[duaned]

2005 Durango Ltd with premium sound.

I upgraded my head unit a couple months ago and there's a buzz coming from the speakers. Interestingly enough, it's only on the dash and right-rear speakers.

Still using the factory amplifier.

It happens when muted, with the media player as well as with broadcast modes so it's not coming in through the receiver.

It only happens when the lights are on (parking and/or headlights)

At first I thought it was the LED lamp upgrades (I had done them about the same time) but it continued even after unplugging all the LEDs.

There's a 150 Hz pulse on the power wires running to the head unit so I tried powering the head unit (both always-on and switched power leads) from a separate mains powered power supply. The buzz is still there.

Adjusting the dash light dimmer varies the tone of the buzz slightly. If I unplug the "Cluster Battery Feed", fuse cavity 10, the buzz goes away. It looks like it's a PWM signal coming from the instrument cluster.

I'd greatly appreciate any suggestions on what to try next.

Thanks

 

[radioactive]

I think you already answered it. It's likely the dash light dimmer PWM signal. When your lights are off, the dash lights are on 100% duty cycle, no noise. The PWM is getting into your amplifier inputs, at least some of them.

The PWM may go off-board to dim other cabin or dash lights outside of the cluster.

Check your radio chassis grounds. If anything is floating the noise can get in easily. There is no mitigating EMI noise at that low frequency using filters.

Possible that you routed your audio wiring too close to a dash PWM signal. Also make sure that your head unit outputs are compatible with your amp inputs.

 

[radioactive]

Also -- worth a try -- plug in your stock head unit, verify you haven't inadvertently opened a connector or ground in the system. Are there any adapters between your new head unit and your factory harness? Should there be?

Lots of guys bypass the factory amps when installing new systems, if the new unit has it's own amp.

 

[duaned]

The head unit is using the factory wiring and factory amp. It plugs into the same connector as the stock radio.
All connections made by me were soldered and covered with shrink tubing. I suppose there could be a bad crimp in the adapter cable.

The dimmer circuit uses dedicated wiring (for the cluster) and non-dedicated wiring (for door mounted switches, environmental control and dome lighting), Unfortunately the FSM doesn't say what wires are used.

I guess it's time to break out the o-scope again and check the rest of the wiring going to the head unit. I had hoped it was coming in on the 12vdc supply as it could be eliminated, or significantly reduced, with a simple RC filter.

 

[radioactive]

Well you can't really filter a DC power supply with an RC filter, as the R ends up in series with the load and induces a voltage drop. You can use an LC filter. But I'm thinking that your problem isn't on the incoming power -- you already confirmed that.

I mentioned the input compatibility because I recall some discussion about floating/isolated inputs versus ground referenced inputs or some such thing -- I'm not really up on the automotive aftermarket audio scene.

If you don't find a wiring error, then clearly your new head unit has created a scenario that allows the PWM noise to creep into the amp inputs. Maybe it's an impedance mismatch. That's why I recommended trying out your old OEM radio as a test.

 

[duaned]

I meant LC.

The noise is definitely coming from/via the new head unit. It's fine with the stock unit and the noise goes away when I disconnect the amp inputs. I went over the wiring yesterday and it is all correct.

Adding a ground-loop eliminator / filter between the head and amp made no difference.

According to the FSM the factory system uses floating grounds and there's a warning that grounding any of the speaker leads could lead to damage to the "radio". It seems to often refer to the head/amp combination as the "radio" so I'm not sure if that also applies to the amp inputs.

Unfortunately there is very little info available about the factory system.