Building a mixer

[yano]

Here is my current design:



It still needs a bit of work, especially on the Voltage Reference Generator.
Notice that the Output 1 jack is a stereo jack, so that the mixer is only being powered while something is plugged into Output 1.

Some things I need advice on follow:

-For the op-amp, I assume I can use any dual opamp. I've seen the TL072 recommended, but I assume I could use an NE5532 in place of it, correct me if I'm wrong.

-I added the output to the peak detector as though I were adding another output, I assume this is correct.

-The comparator used in the Peak detector is an LM339. The original design used 3 of the 4 comparators, however my alteration only requires one. Is there a comparator that is essentially the same as the LM339 but has less "channels"? I would prefer to use something in maybe an 8pin package rather than 14.

-The Voltage Reference Generator is the thing that I know nothing about, I've never dealt with a reference generator before. The design shown here is from the original peak detector schematic (Dave Johnson's Acceptable Voltage Indicator). This circuit was apparently designed for a 12V system.
My guess is that I can use a 9V controlled power supply to supply the reference voltage, and then just use straight 9V from the jack/battery to supply the 9V.

-I just noticed, the ground on output jack 2 is connected to some other circuitry. It eventually gets grounded, but should I use a more direct ground (i.e. tie it to output jack 1's ground)?

-Should I buffer the per channel outputs in the same fashion as the mixed outputs? (100 ohm resistor followed by 10uF cap)

Thanks again, and if you notice any values that look wrong, don't hesitate to tell me. I'm pretty new to designing circuits.

Design elements taken from
Mini Mixer
http://www.generalguitargadgets.com/v2/diagrams/mixer_sc.gif
Simple Mixer Schematics
http://www.all-electric.com/schematic/simp_mix.gif
Acceptable Voltage Indicator
http://www.imagineeringezine.com/PDF-FILES/voltst1.pdf