[heatmizer]

Hi need some help with a cell phone to motorcycle intercom cable.

I need to dc isolate the output of the cellphone to the input of the intercom and am unsure how to do it.
Should i use a capacitor, a capacitor and resistor, an audio transformer?

If i use capacitor what value would be good. like .22 film and 100k resistor like a cmoy. Looked at output capacitors on various amps and it seems have much higher values. I guess that is because of the higher current flow and lower impedance

 

[mono]

The intercom has some impedance value you would input into an equation (see "RC Filter Weakness 1" section on Tangent's Input Capacitors webpage.) to determine the rolloff vs capacitor value. If you don't know the impedance (no specs on it), you might measure resistance between input and ground as it probably has a resistor in the circuit there. Maybe even a coupling cap already or had you tried or examined it and found it did not?

Try with just a cap first.

 

[heatmizer]

Looks like all the equipment, radio ,cd player, and cb has output capacitors. The intercom amp does not have input capacitors. It does have output transformers
trying to find info on the amp itself but the chips are potted,

 

[mono]

What is the intercom driving, a speaker? I'd suspect it has a lower input impedance than a CMOY headamp, would need a larger value capacitor. If we took a conservative figure like 50K from an LM386, with a 0.2uF coupling cap it would result in corner frequency around 16Hz. It may not be safe to assume 50K input, measure that with a multimeter. If it were 50K I'd shoot for larger than 0.22uF though, as large as affordable and still small enough to fit in available space while staying with a film instead of 'lytic cap. That's without adding any resistor yourself.

 

[heatmizer]

the intercom is driving 2 75ohm headsets
I get 127k from input to ground of the amp

 

[heatmizer]

The intercom amp has 2 trimmer pots on the input that is where the 127K reading came from. did not adjust them to see highest value. So if i use a 0.22mF film I should be good. Filter would be at 6hz

 

[mono]

Based on available evidence I would say yes, try it and see if it's satisfactory.

 

[heatmizer]

gave it a try with the .22 film still haveing problems. the .22 cap works but still have alot of noise it must be from the uneven grounds in the circuit
check with the ipod to neither it nor the phone are happy running into the (I think) single supply Intercom. Btw this is a 8 year old autocom intercom in a 94 goldwing the original intercom was replaced at somepoint in the past. I think I am haveing ground issues cellphone and ipod seem to be virtual ground and the intercom seems to single supply. I dug into the epoxy of the intercom in some places power in to headset ground did not find a virtual ground type circuit.

 

[AndrewB]

Saw this earlier today. Thought I'd pass it on.

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