RTA 11t Crossover service

[lovinthehd]

It looks like my old multimeter does measure up to 20uF max so I will indeed measure those caps when I take them off and compare to the new ones.

More interested in how they compare to original spec....

 

[ryanosaur]

Why are we rebuilding the XO, anyway?
30 yrs old?

May get it working just to watch the Surround disintegrate. *shrugs

Could just rebuild the whole XO. Air core instead of iron core... it's only a .4 inductor, unless it is a bizarre resistance factor.

Key thing still is, without testing the individual components to see if they are out of whack and by how much, you aren't necessarily achieving anything.
Have you run the speakers at all since your OP?

 

[59ctd]

Why are we rebuilding the XO, anyway?
30 yrs old?

May get it working just to watch the Surround disintegrate. *shrugs

Could just rebuild the whole XO. Air core instead of iron core... it's only a .4 inductor, unless it is a bizarre resistance factor.

Key thing still is, without testing the individual components to see if they are out of whack and by how much, you aren't necessarily achieving anything.
Have you run the speakers at all since your OP?

Yes, they have been used on and off since my receiver went out several years ago. My son used them on his receiver for playing games but they just sound different now. He never overloaded them. I have checked the surrounds and they are soft like they should be. They do work but just seem to be a bit 'flat' to me with the new/used aventage receiver I purchased recently.

 

[Uncle_Big_Green]

I have a pair of Polk RTA 11t towers that I've owned for nearly 30 years now. I am getting my sound system back into working order with new receiver, etc. I think I heard somewhere that the caps on a 30 year old loudspeaker crossover likely need replacement. How do I know and is there anywhere with a bit more info on what to look at and replace? I am handy and able to take them apart and solder in replacements, etc but I am unsure if this is really something to worry about.

Thanks for posting. I was thinking about this. I have RTA 11tls. They definitely don't sound right.

 

[Verdinut]

Why are we rebuilding the XO, anyway?
30 yrs old?

May get it working just to watch the Surround disintegrate. *shrugs

Could just rebuild the whole XO. Air core instead of iron core... it's only a .4 inductor, unless it is a bizarre resistance factor.

Key thing still is, without testing the individual components to see if they are out of whack and by how much, you aren't necessarily achieving anything.
Have you run the speakers at all since your OP?

The air core inductor has more resistance than the iron core one, and that might cause an imbalance between the drivers outputs. Additional resistance most likely would have to be put in series with the tweeter.

 

[kevintomb]

It looks like my old multimeter does measure up to 20uF max so I will indeed measure those caps when I take them off and compare to the new ones.

I did a set of Rta11t's years ago, but redid it slightly differently.

I twisted the Protection circuit together to effectively remove it from the circuit. It normally has about a half ohm of resistance and increases a big amount with power spikes to protect the tweeter.

Well over time, its resistance can increase and lower the level of the tweeter ever so slightly, so I figured just get rid of it.

I also modified the actual components, but going from memory, instead of (2) 12uf caps, summed them to one 6.2uf and removed the one resistor that was across the first 12uf. Not so much to save money in parts, but to simplify things overall.

The one 6.2 minus the one resistor, actually has a very similar filter characteristic and sounded for pretty much identical and measured nearly identical. Yes I tried it with all the ACTUAL parts also. More for the sake of simplicity and easily being able to locate the caps on the board.

Why Polk used (2) 12uf caps is beyond me. Oddly complicated with no real reason. Yes I get it "Slightly" alters the filter slope, but it is not audible really.

 

[Eppie]

I did a set of Rta11t's years ago, but redid it slightly differently.

I twisted the Protection circuit together to effectively remove it from the circuit. It normally has about a half ohm of resistance and increases a big amount with power spikes to protect the tweeter.

Well over time, its resistance can increase and lower the level of the tweeter ever so slightly, so I figured just get rid of it.

I also modified the actual components, but going from memory, instead of (2) 12uf caps, summed them to one 6.2uf and removed the one resistor that was across the first 12uf. Not so much to save money in parts, but to simplify things overall.

The one 6.2 minus the one resistor, actually has a very similar filter characteristic and sounded for pretty much identical and measured nearly identical. Yes I tried it with all the ACTUAL parts also. More for the sake of simplicity and easily being able to locate the caps on the board.

Why Polk used (2) 12uf caps is beyond me. Oddly complicated with no real reason. Yes I get it "Slightly" alters the filter slope, but it is not audible really.

So the 12uf electrolytic and 12uf mylar capacitor are connected in series? 6.2uf is still within 5% of 6 and would work fine if that is the case, but that is an odd design choice if it's wired that way.

 

[Swerd]

I've been reading this thread with mild amusement for some two days. So far, I've resisted the urge to respond. It seems like yet another one of those threads where someone discovered electrolytic caps on the crossover board in his old speakers, and he wonders if he should 're-cap'.

I haven't read this entire thread carefully. Has the OP already checked to see if his tweeters are working? If they're dead, no re-cap effort will help them.

