HDX and SV-COM-T25 audio summing resistors

[John Bright]

The audio panel I am installing, GMA 245, has inputs for audio alerts and for failsafe audio alerts. The failsafe audio alert input shorts to com 1 audio when in failsafe which is power off or audio panel failure.

Garmin mentions using "summing resistors" in series with the com and alert audio sources. Com 1 is SV-COM-T25. Alerts come from Dynon HDX.

Someone on VAF mentioned there are resistors internal to HDX in series with audio alert outputs. If SV-COM-T25 audio out also has a resistor built in I suppose it would be OK for com 1 audio and HDX audio to short together.

Does anyone here know this?

 

[Rhino]

The HDX audio output is adjustable by presets, so there are varying resistance levels offered. I imagine that's what was being referred to at VAF. As to whether or not this resistance is in serial, and is a suitable mixing source, I'd defer to Dynon on that.

The T25 output is not variable. So, to my knowledge, there is no inline resistance, and a summing resistor may be needed. Your issue will be equalization. Without knowing the various resistance levels out of the HDX, you'd be guessing at the resistance level for the T25 to get balanced equalization. That would likely just result in different volume levels rather than presenting a problem though, so you could experiment. Personally, I'd use a potentiometer rather than a resistor, that allowed me to adjust as needed to get what I want. 0-10k should work.

You might also want to consider a mixer, so summing resistance won't be an issue. That's what the GMA 245 uses internally to combine audio when it's functioning normally, but obviously that's not available if the 245 is down. I'd suggest looking at the AMX-2A or the AP-60 mixers. Those have adjustable outputs, and are designed for aircraft. The designer of the AMX-2A is also a member here (Vern).

 

[John Bright]

Thanks Rhino,

I understand what you're saying about equalization.

I don't understand when you say T25 output is not variable... there's a volume knob on SV-COM-PANEL. That's OK, I decided to KISS and not use the GMA 245 failsafe alert input that shorts to com 1 audio in failsafe mode.

Now I see in the the Skyview System Installation Guide:

• "SkyView’s audio output cannot be connected in parallel with a radio to a headset. Doing so will significantly reduce the volume of the radio output, possibly to the level that it is not usable. An intercom such as the SV-INTERCOM-2S, audio mixer, or audio panel is required to utilize this output."
• I imagine one could do a work-around with resistors or a mixer but since Skyview has visual alerts and visual AOA I'm gonna say if audio alerts are lost it's not a problem.
• And I found the paragraph called "SkyView Audio Output Settings" where as you mentioned alert volume can be set.

In the Skyview Pilot's User Guide I found... Table 16: Messages, Alerts, and Audio Alert Details. I have not flown behind Skyview so things are new to me.

 

[Rhino]

The output from the T25 is not variable. The output of the SV-COM-PANEL is. But you didn't mention you were using an SV-COM-PANEL, so I assumed you were connecting the T25 directly to the GMA245.

The key phrase in that sentence is "to a headset". A headset has no mixer, and can't equalize. You can send HDX and radio audio to separate (non-failsafe) inputs on the GMA245, because it has an internal mixer. This makes failsafe no longer a factor since you aren't connecting the HDX to the failsafe input, which is probably what I would do.

 

[John Bright]

Thanks Rhino.