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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,

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
srosenberg's avatar
srosenberg OP
209 posts
 
@ 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.
marakanetz's avatar
marakanetz
5,473 posts
 
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.
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
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