Anatomy of a Polk RT3

[mazersteven]

That's him, we should be able to get a free driver out of this deal!

There is also

RuSsMaN
http://forums.audioholics.com/forums/member.php?u=10195

 

[Sheep]

I mean this in the best possible way...

Get a new job and get what you want. You're never going to be happy with second.

SheepStar

 

[Guiria]

You're never going to be happy with second.

There is a lot of truth to this. After tonight I thought maybe I'll just hold out.

I removed my polks from the room and brought up my DIY 3" Tang Band bamboo speakers I had in my bedroom and hooked them up as my mains. After listening to a few cd's and watching a movie I have made the following subjective conclusions:

The polks are better for movies, more midbass, not as harsh or forward on the highs, and definitely more dynamic. They also blend better with my infinity center channel.

The TB's however have a very smooth, natural and accurate midrange that I do not have at all with the Polks, polks midrange is muddy by comparison. I actually really like the sound of the TB's and ended up listening to several songs from various cd's instead of just a couple like originally planned. Of course given the constraints of a 3" extended range driver it is not dynamic at all, the highs seem more forward than the polks, which I didn't mind, and at times more harsh. The midbass is not very strong which made some music seem rather thin.

Now I want to hold off and find something with smooth midrange or hack my bedroom speakers up and make a pair of three way bookshelf speakers for my theater.

I always liked the sound of the TB's in my bedroom. I never thought they could sound decent up in my bonus room which is close to twice the cubic footage as my bedroom.

There will always be decisions to make...

 

[TLS Guy]

When it rains it pours. I had plans for dual subs, plasma, etc then right before I commit I lose my job...sheesh. Needless to say I'm on a pretty tight budget now. To top it off my right main channel (Polk RT3) is now blown. There is very audible noise like flapping / fuzziness coming from this speaker in the lower audible range of the driver (currently crossed over at 80 Hz at the receiver).

I thought I'd open it up and take some pics. The cabinet is actually not bad with a 1" mdf front baffle and 3/4 mdf throughout.

I'm looking to replace the driver in the right only, or replace all sets of drivers in both speakers to have essentially a new pair of speakers in an old cabinet.

Any one with experience in this arena? I'm unsure how to go about looking for new drivers given the box design. Two ports with different lengths? How do you go about computing what frequency those ports are tuned to?

I'd be willing to make some cabinet modifications (internally) to make the cabinet work for a different set of speakers if necessary.

One more thing, these speakers are 10 years old

These are specs pulled off the Polk Audio Website regarding the RT3

--------------------------------------------------------------------------------

Driver Complement
Mid/Woofer 1 - 5-1/4" Diameter (13.34cm)
Dynamic Balance® driver
Tweeter 1 - 1" Diameter (2.54cm)
Dynamic Balance® polymer dome tweeter

Electrical
Overall Frequency Response 50Hz - 25kHz
Lower -3dB Limit 63Hz
Upper -3dB Limit 23kHz
Nominal Impedance 8 ohms
Recommended Amplifier Power 20 - 100 w/channel
Efficiency 89 dB

Dimensions
Cabinet Size 12-1/2" H x 7-1/4" W x 8-1/2" D
(31.75cm H x 18.41cm W x 21.59cm D)
Enclosure Type Vented, dual port


Any help would be appreciated.

http://www.racecyclesonline.com/RT3

It is generally easiet to build a speaker from scratch. adapting a speaker to a different driver, just about never works. All drivers need a specific enclosure size and custom crossover.

That is a common enough speaker, I bet you could get it reconed. You seem handy and if you want I bet you can recone it yourself. See if one of the reconing services, like Orange County Speaker. Polk are on their list.

While you are about it, upgrade the crossover to decent parts. Those crossovers are miserable. Electrolytic caps like that have no place in crossovers. They won't be the right value by now anyway. Upgrade them to polypropylene caps.

 

[dorokusai]

I do think they design them for certain but probably don't make them in house any longer, or at least not all of them for sure. The ring radiator tweeter on the LSis, for example, is a Vifa tweeter made to their spec.

True. Designed in Baltimore, manufactured in China. They mock up drivers for testing and design but the production stuff is overseas and has been for awhile.

Mark
Polk Audio CS

 

[dorokusai]

I talked to Polk this morning and they want $70.00 + shipping for a single driver.

If I join club Polk I can get the driver for 56 shipped because of a 20% discount but still...

It seems to me that running a pair of speakers one with a new driver and one with a 10 year old driver would not be ideal. I guess I'm not in an ideal situation anyways

You shouldn't hear anything drastically different. Email me. mln@polkaudio.com

Mark
Polk Audio CS

 

[TLS Guy]

True. Designed in Baltimore, manufactured in China. They mock up drivers for testing and design but the production stuff is overseas and has been for awhile.

