Woofer Disttortion

[davet753]

I ran across something new this afternoon, and was wondering if anyone else has had this happen.

A few months ago I restored a 50 watt (25 per channel) Magnavox Astro-Sonic console stereo. I ended up replacing every single #%*@ electrolytic capacitor in the stereo, and even the non-polarized cap in the crossover. The stereo sounded great, except for a very slight distortion in the woofers on certain passages. It was very dependent on the type of music being played. For instance, a string bass seemed to make the distortion more noticeable, or even some voices. A male announcer on a local station I listen to has a pretty deep voice, and I seemed to notice a little muddy "buzz" when he spoke. I hooked up a pair of vintage University Diffusicone 12 speakers to the external speaker jacks, and there was no distortion at all.

I read an article somewhere online last night about distortion in old woofers. The guy who wrote the article said that on some of the late 50's-early 60's AlNiCo woofers, the cone will start to "sag" a little bit and cause the voice coil to rub slightly. He claimed this was more common in the larger size speakers (mine are 15"). The article claimed this causes a muddy, distorted sound.

Just for the hell of it, I took mine out, flipped them upside down, and re-installed them.....low and behold, PROBLEM SOLVED! Not only did the distortion disappear, but the speaker sounds a little brighter (to my ears).

I have never heard of such, until now. It does make sense, though. A big, heavy cone sitting there for 50 years with gravity pulling it down could eventually cause some settling. I suppose the spider could also start to loose it's form a little and contribute to the problem. At any rate, I guess turning mine over has pulled it back to center just enough to stop the distortion.

You learn something every day

[Username1]

Good story, I'm going to have to remember that.....

I guess humidity over the years could also have contributed to it as well....

[Paul Knaack]

Yup, definetly going to file that one. Thanks

[old_coot88]

Dave,
Before reading halfway thru your post, the very first thought was voice coil rubbing. And sho'nuff it was.
Hadn't ever encountered it caused by cone 'sagging under gravity' before.

You learn something every day.

Yup.

[Reece]

Yes, I had heard of this trick for sagging speaker cones. Wonder if it would work on people? I'll go try standing on my head for a while...

[davet753]

In a way, I can't believe turning the speakers upside down could solve the problem...but it has. I have a Teac reel-to-reel hooked up to it, and have been listening all afternoon.

[Reece]

I've seen it used even on little table radios. Just think, after another couple of decades, you can just turn the speakers back over again!

[old_coot88]

There's the old tried-and-true "thonk test" for a rubbing voice coil. You unmount the speaker, hold it up by the rim, and thump it smartly on the back of the magnet with the ball of the other hand. The cone should emit a clear, resonant "thonk!". Any trace of fuzziness indicates rubbing.

One caveat - if the output xfmr is mounted on the speaker, be sure to disconnect one side of the vc from the secondary. Otherwise the loading will prevent a clear thonk.

[DavGoodlin]

That's an interesting one old coot 88. I have to try that.

I have a gently-used 1969 Zenith console with 10" woofers, and a spare pair of the same exact speakers from a similar console I scrapped. Which was used pretty hard by a friend of mine, it was his parent's Zenith and rocked many house parties etc.
I subbed in the hard-used speakers and there was a similar, but not always obvious, degradation in the sound. You can also feel the voice coil rub with your thumb*.
I assume the gently-used ones are not rubbing.

* Gently, using your thumb to push (in about 1/4 inch) on the coil, usually covered with a felt disc.
Any rubbing usually is transmitted to the cone as audible and you can feel it too.

[old_coot88]

The advantage of the thonk test is that it avoids any misalignment such as might occur from manually (thumbually?) pushing on the cone.

[Robb]

Good info. I may give this a shot on my Granco FM Radio.

[Reece]

I've always used the thonk test when choosing speakers new or used. As well as telling about the voice coil rubbing or not, it shows the speaker's resonant frequency giving an idea of the bass response.