Native IDE Optical Drive Audio...

[Castellen]

can I just connect the audio out from the board to the audio out RCA connectors on the miggy without blowing anything up? Will that even work?

That wouldn't work well because the output impedance of the A1200 audio stage is 1k Ohm and the CDROM drive line output (not the headphone jack) will be something in the order of 10k.  i.e. you'll get practially no CD sound out, but you won't do any damage either.

The better way would be to connect the output from the CDROM audio to the A1200 op-amp U15 as a summing amplifier.  Without knowing the output impedance of the CDROM drive I can't do the maths to give you the correct levels out, but as a start you'd essentially do this:

Code: [Select]


CDROM L   >-----[680]----||-----> U15 pin 13
                        4.7uF

CDROM R   >-----[680]----||-----> U15 pin 9
                        4.7uF

CDROM Gnd >---------------------> A1200 audio GND  



You'll probably need the capacitor or you'll upset the DC bias of the op-amp, unless the CDROM drive output is AC coupled, in which case you won't need the capacitor.  No harm in including it anyway.  If you use a polarised capacitor, the + side obviously goes towards U15.  The DC bias voltage on this is +2.4V, so any capacitor voltage you pick will be fine.  e.g. 16V

The resistor value of 680 Ohm is a bit of a guess, based on the output impedance of the drive being fairly high.

Connect the CDROM audio ground to the A1200 analogue ground.  The - side of C303 or the + side of C304 would probably be convenient.

Mount the four components on a small piece of veroboard or something to keep it tidy.  When it's tested, put a piece of clear heat shrink over the entire thing.

[Castellen]

I know the miggy audio is real hot

If the analogue audio stage is getting hot, you have a major hardware problem.  The op-amp U15 will dissipate a few milliWatts of heat at the most.  Also, the output power of the audio stage will be in the order of (2Vsq / 1k Ohm)= 4mW at the most.

If you're talking about the 8364/Paula being normally warm, that's just due to the relatively high quiescent current of the device due to the technology used at the time.  It doesn't dissipate any more or less heat while the digital to analogue converters are being used.

[Castellen]

LOL! I meant the miggy audio is real hot as in LOUD at low volume.

Noted  I don't think it's loud at low volume however, it's a line level output of about 0.75V RMS, which is pretty standard.  The software volume control works normally and gives expected levels out.  I have an audio tone generator here if you want to play around with it.  Keys 1-9 set the audio output level.  If low volumes are still too loud, then whatever you have receiving the audio is probably expecting a different range of audio.

Makes a guy wonder why C> didn't add a small circuit and input pins like that on the MB (they thought of everything else!).

Mainly due to the fact the A1200 was never intended to have an internal CDROM drive.  Would have been cost related as well.  By the time you put in all of these little extras "just in case" into the design, the final price ends up being higher.  It's not just the cost of the components, but also it's another thing that needs to be tested during production.

I mean, did the A3K/A4K have a proper on moherboard preamp to mix CD audio with? If so, maybe an A3K/A4K schematic, or even a CD32 schematic, would provide proper circuit values.

The audio stage in the A4000 is quite different as it has a dual op-amp following essentially a copy of the A1200 audio stage.  This was added as a summing amplifier to essentially do what I described in the first post.  You can't really make your A1200 do this without modifying the hell out of it and having nasty add-on bits, which would be especially ugly.  But the information I gave you is probably the easiest, tidiest solution.  Don't get hung up on the resistor values, if my estimated value ends up with audio way too loud, just increase the resistor by a factor of two or four and keep going until you're happy.  It's a summing amplifier, not rocket science.  Or you could use 10k pots instead, but personally I wouldn't bother.

Ian's mixer will work, but from a quick glance at the design, it's intended to *replace* the Amiga audio output and probably be mounted in a desktop or tower case.  What you want is to mix put audio from CDROM drive onto the A1200 standard RCA connectors, right?  In this case, this isn't what you want.

[Castellen]

I haven't looked at an A1200 motherboard in a while, but there is an audio input on the trapdoor slot. 146=audio gnd, 147=audio left, 148=audio right.
 
You might be able to trace them to a point where you can solder something in.

Unfortunately not, because these lines essentially connect to the RCA outputs.  The connections at the CPU slot would be for audio in to a CPU card, or from a low impedance audio out of a CPU card.  The CDROM drive output will be high impedance as already described, so you'll get negligible audio out.

[Castellen]

Just one other question... can I get to these pins on U15 on the top side of the Mobo?

Yes, all the points I mentioned are deliberately on the top side of the main board.  U15 is the SO14 package located roughly rear of centre of the board.