Subwoofer Hum

Status
Not open for further replies.

lexmedic157

Member
Joined
Feb 4, 2009
Messages
191
Location
Lex, VA.
I have a low freq hum from the subwoofer in my car when I transmit on my UHF mobile radio. Just wondering how I can get rid of the hum. Antenna is mag mount on trunk lid. I don't notice any humming from any of the other speakers in my car. Curious.
 

pogbobo

Member
Joined
Jan 9, 2007
Messages
644
Location
VA
I used to have a 65W icom 2m mobile in my suv ... whenever I would tx at the full 65 it would hum/thump my subs pretty bad (scared the crap out of me the first time haha)

I cant tell you exactly why this happens, but I do know that when I decreased my tx power down to 25W or lower it stopped ... more than likely something to do with the amount of current being sucked out

fwiw, I did have a direct-to-battery connection for both the sub amplifier and the mobile radio, probably need some sort of isolation in the mix, i dunno

maybe the RR braintrust can shed some light on the physics of this
 
Last edited:

petnrdx

Member
Joined
Feb 5, 2004
Messages
253
Location
Sparks, NV
Many sub-woofers are powered direct from the battery (if they are high power), so even with the sound system "off" they will still hum or thump.
I have had this problem also (on several cars) especially with non-factory installed amps.
It has to do with the class of amp, and the need for LOTS of current on amplifying bass of any loudness when at VERY low frequencies.
A gross oversimplification of this is that many of the amps "modulate" the power supply output to "drive" the amp transistors.
Remember all you technical folks, I am oversimplifying the explanation here.
The current draw is high, but "on and off" with the freq of the sound, so at any given second, it may average low enuf to not fry wiring.
Because of that need and others, they usually connect direct, and they almost NEVER have any effective radio frequency filtering.
So what can happen is RF getting into the leads of the amp pass thru and become unwanted audio, or RF gets into the speaker wiring and "backfeeds" the amp, and you get rectification noise.
If lowering the TX power reduces or eliminates the "interference", that is a good sign that you could filter or isolate the RF getting in.
You might clear it up by doing any or all of the usual RF interference techniques.
Even just twisting the power lead to the amp or re-routing the leads away from the antenna or radio may work.
Sometimes just using duct tape to hold leads (power in or speaker out) tightly against the car sheetmetal parts will fix it.
Just think of all that wiring being like antennas everywhere, and try to make them be poor antennas.
Even clamp on ferrites at the amp may be worth trying.
Just depends on how bad the trouble is.
I had one Explorer with a really high end system that I had installed, with a 1 Kw subwoofer that I never could get the hum out of, so I just hooked up a remote relay to disconnect the power when transmitting.
 

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,131
Location
Point Pleasant Beach, N.J.
"Sometimes just using duct tape to hold leads (power in or speaker out) tightly against the car sheetmetal parts will fix it."
You must be from 4 Land.
"I had one Explorer with a really high end system that I had installed, with a 1 Kw subwoofer that I never could get the hum out of, so I just hooked up a remote relay to disconnect the power when transmitting."
Like I said, shut off the sound system.

All sound ideas (sic) and thanks for the lulz, please pardon me for pointing them out. (;->)
 

lexmedic157

Member
Joined
Feb 4, 2009
Messages
191
Location
Lex, VA.
So I have to revise my statement about what happens when I xmit. It's not a hum so much but more like a thump from the subwoofer when I PTT and then again when I release. during transmission I don't hear and feedback or noise. I have since put all the antennas on lip mounts. Same as above apply? Anything to put on the cables themselves to shield them? I don't really have the ability to duct tape them to the sheet metal.
 

CLynch7

Member
Joined
Apr 13, 2009
Messages
183
Location
North GA
You must be from 4 Land.
I wish I was in the land of cotton,
Old times they are not forgotten.....
Look away! Look away! Look away! Dixie Land.

Oh, I wish I was in Dixie, hooray! hooray!
In Dixie Land I'll take my stand to live and die in Dixie,
Away, away, away down South in Dixie,
Away, away, away down South in Dixie.

;)
 

BonziBuddy

Member
Joined
Oct 9, 2008
Messages
166
I have the same problem. There is nothing you can do, the speaker is unshielded. Take any handheld and tx near a speaker that is on... it will cause it to make noise. Transmitting on 2 meters makes the speakers hum/thump in the room directly below where my antenna is mounted. 440 doesn't do anything to them. Because of this, I always use the 440 inputs to my favorite repeaters.
 

k8mcn

Member
Premium Subscriber
Joined
Feb 10, 2005
Messages
241
Location
Old Monroe,MO
i had the same problem with an amplified speaker----seemed no amount of shielding would work. I ran a wire from the speaker power feed under the carpet over to a micro switch by the drivers seat to completely cut of the power--problem solved, unless you like to listen to rap while you talk on the repeaters :)


and if you think you've got a problem -here is mine----when transmitting on 20 meters from my truck, any time you press the horn after i have transmitted, the laser warning on the Escort detector chimes.....unplug the escort to reboot it problem gone--why does it do it? no clue and dont really care-just thought it was a prime example of the weird things RF can do.........and how the first time it happens you may have to change your pants, as i was doing 80 in a 55 when the usually "always right" Escort laser beeper went off................
 
Last edited:

lexmedic157

Member
Joined
Feb 4, 2009
Messages
191
Location
Lex, VA.
So it looks more and more like I just have to add another power toggle to it... Any known adverse reactions if I just leave it alone and deal with the thumping? It's not terribly annoying just wanted to know a way to suppress it.
 

OpSec

Code 21, Status 7
Database Admin
Joined
Dec 19, 2002
Messages
1,635
Location
Monitoring the database
Get rid of the mag mount antenna, move the antenna away from the sub, don't have the sub powered when using the radio?

All things that come to mind...
 
Status
Not open for further replies.
Top