Rebuilding Crossovers in Polk RTA12's

[Michael8it 49]

I am getting all the parts together to do a major overhaul of my Polk RTA 12's.  there are quite a few capacitors in the crossovers, which are mounted on top of the cabinet, along with the tweeter.

 

I have looked at several different manufacturers of caps including Solen, Clariticaps, Sonicaps, and Erse.  Erse seems to be the best cap for the price, based on several reviews I read.  Clariticaps and Sonicaps would simply be too costly as each crossover has six very large caps and 2 small ones.

 

However, one of the things I would like to get out of this is improved (tighter) bass response, and that may lead me to a different cap for the LF side (it's only 2 caps per speaker), and if I can get significant improvement with a different cap I may do that.

 

Does anyone have any feedback on Erse caps in general, or on caps specifically for the LF module ?

 

Thanks,

Mike

[zumbini 6,147]

Why rebuild the passive XO's? GO ACTIVE YOUNG MAN!

[Michael8it 49]

Primarily because they are 31 years old..... and from all that I have read both here and at the ClubPolk.com forum, the caps dry out and change their values with time, and it is well worth it (if you intend to keep the gear) to replace the caps.  It's not a particularly hard job in this case because the crossover is mounted on top of the cabinet. But it can get extremely expensive if you start using high end audio caps.

 

From what I have read, any modern day cap is going to out perform the original, made in Me-hi-ko caps that were on these.  Although they were good for their day, there have been some great improvements in the manufacture and design of them.  New caps (plus some TLC to the interior of the cabs themselves) would make these speakers come alive even more than now.   

 

Using Erse caps and Mills resistors I can redo both crossovers for $80 total.  That's in the budget.....

[Michael8it 49]

Oh, I forgot, THANK YOU for the 'young' part..... I don't hear that much anymore...

[RichP714 3,163]

 

 

.....However, one of the things I would like to get out of this is improved (tighter) bass response.....

 

I went down the same road with my Klipsch Cornwall II.  I finally decided that, rather than waste money on exotic caps in the passive crossover, I wanted to bypass them and run an active crossover.

 

I'm glad I did.  The benefits, as compared to any passive crossover, are easily heard

 

Bi-amping your speakers ----just do it!

 

Active V Passive Crossover design

 

[Michael8it 49]

Any suggestions on active crossovers ?

[zumbini 6,147]

 

 

Primarily because they are 31 years old..... and from all that I have read both here and at the ClubPolk.com forum, the caps dry out and change their values with time, and it is well worth it (if you intend to keep the gear) to replace the caps.  It's not a particularly hard job in this case because the crossover is mounted on top of the cabinet. But it can get extremely expensive if you start using high end audio caps.

 

From what I have read, any modern day cap is going to out perform the original, made in Me-hi-ko caps that were on these.  Although they were good for their day, there have been some great improvements in the manufacture and design of them.  New caps (plus some TLC to the interior of the cabs themselves) would make these speakers come alive even more than now.   

 

Using Erse caps and Mills resistors I can redo both crossovers for $80 total.  That's in the budget.....

 

Maybe I wasn't clear. I understand that capacitors age and need to be replaced.

I was suggesting that you'd be better served to eliminate the passive components in favor of an active XO.

 

An active crossover placed between your preamp and amps is more efficient (doesn't waste power).

In addition it allows you to adjust crossover frequencies and balance levels without changing components.

[Michael8it 49]

Zumbini, I picked that up when I re-read your reply.

 

Any suggestions on active crossovers?  I have been looking around on the internet and all I find are pro audio systems, plus a DIY which I am open to,  but not a complete kit. 

[Michael8it 49]

 

 

 

.....However, one of the things I would like to get out of this is improved (tighter) bass response.....

 

I went down the same road with my Klipsch Cornwall II.  I finally decided that, rather than waste money on exotic caps in the passive crossover, I wanted to bypass them and run an active crossover.

 

I'm glad I did.  The benefits, as compared to any passive crossover, are easily heard

 

Bi-amping your speakers ----just do it!

 

Active V Passive Crossover design

 

 

 

I read the attached articles, all of which talk about LF crossovers.  The Polk RTA12's have 2 mid-bass drivers, and tweeter mounted outside the cabinet (on top) and a 12" passive radiator.  So I would only be looking at a crossover between the mid-bass and tweeter - and I cant find one .  All the one's I have looked at have 2 way (bass vs mid-high) or three way (bass-mid-treble).  Nothing I see has the crossover I am looking for, which means I am going at it from scratch, or I am not looking in the right place...

[zumbini 6,147]

Any suggestions on active crossovers ?

 

I picked up a used Behringer CX3400 on ebay for $80 but you can buy a new one for $120.

It can be configured for 2 or 3 way stereo or 4 way mono.

 

Some members favor Ashly or Rane which are sturdier but cost a lot more.

 

The problem is that all of these XO's have balanced inputs and outputs so you'll need to convert.

Sometimes you can get away with adapter cables but a balance box or balanced preamp is better.

I ran an Aphex 124a balance box initially but now use a modified C-11 preamp with balanced outputs.

 

BTW - Unbalanced XO's exist but they are limited to bass only (Audio Control Richter Scale), are hard to find (DBX), or are expensive (Marchand).

[BillD 239]

I swear by Rane's.  The company was formed by Phase Linear guys.  Another choice might be a Marchand.

