My RTA 12b mod/upgrades so far...

cwsmith68

Let me start by saying that this is one of the most helpful and informative forums I have ever been a member of and that I am glad to be a Polkie!

Sometime late last year as I was scanning clist for any old school/vintage stereo equipment I saw a pair of Polk Audio Monitor 10's for sale. I gues that was the first I had seen though I had heard of Polks for years. I was intrigued by the 2 small drivers and the bigger passive so I set out to obtain some vintage Polks. Finally after watching clist daily, came across a pair within a decent driving distance and a price within my range. My girlfriend and I drove up to Collierville, TN early one Saturday morning in late November, 2011 and met a guy that had just moved down from Iowa. He took me into his garage and showed me a mint pair of Monitor 10b's with factory stands and original boxes. He had them hooked up to an old, small, Onkyo reciever. He cranked them up and I handed him the money and started to load them before he could change his mind, I had talked him down from his asking price of $175 the $125 and was afraid he may back out. Anyway, I took them home and hooked them to my HK3490 and fell in love! This was only the beginning of the illness that has taken me over since.

For the next month or so I was online looking for more deals like this, but only Polk Audio from the same era. I came across a pair of RTA 12b's on that auction site but the seller had them listed as something else which I forget but I thought that since I could find nothing on them, that they were truly rare and I had to have them! Well after I finally came to an agreement in the form of several back and forth offer's I had purchased those "oh so rare":surprised: Polks. I was ecstatic! Several days later they arrived and the seller did not pack the tweeter module and top hat like I had asked him to do and Fed ex evidently brought them cross the whole country upside down even though they were clearly marked "this side up" on all four sides and "fragile" as well. So then I start a claim and begin looking for replacement parts and stumbled upon this site.

I emailed with a member that had totally rebuilt and modded a pair of these for another member a while back and he sent me a list of what would be needed. Sooooooo... since last December I have been working on these a little at a time and here is where I am:

SL-1000's were damaged in shipping. A member from here Karma'd me a pair of Peerless til I can fork out the dough for the RDO-194 upgrade. The Peerless sound pretty dang good:biggrin:

All caps replaced and I have just approached 250 hours of break in time

Resistors replaced with 12 watt Mills

Binding posts replaced with gold plated posts

All drivers and passives have been dynamatted and JB welded

Removed the riser from the bottom and put them on the factory Polk stands I purchased with the Monitor 10's

Still left to do:

Waiting on the Sonic Barrier, speaker gasket tape and resistors that I am going to replace the tweeter fuses with.

I have hurricane nuts and the stainless black oxide screws that are mentioned in an upgrade thread somewhere around here.
After all of that I will try my hand at taking off the old vinyl laminate and smoothing everything out to put on some veneer.

These things sound so good as I listen to Martin Sexton's "Black Sheep" right now! I can barely wait to get the rest of the mods done and hear them!

Since the purchase of the RTA's and Monitor 10's I have also picked up a set of 10a's (my girlfriend had to have her own) and Monitor 11's.
When I am done with the RTA's I will probably start on the Monitor 11's.

Thanks to all of the fine and knowledgeable folks here in the club! Without you guys and all of your experience and skills I could not have done this myself!

TNHNDYMAN

Nice story. Glad you are enjoying and upgrading the vintage Polks. Monitor 10's were my entry into this wonderful hobby and forum.

cwsmith68

Ok. So last night I installed the hurricane nuts, armacell and Sonic Barrier. The cabinets are definitely more airtight than before but I am a bit disappointed in the bass response after adding the Sonic Barrier. I put it on the sides, top and back wall behind the midbass drivers. My inspiration for this upgrade came from a write up a member did when he modded a pair for another member. The pics I saw had what looked like the entire inside of the cabinet cover in the 3-layer, 3/4 inch Sonic Barrier but I had read in another post or 2 where others said to NOT put it behind the passive at all. Should I cover it all? The bass is gone as it is now.

drumminman

Definitely do not put it below the MW's.

I recommend a more effective acoustic foam that I've had great success with in both my RTA 12C's and SDA 2.3TL's: Black Hole damping sheets. For your 12B's you only need a 3-4" wide strip placed on the inside back wall horizontally behind the MW's. For my review go here: http://www.polkaudio.com/forums/showthread.php?128159-Black-Hole


It's expensive, but you don't need much either. Buy the smallest sheet you can find and it should be more than enough. I bought mine here: http://www.soniccraft.com/products/damping/blackhole5.htm though I've seen smaller sheets for less money elsewhere.

Have you done the test for how well your cabs are sealed?

cwsmith68

I already have the money invested in the Sonic Barrier and would like to use it. Just want to know how much to use and where to place it. Like I said in the earlier post, I saw pics of an entire cab covered with this. Just wondering what the results from that sounded like.

Yes. I did the test. Seems to be pretty well sealed. What else should I test for?

drumminman

Try removing the sonic barrier from the top, bottom and sides so that it's only on the back wall directly behind the MW's. Add more as needed, but be careful. Several members here reported killing the bass response on their larger SDA's by using too much, covering the entire interior of the cabinet.

Not really a test, but should bring back your missing bass.

cwsmith68

Would you also put the stuffing back in behind the MW's as well?

drumminman

Yes, absolutely. The polyfill as well should be no lower than the bottom of the MW's.

geoff727

cwsmith68 wrote: »

Like I said in the earlier post, I saw pics of an entire cab covered with this. Just wondering what the results from that sounded like.

I'm assuming you're talking about the speakers I worked on for Roger. On those, 3 walls were covered with S.B., side, side, and back, if I remember right. It was not the 3-layer stuff, just one layer of foam.

I'm not sure what else you've got going on with your project, system, or room placement. But, I do remember Roger's comment after we had played the speakers for a while and he was starting to get an impression of how they'd changed....."They never did THAT in the bass before."

