Bose 501 Series IV pops at high volumes....HELP!

Hey all,
I just purchased these old 501's yesterday for $30. Both speakers sound really great with my Pioneer......until I reach a certain volume, then they make an ear piercing popping noise. I don't think the receiver is at fault because it works fine with other speakers. The pop seems to be just coming out of the woofers.
They don't really act like they're blown to me (at least I hope they're not). They have a really nice and clean bass response. I've taken the woofers out and noticed that the circuitry contains what looks like a light bulb. I assume this is supposed to help absorb power in order to protect the speakers (just sorta guessing). I've noticed this "light" doesn't light up even after putting quite a few watts through it.......maybe it's not supposed to? Just curious if either that or the caps could possibly be the culprit. Any help is GREATLY appreciated!
These sound so much better than my Kenwood KL-888Z speakers that I had been using.

the bulb if im not mistaken is protection for the tweeters, ive had woofers do that before, they were bottoming out, check the spider, have they ever been re-foamed or even re-coned? pics may help others who may have experience or opinions.

Thanks for responding! I'm really not sure whether or not they have been re-foamed or re-coned, however the pop sounds like an electrical pop as opposed to a bottoming out sound. The speaker looks to be in pretty good shape. The foam and the cone doesn't look damaged at all.....no tears or separating anywhere that I could see. The speaker apparently had something spilled on it, so the pic really brings out the stain...lol. I haven't pried off the front cover of the other speaker yet to check it out.

The marks on the edges of the foam are just dark spots where something got on it....everything is in tact......looks worse in the pic than it is.

i had a pair of infinity bookshelf speakers once,the guy i got them from said he re foamed them but i found out later that he did not shim the voice coils and they made a popping sound at high volumes,it could be that.

Ok, thanks! If that's what it is, I may just have to retire these things. Is there a way to troubleshoot to see if that's the problem?

Tomorrow, I'll try them out on another stereo and see if it still happens and I'll post what I find out.

How powerful is your receiver and just how far are you turning up the volume ? Past 12 o'clock ?

ehc68 said:

Thanks for responding! I'm really not sure whether or not they have been re-foamed or re-coned, however the pop sounds like an electrical pop as opposed to a bottoming out sound. The speaker looks to be in pretty good shape. The foam and the cone doesn't look damaged at all.....no tears or separating anywhere that I could see. The speaker apparently had something spilled on it, so the pic really brings out the stain...lol. I haven't pried off the front cover of the other speaker yet to check it out.

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What series? Should be on the terminal plate. Look like II's but can't be sure.

These buggers CAN tax a low power amp/receiver. I believe mine say 4-ohm on the back and they WILL draw that. So, if you are running an amp that is NOT rated down to 4-ohm (clearly marked or says so in the manual) do NOT continue using that amp, you will damage it. You could actually be driving the amp (receiver) into clipping.

A voice coil banging against the back plate of the magnet assembly can make a terriblly loud sharp snap noise. My guess would be slightly stretched spiders. A good shop can change them but good used woofers from eBay would probably be cheaper.

That was my thought, if there is nothing wrong with the speakers it may be the amp going into clipping.

The light bulb is designed to light up only when high power is going through it. A filament changes resistance with temperature, so it works well for that.

To see the light bulb you would have to have the speaker open which changes things for the woofer, particularly if the cabinet is not ported and is totally sealed. Avoid more than a little bit of volume if you have the cabinet open. In which case you really won't see the filament light up.

But I assume this happens with it all sealed up as well, which points to the amp.

mikeybc said:

A voice coil banging against the back plate of the magnet assembly can make a terriblly loud sharp snap noise. My guess would be slightly stretched spiders. A good shop can change them but good used woofers from eBay would probably be cheaper.

