Genesis mode 1 Audio Mod

[blackevilweredragon]

Ok, because putting an amplifier into an amplifier is a bad idea, and creates a buzzing sound, I want to try this modification to my model 1 Genesis..

I imagine the CXA-1034 is an audio amplifier, correct?  Now, if I just snag the audio from pins 3, and 6, would I get "line-level" audio, that's safe to go into my amplifiers "Game" input? (or the TV for that matter?)

or would it be messed up?

http://cgfm2.emuviews.com/img/mega3.gif

[ido8bit]

I can't find a datasheet for the cxa1034, but looking at the schematic pins 3 and 6 are tied together so they are not the input.

Try pins 1 and 8 (I think the number is hard to read, the pin that leads to R43). This would take the signal just before summed to mono, not sure about the level though as the mono signal is buffered through the LM358 before arriving at the mono output.  


The LM358 is a dual opamp.  If the other half is not used elsewhere in the circuit it could be used for the other channel.  This is probably the better approach if you don't need to keep the original mono output.  Disconnect pin R43 from pin 2, connect it to pin 6.  Connect pin 5 to pin 3.  Connect an additional 33k resistor from pin 6 to pin 7.  Connect the negative side of an additional 10uf capacitor to pin 7.

Disconnect pin 8 of the cxa1145.  Connect the positive side of C30 to the audio jack for one channel.  Connect the positive side of the additional capacitor to the audio jack for the other channel (the schematic doesn't specify which channel is left or right, it just has channel a and B).

This is off the top of my head, so recheck the schematic and lm358 pinout before building anything in case I've made any errors.  

[GZeus]

That hum's coming from something else.
The output from the Gen's phones output is pretty weak for a phones out, so you wont get clipping(if you did, you can turn it down)
The ouput impedence is going to be fairly low, but the difference between that and line level is relatively small(and again, the slider would fix that).

The only difference between line and speaker level is output level and impedence, so...

I'd try plugging the genesis into another outlet. There's probably a ground loop going on.
I had the same problem with my turntable, plugged it into a different outlet, and the problem was gone.

[blackevilweredragon]

oh the hum is without a doubt coming from the genesis..  if i plug in headphones directly into the headphone jack, with no volume, no hum, but with volume, there's hum..  this is on a stocked unit too...  it's coming from it's amp..

[GZeus]

oh the hum is without a doubt coming from the genesis..  if i plug in headphones directly into the headphone jack, with no volume, no hum, but with volume, there's hum..  this is on a stocked unit too...  it's coming from it's amp..

Like I said, try using a different outlet. What you said has no bearing on whether or not it could be a ground loop.

If I have a ground loop, and turn my guitar amp's volume knob down(which is pre power amp and post preamp) I get near silence. When i turn it up again, I get the hum again.

What you're describing has nothing to do with the fact that you're running through another set of op-amps.
You can run op-amps for days any the only reason noise would be introduced is there's alot more stuff in a longer chain.

I know analog gear. It's my speciality.

[blackevilweredragon]

the humming happens no matter where you are, even if it's being powered by a UPS..

the humming is the audio amplifier...  this isn't no 60Hz hum..  this is a general hum found ONLY via the amp..  when the audio is grabbed from the Sega CD audio out, there is NO hum at all...

[GZeus]

Then...YOU HAVE A GROUND LOOP of some kind.

or your ground is bad in the jack and isn't making contact to the plug.

'General' hum?
What's a 'general' hum? Hum has at least one pitch. If it's more than one, it's bad shielding or some kind of dying transistor.
Maybe you fucked something up inside it during a mod. It's possible that a wire is now located along an audio line.

In any case it's not because the amplifier's THERE, it has to be because something's WRONG/poorly shielded, etc.

Your logic is flawed.
Op-amps do not inherently cause hum.

[blackevilweredragon]

explain how the buzzing gets introduced AFTER the amplifier then...


especially if this is on a STOCK genesis, an unmodified unit..  and all that i have touched, DO this...

