New project! I have a Honda Goldwing motorcycle and it has a factory audio system that includes a headset jack and an AM/FM radio with an auxiliary input and a CB radio and intercom system. Way cool. Works like a charm.
Honda, however, does not believe that modern motorcycles should have a cell phone interface, for obvious reasons, but many riders are forced to the side of the road to answer the phone, which often means removing riding gear, helmets, etc. It's a major pain.
So...the bike's system jack is pretty straightforward, it likes a 600 Ohm dynamic microphone, and produces a maximum output voltage into two 75-Ohm speakers of about 3.5V AC.
The cell phone, on the other hand, at least the few I've tried, will easily use a similar dynamic microphone (tested using a voice recorder app on my Galaxy and HTC Evo, etc) as the bike. I've even used Thom Engdahl's 'converter circuit' to make a dynamic look like a condenser, and that works too.
It's really easy to safely split the helmet microphone signal to provide a signal (around 10mV typically) to both the bike system (direct input) and the cell phone (isolated input via transformer) using a good quality audio transformer as a 1:1 tap or even a 1:2 tap to the cell phone. Mouser sent me a variety of very nice ones and this part works great. I can speak in the helmet and hear myself since the intercom has sidetone, so you know you're speaking as well to the phone. Testing confirms I can do intercom, CB radio and cell phone voice recording, and hence outgoing phone calls this way. Cool there.
Here's the hitch: the phone has stereo media audio output, not sure the voltage/current level, but assume it's in the 18-20mW range for typical phone headsets. I'm thinking the incoming phone call audio part is also 'stereo' via two mono streams since my bluetooth and wired headsets play calls in both ears.
Problems come in when I combine the stereo audio output from the bike (which has a convenient mute function button on the handlebar) with the stereo audio output from the phone.
Since the phone uses a single ground for mics and speakers (using a TRRS 4-circuit plug/jack), but the bike has a separate ground line for the speakers (common gnd) and a separate ground line for the mic. The passive resistive network I used was the typical 10K passive resistor approach. That left a feedback loop that is more than unpleasant when I patched in the phone audio output!
My question is how to combine (actually mix) the two speaker-level audio streams which have differing grounds properly.
I considered transformers on the cell and bike outputs to lift grounds and then combine them with a resistive network, but the resistive loading seems wrong intuitively, and there's really no isolation once you wire them together.
I could possibly use a 4-input mixer to combine the two stereo streams into a single one, but I am not sure if op-amp mixers like you'd typically use on line level signals would do well with higher voltage and current levels that drive speakers, so really the question is whether anyone can recommend a 'speaker level' mixer approach. As long as the levels are inside half of 12 Volts, I could use opamps to combine the speaker outputs even if I have to drop the levels with negative gain, but not sure they can source the current needed, so an output stage is probably called for.
Thanks for any feedback, no pun intended...
Ed
Honda, however, does not believe that modern motorcycles should have a cell phone interface, for obvious reasons, but many riders are forced to the side of the road to answer the phone, which often means removing riding gear, helmets, etc. It's a major pain.
So...the bike's system jack is pretty straightforward, it likes a 600 Ohm dynamic microphone, and produces a maximum output voltage into two 75-Ohm speakers of about 3.5V AC.
The cell phone, on the other hand, at least the few I've tried, will easily use a similar dynamic microphone (tested using a voice recorder app on my Galaxy and HTC Evo, etc) as the bike. I've even used Thom Engdahl's 'converter circuit' to make a dynamic look like a condenser, and that works too.
It's really easy to safely split the helmet microphone signal to provide a signal (around 10mV typically) to both the bike system (direct input) and the cell phone (isolated input via transformer) using a good quality audio transformer as a 1:1 tap or even a 1:2 tap to the cell phone. Mouser sent me a variety of very nice ones and this part works great. I can speak in the helmet and hear myself since the intercom has sidetone, so you know you're speaking as well to the phone. Testing confirms I can do intercom, CB radio and cell phone voice recording, and hence outgoing phone calls this way. Cool there.
Here's the hitch: the phone has stereo media audio output, not sure the voltage/current level, but assume it's in the 18-20mW range for typical phone headsets. I'm thinking the incoming phone call audio part is also 'stereo' via two mono streams since my bluetooth and wired headsets play calls in both ears.
Problems come in when I combine the stereo audio output from the bike (which has a convenient mute function button on the handlebar) with the stereo audio output from the phone.
Since the phone uses a single ground for mics and speakers (using a TRRS 4-circuit plug/jack), but the bike has a separate ground line for the speakers (common gnd) and a separate ground line for the mic. The passive resistive network I used was the typical 10K passive resistor approach. That left a feedback loop that is more than unpleasant when I patched in the phone audio output!
My question is how to combine (actually mix) the two speaker-level audio streams which have differing grounds properly.
I considered transformers on the cell and bike outputs to lift grounds and then combine them with a resistive network, but the resistive loading seems wrong intuitively, and there's really no isolation once you wire them together.
I could possibly use a 4-input mixer to combine the two stereo streams into a single one, but I am not sure if op-amp mixers like you'd typically use on line level signals would do well with higher voltage and current levels that drive speakers, so really the question is whether anyone can recommend a 'speaker level' mixer approach. As long as the levels are inside half of 12 Volts, I could use opamps to combine the speaker outputs even if I have to drop the levels with negative gain, but not sure they can source the current needed, so an output stage is probably called for.
Thanks for any feedback, no pun intended...
Ed
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