stereo to mono procedure

[rider99]

Hi guys

I would like some help with this project please.
I have a stereo headset in my motorcycle helmet which receives audio from a portable 12v mixing device.
I wish to patch into the headset and take the stereo signal to the speakers and direct it into the mono mic jack of my helmet cam recorder.
I have a P3189 line matching transformer to bring the speaker signal down to mic level specs but my question is how do I get the full stereo signal into the mono mic input and still retain the channel separation for music in the headset.
I have seen a few suggestions for people asking about home stereo or high powered senarios but is this relevant for my low powered system.
Obviously Im looking for the simplist solution as I need to have the the solution incorporated in the helmet cam wiring and carried in my jacket.

Cheers

[Speedskater]

Rane Corp. has a paper on how to Wye "Y" stereo to mono:

Why Not Wye?

You could wire a stereo output jack in parallel with the stereo input jack.

[rider99]

Thanks for your reply.
Ive had a thought that I could buy another line matching transformer and run one on each audio channel. These P3189s are isolators arent they?

The circuit I have at the moment picks up the left channel + and runs it through a 100Uf 10V capacitor and then to one side of the P3189, the other side of the P3189 then takes the reduced isolated signal to the + of the mono mic input.

If I was to put another capacitor and P3189 on the right channel and take the reduced output signal and directly couple it to the left channel output from the other P3189 would I have a "complete" mono signal to feed the mic input concidering that I am doing the mixing after the isolation of the transformers? Or would I still need to use the Wye config?

[Speedskater]

The ETAL P3189 transformer appears to be a 1:1 voice only (not music) isolation transformer.

[wakibaki]

I'd arrange 2 opamp unity gain buffers into an opamp summer. You can connect the buffers to the L & R signals without affecting them significantly.

Unity gain buffers and summers are straightforward circuits you can find on google.

More-or-less any opamp will do, it's a fairly lo-fi app.

w

Power supply will complicate things a bit, dual rail would be easiest. What were you intending, to run off 12V?

[rider99]

I was hoping not to have to use anything that requires a power source as this set up is on an offroad motorcycle that it is quite posible the bike and I will depart company on occasions (hence the wish to video record moments of stupidity, hopefully other peoples)

The smallest solution with the least wires is what Im looking for.

The reason I thought about using the extra P3189 is that it doesnt require power, its small, its cheap and I could squeeze another into the box that houses the one I already have.

Why is the P3189 not good for music? As you can probably tell I know very little about this stuff.

As I said the mono circuit I have at the moment does work its just that the music sounds a very 'empty' which I assumed was just because I was recording only half the sound (one channel only). Could this be also that the P3189 will never let the music sound any good?

Remember I am not looking for music perfection as the external noises are quite high, its just that I dont want to listen to the recording playback and hear only one channel of the music.

Is it OK to couple the post transformer channels to make it mono?

Thanks

[Speedskater]

Why is the P3189 not good for music? As you can probably tell I know very little about this stuff.

It's a design trade-off. A wider bandwidth for music would increase:
Cost, weight & size. (cost being the most important)

[Pano]

Tho I'm not quite sure I understand, Edcor makes small, cheap stereo to mono transformers. EDCOR - PC6400

Maybe too big for you. Seems like you should be able to do it with just a few resistors.