This thread has gone on for a number of posts, and I've yet to see a schematic diagram of the crossover. Without that, you just shooting in the dark. There are only a few parts on that crossover board. So, it should be possible to trace all the connections and come up with a diagram. I urge the OP to do that. Or, he can post a photo of the back side of the board and let others try.

A few words on the three caps on that board:

• The large yellow 12 µF is a film type cap, probably made with Mylar film. Keep it, as it won't go bad with time.
• The two smaller 12 µF and 34 µF caps covered in blue plastic, are non-polar electrolytic caps. Replace them if you must, but they don't look bad in the photo. I see no obvious bulges or leaks.
• Be sure that you understand this: Polypropylene film type capacitors (sometimes called metalized polypropylene or MPP) of the same capacitance value will be physically larger than the equivalent electrolytic caps. Make sure you have room on that small board for them, especially a replacement for that 34 µF cap. Look for MPP caps with lower voltage failure ratings, such as the 250V Dayton caps. They will have smaller dimensions than the 400V Solen or Audyn Q4 caps.

The small 0.4 mH inductor in the photo sure looks like it's air core – not iron core. Unless the coiled wire is broken or otherwise visibly damaged, don't replace it.

 

[kevintomb]

I've been reading this thread with mild amusement for some two days. So far, I've resisted the urge to respond. It seems like yet another one of those threads where someone discovered electrolytic caps on the crossover board in his old speakers, and he wonders if he should 're-cap'.

It's gone on for a number of posts, and I've yet to see a schematic diagram of the crossover. Without that, you just shooting in the dark. There are only a few parts on that crossover board. So, it should be possible to trace all the connections and come up with a diagram. I urge the OP to do that. Or, he can post a photo of the back side of the board and let others try.

A few words on the three caps on that board:

• The large yellow 12 µF is a film type cap, probably made with Mylar film. Keep it, as it won't go bad with time.
• The two smaller 12 µF and 34 µF caps covered in blue plastic, are non-polar electrolytic caps. Replace them if you must, but they don't look bad in the photo. I see no obvious bulges or leaks.
• Be sure that you understand this: Polypropylene film type capacitors (sometimes called metalized polypropylene or MPP) of the same capacitance value will be physically larger than the equivalent electrolytic caps. Make sure you have room on that small board for them, especially a replacement for that 34 µF cap. Look for MPP caps with lower voltage failure ratings, such as the 250V Dayton caps. They will have smaller dimensions than the 400V Solen or Audyn Q4 caps.

The small 0.4 mH inductor in the photo sure looks like it's air core – not iron core. Unless the coiled wire is broken or otherwise visibly damaged, don't replace it.

I owned the same exact speakers.....They do not look bad, but will sound quite bad.

Mine made the tweeter sound very muffled, like in the range of about 10db less than normal level. There were ZERO outside signs they were bad at all.....

 

[kevintomb]

So the 12uf electrolytic and 12uf mylar capacitor are connected in series? 6.2uf is still within 5% of 6 and would work fine if that is the case, but that is an odd design choice if it's wired that way.

Yes they are in series, but with the 2.0 ohm across the first 12uf, so a "Slight" change to the response. I plotted all this in a crossover designer years ago, and measured it back then also.

Just using a 6.2uf emulates the components polk used, (Circled in green) very closely.

 

[Eppie]

Yes they are in series, but with the 2.0 ohm across the first 12uf, so a "Slight" change to the response. I plotted all this in a crossover designer years ago, and measured it back then also.

Just using a 6.2uf emulates the components polk used, (Circled in green) very closely.View attachment 54567View attachment 54568

OK, that makes more sense. Thanks for posting the schematic.

 

[kevintomb]

OK, that makes more sense. Thanks for posting the schematic.

So EVERYTHING circled in green can be replaced with ONE 6.2uf cap. The response will be SLIGHTLY different but for all intents very close to the original sound. I did that years ago, and it worked quite well.

 

[Eppie]

There is software for crossover modeling so one could predict the change in removing that 2 ohm resister if you could find the parameters for the speakers. A bit beyond what I have time for though. As Swerd mentioned above, the mylar cap should be fine, though.

I rebuilt a pair of KEF 103.2 speakers from 1984 that also had protection circuitry. I ended up leaving the crossovers original. KEF used very good components and matched each crossover to its drivers for that speaker line and the soft highs of that model were actually very pleasing to my ears.

 

[Verdinut]

I've been reading this thread with mild amusement for some two days. So far, I've resisted the urge to respond. It seems like yet another one of those threads where someone discovered electrolytic caps on the crossover board in his old speakers, and he wonders if he should 're-cap'.

I haven't read this entire thread carefully. Has the OP already checked to see if his tweeters are working? If they're dead, no re-cap effort will help them.