[NFG]

Keeping it civil, right?  Good stuff.

[blackevilweredragon]

i hope it can remain civil..

lawrence, you got a model 1, right?  when i plug in loud headphones into it's jack, and crank the volume up loud as it can go, even with no cartridge, don't you get the hum too?  every genesis model 1 i've used, does just this, and it doesn't seem to be a ground loop, because it happens right after the amplifier..

[NFG]

I don't have a Gen1 anymore, but I do recall the behaviour you describe.  The Saturn does it too.  It's a ground loop.  On the Navi Saturn I could eliminate the effect by using the headphone port, which seems to have better grounding.

[blackevilweredragon]

I don't have a Gen1 anymore, but I do recall the behaviour you describe.  The Saturn does it too.  It's a ground loop.  On the Navi Saturn I could eliminate the effect by using the headphone port, which seems to have better grounding.

if it's a ground loop though, why does it happen only after the amp for the headphone jack?  the AV out on it has no humming at all..

if it is a ground loop, it's not one i normally experience in the music studio, and i apologize...

[blackevilweredragon]

ahh, it IS a ground loop..  I only noticed this AFTER recording it on studio equipment (mono recording, amplified 3x normal volume)

after i turn OFF the Genesis, notice you hear mexican music..  so it IS..  i am sorry..  now, how do I bypass this with line-level sound?

http://blackevilweredragon.spymac.com/hum.mp3

[GZeus]

explain how the buzzing gets introduced AFTER the amplifier then...


especially if this is on a STOCK genesis, an unmodified unit..  and all that i have touched, DO this...

You're completely ignoring/not understanding me.

There's noise being introduced somehow, either a bad ground(Jacks can wear out pretty easily, sadly), ground loop that doesn't exist in the SCD or something else is going in inside. IE a broken/loose part or noise introduced by a mod.

I don't know why you though of something being 'after' the amplifier.
It could be at, before or after it, so long as it was before the potentiometer.
The ground loop has not been ruled out, it's just less likely.

[blackevilweredragon]

please read above, i apologized because it was a ground loop..  but it was happening post amplifier stage..  the amp may be going wonky..

[GZeus]

ahh, it IS a ground loop..  I only noticed this AFTER recording it on studio equipment (mono recording, amplified 3x normal volume)

after i turn OFF the Genesis, notice you hear mexican music..  so it IS..  i am sorry..  now, how do I bypass this with line-level sound?

http://blackevilweredragon.spymac.com/hum.mp3

This mexican music you here is an indication of RF interference or an odd bad connection elsewhere, not a ground loop.
Unless you have mexican music somehow being sent to the ground of something connected to your Genesis in some way...
That would mean the hot lead from something playing the music was connected to a ground point.

Generally inductors are what become unintentional antennae, not sure what's going on here.

So, while your conclusion may be correct, your method of reaching it was incorrect.

[blackevilweredragon]

there's an AM station 15000 a city over...  they play mexican music non-stop...

EDIT:  im gonna say the 15KHz channel number is just a coincidence..

[GZeus]

EDIT:  im gonna say the 15KHz channel number is just a coincidence..

Are you being ironic/sarcastic?
Because that's a perfectly logical course to follow.

[blackevilweredragon]

EDIT:  im gonna say the 15KHz channel number is just a coincidence..

Are you being ironic/sarcastic?
Because that's a perfectly logical course to follow.

well, being there's no TV active during that recording, and you only heard the mexican music when the Genesis was OFF, not outputting the 15KHz scan rate, it's a little fishy to be THAT...

[Segasonicfan]

I did some work on this in the past with a model 2 and model 3 Gennys.  From what I found, the audio is generated in the main Sega CPU and fed directly to the LM358 op amps which bring it to a useable line level output.  Just isolate the outputs and they should be fine.  If not you can always use another LM358 (or better IC) to increase the amplification.