Mark
Polk Audio CS

And while you are about it please try and spin the reason for using those electrolytic caps. We seem to have made a decent into sub mediocrity trying to meet price points. There comes a point where a product is not worth producing at all.

You have good company, I have seen some similar items in lower end B & Ws. There is just no shame anymore!

 

[Guiria]

That is a common enough speaker, I bet you could get it reconed. You seem handy and if you want I bet you can recone it yourself. See if one of the reconing services, like Orange County Speaker. Polk are on their list.

I checked with them and they do not service this particular driver...oh well.

 

[Guiria]

And while you are about it please try and spin the reason for using those electrolytic caps. We seem to have made a decent into sub mediocrity trying to meet price points. There comes a point where a product is not worth producing at all.

You have good company, I have seen some similar items in lower end B & Ws. There is just no shame anymore!

So if I were to upgrade the crossover how do I tell what polycap to buy? Can I measure the uF of the cap using a Fluke or whatever they are called (Is it micrometer?). I have access to a couple electronic measuring devices at work even an oscilloscope if required.

If the cap isn't going to have the correct value by now anyways then how do I determine the original value?

Also while I'm at it I wonder if I could work in a BSC network???

Honestly I've never tampered with crossovers but I wouldn't mind getting my feet wet. Why not on a speaker that is already broken, sounds like a great place to start.

 

[Guiria]

I have to ask:

Are the two ports tuned to two different frequencies or is the total length used for a single frequency? If tuned to a single frequency then why did they make the port lengths different?

Since I have the boxes open, would a different internal dampening material help improve the sound? I guess you would change Qts of the enclosure if you packed too much in. Any way to figure that one out on existing speakers?

If I end up buying the driver I'd like to improve the overall sound of both speakers by improved dampening material, new crossover components, or internal bracing.

I would need some knowledgable support however since this is pretty new to me.

 

[TLS Guy]

So if I were to upgrade the crossover how do I tell what polycap to buy? Can I measure the uF of the cap using a Fluke or whatever they are called (Is it micrometer?). I have access to a couple electronic measuring devices at work even an oscilloscope if required.

If the cap isn't going to have the correct value by now anyways then how do I determine the original value?

Also while I'm at it I wonder if I could work in a BSC network???

Honestly I've never tampered with crossovers but I wouldn't mind getting my feet wet. Why not on a speaker that is already broken, sounds like a great place to start.

The value of the cap should be stamped on it. Madisound sell French Solen caps at very reasonable prices. Just buy a Solen cap of the same value.

However you need to buy a new driver, or find a source of the parts and correct shims, before it is worth doing anything.

 

[Guiria]

The cap is 10 uF +/- 10% with 50 VDC capacity. The Solens TLS_Guy pointed out are +/- 5% with 400 VDC capacity for around 5 bucks. I'll definitely upgrade that if I get the replacement driver.

What about the coil and resistors? Is their an audible advantage in replacing just the cap in a crossover or should I be modifying the whole thing to really make the upgrade worth it?

 

[TLS Guy]

I have to ask:

Are the two ports tuned to two different frequencies or is the total length used for a single frequency? If tuned to a single frequency then why did they make the port lengths different?

To make people think they have some new groundbreaking piece of engineering. You can only tune 1 cavity to one frequency. Another case of engineers prostituting themselves to useless non productive maketing types, instead of giving them the finger they deserve.

Since I have the boxes open, would a different internal dampening material help improve the sound? I guess you would change Qts of the enclosure if you packed too much in. Any way to figure that one out on existing speakers?

If I end up buying the driver I'd like to improve the overall sound of both speakers by improved dampening material, new crossover components, or internal bracing.

You can't use too much fill in a reflex or you kill the resonance. A bit of Rockwool like product behind the driver might help. It is hard to add bracing to an existing enclosure that does much good.

To be honest, I'm yet to hear a Polk speaker that is any good. That crossover is a joke. It is clearly first order low and high pass. It looks like an inductor in series with the woofer, with a zobel across the woofer. That 10mfd electrolytic is in series with the tweeter. If that is an 8 ohm tweeter it puts the crossover at around 2.5 KHz 6db roll of per octave. So the tweeter won't be down 24 db until just below 200 Hz. It will be driven hard into its area of resonance. It is a wonder it can produce any detail at all.

There will be a big lobing error also. That plan never works for me.