This thread has gone on for a number of posts, and I've yet to see a schematic diagram of the crossover. Without that, you just shooting in the dark. There are only a few parts on that crossover board. So, it should be possible to trace all the connections and come up with a diagram. I urge the OP to do that. Or, he can post a photo of the back side of the board and let others try.

A few words on the three caps on that board:

• The large yellow 12 µF is a film type cap, probably made with Mylar film. Keep it, as it won't go bad with time.
• The two smaller 12 µF and 34 µF caps covered in blue plastic, are non-polar electrolytic caps. Replace them if you must, but they don't look bad in the photo. I see no obvious bulges or leaks.
• Be sure that you understand this: Polypropylene film type capacitors (sometimes called metalized polypropylene or MPP) of the same capacitance value will be physically larger than the equivalent electrolytic caps. Make sure you have room on that small board for them, especially a replacement for that 34 µF cap. Look for MPP caps with lower voltage failure ratings, such as the 250V Dayton caps. They will have smaller dimensions than the 400V Solen or Audyn Q4 caps.

The small 0.4 mH inductor in the photo sure looks like it's air core – not iron core. Unless the coiled wire is broken or otherwise visibly damaged, don't replace it.

Solen also have 250V caps which are smaller than their 400V ones. They also have 630V Polypropylene and very expensive audiophoolry metallized Teflon Film caps:

Solen

solen.ca

 

[kevintomb]

There is software for crossover modeling so one could predict the change in removing that 2 ohm resister if you could find the parameters for the speakers. A bit beyond what I have time for though. As Swerd mentioned above, the mylar cap should be fine, though.

I rebuilt a pair of KEF 103.2 speakers from 1984 that also had protection circuitry. I ended up leaving the crossovers original. KEF used very good components and matched each crossover to its drivers for that speaker line and the soft highs of that model were actually very pleasing to my ears.

Yeah, I actually did that several years ago with my long gone Polks, that were the same model the OP is talking about. The difference was so small, it made me wonder WHY Polk used something more elaborate for no real gain.
If anything, it made the Polks response slightly better overall. With the stock crossover components, there was a fairly distinct dip around the crossover region. And that response was also seen when STEREOPHILE measured them back in the day. The removal of the resistor, filled in the dip at aroudn 2,500 hz a bit. I no longer have those measurements, they are on my dead older comp.

Polk RTA 11t loudspeaker Measurements

Sidebar 3: Measurements Regarding measurements, I use a mixture of nearfield, in-room, and quasi-anechoic FFT techniques (using the MLSSA system from DRA Labs) to investigate objective factors that might explain the sound heard. (Stereophile's measuring microphone is a calibrated B&K/DPA 4006.)...

www.stereophile.com

Unless they had a deal on 12uf caps??

 

[59ctd]

Yes they are in series, but with the 2.0 ohm across the first 12uf, so a "Slight" change to the response. I plotted all this in a crossover designer years ago, and measured it back then also.

Just using a 6.2uf emulates the components polk used, (Circled in green) very closely.View attachment 54567View attachment 54568

Thanks much kevintomb, and thanks to others for replies as well. The schematic really makes it easier to understand what is going on here. I am planning on getting non-electrolytic caps of the original capacitance values and go with it. As many can see since my OP was quite a while ago I am not in a huge hurry to get them fixed but now that I understand these details and I've located some new caps online I think I will move on it.

 

[59ctd]

For replacing the 34uF electrolytics, is 33uF close enough? It is about a 2.9% diff. Lots of 33's from several mfgrs online but exactly 34uF I have not found them yet except for some from Sonic Craft and they are way expensive.

 

[ryanosaur]

For replacing the 34uF electrolytics, is 33uF close enough? It is about a 2.9% diff. Lots of 33's from several mfgrs online but exactly 34uF I have not found them yet except for some from Sonic Craft and they are way expensive.

You should keep that as a non-polarized electrolytic. Switching that to a poly-cap is of no consequence.
Running it as a 33uf shouldn’t be a problem, of course you can parallel 2 caps to achieve the appropriate recommended value.
You can, if concerned, run a 33uf NPE with a 1uf poly. *shrugs

 

[Swerd]

For replacing the 34uF electrolytics, is 33uF close enough? It is about a 2.9% diff. Lots of 33's from several mfgrs online but exactly 34uF I have not found them yet except for some from Sonic Craft and they are way expensive.

I wouldn't worry over using a 33 µF cap instead of 34 µF. The rough rule-of-thumb is that any cap within 10% of the value called for in the schematic will be OK. At 34 µF, ±10% means anything from 31 to 37 µF.

I wouldn't bother using two caps wired in parallel to get to exactly 34 µF. It will take up more space that on that small board.

 

[ryanosaur]

I wouldn't bother using two caps wired in parallel to get to exactly 34 µF. It will take up more space that on that small board.

Compared to what? That 33uf poly cap will not be small. (Granted, much smaller than the 100uf cap I played with in my XO mod, but hey... who's counting.

I do agree that the 33uf value is sufficient and running multiple caps in parallel is not necessary.