Monitor 7A passive radiator replacement?
jrotten2
Posts: 8
Hi all, new here, just entered the Polk world and need some help. Just picked up a pair of Monitor 7A's of which one has a damaged radiator. Only identifying numbers are RR-1004 on the outer rim of the cone and 75751 printed on the inside of the cone. They have the smaller front dust cap + a steel washer glued to the inside of said dust-cap. Also are the zinc plated instead of black metal frame. Question is, will SW100/SW101's work? (as they seem to be common) or should I look for something more similar to these? Looking forward to using these as I've heard so much about them!
Post edited by jrotten2 on
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The PR works in conjunction with it's matched MW, so swapping in alternate part numbers can/will affect the output.
Now, when you say damaged, what exactly constitutes damage? Pics would be helpful to determine if it is repairable or not. -
Damage is a 1" tear emanating out from the center cap. It has been repaired prior to my ownership but I am interested in putting a clean PR in if possible. Sorry I posted without pics but just picked them up and ran out of time today, I will post some when I get time over the next day or so.
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Here's pics, you can see the cone is a little distorted from the repair, they probably work fine but would prefer to get a decent condition PR to swap in if I can, other pics for reference
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Yes, bit like that... There is one on the bay right now but it has a similar tear, wonder if it's a defect or just clumsy folks. Anyway, I'll keep an eye out for a replacement, but at the very least I got 2 Peerless tweets out of the deal. I read somewhere that the crossovers are virtually the same between certain models, I wonder if I can swap in matching drivers and PR's, may not be worth the effort. IDK.
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Clumsy (idiots) folks.Political Correctness'.........defined
"A doctrine fostered by a delusional, illogical minority and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a t-u-r-d by the clean end."
President of Club Polk -
That one on the bay has a smaller tear, might be able to put a very small seam of urethane down the crack, urethane will work well with the paper and possibly be minimal to the eyes and added mass, I'll think about it as I continue to scan the bay.
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Series 1 peerless post "model" era parts should work.
* Just looked at the schematic for what I'm talking about, and a mw6502 with it's PR should work, unless you have the 8" PR (very early version). Most 7s use a 10" PR. -
Thanks festyboy, it's a good suggestion. I did look at the schematics and on one forum someone modified the original 7 schematic to match the 7A - very little difference. Looked at mw6502's on ebay. They have a "engineered for use in" on the label then a stamp from m4 to m7. Not sure what the differences would be, resistance seems to be similar between them. Only saw one made for the M7. There is also a NOS "POLK AUDIO Monitor 7/10 passiv woofer"(sic) from a seller in Canada, which looks almost the same as mine except the frame is more conical than the flat look mine has. Might be an 8", I messaged him but but haven't heard back yet. I have the 10".
FYI - he's doing a "Liquidation of a specialized service workshop in audio after 45 years." and has a bunch of NOS parts including at least one M7 crossover. -
The important parameter (and, to my understanding, there's really only one) is the moving mass of the passive radiator diaphragm*.
A passive radiator takes the the place of a port; the mass of the PR is supposed to match the mass of air that would be contained in the port of a bass reflex (Helmholtz radiator) for a bass reflex enclosure with the appropriate tuning.
As long as it is air tight, and that charming repair doesn't much alter the moving mass significantly (which, I realize, is not a given), the PR is probably fine as is.
You need to replace like with like, in terms of the PR, to get proper loading of the "energized" drivers ("midwoofers", as Polk called them in those days).
If the mass of replacement is too low, mass can simply be added to tune the system, though.
In fact, 'generic' PRs (e.g., for DIY work) often have a threaded rod attached to the "dust cap" to which threaded weights (ahem, nuts, e.g. ) can be
attached to tune the PR.
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* Common sense tells me that the compliance of the surround must have some effect as well, but I've never read anything that discusses this. I am not an expert, though, by any means!