"THAT" was greater clarity, definition, and no loss of bass quantity. Nothing at all was 'killed', and they were rendered much more 'alive' in terms of the realism of what was happening in the bass.

I have to be honest, I'm very resistant of hard-and-fast rules that suggest never putting S.B. or polyfill or whatever in one place or another, with the exception of not wanting to choke off a port with fill, etc. In fact, I have almost always ended up with about 70% filled with polyfill in the enclosures of these speakers. None of this was arbitrary; it was all arrived at by what my experimentation told my ears was the most musically correct. (Yes, it involved removing and reinstalling the passive radiator a lot of times!)

What others hear, I really don't know. Is it an alteration of the Q of the response curve that leads to an impression of 'killed' bass? I don't know. Maybe. Everything we hear has a perceptual bias assigned to it by our own individual brains and our own experiences. My own perceptual biases led me to believe that I arrived at a good solution for Roger's upgrade (and others, too). Was what I did wrong? No, not at all. My own and Roger's opinions suggested exactly the opposite.

Well, let the lambasting begin, I won't even try to defend myself. Take care, guys. And good luck with your project.

G~

drumminman

CWS, Geoff is one of the experts here on CP and his rebuild of the 12's was masterful. It's what I used to do mine, though I didn't follow it exactly.

I think the takeaway here is try different approaches to get the sound YOU want. For example I've read many times how awful the SL1000 tweeters are, and yet mine sound good to my ears. Maybe it's the components I used in the Xover upgrade, maybe I got a "good set", or maybe I just hear differently. Still thinking about trying the RD-0194's, but not in a hurry.

With the way I did mine as described above, the improvement across the board is impressive. The bass is tight, goes low and is musical, i.e. not boomy at all.

You have a several of things to try - have at it and let us know what you find.

cwsmith68

Thanks guys.
Yes, Geoff. I was speaking of your build for Roger. Not sure if you remember but I discussed having you build mine the same way but after going over and over the build option list you sent me I decided to take on the project myself. (Shipping to and from you would have been very expensive)
I bought the 3-layer that was on the list. I am going to keep experimenting with these and see what I come up with. I was about to get discouraged and put them up for sale and start a new project but you guys give me hope that I may find the sweet sound I am looking for.

Thanks again,
Coley

geoff727

Yes, that's it exactly. Don't worry about what we 'say' is right, or what we tell you to 'do' or 'don't do'; do what the music says is correct. Experiment, come up with your own answers, do your homework, and you will be successful.

Looking back, I see that you put them on stands. I will make this recommendation....implement the speakers so they have zero back-and-forth, side-to-side rocking motion. This is an action-reaction principle; when the woofer goes out, the speaker wants to go backwards. If it's able to do so, even some microscopic amount, you will commonly lose some definition and dynamics in the bass. The woofer is essentially trying to produce sound by pushing against a surface which is not quite rigid enough to be pushed against, like trying to brace yourself against a wall of jello vs. a wall of solid steel. Play around with this concept and see what you think.

I am wondering, part of your perception of the bass going away may be because the speakers are mounted on the stands, and may not be transferring some bass energy to the floor (which then re-radiates the energy), and the speakers may be susceptible to rocking motion. I don't know exactly how high the stands are, but it's also possible that you've removed a 'wall' (so to speak) from between the passive radiator and the floor, and replaced it with the relatively empty space of a stand. This could lead to a small loss of contribution of bass response. Get in the books and read up on 2-pi vs. 4-pi radiating space of bass drivers, and baffle-step loss, and this will start to become clear.

Taking on the project yourself is the gest way to do it, that's how you learn the most. And thank you guys for not lambasting me!! Somehow, each time I contribute to a forum, I expect to be roasted.:rolleyes: Which is why I look a lot...but don't say much!

G~

cwsmith68

Thanks, Geoff. I put them back on the floor and its seemed to let them go lower than when placed on the stands.
Now I will go back to experimenting with the S.B.

geoff727

This may or may not be instructional. Since you have me curious, I did a very quick & dirty simulation of two different baffles. The first, a pseudo-RTA12 on a tall stand (in other words, below it is free air). The second, a pseudo-RTA12 with a really tall enclosure. This will start to approach the bass reinforcement afforded by the floor boundary. But, since this particular program cannot simulate complex shapes with many different corners (like the boundary of a speaker and the floor), then we have to look at what a much larger (in this case, taller) enclosure would do and start making some inferences from there.

Now, keep in mind that none of this is going to be very accurate, and it's just for a general look at things to help us piece together a 'real-world' understanding. But, notice a couple of things here....

First, the entire region from about 80Hz on down to the lowest response of the speaker is shelved down by about 1/2 dB when the speaker's up on the stand.

Then, from 300Hz to 1kHz, the filtering/diffraction action of the larger baffle shelves the response upward from 1/2 to 1 dB. So, just by changing the baffle size vertically, we've effected a slightly different balance in the speaker. On the stand....a little less lower end, a little less lower midrange, but comparable response through the midbass.

This may or may not mean anything, and it's just one small piece of a bigger puzzle....the size of the 3 drivers is not accurately simulated, the responses of the individual drivers/passive are not taken into account by the program, etc., etc. But, it does help to start paint a picture of what happens when a floor boundary is removed and the speaker moved into more 'open air'. Don't put TOO much stock in it, though; it's not done accurately enough to make a hard-and-fast judgment on.

Another thing you have me curious about....the 3-layer SB. I wonder if that middle vinyl mass layer, used in enough quantity inside the enclosure, is loading the MW's in a similar fashion to the way a port or passive does. Hmmm, need some further thought here.

So, I'm going to follow this and see what your experimentation yields.

G~