Polk 7A Sound Problems, maybe?

ajt528 · March 1

First post, so appreciate any assistance: I recently picked up a pair of Polk 7A’s off the FB, dragged all my old CDs down from the attic and set-up a listening room in my spare bedroom. I thought the 7A’s sounded really good. I’m not driving them with anything special right now - a cheap Yamaha 100W 2 channel receiver. I also picked up an inexpensive Fosi amp (also 100W) just to test what I thought I was hearing. I can also always drag them downstairs and hook them up to a Denon AV receiver I have to do more testing too. (I use my RTA-8ths as fronts in that system)

I was trolling through the marketplace again last weekend and a guy was getting rid of some 5JRs for 50 bucks. I couldn’t pass them up. He was the original owner and it was apparent he cared for his equipment. The guy I brought the 7As from I’m pretty sure just grabs stuff at flea markets and was a bit of a hoarder of electronic gear when I got there. Lol! So their history is unknown, but the drivers and cabinet overall look really good (nothing visibly wrong anyway).

I thought the 7A’s sounded fine UNTIL I heard the 5JRs. Now the 7A’s sound a bit on the “muffled” side, maybe that’s the best way to describe them. The 5JRs just sound “brighter” and even the bass sounds a bit cleaner (less “thumpy” maybe?). As near as I can tell, sound is coming out of all the drivers, but not as clean as the 5JRs. I don’t know if that’s a speaker issue, a power issue, or what and TBH, I don’t really know what to even search on the forum to try to diagnose the problem (if it is even a problem with the speakers… maybe it is just my ears). I’ve tried different audio cables and connectors I have around here (everything from 16 AWG to 12 AWG) and I’m not noticing any real difference with cables. The 5JRs just sound better.

Any thoughts or suggestions on what to search would be helpful.

-A

Gardenstater · March 1

Start by cleaning the fuse holders. Do your 7As have the dual fuses? So many other things could be going on because 7As should sound better. Check to make sure there are no cabinet leaks and scraping voice coils. The 7As are much older and therefore the electrolytic capacitors are more suspect.

ajt528 · March 1

Thanks, George. Has dual fuses. TBH, I’ve never had speakers with external fuses. I don’t even know what they are rated for. My first foray into “hi-fi” was my Polk RTA 8t’s so no external fuses. Rest of my surround sound set up is early 2000s Polk. I’ll check the 7A cabs and voice coils, The cabs overall seemed solid, but at the same time, I wasn’t looking for leaks. I honestly don’t know how to check voice coils, but assume there is a forum post somewhere around here.

Gardenstater · March 1

We ought to have stickies for checking for voice coil rubbing and checking for cabinet/gasket leaks.

To check for voice coil rubbing you can lightly and carefully press on the area where the dust cap meets the cone with 3 fingers and a thumb equally spaced as possible. Listen/feel for the scraping. If it is scraping you can try to find the quadrant that, when pushed there, doesn't produce the scrape but that would just give you an idea of where it is rubbing. The bottom line at that point is that it needs to be repaired or retired from service.

If I remember correctly, the tweeter fuse is 1A fast blow and the other one is 2.5A. Needs verification. I know for a fact that my single fused 7B has 1A for the tweeter fuse.

Checking for cabinet/gasket leaks is basically a matter of pushing on the passive radiator and holding it there and counting off the seconds that it takes for the Midwoofers to settle back somewhat after they are maximally displaced. Should be more than 3 seconds. My 7Bs are much longer than that though. The 7Bs have the MW6500 though and it is a felt dust cap. I believe the dust caps on the MW6502 (in the 7C) are more permeable to air.

ajt528 · March 4

Hey George, thanks again for the advice. Sorry took me a day to sit down and take a look at these where I could think through it and really listen. Cleaned the contacts first, and I could see a little corrosion. That did help a bit actually. They seemed to perk up. Still not as loud as the 5 JRs. Wouldn’t think it should be that way, but I dunno, maybe it is more speaker to drive so it won’t be as loud at the same volume? I checked the voice coils and the one midrange has some pretty apparent scraping sounds, so that may be part of the problem.

Flipping the speaker switch from 1 to 2, to 1 + 2, the 5 JRs are noticeably louder. When you add them into the 7a’s, the whole room fills more and things get clearer. Seems like the only thing the 7a’s add is the bass. So that’s where I’m at with them, but nothing seems insurmountable to fix. Thanks again for the guidance!

F1nut · March 4

The 5jr has a sensitivity of 91dB while the 7 is 89dB. Therefore, the 5jr will sound louder than the 7 at the same volume knob level.

ajt528 · March 4

Ah thank you. That makes sense now that you mention it but up until this past week or so, the only thing I’ve listened to regularly for the past 30 years is the RTA-8t’s. 😂 So that’s kinda what I know.

Gardenstater · March 5

Polk didn't report on sensitivity in the sales brochure that I was given when I bought my 7Bs in late Oct 1979. I don't know why they left that spec out but it has made me curious as far as what the actual value is so I did some research.

Official data direct from Polk would always be best. They didn't start doing the multi page informational advertisements in Audio magazine until the time of the 7C, but in Oct 84 they reported efficiencies: 7C = 91dB and 5jr = 92 dB. They reported the same in Oct 85.

As far as the earlier models (7A and 7B) here is what they reported to Audio magazine for the end of the years buyer's guides, FWIW and subject to human errors no doubt:

78 (7A) = 94dB, 79 (7B) = 92dB, 80 (7B) = 91dB, 81 (7B) = 91dB, 82 (7B) = 91dB, 83 (7B) =91dB, 84 (7C) = 91dB

I think the 94dB is highly doubtful for the 7A since it is basically the same crossover and is the same drivers as the 7Bs. They definitely settled on an efficiency of 91dB for the 7B and that is what I would assume is the proper spec. for the 7A.

So maybe the 7A is about 1dB less sensitive than the 5jr.

If you factor in that the electrolytics are 6 to 7 years older and the fact that ESR (effective series resistance) of electrolytics increase with age, maybe that is making the discrepancy of volume you are hearing even greater.

Also, you didn't mention if you did the cabinet/gasket leakage test or not.

Gardenstater · March 5

Of course, depending upon how much that one midwoofer is scraping, that could be producing audible distortion, adversely effecting clarity as you mentioned. At some point it would drop the output of the driver noticeably if it was really bad.

I'd put some adhesive on both sides of the magnet on the good one. The MW6500s are more prone to rubbing than later MWs because of the tight clearances. My recommendation would be to use what I used since it worked great and bonds tenaciously and strong at 1500 psi tensile strength. It isn't unremovable should the driver ever need work in the future and if you were so inclined. Gorilla Max Strength Clear.

ajt528 · March 5

Thank you for all the info. I tested for cabinet leaks as you suggested and the midwoofers both take over 3 seconds to settle back. I tested the voice coils again and maybe I was hearing something the first time because I actually don’t hear it this time.

It could be that I was just thrown off by the loudness difference between a larger and smaller speaker, not appreciating, as F1nut noted, that the sensitivity between the two is different. I do think cleaning the fuse contacts helped and evened the loudness out a bit.

But again, I appreciate all the guidance and input on questions that I’m sure are a regular occurrence on the forum.

Polk 7A Sound Problems, maybe?

Answers