New 12 Volt Amp = Buzz in AM Sound System

I'm not sure it will help, but maybe an inductive choke in the 12vdc power supply would clear up ignition noise and the like from the vehicle's system. My recall is vague and old, but I think I did this once with success. FM is great when you're close to stuff, but AM still has a real home on the open road. Good Luck!

May be horizontal orientation of whole system can make it work, I remember the time when we use to Orient small AM set on our ears for the cricket commentary. And BBC news.

Filter can only work if there is voice+noise, If there is only noise filter will give you nothing.

What frequency is the "buzz"? You have a bare ant. wire touching the amp?

Engine on or off. Trailer attached to vehicle?

Hi 4123:

Make sure the integrity of the antenna wire was not compromised, that its shielding is grounded, and try to slide-in a sheet of foam or other insulation between the amp and the antenna wire, just to provide sepparation.

Yahlasit

I would expect the source of the AM interference to be the class "D" amp. To begin to trouble shoot the how I would first disconnect the antenna from the tuner and turn the amp and tuner on. If the buzz is still there (I don't expect it to be) then the buzz is coming in through the power supply and you can apply the appropriate (I recommend a pi network) filter. If the buzz is gone then it is being radiated from the amp and is being picked up through the antenna. At that point the only options are to shield the amp and or to move the antenna away from the amp using shielded cabling. I had a similar installation several years ago with a power amp using a voltage noisy doubler. We moved the antenna to the roof/ceiling using coax and a balum. The buzz was reduced enough to make the installation usable for AM reception. I hope this helps you to at least isolate where the buzz coming from.

I too suspect that the EMI is getting coupled into the tuner through the antenna. The question then becomes how does the the EMI energy from the amplifier get into the air itself. If the amplifier and its chassis is so poorly designed that it is radiating itself then an extensive redesign of the amplifier is required. So while there's nothing that says that this cannot be true, there is no easy fix because the four or five meters one can get from relocating the antenna placement will not significantly reduce the energy entering the antenna. I suspect the coupling to free space is actually the incoming power wire is acting as a radiating antenna, not a current path into the tuner's power supply circuitry. This is why I recommend the placement and minimum value of the inductor on the power supply line. The common practice of having a widely distributed chassis return path and a single supply current path makes coupling to free space plausible. The other possibility of the speaker wire path coupling to free space seems very unlikely to me for the speakers themselves will be a very inductive load and thus a high impedance to the 500kHz minimum of the AM band. In contrast the battery will be a very low impedance to 500 kHz and thus high frequency current will flow easily along the length of that possible radiating path.

If, as I suspect, the amp is radiating then, depending upon what the chassis and cabinet is made of the answer could be as simple as grounding them both properly after moving the antenna. Grounding and propagation are factors that can be overlooked easily.

Often, when installing auto power amps, the use of "Stiffening Capacitors" is required. These usually range from 1 Farad to 30 Farad, (not uf). A 400 watt amplifier, running on a 12 volt power source uses a step-up switching power supply to create the needed voltage for the power amp. This may be causing the AM noise. The suggestion of chokes is valid, and don't forget to ground each unit separately. Also, close proximity of the amp to the tuner, or antenna/cable can cause the problem.

You mentioned there is no way to separate the 2 units more than 3 inches. Try an extra grounding strap between the units, chassis to chassis, ground loop problems have a way of disappearing this way. Next, how far from the battery source is the system, and what guage power wire did you run?

Don't forget to try the capacitor, it's an industry standard, mount it as close to the amp as possible.

I think that you have somehow lost a good aerial connection to the tuner, its switching into maximum "receive" and picking up some interference from somewhere, that it would not have picked up if a good strong AM signal was available.....

I assume the buzz was not there before when you could receive AM stations and its still not there on FM.

Check your aerial out or take a length of wire as a temporary aerial, but make sure that it can go outside for a meter or three....

Adding to the fine comments above another possible noice path is the amp output where all the chopping happens.(The power line is somewhat filtered.) If using far positioned speakers temporarilly connect them with short heavily twisted wires and see if noice reduces. Another remedy is to replace the cheap inductors in the cirquit with (no-gap) torroids, but this will ruin the looks of the amp. Using bigger output filter capacitors will also reduce interference but will ruin sound. Tell us how it goes. S.M.

Wow! I'm overwhelmed with different answers. I suppose I should have added the fact that both the tuner and the amp are mounted on a wood cabinet. The only ground used is the ground connection on both components going to the negative side of the battery via a long wire. Should I run a ground wire from both chassis's to ground as well?

Be careful that you do not make any ground loops, and that you have not already made one, as that could be the reason for the problems you have....

Grounding must be done in a "Star" configuration only....grounded signal cables can be a problem as well....stereo connections where the grounds are connected together can be a problem as well.....if you run large grounds, you are often better off in cutting the ground from a signal cable in the equipment at the "far" end......

Its an involved subject and rather difficult to cover well in a blog......simply no loops and cables only grounded at the "near" end.....

Let's get more specific about this situation. You have a HARD switching device at frequencies centered at about 650 Khz and upto more than 1Mhz (That's what the TC2000 which is the switching modulator on your board does). And you expect to cleanly tune AM ~(540KH-1,6Mhz) on a tuner 3 inches appart. The interference path can be from supply, from analog input and from the strong radiated field from before, through and after amp's output low pass filter. Your only chance is to control all of them, plus not allowing antena input to cach any transmission near the tuner. The last one can only be accomplished if the tuner is metal-enclosed, it's input is single-ended and good shielded wire is used, grounded only at the tuner side. (This will probubly weaken FM reception.) If you're still interested come back. S.M.