Line Level Stereo Output on Genesis 1

[NCrouther]

I am planning on doing an s-video and rca jack mod on my Genesis. The mods I see done seem to all use the headphone jack as the source of the stereo sound. Is this the only way? I would prefer to have the output not be controlled by the volume slider. Is it possible to tap the signals before they go to the amp and route them to my own (fixed-gain) amp? Any help/advice is much appreciated.

[crazzywolfie]

http://www-unix.ecs.umass.edu/~dhowland/mod/
this website might be exactly what you looking for.

[NCrouther]

Thanks, but that site uses the headphone jack for the stereo sound, which I am trying to avoid.

[jb143]

Can you use Audio output from IC-13?

That would be pin 9 of CXA-1145.

I've never messed with it or anything, I'm just going by this schematic I found here:
http://emu-docs.org/Genesis/mega2.png

EDIT--
Tracing the circuit, it looks like PSG, pin 95 on the main processor, is the root of the audio output. I'm not sure if that will be a useable signal to you though. You might also want to try after the amp, IC-14 (or audio in of IC-13).

[NCrouther]

Thanks for the schematic, it is very helpful. It looks like the IC-13 audio output is the mono signal going to the DIN, but IC-12 pins 11 and 14 look to be a good source for the signal.

This gets me much closer to the goal. Now I have to determine what FB11 and FB12 are (they appear to be the volume slider) and with what to replace them for fixed-level output. Does that symbol mean variable inductor?

[jb143]

I wouldn't mess with anything on the output of IC-12 if you don't want the volume knob affecting it. If you want stereo I'd try to 2 inputs to the amplifier IC-12 because that's what would be miing the 2 signals to mono. Pins 1 and 8 of IC-12 would probally work.

[NCrouther]

Ok, thanks, I must be wrong about the volume slider. Is the amplification conrolled by IC-12 with VR-1 as the slider. That would make sense based on what you have said. Unfortunatly, it makes things more difficult as I have to mirror what happens to the signal inside the IC (hopefully just amplification).

[jb143]

That's what it looks like. The mono part is mixed at IC-14 though. It's just a standard op-amp audio circuit. Here's a simple schematic of a 3 channel mixer.
http://www.hobbyprojects.com/operational_amplifier/images/opamp9.gif

This might be helpfull though...
http://en.wikipedia.org/wiki/Yamaha_YM2612

That's IC-11, the actual audio chip with left and right outputs. The only thing is that it looks like output from IC-8 is being mixed in with one of the signlas.

I would say that your best bet would be to tap right before R42 & R43. Run those signals through caps and to their own pre-amp. It may not be the exact same L&R signal comming off the headphone jack but it'd probally be close. Otherwise you may have to find out exactly what IC12 does.