Help for Amplifier/buffer circuit design

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humbug

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Joined 2010
#1
Hello everyone.

My name is Brendan and I have been have some difficulties with designing an audio system.

I have come from gaming forums, but couldn't find enough help in regards to my problem.

My project is that I am trying to get digital audio from my old Sega Megadrive. I have done the same mod on other systems, but that was much easier than this.

Everytime I learn abit more, I dig a little deeper into something I don't understand. I need guidance from someone with experience.

anyway, enough talking and onto the problem:

I have posted the same problem on maybe 4 other forums but to no avail. The origional thread: SpritesMind.Net :: Log in

if you read into that thread, I have considered many ideas and havn't had much luck, until... I posted on the sega-16 forums.

Thanks to Jorge Nuno from Sega-16 forums

Link:
Help MD1 audio w/ mCD & 32x buffer~amp circuit design (digital audio) - Sega-16 Forums

The output impedance of the YM2612 is 1kOhm or more IIRC, you really should use a voltage follower/or a non-inverting amplifier configuration.
I don't remember it's output level, I could try to measure that if you want, but I need some program to set it's channels to the maximum level, or write the full scale to the DAC (I don't know much about the YM, software side, so don't count on me).

The PSG, on the other hand, is around 6dB (you wanted dBs, you get dBs ), and has like 1kOhm too.
Looking at the MD schematics, you'll see that the audio is mixed together with relations of 0.0431 for the PSG, 1 for the YM and 0.0468 for the external audio sources (both 32x and CD)

Assuming these figures are correct, (I hope they are ) you get that the output of the YM is around -27dB below PSG.

What I did with my audio amp was tapping all signals before going through the electrolithic capacitors (and removing those caps, and everything after them, resistors, the sony amp, everything), Amp'ed the YM outputs by 28, and mixed them with the straight PSG and Ext audio, "without" amplification (actually there's a trimmer to adjust the PSG level ), then added an output stage with a voltage follower, then removed the DC offset with 1500µF capacitors to the headphone jack.

Also you either need a polarization voltage of Vcc/2 as a gnd reference or use symetrical supplies for the opamps.


About the ADC filtering, a first-order low pass filter at 61kHz should be enough. It will give you 40dB attenuation for the 6.1Mhz replica. This means that the spectral components around 6.1Mhz (+/- 61kHz) will have a maximum of 7mV (ignoring noises and crap)
Click to expand...

Anyways...

I just drew up a quick schematic... but I need someone to check over it.

here is what I came up with. Note: the total output voltage is not allowed to exceed 6.7v otherwise the signal will dump to the ADC overflow to gnd.


please post a markup

Kind regards

Brendan

An externally hosted image should be here but it was not working when we last tested it.
Click to expand...

And thats how its gone from conception on racketboy.com... quite the journey. >_<

Anyway... this is going to suck abit of time for anyone who wants to help, so I appologise for that in advance.

Basicaly I need someone to mark up my latest sketch and recomend an amplifier ic chip and possibly a better ADC + a better spdif transmitter chip (CS5361 and CS8406).

I am looking forward to reading all the nitty gritty (and usefull) posts!

Cheers

Regards

Brendan
 
#5
Hi Brendan,
I glanced at this when you posted it and I think the reason your not getting replies is because the question is far too long and complex to sort through, and of very limited appeal 🙂
Perhaps break it all down into one off questions and post those.

VQ looks like signal ground to me, looking at your fig 24 CS5361 it can't be anything else if using split supplies, and VCC/2 if not.
 
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