Rebuilding Crossovers in Polk RTA12's

[Michael8it 49]

And the tweeter.  No coloration from being in the cabinet, and they have a really open sound.  Are these yours?

[Guest RodH]

Not tryin' for a derail here - but I looked into the Marchand thing and got bitten by the DIY bug once again.

 

The XM-9 boards at $70 a channel look like a good place to start. I've read on some hardcore DIY sites that they are vastly superior to the XM-1 model, and for anther 30 bucks...

 

Anyone hee have experience with them?

Fits my goal of an active X-over with unbalanced (RCA) in and out.....!

[BillD 239]

I would have thought that the XM26 would be more your style, Rod.

XM26 

[Michael8it 49]

RodH - let me know how it goes   My RTA12's have a very complicated crossover and I am going to just rebuild these.

However - I did seriously consider the bi-amp solution.

 

 

[Guest RodH]

Oh, I looked at it.... way out of my budget.

[zumbini 6,147]

And the tweeter.  No coloration from being in the cabinet, and they have a really open sound.  Are these yours?

 

They are not mine Michael, just something I found while searching the internet.

 

My mains are custom sub/sats from Legacy Audio. (Real-To-Reel Designs when I bought them in 88/89.)

I recently put new drivers in the satellites and bi-amped them (tri-amped if you include the subs).

The new drivers are 6.5" Kevlar mids with a 1" ribbon tweeter in a D'appolito configuration.

(There's also an adjustable 1" dome tweeter on top.)

 

[Michael8it 49]

After researching how these crossovers work, I would need to Tri-amp these to get it to work right, and that's just not in the budget. 

 

 I can replace all the caps with Erse caps for $50 per speaker.  I can replace the entire x-over, including inductors, for $100 per speaker. So.......  Is there any point in replacing the Inductors?  Do they age, and are inductors made today any higher quality that the original ones?  I am sure there will be a performance gain replace 30 year old caps, but I don't know about Inductors.

 

Thoughts anyone? 

[madmike46 398]

Is there any point in replacing the Inductors?  Do they age, and are inductors made today any higher quality that the original ones?  I am sure there will be a performance gain replace 30 year old caps, but I don't know about Inductors.

 

Thoughts anyone? 

 

I think the Inductors should be OK, they are a coil of wire and if it doesn't smell burnt it should be OK.  About 1-1/2 years ago I re-built the cross overs in my Polk Audio 5B,  I used Carity SA series caps and Mundorff resisters, I also replaced the SL-2000 tweeters with the newer RDO-194 (had a blown tweeter). it took about 50-60 hours of playing time to break in the tweeters but now they sound better then when they were new.

[Michael8it 49]

Do you think it was the caps or the new tweeter, or both?  I ask because there seems to be two prevailing theories in recapping corssovers:  A - it works, don't fix it.  B:  Caps have a shelf life, and yours have long expired.  Replace them and your speakers will sound 10X better.

 

I have the Denmark Peerless tweeter in these, and have been told not to change them (they supposedly sound better than the RDO).  

 

I am looking at Erse caps because they have the right values, and Clarity, Solen, et al do not, so I would need to do alot of parallel caps to get the value right, and at 7 capacitors per x-over that is a lot of $$$$$ when you start running them in parallel.  

 

My inductors look fine, so I think I will leave them alone.  I can always do that later.

[madmike46 398]

You definately want to keep the Peerless tweeters, I have a pair of  Advance A+ that have 2 Peerless tweeters and they sound great.  I think the difference in sound comes from both the tweeters and the re-cap.  The RDO-194 are not as shrilly sounding as the SL-2000.

[hipmomk 1]

On the cap replacement front, replacing caps is often an insurance policy, if one of the caps shorts, pop goes the tweeter, midrange, or worse. It is also possible that after the new caps “wear in” that it will improve sound quality and/or return the overall frequency response back to where it was designed to be, as you have mentioned cap values can change in 30 years. Highly unlikely inductors should be replaced, ones part of a well designed system (Polk) probably will not be improved upon dramatically.

