Woofer induced hum?

[marting]

I'm experiencing an odd problem. I've got 1980 Heresys hooked up to Bottlehead Paramour monoblocs and a Bottlehead Foreplay preamp. The pre and the amps are unmodified. There is normally very little hum, a little, but not much. I hadn't listened to this system lately for one reason or another. I got a chance to listen this weekend and found more hum than I'm used to. I shut everyting down and checked all the cables and tube sockets. It still hummed. I pulled the grills off the Heresys to try to get a better listen. The hum seemed to be comming mainly from the left side. I gently tapped the cone of the woofer on the left and the hum dropped down to the very low level that is normal for this system. It didn't come back during that listening session but returned the next time. Again, gently tapping on the left woofer made the abnormal hum go away.

Anyone got any ideas about what's going on here?

[Daddy Dee]

I'm not sure what is going on.

Wasn't reading that you had swapped your speakers L to R. If the new Left speaker still hums, it is something with the amp or tubes. Swapping the tubes could tell you if the problem is a particular tube. If the hum follows the swap, there's the problem component.

BTW, I'll bet the Heresy's sound sweet with the Bottlehead pre and poweramp.

(If one can call the power )

[Erik Mandaville]

This might suggest a loose connection on the woofer, itself, although we can't be certain of that. I leave the amps and preamp alone for the time being, and perhaps take a very close look at ALL connections related to the drivers, particular the woofer. Of particular interest might be the connections from the output of the crossover to the terminals on the woofer. These probably consist of crimp-on connectors, and one of them may be very loose.

Let us know what you find, and good luck.

Erik

[WMcD]

That is strange.

I have some strange theories.

Probably the amp is putting out a hum. You can check that by rewriring the left and right speaker feeds.

Theory 1 . . . maybe you have a partially damaged voice coil. It could be hanging up from time to time and you don't hear the hum when it is hung up. The problem with this is that tapping would be expected to free it, and not put it back in a bind.

You can check. With fingers spread, try pushing in gently on the cone by 1/8th inch. You might find a bind or just gritty-ness. It will go back to the normal position as quickly as the other one does.

Theory 2 . . . you have an air leak. When the leak is sealed, the resonance of the system is such that the woofer does reproduce the hum. But with the leak, system resonance is low enough to not reproduce the hum. This might show up as a less forceful return to normal because the spring of the trapped air in the box is lacking. Knocking it seals the leak.

Theory 3 . . . you have a loose connection, maybe in the tinsel leads from the terminal block or at the terminal block. The woofer does respond to the hum but knocking it breaks the connection.

There is some chance the hum is being induced into the inductor in the woofer crossover, and that is the source. Still, I'd think something else, per the above, is turning it on and off.

If it is an induction issue, the switching the amp leads will not help. But you can disconnect the leads from the amp and short them. That will make a path for the induced hum which otherwise does not exist (except through the amp). With the amp off, the output transformer may have enough inductance to choke off the induced current.

Other than that, I'm scratching my head.

Let us know.

Best,

Gil

[Erik Mandaville]

As William says, it's even possible to 'massage' the motor, where you may have a slightly binding or loose voice coil. If this were the case, though, distortion would probably be heard in addition to hum -- more like scraping or scratchy sounding noises. If just light tapping improves the hum, I wonder if there is some problem or issue with very fine voice coil leads. This would have nothing to do with the actual connection on the terminals of the driver, but would be inside the motor area and difficult to reach and repair.

[Scp53]

Erik, that was my first thought too.

[marting]

I've swaped cables and tubes side to side with no change. I'm pretty sure its in the left side speaker. I'll try to get some time to open it up tonight and check all the internal connections.

Thanks for the input so far!

[D-MAN]

Wow! that IS a strange one!

I was thinking about what could possibly do that, so here's my conclusion...

the problem "goes away" when the woofer cone is "tapped" -

I would say that it has nothing to do with the woofer itself, although it could be the wires are touching the back of the cone essentially rubbing or scraping which I suppose could create a "hum". Look for that first.

Second, in lieu of that being the problem, I would go for something loose in the cabinet. If the woofer cone moves from being "tapped" the resultant change in air pressure causes something to either reseat, reconnect or move out of the way. Check for internal insulation being out-of-place or something rattling around inside the cabinet.

Third, I would go for a connection problem, although I don't know exactly how that would cause excessive humming. It could be a oxidized connection, though. It would add a deal of resistance to the circuit and who knows what the audio ramifications of that could be but it can't be good.

That's all I can think of.

Good luck, and let us know what you find.

DM