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PostPosted: Sat Sep 05, 2015 6:55 am 
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Joined: Sun May 24, 2015 6:16 am
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A problem with audio on the gpm-02 is that original audio come out too low when mixed with cart audio. The audio circuit on gpm-02 vs earlier revs differs in the R7 value of 43K (instead of 100K).

Whats the best way of adjusting to the right level?

I am planning to add a slide resistor of 100K (physically limited to max at 70K ohm) in line after the 43K resistor so that the value would span from 43K to 113K.
Another way to adjust would be to change the values of R4 and R5 coming out of the cpu.

I don't even know yet if my initial plan will work, but it seems logical that it will.
By changing R7, am I in that way lowering the impedance of the circuit and if so, could it be harmful in any way?

Couldn't find any mod on this issue (it seems to me that the general discussions on mixing audio regards either the NES and powerpak or changing the cart level itself).


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PostPosted: Sat Sep 05, 2015 8:17 am 
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Joined: Sun Jan 22, 2012 12:03 pm
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Location: Canada
It is normal to use resistors like that to adjust the level of audio mixing. Replacing it with a trim pot should not cause a problem; it should adjust volume, though, I believe this resistor is feed back on an inverting amplifier, so higher resistance will actually raise the volume? (You might want a pot that goes a bit higher instead.) The signal might clip/distort if it becomes too loud, but I don't think that's harmful to the circuit, just the sound.


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PostPosted: Sat Sep 05, 2015 8:27 am 
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Ok. I can see that the volume rises as I lower R4 and R5. R7 on the other hand sits across the 40H368. My guess is that the ic works as a line driver and resistor sets the amplification. How that ic work as an amp i don't tough (if so) and if messing with impedance can result in echoes or other worse stuff..


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PostPosted: Sat Sep 05, 2015 8:29 am 
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Joined: Sun May 24, 2015 6:16 am
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Man, I can't read.. That's what you wrote. Thanks!


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