I'm trying to simulate a gate/trigger for an analog synth, providing a control voltage out if input is below a threshold.
So analog switch (mosfet) driven from the comparator is a better way to go?
Yes that would work. Pull the output up to 15V and add a 2N7000 to the output. The functions of the + and - inputs would be reversed as the MOSFET would perform an inverting function.
Another option would be a comparator with a MOSFET output, although those typically won't work with a +/-15V supply. The maximum power supply voltage of a CMOS comparator is normally 16V.
The reason for pulling up to 15 is because of the current required by the 2N7000 right? (the gate voltage threshold for 2N7000 is much lower)
Would 12 also work in that case?
The 2N7000 only draws a current to charge up its gate capacitance. Once it's on, the gate takes no current at all. I only suggested 15V, because it's the power supply voltage. There's no reason why a lower voltage can't be used. Normally it's desirable to use a much higher voltage than the threshold voltage to ensure a low on resistance, but in this case the drain current is so tiny, it won't matter.
BTW, if I wanted to build a few of these on one board, what would be cheaper/better? A bunch of 2N7000s or a quad analog switch like a CD4066? Are there newer alternatives?
The 2N7000 will be better than the CD4066, because it has a lower on resistance, therefore will be able to pull the voltage down more. I don't know about cost. Look at the prices wherever you buy components from.
Another possibility is a CMOS gate with an open drain output, such as the 74HC07 or 74LVC07, but they have a maximum supply voltage of around 5V.
https://biakom.com/pdf/TC74HC07AP_Toshiba.pdfhttps://assets.nexperia.com/documents/data-sheet/74LVC07A.pdfI was going to suggest the 74C604, which will work up to 18V but it seems to be obsolete.
http://www.mouser.com/ds/2/149/MM74C906-356201.pdf