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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.
128x128srosenberg
almarg's avatar
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almarg
9,670 posts
 
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:

http://www.digikey.com/product-detail/en/HLW20R1Z100R0JF/HLWB-100-ND/713741

Best regards,
-- Al
srosenberg's avatar
srosenberg OP
209 posts
 
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?

thanks,
Scott
almarg's avatar
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almarg
9,670 posts
 
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
swampwalker's avatar
swampwalker
5,138 posts
 
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.
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.
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