-Segasonicfan

[blackevilweredragon]

I did some work on this in the past with a model 2 and model 3 Gennys.  From what I found, the audio is generated in the main Sega CPU and fed directly to the LM358 op amps which bring it to a useable line level output.  Just isolate the outputs and they should be fine.  If not you can always use another LM358 (or better IC) to increase the amplification.

-Segasonicfan

well heres the thing, the model 1 has above line level, cause it goes to the headphone jack..  so i think i need to go before that stage..

[ido8bit]

The CXA1034 is the headphone amp IC.  This IC was originally designed for use in walkmans, so it's not exactly hifi (10% THD).  This IC is designed to take it's input directly from a tape head, it's input is not a line level signal.  

For a line level output split the signal before it is summed to mono at the input of the LM358 and take the output from LM358 as described above.

[blackevilweredragon]

oh ok, cool...  i hope this mod works out..

[Epicenter]

I did some work on this in the past with a model 2 and model 3 Gennys.  From what I found, the audio is generated in the main Sega CPU and fed directly to the LM358 op amps which bring it to a useable line level output.  Just isolate the outputs and they should be fine.  If not you can always use another LM358 (or better IC) to increase the amplification.

-Segasonicfan

The 'Main Sega CPU'? Audio is generated in the GLU chip (PSG) and the YM2612 (FM). The Z80 processor is more involved in audio most of the time than the 68000 is unless you mean passing along the occasional PCM sample.

[blackevilweredragon]

I did some work on this in the past with a model 2 and model 3 Gennys.  From what I found, the audio is generated in the main Sega CPU and fed directly to the LM358 op amps which bring it to a useable line level output.  Just isolate the outputs and they should be fine.  If not you can always use another LM358 (or better IC) to increase the amplification.

-Segasonicfan

The 'Main Sega CPU'? Audio is generated in the GLU chip (PSG) and the YM2612 (FM). The Z80 processor is more involved in audio most of the time than the 68000 is unless you mean passing along the occasional PCM sample.

I can't find the 2612 on the model 2...

[Epicenter]

It's integrated into the large custom SEGA ASIC with ID# prefix 315-xxxx. The Z80 is discrete but the YM2612 is not.

[blackevilweredragon]

ok, thanks, that's what i figured..  cause I saw the Z80...

(crap, so that must mean it's exactly like I thought, 3rd party 2612, that might be why it's not as clear as the real one)

[blackevilweredragon]

i JUST got around to doing this mod, and ran into a big problem..

I passed those audio lines to the EXP connector, and now CD audio is TWICE as loud..  as usual, i found out why..  because now CD audio is going BACK into the Sega CD out it's audio connectors..

how do pass the audio to the Sega CD without it's own Sega CD mix?  do i use diodes or something?

EDIT:  I never did get to say that the Genesis sounds did NOT work..  they were there, but VERY weak..  Also, with them wired up that way, the headphone jack and TV out, no longer worked (audio of TV out), without cranking the volume..

i might have to use the op amp, but i still want the functioning mono out (don't ask, lol)...   i might just purchase that LM chip on ebay or something..

[Joe Redifer]

Maybe try eliminating the audio inputs from the Sega CD to the Genesis through the expansion port?  Not the ideal solution, obviously.  Also, try plugging one end of a random cable into the mixing jack on the back and see if that changes anything.

[blackevilweredragon]

Maybe try eliminating the audio inputs from the Sega CD to the Genesis through the expansion port?  Not the ideal solution, obviously.  Also, try plugging one end of a random cable into the mixing jack on the back and see if that changes anything.

two problems..

first one, if i do that, then the headphone jack has no CD sound..

second problem, mixing cable=forget about it completely, this is to make the Genesis 1 act like the Genesis 2, without needing, or doing anything crazy with the mixing jack...

i see the contacts, they are there, sega just never implemented them (they are literally not connected on the model 1 genesis)

[arfgh]

what contacts you mean ?