NeoGeo MV1B Stereo Mod
MV1B Audio Schematic
Main audio
According to these Neo-Geo.com forum posts you have to remove some resistors to get a stereo line level feed out of the MV1B board. I don't think that's really necessary;
If you take a look at the application manual (Japanese) for the YM2610 you can see it needs an external DAC to produce any analog signal (apart from the noise channel apparently). The DAC on the MV1B is a YM3016, which then goes to the C844G opamp to pre-amp (Hmmm.. Seems to be configured as a unity gain buffer actually.. there are no resistors around the C844G, maybe the YM3016 can't drive all that hard on it's outputs?) the signal before it's feed into a mono power amp. The signals going from the DAC to the C844G opamp aren't mixed, they are mixed after they come out of the opamp,… so if you want you can break the circuit after the opamp (And loose the JAMMA output) and take the signals from there. You shouldn't directly tap the signals coming from the YM3016 as it seems the YM3016 can't drive very big loads; When I tried this the sound became distorted on both the JAMMA output and the tapped signal.
The solution is to build a small buffer from an op-amp (I used a NE3352) and tap this onto the output pins of the YM3016. I have uploaded the data sheets for the YM3016 and linked them below, you should be able to guess the pins you need to attach your buffer onto. Note that the labelling on the pcb silkscreen is wrong, someone at SNK can't count and added two extra phantom legs to the YM3016 (The chip has 16 pins, the markings suggest 18), the diagrams in the datasheets are correct just ignore the numbers on the silk screen.
Noise Channel
It it appears you can't just tap the noise channel and sum it into your stereo channels either. When I did this I got the noise channel out of my line level circuit but it distorted the output on the JAMMA connector, Maybe the YM2610 isn't up to driving it… So What I did was this; Built another set of buffers from another NE3352 and hooked these both up to the noise channel (I soldered onto the pins of connector that the cart-slot-board goes into..) creating two buffered copies of the noise channel. I then summed a copy of the noise channel onto to each of the main channels via some 4.7k resistors. This gives me sound coming out of the JAMMA connector and a stereo line level output. There is one draw back however; A small amount of the mixed signal (The one that goes into the power amp) leaks back into the noise channel and thus into the line level output. I found this out when testing the noise channels buffers, I didn't have the main channels mixed in but I could just about here them in the signals coming out of the buffers, my amp had to be cranked up to full blast to hear it though. I guess you could remove the SMD resistor where the noise channel enters the summing circuit, build yet another buffer, connect the input of the buffer where one side of the SMD resistor was and connect it's output via a small resistor to the other side. But the amount of signal leaking in is pretty small, and I don't really want to take my MVS apart again so I can't be arsed to do that. You probably don't need as many buffers.. maybe one virtual earth amp would do.. but I had loads of NE3352's around so I just built a buffer for each signal I wanted. :)
Modified Schematic
