Built in Line Mixers for a 4 x SID

[Altitude]

I have been hunting for some easy mixers to sum the 4 audio outputs since I dont have 4 channels free on my console and came across this:

http://generalguitargadgets.com/diagrams/mixer_sc.gif

Now my question is, what would be the most efficient way to power it from the C64 PSU?  The ins and outs of the TL072 are a little over my head so i am not sure whats acceptable for supply voltages and how that impacts the audio

[Jaicen]

That schematic should work just fine for your needs.

If  it was me, i'd power it from the highest voltage you can, a TL07x can handle +/- 18v so you should be fine running it at 9-15v. The more voltage you have, the more headroom is available so that's something to consider.

edit: If you're confused by the pinout you can always build it using the PCB layout on that page, but it's not that hard to figure out.

[Altitude]

So running it off the same 14 VDC rail as the sid modules is fine with the schematic as is?

[Jaicen]

Yeah that will be fine, it's set up to be self biasing as it is (R11 & R12 form a voltage divider, so the bias point is always half the supply voltage).

You can make R13 a variable resistance if you want a master volume control, higher values increase the gain, but bear in mind this may introduce noise at higher gain settings. It may also be an idea to increase the 0.1uF caps, as i'm not sure how low the output impedance of the core is. 

[Altitude]

Thanks for the help, btw

What would be a indictation that there is a mismatch in impedance and what size steps should I increase the caps if there is?

[Jaicen]

If the output impedance is too high, you're going to have a rolloff of low frequencies with a smaller cap. However, i'm fairly sure that the SID output is buffered to prevent this, but i'd still use larger caps just to be sure. They're only there to block DC, so as long as you don't use a huge electro cap, then you should be fine. I'd go 1uf film caps on the input and 10uF on the output.