i know this might sound dumb but i am lost with this problem.... when i turn my heat seat on the subs start to hum.... not sure why... from what i read the grounds might be fighting for the lower voltage but i cannot figure out where to ground out the amp.... the problem occurs when u turn on the heated seats on the lower level... when i hit level 2 aka both light are visible the humm goes away... can anyone with experience with amps/subs help? thanks in advance....not sure if someone has already posted this but i have searched and found nothing....

 

Could you give us a bit more information on your setup? I am assuming you have factory seat heaters, and aftermarket subwoofers? If so, where are the subwoofers and amplifier located?

Perhaps something isn't electronically isolated well enough. How is the shielding on the cables to the amplifier? Do they run too close to the seat heaters?

 

heated seats are factory installed.... aftermarket sub and amp... 1 12" alpine type r, 1 rockford fosgate 750W amp more like 300w rms, and a capacitor... power cable is isolated and runs from battery through firewall and follows the carpet along the passenger side... the ground is located under the rear seats on a bolt... is there maybe a better place to ground the amp? note i have just replaced the headunit (which is aftermarket) so i don't think that is the problem...

 

Try running the ground to the battery, that's the way I've run mine, cleanest ground you can get.

 

You have determined the source of the hum, the pwm controller of the heated seats. Now you need to determine the path of conversion to sound in your sub. Three common paths are through the DC power system, through the air to the low level portion of the sub amp, and through the wires connected to the sub amp input.

A few minutes with an oscilloscope will tell a technician what path the undesired signal is taking.

Let's consider the DC power system. When the seat is operated at half power, the current is switched on and off by a transistor in the seat controller. Each time the seat is switched off by the transistor, back electromotive force is generated by the inductance of the seat coils. If the bemf is not properly damped by the controller, it can impress an AC current on the DC power lines. The most usual cause is a poor ground connection from the seat heater elements to the chassis. This creates a high impedence connection between the damper in the seat controller and the seat heating element. Fix is simple, pull up the teppich and tighten the ground nut.

Sometimes the ground connection is a good low impedence, but the hum is still audible. This is most often the result of a poor design in the sub amp. There is a lot of poor design out in the wild. The solution is a bit more complicated, but it boils down to isolating the seat heater controller from the DC supply in the car using an inductor and capacitor to block the bemf from the seat coils passing back onto the DC supply. It is also possible to install a similar circuit on the power input to the amp, but this is not as desirable as nipping the problem in the bud at the seat heaters. The reason is the current requirements of the amp are constantly changing with the intensity of the audio source. Any compoments selected to smooth the DC entering the amp would be like socks marked one size fits all. The guy with the big foot thinks his socks are too tight, and the guy with the little foot has to deal with a wad of cotton in the toe of his shoe.

Hum coupled into the amp through the air is one of those "black magic" arts that sometimes confounds sound engineering practices. Time honored solutions include moving the amp away from the source of the hum. The problem here is you must keep the amp with the car, and while you might know what system is creating the hum, but you don't know the pinpoint source. Sometimes separating an audio wire from a power wire in a harness by a half inch causes radiated hum to cease.

If the hum is coming into the amp on the input wires, it might be a poor installation. Many subs connect to the amp at high level by tapping into the speaker wires to the rear doors. None of the wires to the speakers should be connected to ground. If the installer connected the speaker common wires together, or the sub amp connects them together, that can be the problem. There are a lot of sub amps that don't use proper instrumentation preamps (or equal in a DSP) and they should not be selected to be used in a Mercedes car. You must not have the speaker common wires connect to ground in the sub amp. In some cases, the speaker current might have the AC component of the seats on it, but because of the relative low impedience of the speakers, they do not respond to the hum. The sub amp, however, has a high impedience input, and the signal is multiplied for the subwoofer to the point it is audible. In this case the solution above to place an inductor and capacitor on the seat controller input will work the best.

Best of luck to you!

 

With this information, I must make an editorial comment. RF amps are like a nest of hungry canaries, "Cheap, Cheap, Cheap, Cheap."

How is your power wire isolated? Isn't it connected directly to the battery? This is where the heated seats get their current from. Isolation requires a couple of diodes, or inductors and capacitors. If the hum is travelling down the power wire to the amp, your capacitor by itself is not helping you. It is acting as a charge pump. This is transferring the hum into your amp. If you think of the pwm transistor output of off and on as a digital signal, the charge pump converts this to an analog signal and dumps it into your amp. Running the wire under the teppich next to a seat is probably not the best routing. I would have preferred to use the door sill channel for cables.

What to do?

Try connecting the sub amp to the fat # 8 wire on the rear fuse panel in the trunk. Plenty of current to run a Rockgut Fuzzgate amp there.

Try moving the ground wire to the ground point nearest the right wheel arch in the trunk.

Replace the stock head unit, and connect the amp using the high level inputs. I am assuming the amp has them, so you can connect the speaker wires to the amp input.

Purchase an audio isolator. This is a couple of audio transformers that take a line level output from a head unit connected to the metal in the dash, and isolates the output to float above ground. Don't pay more than $ 20. Most have RCA jacks and plugs.

If your sub does not have a tuned enclosure, consider an Infinity Basslik II system. It has a passive radiator, is supplied with tunable controls, and has a class D amp that thumps with the best of them.

 

Great advice Moviela - this should be placed on a sticky!

 

Excellent writeup. The section above is my initial suspicion. If the amplifier input wires (or a portion of those wires) are unshielded, stray RF can enter the system. It is then stepped up by the amplifier, resulting in an annoying hum. It could be as innocent as the speaker wires (amplifier input) coming out of the back of the head unit, which happens to be close to the heated seat switches.

 

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