AP74-HS34 causing noise on intercom when dimming

[Brantel]

I have noise over my intercom when I dim my system and the white lights to come on inside the HS34 and AP74.  The buzzing sound changes with different dimming levels.

Has this ever been reported and is there something I can do to reduce this noise being introduced into my intercom?

I used a common firewall ground point and normal shielding techniques when building my panel....

 

[dynonsupport]

Re: AP74-HS34 causing noise on intercom when dimmi

First thing is to find the conduction path for the buzzing. Are you using the audio out from the HS34? Does it go away when you remove that connection from the intercom?

 

[Brantel]

Re: AP74-HS34 causing noise on intercom when dimmi

I am using the audio out from the AP74.  Same difference I guess...

I will have to test to see if it goes away. Not easy to get too at this point.

 

[ChiefPilot]

Re: AP74-HS34 causing noise on intercom when dimmi

I am using the audio out from the AP74.  Same difference I guess...

I will have to test to see if it goes away.  Not easy to get too at this point.

Did you by chance ever figure this one out? I'm seeing (well, hearing) the same thing. Like you, I have a single point ground system and still have the problem.

 

[Brantel]

Re: AP74-HS34 causing noise on intercom when dimmi

No, it still does it...I don't do much night flying so the system does not dim much but when it does, it makes noise in the intercom.

 

[ChiefPilot]

Re: AP74-HS34 causing noise on intercom when dimmi

No, it still does it...I don't do much night flying so the system does not dim much but when it does, it makes noise in the intercom.  

Thanks - I still have my forward top skin off and so have easy access to everything so I'm going to poke into this a bit more. I'll post back if you're interested.

 

[Brantel]

Re: AP74-HS34 causing noise on intercom when dimmi

Thanks,

A filter cap on the power leads to the AP74 might do the trick.

Van's put a filter cap on the power leads of the servos for the RV12 to stop similar issues.

Let me know if you figure out a solution.

 

[ChiefPilot]

Re: AP74-HS34 causing noise on intercom when dimmi

Thanks,

A filter cap on the power leads to the AP74 might do the trick.

Van's put a filter cap on the power leads of the servos for the RV12 to stop similar issues.

Let me know if you figure out a solution.

I can report that a 2200uF cap across the power leads had little effect, but that wasn't entirely unexpected as it's a large cap and the frequency is probably too high for a cap like that alone to resolve...but that was all I had in the hangar. I'm going to try a couple of smaller caps in parallel - 470uf and 0.1 uF - to see if that blocks more of the noise.

More when I find out...

 

[ChiefPilot]

Re: AP74-HS34 causing noise on intercom when dimmi

So I hooked up the scope today to see what was causing the noise, and just as you'd expect it's a PWM signal apparently associated with the white lights which come on as the brightness is decreased via the EFIS. I found that when the lights come on, a 100hz signal of approximately 40mv is injected onto the audio out line. A similar signal can be seen on the power line as well. You could clearly see the duty cycle, but not the frequency or amplitude, changing as the brightness was adjusted.

A 2200uf cap in parallel with a 0.1uf cap across power & ground took the noise out of the power line, but the signal still remained in the audio line.

A question for Dynon - is this expected? It seems odd that anything other than audio would be injected into the audio output stream. My AP74 serial # is 1622 - has there been any changes to the design since then that might address this particular issue?

 

[Brantel]

Re: AP74-HS34 causing noise on intercom when dimmi

Just as I expected....Thanks for doing the investigation!

Sounds like half the battle is won with the caps on the power lines but not sure what to do about the audio line!!!

I also hear some high freq noise when the AP is turned on and it is definately coming from the servos. (no where near as audible as the dimmer noise) I suspect that a filter cap out there on their power leads at the servos will stop that problem.

 

[ChiefPilot]

Re: AP74-HS34 causing noise on intercom when dimmi

Just as
I also hear some high freq noise when the AP is turned on and it is definately coming from the servos. (no where near as audible as the dimmer noise)  I suspect that a filter cap out there on their power leads at the servos will stop that problem.

I had this issue as well. Another pair of 2200uf/0.1uf caps solved that problem, or at least reduced the noise level to the point where I don't think it will be noticable with the engine running. I guess I hold out a tiny bit of hope that the tantulum cap I ordered to build the real filter (instead of the ceramic disc I used to try it) will do better.

I'm looking forward to hearing from Dynon on the AP74 noise issue. If there is not a solution available from Dynon on that issue, the options are to live without the audio annunciators, put the thing on a switch input on my audio panel and enable it only during the day, or open the AP74 up and disable the dimming / backlight circuit. I suspect a notch filter for a 100hz square wave would have unacceptable effects on the rest of the audio so I don't think that's an option but I've been out of EE school and writing software for far too long to remember the details of why I think that