@Frame The Mic/ext mic input is via IC4201 (20 pins) which has its own PCB. Maybe you can find out more about the IC4201's pin-outs. Sorry, I haven't opened up my GH2 to look at that IC. Maybe you can look up the GH2 photos to see what's written on the IC. [Edit 19 May]: larger diagrams added.

@DirkVoorhoeve

Looks like you've got the experimenter's bug! Please do be careful - think of your own safety first.

Always remember to discharge the FLASH terminals before poking around.

Next, try not to fry anything in the camera: take a good look at any PCB to make sure you're only messing with analogue circuitry, not logical, that way you won't find the camera defaulting into a "take me to a Panasonic service centre" mode.

Manuals, service-only parts and testing tools are available to licensed repairers. You just might find a friendly independent repairer in a small town ;-).

That said, to my eyes, (not a Panasonic repairer), what you're proposing looks do-able and I am looking forward to your results.

@svart

Thanks for your help. Updated pics in previous post. As I said, a look inside may reveal a proprietary name on what Panasonic calls " IC4201" and just possibly more info on what this IC does. There may be unused functions, we could use alternative ICs, etc.

@DirkVoorhoeve

If the camera is still open, can you take a closer look at the mic PCB and see what's written on the main IC? (I think it's the blurry pic below).

@DirkVoorhoeve, @Roberto: So the main IC you see in the pic is an NJM2747SCC, which is a generic quad opamp. The configuration is strange because they only use 2 of the 4 opamps in the package. If you look at the schematic on connector FP4201(right side), pin 1 is labeled as what I think says PW_REG4R8Y. It's still a bit hard to read though. Anyway, this looks like some kind of bias/low voltage phantom power as it goes through 3K3 resistors to ECM_LIN/ECM_RIN and is AC coupled(DC blocked) by C4201 and C4204. 4R8 would typically mean 4.8VDC. This would be enough for back electret mics but not true condenser mics. The NPN transistors Q4201 and Q4202 are common collector configurations, which is generally used as voltage buffers to drive low impedance loads and won't have much voltage gain. So in other words, the opamp does the gain and the BJTs do the driving.

Also: do NOT force 48V phantom into these inputs from an external source, there is no true overvoltage protection for the opamp, which is also fed from the 4.8V source and phantom could be forced back through the 3k3 resistors and into the IC.

@DirkVoorhoeve, yes, that's essentially how the circuit connects to the electret mic elements. The mics connect to the connector on the left side, are gained and buffered and then go out to the rest of the circuit through the connector to the right.

This is all fixed gain on this board. There is no AGC(automatic gain control) in this circuit at all, so I expect the 3 steps of gain are done digitally, or done through an IC on another board before being digitized.

It's possible that you can connect your mixer! I'm not sure what kind of levels the MIC_RIN/MIC_LIN need to see though but that should be pretty easy to figure out. Just start low and work upwards. It looks like the most the opamps can do is around 3-4vpk-pk in this situation at full gain.

IC9101 to AV out!?

I was sort of hoping the op amp might be switchable to comparator mode; ie if it senses a line level it bypasses its pre-amp so you can just send your own pre-amp signal through the mic terminals.

My Sony engineer friend says camera pre-amps are crackly because they typically use cheap, power-saving chips - and that you'd really have to be a Panasonic engineer to know what cameras' sophisticated circuitry does. Until we get one to talk to, we'll have to limit any testing to, say, analogue trials with the mic PCB on its own on the bench top.

I certainly wouldn't subject a perfectly good GH2 to a smoke test.

@kavadni, no, the unused parts are terminated to ground, likely right at the pads.

@Roberto, the preamp circuit is pretty simple. I've done a fair bit of work designing audio circuits before. The NJM parts themselves are pretty clean. The problem comes with the amounts of gain needed, which will cause any impedance mismatch or unclean signal/power from the mics to become huge amounts of noise after amplification. This is compounded by the unfortunate fact that audio is almost always an afterthought in cameras. I think the hissy/noisy nature of the audio in most of these cameras comes from the electret microphones. These are very good devices for fidelity and sensitivity, but are generally noisy. I've done some designing using these mic elements for pro-audio use and they are capable of extremely good sound but are not usable for low signal levels. Good for drums and electric guitars. Not good for voiceovers or background mics.

Now, you can modify the electret mics to get better performance. The most prevalent one is called the "Linkwitz" mod, after the famous speaker designer who came up with the mod.

Scroll down to "microphone" section and check out some interesting modifications: http://www.linkwitzlab.com/sys_test.htm

http://www.mp3forkidz.com/mic/mod.html

This would change the way the mics work and a lot of testing would be needed, but it could improve the overall sound of the internal mics of the unit, possibly.

Here's what's actually in the mic: http://www.pitt.edu/~szekeres/mic/wm63.jpg

Most important problem is Remote and Outside Mic work in same time. It will be nice..

@svart

the main IC you see in the pic is an NJM2747SCC, which is a generic quad opamp. The configuration is strange because they only use 2 of the 4 opamps in the package. If you look at the schematic on connector FP4201(right side), pin 1 is labeled as what I think says PW_REG4R8Y. It's still a bit hard to read though. Anyway, this looks like some kind of bias/low voltage phantom power as it goes through 3K3 resistors to ECM_LIN/ECM_RIN and is AC coupled(DC blocked) by C4201 and C4204. 4R8 would typically mean 4.8VDC. This would be enough for back electret mics but not true condenser mics.

One thing which keeps going round in my mind is the small print in the manual of my 1981 Radio Shack condenser-mic 33-1080 "1-12V!" In the place of a single AA battery recommended I fitted two x 6V batteries of the same diameter - and got a totally different, 12V mic!

OK, it's a long-shot, brute-force solution, but do you have info into the voltage ratings for the IC and op-amp inputs?

@svart

Thanks for looking into it. It's a more comprehensive answer than I'd expected but still makes me want to try something on the bench.

BTW, I just saw this GH2 top assembly for auction, (new). I can't see if there's anything in there besides the switches, tho..

http://cgi.ebay.com.au/ws/eBayISAPI.dll?VISuperSize&item=280917397638