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As an example: This generic PR has a threaded hole to which a bolt with or without added weight can be added
https://www.parts-express.com/dayton-audio-ds270-pr-10-designer-series-passive-radiator--295-494
One does note that other parameters besides mass are supplied for this PRDayton Audio DS270-PR 10" Designer Series Passive Radiator
The Dayton Audio Passive Radiators are excellent general-purpose passives suitable for replacement or for new construction. A built-in mass disc offers a good starting point for most designs, and a convenient M5 threaded hole (screw included) allows different types of weights to be added if desired. Cosmetically and mechanically, the passives match the Shielded DVC line of subwoofers, with a poly coated paper cone and medium-roll rubber surround for clean excursion and long life. The possibilities with these passives are endless, from floor standing two-ways to small active/passive subwoofer systems.
Specifications: • Fs: 21.9 Hz • Vas: 3.73 cu. ft. • Qms: 3.98 • Cms: 0.60 mm/N • Rms: 3.1 kg/ • Mms: 88.4 g • Sd: 353 sq. cm. • Xmax: 11 mm • Dimensions: Overall diameter: 10.63", Cutout diameter: 8.94", Depth: 2.81".
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Looking at the link they have a tip:
"As a general rule the passive radiator(s) should be able to move double the volume of air as the active woofer(s)/subwoofer(s) in the system. For example: A passive radiator system for a 10" woofer/subwoofer that has 10mm of X-max would require 2-10" passive radiators with 10mm of X-max. For single passive radiator systems, it is common to use a passive that is larger than the active woofer(s)/subwoofer(s)."
And a note:
"Adding mass to the passive radiator will decrease Fs and increase Qms."
So the excursion + diameter govern the volume moved by the thing which, by them, should be double that of the active woofer. Then the weighting to vary the resonant frequency and Q factor. Unfortunately, I'm not about to run off and purchase a spectrum analyzer although my interest in speaker dynamics has been piqued.
Magically, that NOS PR in Canada is 10" and from the front looks identical to mine, meaning the "dust-cap" size, cone and the rubber suspension look the same + the steel washer (I mean, accurately manufactured weight) glued to the back of the cone with what looks like red Glyptal, looks the same size. The different shaped frame may indicate a smaller spider, but I'm thinking that shouldn't affect things too much. Being from an ex-shop, it could be a Polk replacement PR which would explain the different shape.
Anyway, I think I'll give it a shot, spectrum analyzers aside, the real test is in the hearing and if it turns out good to my ears, then I'll be happy. Thanks for your help!
Just had a thought that the joys of these old speakers are sometimes due to the fact that they have their own sweet resonance that gives them magic, if we messed around and tuned them, they may just lose that, who knows... -
I would not be surprised if the woofer is a MW6500, not a MW6502.Home Theater/2 Channel:
Front: SDA-2ATL forum.polkaudio.com/discussion/143984/my-2as-finally-finished-almost/p1
Center: Custom Built forum.polkaudio.com/discussion/150760/my-center-channel-project/p1
Surrounds & Rears: Custom Built forum.polkaudio.com/discussion/151647/my-surround-project/p1
Sonicaps, Mills, RDO-194s-198s, Dynamat, Hurricane Nuts, Blackhole5
Pioneer Elite VSX-72TXV, Carver PM-600, SVS PB2-Plus Subwoofer
dhsspeakerservice.com/ -
westmassguy wrote: »I would not be surprised if the woofer is a MW6500, not a MW6502.
I agree with you Dave. And notice the gasket material being used in the pictures. Appears to be an early Monitor 7A. -
If you are still looking for a radiator i found this one on ebay that appears to have the silver basket and gasket around the surround. https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/264232692974Monitor 4.6, 5jr, 5, 7, 10, 11. RTA 8TL. RTA 11T. SDA 2B.
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Just to finish this off, received the Canadian radiator and it was as I thought, the cone is identical to the original with the same washer weight attached, but the frame is different with a slightly smaller spider. It was close enough for me, and sounds between the 2 speakers seem equal to my ears (upper pic is the new radiator).
For future reference should anyone pass this way, Polk said the SW101 radiator was installed in the 7A. Being as my speakers are early production, they have an earlier type radiator, therefore, as the cone is different, I'd probably have to get 2 SW101's should I go that route. Thanks again for all the help. BTW, they sound great!
