mb6582 J70

[Davo]

Wilba, what's the purpose of the J70 header? ???

[Wilba]

This is a passive mixed output of the four stereo channels, which you can connect to the small phono jack above the power switch. Totally optional.

This was a late design idea I threw into the prototype, the resistors below each audio socket are used to connect the audio signals together when there is no plug in the switched audio socket, i.e. it will only mix those sockets without plugs. You need to connect it together with insulated wire under the board though.... I didn't want to mess up the ground plane with tracks. Basically join four unused pads together in two rows.

I used 10K resistors there because that's what I've seen before in passive mixer designs, but the output is very attenuated, and I am guessing that you could drop these to 1K or less, as the outputs of the audio buffers after each SID can probably handle that. Someone with more audio electronics (and SID!) knowledge can probably answer that question.

[Davo]

I was wondering what those resistors were doing up there.  What do you think about an offboard buffer/amplifier and leave the resistors at 10k?  It'll be tiny enough to do point-to-point on some perfboard and would provide more protection for the SID.

What part number would you recommend for the trimpots?  The 1/4" ones I got from Jameco have a way-too-small footprint.

[stryd_one]

What do you think about an offboard buffer/amplifier and leave the resistors at 10k?

I would do a bit of both. No point losing amplitude unnecessarily then amping it up again.

There's some formula for getting the R's right, but you could probably just as easily tie 10k pots to each, and feed full volume through all them and turn them up until a bit of headroom before clipping. Round down to the nearest value to be safe... sweet.

Every time someone talks about protecting the IO I can't help but think "what are you guys plugging these in to?" hehehe

[Wilba]

The SIDs are already well protected by an audio buffer. This mixer happens after the audio buffers.

I thought about adding an amplifier but an eight channel amplifier off a single supply is a bit out of my league... and since most of the time people will want separate channels anyway, there wasn't much demand for it either. If you really want to treat the SID as one stereo device then plug it into a mic input with some more gain and be done with it (might as well use a "real" amp).