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.
very simple. the amp output resistance is too high to the input of sub. in this case you should piggy-back your mains.
thanks, Marakanetz. unfortunately piggy backing the mains isn't really possible in the current setup - is there any way to reduce the output resistance of the amp?
try using cheap interconnects such as radioShack and see if problem's still there.
Hi Scott,

Are you saying that the hum occurs even when the sub is unplugged from the AC and the amp supplying the signal to the sub is also turned off? If so, are you sure that the hum continues to occur after several minutes have elapsed, when the power supplies in the amp and in the sub's internal amp can be presumed to be fully discharged?

Also, a way of implementing Marakanetz' suggestion about lowering resistance, without changing to the main outputs of the amp, would probably be to put a suitably chosen high power resistor between the + and - terminals of the sub inputs, for each channel. Let us know the make and model of the amp that is driving the sub, and I should be able to recommend a specific resistor.

Best regards,
-- Al
@ Marakanetz...
thanks for the suggestion - i've got some inexpensive IC's i can try, as well as a 'subwoofer' IC. i didn't bother messing with them as when i removed the IC entirely, the noise was unchanged. i will certainly try swapping a few to see if that helps.

@ Al...
as always, my sincere thanks for your inputs. yes, with the sub unplugged from the AC and the amp supplying the signal to the sub turned off, the hum occurs. in fact, it is quite a bit louder this way then when the sub is plugged into the AC and the amps supplying signal to the sub are turned off. turning on the amp supplying the signal to the sub eliminates the hum entirely.

the amp is a McIntosh MC7100. i am not sure how long a full discharge takes, but i can confirm that the hum does persist for at least 30 minutes after the amp is turned off. the amps have been off all evening, so i will check when i get home tonight if the sub is still humming.
It's been quite strange to this subwoofer since 2wq has 47kOhms input impedance. Don't know about 2w though which may differ.
Some of the 'directional' interconnects have their shield disconnected from the negative terminal so that's why I've suggested to use something simple and conventional.
Hi Scott,

Yes, that's certainly a puzzler. I see that the 2w series of subs is described as using some kind of feedforward error correction. Perhaps that is allowing small amounts of leakage between the AC line and the circuit ground of the MC7100 to find its way past the sub's amplifier to its driver. When the MC7100 is turned on, the low output impedance it has when powered up kills that leakage.

BTW, I see that the MC7100 was first released about 20 years ago. If by any chance it has a non-polarized two-prong power plug, I would try it with the opposite orientation from the one you have been using.

If it has a polarized two-prong power plug, temporarily try reversing its orientation using a cheater plug that has non-polarized (identical) prongs.

Beyond that, assuming that you are using the MC7100 in stereo (non-bridged) mode, for which it is rated at 100 watts into 8 ohms, purchase 2 of the following 100 ohm 20 watt resistors, and connect them between the + and - input terminals of the sub (1 resistor on each channel). I suspect that this will resolve the problem, with no sonic side-effects:

Best regards,
-- Al
Al, after having the Mac amps off all night, the sub is still making the hum noise.

the AC plugs on the Mac amps are 2 prong polarized. reversing them using a non-polarized cheater plug did not change the hum at all. one thing i did notice is that if i touched the banana terminals at the Mac amp, the hum got louder, but if i touched the banana terminals at the sub amp, the hum went away. not sure if that's meaningful.

there are inputs for both the left and right channel on the 2W sub amp, and i am only using one, leaving the other open. i switched to the unused input, but that didn't change the hum. i thought perhaps using only one input on the sub might be causing this, but according to the manual, when stereo 2W subs are in use, they are connected using just one input.

regarding the resistor - sorry for the rookie question, but do i just insert the leads into the inputs on the sub amp, then insert the banana terminals on the speaker wire along with them? the amp on the sub can only accept bananas. also, i assume since i'm only using one input, i only need to add the resistor to the input in use?

Hi Scott,

I hadn't been envisioning that the sub only accepts bananas. It looks like at least some versions of the MC7100 have screw type terminals, although I see that you referred to banana terminals on it. If the terminals on the amp are either screw type or multi-way binding posts, I suspect that if you connect a resistor there it would be as effective as if it were connected at the sub. Otherwise, you'll probably have to kludge up some connection arrangement, as I doubt that a resistor lead and banana plug would fit into a banana jack simultaneously, at least in a secure manner.

Also, another thing to try (first) would be shorting the + and - terminals of the unused sub input channel together. Although the manual seems to imply that leaving an unused input open is ok, I wouldn't consider that to be definitive for all systems. It may depend on the output impedance characteristics of the amp when the amp is in the powered down state, which are unpredictable.

Best regards,
-- Al
Al can usually sort this out, but if all else fails, try calling RV. If you're not asking him a stupid question, he's usually very helpful.
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?

i tried calling Vandersteen prior to posting - they are closed this week. will try again next week just to see what they think.
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
Thanks, Al... i'll give that a try as soon as i can. i really appreciate the help!

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?
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
ARGH!! i spoke to McIntosh today and they said a grounding wire should be connected to one of the screws on the bottom of the chassis of the amp. i went ahead and did this, connecting from a screw on the bottom of the chassis to the grounding screw on the sub and it did not change anything.

one thing i did notice that i need to correct from my earlier posts is that when the sub is unplugged from AC, the hum does in fact get louder, however, after 30 seconds it stops entirely.
Hi Scott,

I still feel that the things to try are the short on the unused input, and the resistor on the used input. Or, alternatively, a resistor on both of the inputs.

Best regards,
-- Al
Hi Al,

ABSOLUTELY! just trying some alternatives while waiting on parts.