As a comment, a few years ago I replaced the caps in one of my AR-LST’s ( built ~1973) as a preventive measure, was a very tough job (well, for me) based on the way the x-over board was located… got busy, did not do the other. AR used high quality components, I was just afraid that one of the electrolytics would die while I was redlining my M1.5 during a party. If memory was correct, I used Solen’s for the mid, and a more esoteric cap for the tweets. Long story short, I think I would say that today, the speaker I changed the caps in is a little “brighter” than the one I didn’t, not that either can be criticized as being “dull”. I would not say there is a huge difference, but there is at least some difference. There was a big sprague that I missed on the schematic (~2500mfd) that was not replaced (no parts) as I had not done the proper research at the time.

 

On the active crossover front, another good active crossover is the TDM Design series, TDM CX-2, CX-3, CX-4 (stereo 2 way, 3 way, 4 way). They have been sold both as pro & consumer, Madisound sold them at one time as their “Sledgehammer” active crossovers.   They have the usual 24db Linkwitz alignment.
Pretty easy to find on ePay for a reasonable amount, I picked up my CX-4 for less than $80. In my case, am only using for stereo subwoofers, normally a CX-2 would fill the bill. Note that the vast majority of the units are pro-sound oriented, meaning the connections will be XLR etc, which you can use with home audio with simple cable converters, I happened to get one of the comparatively less common RCA jack versions.

 

If you decide to go the active route, you may want to keep in mind the existing crossover driver slopes of your speakers when shifting to an electronic crossover – many speakers used either 6db or 12db slopes because of the difficulty in designing a 24db slope in a passive (cap & coil) crossover. So it may sound a little different just because of the slope differences. You will also need to find a way to balance the relative volume of the tweeter section with midrange with woofer, etc – an equalizer with a RTA & mic (like above, I have a C-101) or some other PC RTA software.

 

Just replacing the caps with good quality replacements is probably the best place to start – JMO of course.

Joe

[Michael8it 49]

Joe, thanks for the input.  Due to the way the x-over in this is designed, I am going to stay with a passive x-over.  In essence, each driver has it's own x-over, which means I would need three amps to drive these.  I am just not that 'hi-fi'.  So I placed an order today for Erse caps, which have the right values and a reputation for being quality audio caps.

 

I'll see what a difference it makes.  I think doing the BillD upgrade on my C-1 and the MKII upgrade on my M500t will have as much impact as anything else. 

[Michael8it 49]

Call this one 'in the bag'.  I received the Erse Capacitors in the mail yesterday.  They were my choice because they had the right values and were quite economic compared to other makers of Audio Caps.  Total cost was just over $100.

 

Here is the work area, yes, the dining room table .  The first victim is on the left of the photo.

 

 

Here is the Lower Crossover that mounts in the cabinet.  The posts are on the back of this.  I pigtailed the wiring and reversed the connections so that I cannot wire it incorrectly if I need to remove the x-over again. 

 

 

Here is the top crossover.  I removed the fuse holder.  I had jumpered out the fuse before I changed the caps and noticed less distortion in the tweeter so I removed it completely when I did the caps.  I had to drill some new holes to make them look good but that was all pretty straight forward.  All that is missing now is the tweeter.

 

 

The markings on the tweeters say 3514.  Supposedly Polk only used 5514's from Denmark and the US before switching to the SL1000 and eventually SL2000.  I have been told these are the best sounding of all the tweeters.

I have a pair of 5514's as well and will do some A/B testing between the two.

 

Impressions.  Much more detail.  The subtle instruments come through.  The bass is tighter, and I can drive them harder without wincing at the sound.  Huge sound stage, even without SH.  

 

I have the parts on the way for my C-1 Upgrade and M500(t) overhaul.  I think when I am done I will be one very happy camper.