[zumbini 6,147]

The Polk RTA12's have 2 mid-bass drivers, and tweeter mounted outside the cabinet (on top) and a 12" passive radiator.  So I would only be looking at a crossover between the mid-bass and tweeter - and I cant find one .  All the one's I have looked at have 2 way (bass vs mid-high) or three way (bass-mid-treble).  Nothing I see has the crossover I am looking for, which means I am going at it from scratch, or I am not looking in the right place...

 

There is a button on the back of the CX2300 and CX3400 that multiplies the low XO frequency by 10.

I assume the Ashly and Rane units have this flexibility too.

 

Here's a photo of the front panel of my CX3400. Supplementary XO values are in the white boxes.

 

[RichP714 3,163]

.....I read the attached articles, all of which talk about LF crossovers. ...

 

The articles are not step-by-step how to, but rather proof of concept.

 

In that, yes, they do focus on a bi-amp situation, but they leave it up to the reader to expand and adapt the concept to fit their needs.

 

For example, I'm tri-amping my Cornwalls

[Michael8it 49]

I'm thinking possibly a DIY - one of the links Rich had does have a PCB and schematic for a crossover.  I am assuming that the crossover point is simply determined by the component values.  If I am right and I pick the right values then I am getting the right cross over frequency.  Otherwise I have to mod my CT-SEVEN and I am not ready to get into that now.  Later, yes.  But I am focusing on the speakers and amp right now.

 

Plus, bi-amp means.... one more amp..... and more cables..... and more $$$..... and more $$$$$$.... and more $$$$$$$$.

This hobby is getting as expensive and target shooting, and real fast too!  

 

So for $80 I think I will stick with plan A and just rebuild the crossovers, unless I can find a DIY for a resonable price that is, and another amp, and more cables, and more $$$$$$$$$$$.  

 

[zumbini 6,147]

Beware of OCCD (Obsessive Carver Collecting Disorder)....

[Michael8it 49]

I am hoping that it will be offset by WIFC - Wife Induced Financial Coma.  This is also translated sometimes as Wallet Induced Financial Coma.  Unfortunately Government Bureaucracies seem to be completely immune to WIFC of any form

[Michael8it 49]

Ok, now you guys got me thinkin' and that is always dangerous, and generally leads to yet another outlay of cash....

 

I found this at Marchand  , and electronic crossover for $46.00, no case. 

I am thinking.... mount it inside my CT-7 case (assuming there is room, I have not looked), and steal one of the tape loops (I will never use two tape loops again) for the output(s).

 

• I don't have to drill any holes
• I don't need extra cables (except one pair to another amp)
• I just need space in the CT-7 to mount it
• I need 15v to power it. 
  
• Once I dial in the frequency it is very unlikely I will ever change it so not having 'instant control' is not really an issue.
• I overhaul the M-200t I have right after I over haul the M0.5t, use the M0.5t for the mid-bass and the m200t for the treble.
• The M200t is actually a WM200t with tone and volume controls - allows me to adjust the volume to the tweeter to match the mid-bass.
  
• I only spent $100 (I'll need some mounting hardware and some components to tap the power).
• I don't have 'yet another' component in my rack which is already full  
  
• I completely avoid any possibility of WIFC syndrome....    

   SO...... does anyone know if there is room in a CT-7 to mount an additional board like this?

[zumbini 6,147]

There should be room for a 2" x 3" board but I think you will need two.

"One crossover network is needed for each channel of a bi-amplified system."

 

Here are some photos of the interior of one of my CT-Sevens:

 

Top View (there's about an inch of height available above the middle board and between the PSB and tuner boards)

 

Bottom View (no room here)

 

[Michael8it 49]

Do you know the dimensions of the middle board?

[zumbini 6,147]

Do you know the dimensions of the middle board?

 

Not off the top of my head Michael and that CT-Seven is now double-boxed for shipping.

Scaling off the 19" faceplate the exposed green rectangle (part of the foil side of the main PCB) is ~5" x 8".

(The main PCB is twice that size as it extends under the tuner board all the way to the right side of the case.)

 

I think the issue is going to be overhead clearance (from the foil side of the main PCB to the case).

The height of that plug-in resistor network has me concerned and it's not spelled out by Marchand.

You will also need to add the height of the standoffs you mount the XO boards on.

Perhaps this would be a good time to pop the lid on your CT-Seven and take accurate measurements.

[Michael8it 49]

I agree. THANK YOU for your help.  Time to 'pop the hood'.....

[Michael8it 49]

 

A second option - Run the preamp out to my Audio Control C-101 and take outputs from there.  I could fabricate an extension to that case fairly easily (it's half the depth of any other components).  As long as I can get the voltage I need from it, space is not an issue. 

 

So, instead of putting the AC101 on a tape loop, I take the Pre out from the CT-7 to the input of the AC-101, and have two outputs from there.  I think to test the system that would be the most productive way.  

[zumbini 6,147]

I found a spec sheet for the RTA 12B. If your speakers are similar there may be more to the crossover.

Here is an excerpt that describes the crossover.

 

 

[Michael8it 49]

Mine are an earlier version of this.  There was RTA12, 12A, 12B, and 12C.  I have the originals, with the entire crossover on the board (pictured to the left).  The later model (12B etc) moved two of the caps to a separate board mounted to the terminal post connector, which is what you see here.  Polk is amazing in that they still have technical data for these and I was able to get the schematics from them.  They even still have drivers! 

 

So I am still leaning toward rebuilding these as the best use of the capital at hand.  I am still amazed that when I played these next to a pair of KEF 103/3's there was just no comparison at all.  It will be exciting to rebuild these and hear what these really have to offer! 

[zumbini 6,147]

It's an interesting arrangement (having the crossover mounted externally).