I would say those speakers actually need a decent burial. I think you will be better served trying to build a nice two and a half way speaker from scratch.

To be fair to Polk, they are not unique. Unfortunately there is, and always has been boundless quantities of dreadful junk in the speaker market. This is another in an almost endlessly long sorry list.

 

[Guiria]

To make people think they have some new groundbreaking piece of engineering. You can only tune 1 cavity to one frequency. Another case of engineers prostituting themselves to useless non productive maketing types, instead of giving them the finger they deserve.

You can't use too much fill in a reflex or you kill the resonance. A bit of Rockwool like product behind the driver might help. It is hard to add bracing to an existing enclosure that does much good.

To be honest, I'm yet to hear a Polk speaker that is any good. That crossover is a joke. It is clearly first order low and high pass. It looks like an inductor in series with the woofer, with a zobel across the woofer. That 10mfd electrolytic is in series with the tweeter. If that is an 8 ohm tweeter it puts the crossover at around 2.5 KHz 6db roll of per octave. So the tweeter won't be down 24 db until just below 200 Hz. It will be driven hard into its area of resonance. It is a wonder it can produce any detail at all.

There will be a big lobing error also. That plan never works for me.

I would say those speakers actually need a decent burial. I think you will be better served trying to build a nice two and a half way speaker from scratch.

To be fair to Polk, they are not unique. Unfortunately there is, and always has been boundless quantities of dreadful junk in the speaker market. This is another in an almost endlessly long sorry list.

No really...tell me how you really feel

 

[dorokusai]

Yea, no kidding but you can't satisfy everyone in the world. That's the way it goes and thankfully the majority is covered by our brand or another. I like Magnepan...go figure.

Again, if you want to replace the driver...email me. If not, part them out or make planting boxes out of them in honor of TLS.

Mark

 

[TLS Guy]

No really...tell me how you really feel

What is the internal volume of those enclosures and the dimensions of the speaker openings? We just might get lucky and find a woofer that could be fitted and matches the volume. Then find a tweeter and design a crossover.

 

[Guiria]

Yea, no kidding but you can't satisfy everyone in the world. That's the way it goes and thankfully the majority is covered by our brand or another. I like Magnepan...go figure.

Again, if you want to replace the driver...email me. If not, part them out or make planting boxes out of them in honor of TLS.

Mark

I sent an email two days ago. Did you not receive it?

 

[Guiria]

What is the internal volume of those enclosures and the dimensions of the speaker openings? We just might get lucky and find a woofer that could be fitted and matches the volume. Then find a tweeter and design a crossover.

The internal dimensions are: 11" H x 5.75" W x 6.75" D. Gross internal volue is .247 cubic feet.

The two port lengths combined are 12.25" long x 1.25" diameter.

The hole cutouts are as follows:

Tweeter recessed cutout is 3.75" round. The hole cutout is 2.75" and it's kind of a triangle cutout with rounded ends, if that makes sense.

Woofer cutout is 5" even. The driver recess has the square shape to it with 4 mounting holes, one in each corner.

I thing that is all the info we need to get searching right. I'll have to poke around partsexpress and see what I can find.

 

[TLS Guy]

The internal dimensions are: 11" H x 5.75" W x 6.75" D. Gross internal volue is .247 cubic feet.

The two port lengths combined are 12.25" long x 1.25" diameter.

The hole cutouts are as follows:

Tweeter recessed cutout is 3.75" round. The hole cutout is 2.75" and it's kind of a triangle cutout with rounded ends, if that makes sense.

Woofer cutout is 5" even. The driver recess has the square shape to it with 4 mounting holes, one in each corner.

I thing that is all the info we need to get searching right. I'll have to poke around partsexpress and see what I can find.

I think this driver will work. Your box is just the right size. You would have to enlarge the woofer cut out a couple of mm. You can adapt the ports easily.

This tweeter will work, you would have to enlarge the recess about a quarter inch.

The crossover works well with a both LF pass and HF pass second order Linkwitz-Riley filters.

If you think it will work, I can put the package together.

 

[Guiria]

Thanks for researching out those drivers. $90.00 per woofer is a little steep for my budget. I could spend up to about 200.00 (kind of a don't ask don't tell with significant other ) total for either direct replacement speakers or a brand new set of DIY front speakers. At the price point of $200 I'd rather not consider brand name pre-built speakers.

I currently have a pair of DIY TB 3" bamboo drivers

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=264-844

I really like the midrange they produce. I've never heard a carbon fiber cone driver. How do they sound in comparison to bamboo drivers I wonder???

I'll take your word on suggested drivers TLS_Guy I just wonder if we can find another match that is a little less $$.