible and definitely worth it. Mad props to geoff727.

Michael8it

Agreed. Great Job geoff727

geoff727

Once again, I'm gratified this thread has gotten so much positive response. Thanks, guys!

cwsmith68

Wow!!
Just joined the club a week ago after I got my first set vintage Polks (monitor 10b's) and have been looking around for parts for my damaged RTA 12B'S. Came across this and read every single post and looked at every single pic.
Am I stepping over any lines asking how much this project cost? What are the chances of getting you to upgrade my crossovers and wiring?

cwsmith68

Another mod question. Has anyone ever done the bi-amp mod on these?

geoff727

cwsmith68 wrote: »

Am I stepping over any lines asking how much this project cost? What are the chances of getting you to upgrade my crossovers and wiring?

Not at all (to the first question). To the 2nd question, if you're serious about doing them, send me a Private Message when you get ready. The price level on these can run a huge range, due to the fact that there are quite a few capacitors on the boards and there are so many out there to choose from with a huge variety in price. Somewhere I have a list with various upgrade levels that I generated for Roger (the owner of the speakers on this thread). Always happy to do work for a fellow Polk fan.

Geoff

Sea

Thank you to geoff727 for such a detailed description of the this RTA 12C upgrade/restoration. My question is what changes would you make to this, if any, given a couple of years has passed?

Specifically, I am curious about the Sonic Barrier vs more recent discussions about Black Hole 5. It looks like you covered the entire cabinet inside with Sonic Barrier and many are now advocating not putting Black Hole 5 behind the passive radiator.

I am asking because I have a pair that I bought new in 1984 that I am planning to upgrade. I am interested in advice from you and any other member who would like to chime in.

geoff727

Sea wrote: »

My question is what changes would you make to this, if any, given a couple of years has passed?

Given the cost parameters I worked within, I wouldn't do anything differently. If they were my own speakers with an unlimited budget, I would have done a lot differently.....like build new, better enclosures out of something other than particle board.

As far as Sonic Barrier/Black Hole 5 go, they are two different products, in different price ranges, meant for different purposes. Personally, I have never experienced any degradation in putting a product behind the passive radiator, as long as the motion of the radiator itself is not obstructed. I don't know what others experience in this regard; I have theories, but it's probably best I keep them to myself. If one particular solution helps someone enjoy the music better, then it's probably a good solution. On those 12's, I remember increasing the quantity of polyfill and lining the enclosures with S.B. Along with the other improvements, I remember Roger's impression of the resulting bass response and quality was extremely positive compared to the stock speaker.

Sea

Thank you for your response.

Sea

I have another question for geoff727 or anyone else who understands these issues better than me (and that would be probably all of you). According to a schematic and my 12c's there are two 4.4 mfd capacitors. I am clear that geoff727 replaced one of these with 2 - 2.2's in parallel. It would appear that one capacitor was used in the other spot. I don't understand.

geoff727

It took me a little while to remember this....

See on the crossover schematic, there's the first 4.4uf cap in series with the tweeter? And then there's a paralleled combination of 4.4uf and 55uf, which is additive to 59.4uf. In real life, it won't come out to be 59.4 exactly, unless you have capacitors of very tight tolerance. But, that's the stated value, so that's what we try to go with here, as closely as possible.

Those two 750pf caps, you can just forget about those if you want. Some people feel that when you replace original caps with high quality modern alternatives, those original small-value bypass caps are not necessary. I've used them in certain applications, and not used them in others. Some people absolutely obsess over those little things and lambast anyone who uses them, but that's another story for another day.

Anyway, when you replace old caps with new, you have to look at the values in which the capacitors are available. There are a number of values which are fairly standard, and one always has to consider what values are available in the price range for which you are budgeted. In other words, there might be a value which is just what you want, but it may be something that's out of your price range, or way below the quality you desire. If the level of price/quality does not have the exact value you want or need, then you have to do some mixing and matching of capacitors to get what you need.

Back to the schematic, and the 59.4uf value. See the AXON cap that is 56uf? In this application, that cap is paralleled with a 3.3uf ClarityCap (the red one), giving a value of 59.3uf. Almost dead-on for what's printed on the schematic, and it's a film cap/film cap combo that is probably much closer in tolerance to the stated value than the original combination which was made with a non-polar electrolytic cap (these tend to be looser in tolerance). Had I made 59.something microfarads out of those red ClarityCaps, then the cost would have exceeded the budget, and it would have been very difficult to mount the resulting huge stack of caps on the crossover board.

There was probably not a 4.4uf available, so we went with the 2 x 2.2uf. And then 56uf + 3.3uf gave us the other value. Does that make sense?

geoff727

Looking back at the photos, I forgot that we used the 3-layer Sonic Barrier. I thought we used the 1-layer.

One more thing about cap values...there is a well-regarded cap-maker, Sonicap by Sonicraft, who will do custom cap values up to a point. All you have to do is call them and tell them what you need. These are film caps of good quality, and to use a lot of them takes space on crossover boards and money. To use them on Roger's project would have exceeded the budget, though, by quite a bit.

Sea

i am very appreciative of the time geoff727 has spent helping me understand this recap. Now, I get it. Thank you.

xjghost

Amazing guide with amazing detail. Love all the pictures. I have several sets of vintage Polk's that I hope to one day start upgrading. I do have a question about the magnet shift and where you put the JB weld. Is JB weld the preferred material and do you have any better other pictures of where it went or pictures of the driver with the weld and without?

geoff727

I do have a picture somewhere. it's probably on the computer that crapped out a few weeks ago. All you have to do is run a bead around both sides of the magnet. Smooth it with your finger into the seams between the magnet and the (silver-colored) plates.

kdblove_99

geoff727 wrote: »

Looking back at the photos, I forgot that we used the 3-layer Sonic Barrier. I thought we used the 1-layer.

So did you use the 3/4" Sonic Barrier, All i see for 1" is the single layer stuff?

maingey

@geoff727 thanks for this. @drumminman pointed me to this project. I don't understand a lot of it, but I'll have to reread it more slowly when I'm out of work and figure it out.

maingey

@geoff727 when you painted the interior, was it with an acoustic paint or was it just to give the hot glued panels a clean and even surface to stick to?

maingey

@la2vegas thx. Trying to figure out the parts list as it was never really outlined. I'm guessing most of the pictures have values on them so it is a start for me.

geoff727

I haven't thought about these speakers in some time. Looking back, it looks like all I did was paint them black before the application of Sonic Barrier.

I recommend Cascade VB-1X paint- or spray-on acoustic compound, although painting it makes it difficult to get an even surface for Sonic Barrier-type products to adhere to.

I've never tried a spray application of it, so that may be better. Another nice product to use is GR Research's NoRez. It has a thicker vinyl layer that attaches to the enclosure wall to tame vibration, and then the foam layer which cleans up the midrange and mid-bass.
G~

geoff727

la2vegas wrote: »

la2vegas wrote: »

maingey wrote: »

@geoff727 when you painted the interior, was it with an acoustic paint or was it just to give the hot glued panels a clean and even surface to stick to?

He hasn't checked in for over 5 months, it might be a while before you get a response.

Welcome back @geoff727.

Thank you!