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Sub Woofer Hum... This One Puzzles Me

i've got a low level hum coming from my subwoofer, and this one has got me scratching my head. the sub is a vandersteen 2w that's connected via high level inputs (the only option on this sub) from a separate amp channel (instead of piggy-backing off the mains). here's what i've found...

when the system is up and running, there's no hum. so, all the following applies only to when the amp that is sending signal to the sub amp is turned off.
as soon as i turn off the amp that is connected to the sub, the hum starts.
the level of the hum increases and decreases as i adjust the level of the sub (sensitivity on the vandy plate amp).
disconnecting the preamp from the amp does not eliminate the hum.
disconnecting one of the speaker leads at the sub amp does eliminate the hum.
unplugging the sub from the AC receptacle results in the hum getting appreciably louder.
using a cheater plug to lift the ground results in a similar increase in hum loudness as when the sub is unplugged.

any ideas?

the amps are plugged into a running springs audio ac conditioner, the sub is plugged directly into the wall. i was curious about plugging the sub into the running springs unit, but it's too far away.

is this just a normal ground loop? i thought in those cases a cheater plug diminishes the hum - in this case the cheater plug, or simply disconnecting the AC to the sub, resulted in the hum getting quite a bit louder.

any inputs would be appreciated.
srosenberg
srosenberg's avatar
srosenberg OP
209 posts
 
Al,
you are correct - the 7100 has the older Mac screw type terminals, but i have a set of spade to banana adapters that i'm using on the amp. is shorting the + and - terminals on the unused sub channel simply a matter of a short run of wire terminated with bananas inserted at the inputs?

Swampwalker,
i tried calling Vandersteen prior to posting - they are closed this week. will try again next week just to see what they think.
almarg's avatar
almarg
8,673 posts
 
Is shorting the + and - terminals on the unused sub channel simply a matter of a short run of wire terminated with bananas inserted at the inputs?
Yes :-)

It would be prudent to have both the amp and the sub unplugged when you are inserting it.

Best regards,
-- Al
srosenberg's avatar
srosenberg OP
209 posts
 
Thanks, Al... i'll give that a try as soon as i can. i really appreciate the help!

-Scott
srosenberg's avatar
srosenberg OP
209 posts
 
i spoke to Richard Vandersteen yesterday - he's completely stumped. he did say that shorting the unused input is not advisable and that adding a resistor would not correct the situation. his suggestion was to run a ground wire from the ground terminal on the sub amp to the amp or preamp.

is this just a matter of taking a piece of unterminated wire and securing it under the ground terminal on the sub amp on one end and any screw on the back panel of the main amp on the other end?
almarg's avatar
almarg
8,673 posts
 
Hi Scott,

I have no idea why Richard would consider shorting the unused input to be inadvisable, as long as the connection is applied several minutes or more after power has been removed. The sub would see the short as a zero volt signal being applied to it through a zero ohm source impedance. That is essentially no different than how it would see the output of a solid state power amplifier that might be connected to that input, with that amplifier being in the powered up state while no music is being played.
His suggestion was to run a ground wire from the ground terminal on the sub amp to the amp or preamp. Is this just a matter of taking a piece of unterminated wire and securing it under the ground terminal on the sub amp on one end and any screw on the back panel of the main amp on the other end?
Not sure. If the ground terminal on the sub is the same as the negative signal input terminal, or if it is connected directly to that terminal, and if the chassis of the amp or preamp is not connected directly to its circuit ground (for instance, if that connection is made through a resistor or inductor or capacitor in the amp or preamp), doing that would amount to an alteration of the grounding configuration of the amp or preamp, possibly affecting sonics. I don't think it would hurt to give it a try, though, PROVIDED that you are NOT running the MC7100 in bridged mono mode.

And if perchance you are running the MC7100 in bridged mono mode, it is conceivable to me that the hum problem might go away if you were to change the connections and the stereo/mono switch on the amp such that you are running in stereo mode while using just one channel.

Hope that helps. Best regards,
-- Al
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