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I was recently playing my Altec A7's loaded with 4 ohm woofers using an amp that does not even have a published output rating for 4 ohm loads. I wasn't cranking it but well before 12:00 on the volume, I got that slap. When an amp goes into clipping, it can essentially put out a DC voltage which can pull or push the woofer cone too far. When music is superimposed on top of that, Whap! (This may not be a 100% technically correct explanation of clipping, but the point is, it's bad bad bad for both speakers and amp).

ehc68 said:

Hey all,
I just purchased these old 501's yesterday for $30. Both speakers sound really great with my Pioneer......until I reach a certain volume, then they make an ear piercing popping noise. I don't think the receiver is at fault because it works fine with other speakers. The pop seems to be just coming out of the woofers.
They don't really act like they're blown to me (at least I hope they're not). They have a really nice and clean bass response. I've taken the woofers out and noticed that the circuitry contains what looks like a light bulb. I assume this is supposed to help absorb power in order to protect the speakers (just sorta guessing). I've noticed this "light" doesn't light up even after putting quite a few watts through it.......maybe it's not supposed to? Just curious if either that or the caps could possibly be the culprit. Any help is GREATLY appreciated!
These sound so much better than my Kenwood KL-888Z speakers that I had been using.

Click to expand...

OK, one more thing. These are "sealed" speakers. Make damn sure they are. Lightly push on the woofer, there should be a lot of resistance. If not they are bottoming out due to an air leak. This is possible, especially if the woofers had been pulled and not re-sealed correctly. This is quick and easy to remedy.

zebra03 said:

How powerful is your receiver and just how far are you turning up the volume ? Past 12 o'clock ?

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I'm using a Pioneer SX-1050. I believe it's rated at 120 WPC. I'm not cranking quite to 12'oclock , but probably close. They are Bose 501 Series IV's and it says it's 8 ohm. My amp doesn't clip with my Kenwoods but maybe the Bose pull more from the amp.

I've experienced this in car audio applications only.

IMHO, the voice coils in your woofers are somehow damaged and a transient 'dead short' condition is being created. The potential for amp damage may be present.

ehc68 said:

I'm using a Pioneer SX-1050. I believe it's rated at 120 WPC. I'm not cranking quite to 12'oclock , but probably close. They are Bose 501 Series IV's and it says it's 8 ohm. My amp doesn't clip with my Kenwoods but maybe the Bose pull more from the amp.

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Nice receiver. Definitely capable. Sorry, me error, should have caught the IV in the title. Ooops.

Did you check for a good seal with the cabinet?

While checking the cabinet make sure there is a foam gasket between the woofer basket and the front of the box. Some speakers use putty there but most have a thin foam gasket. There should be something.

If cabinet sealing is not an issue, do you have a meter? Check the DC resistance of each woofer. They should be very close to each other, and probably below the nominal rating of the speaker. 8 ohm impedance woofers often read about 6 ohms DCR.

I heard what he's talking about. It's more of an electrical popping noise than a mechanical slap. It's as if the voice coil shorts out. But I don't know what the windings would contact to cause it. I guess the magnetic material could be conductive (?)

BigElCat said:

I heard what he's talking about. It's more of an electrical popping noise than a mechanical slap. It's as if the voice coil shorts out. But I don't know what the windings would contact to cause it. I guess the magnetic material could be conductive (?)

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Ceramic magnets are not conductive. What you describe can still be the VC slamming the magnet pole plate. I had this happen with a couple of my speakers when used for HT duty. Once I switched them out for speakers better suited for the bass heavy work the problem went away. No change in any other equipment.

However, for arguments sake, there is a remote chance you could have a bad lead. If the wire from the VC going to one of the leads has severed it could make intermittent contact at higher volumes. How? Many leads are held in place with epoxy/adhesive to prevent them from rattling about and ultimately breaking. Imagine sufficient flexing of this wire, in the adhesive, and a break accuring. I've had something similar happen, and drove me nuts. At lower volumes you can have contact, where the two pieces make contact so long as motion is limited. But add motion (flexing of the cone) and you get a temporary separation. Problem is, I've noticed this as cutting out and not popping in sound. More often than not the popping I've heard from my speakers has been VC bottoming out.

As for the bottoming out, I've only had this issue with bass reflex speakers and passive radiator speakers. None of my Air Suspension speakers have done this. Of course, I've never subjected any of my smaller (less than 12" woofer) systems to HT duty. But I did this knowing they couldn't handle the bass from such an environment.

As for the leads, from the cone to the driver terminals, those too are know to go bad eventually. Similar to my explanation above, and when I've had such a situation each time there was a cut out of sound, but no popping (that I recall).

Maybe time to measure the DC resistance of the voice coil and compare it to what a known good woofer reads. A partially shorted coil could still work well at low volume but cause the amp to clip at higher volume.