Can anyone recommend a good single supply summing opamp?

[loss1234]

in reference to the schematic i drew at this location:

http://electro-music.com/forum/topic-28219.html

or here


http://electro-music.com/forum/phpbb-files/current_endless_noise_205.pdf


i would like to replace the 2nd to last chip in the chain (currently a dual supply opamp, which is acting as a CV summer/R2R (i believe), with something that can run off of a singe supply 9v battery like everything else in the circuit. when i tried an lf358, it didnt work (i thought that was single supply) but maybe i need to hook things up differently to get it to still work.

the output of the opamp will either be powering another cmos chip or will be sent into the CV in of a synth, so negative voltages are not needed nor encouraged.

any help appreciated.

thanks

[Tim Servo]

Hey Loss,

I'd probably recommend the LM324. Cheap, easy to get, single supply, and I've seen it used all over the place in synth stuff. Other than that, you can still use a dual supply chip but just use a diode to block the negative voltages going to your CMOS logic. On the 566 VCO, Mr. Henry uses a 1N4148 diode in series (arrow pointing towards the CMOS logic) and a 10k resistor to ground after the diode to lop off the negative half of the square wave going to a 4013 (used to create the sub-octaves). Hope this helps.


Tim (lopping off the negative) Servo

[bugbrand]

358 should work good - but I think you've got a wierdy design, no?!

I think you're forgetting about biasing to 1/2 of the supply voltage.

Do you want the opamp in inverting or non-inverting mode?

[I'm kind of guessing your R19 pot is feedback? or is it some level control]

From my slightly lax knowledge you're either:

1 - trying to set it as a non-inverting setup with R19 as gain (though a larger value may be better) - in which case Ref14 (your ground) should be the V/2 bias.

2 - if you're going for a non-inverting summer then Ref14 should still be at v/2

... hmmm, I'm still a bit confused.

To make v/2 either:
1- use two 10k resistors in series between V+ and Gnd -- v/2 is then the middle -- add in a 10u cap to stabilise
or
2 - use the 2nd half of the lm358 as a buffer -- again, two 10k resistors in series, the middle goes to the non-inverting (+ve) input, then tie output and inverting (-ve) input together and that is you're V/2 point (also add a 10u cap here)

Hope that's of some help?!?!?!

[loss1234]

BUG

my r2r came straight from uncle krunkus so you would have to ask him! (he posted it in the lunetta forum..remember?)

though i would rather not have to bi

as anything..so fr the moment i think a 324 might be a better soloution.
i will take your advice though and tinker with the opamp design

and let me double check that R2r

thanks all

[loss1234]

here it is
http://electro-music.com/forum/topic-23215-25.html
looks like uncle got it from ray wilson!!!

my how these circuits travel

does this clarify anything?

anyway, i was afraid if biased to half supply, i would end up with a much smaller voltage swing, which would mean a more restricted CV range


right?

thanks to all for the help

[bugbrand]

Ah, well.... ---- you've certainly got your input pins swapped then!

Dunno about the single-sided approach really though...

I'd say if you're going for a single supply opamp then the ref14 point should be v/2. Anyone else care to chime on that.

[loss1234]

oops

swapped the pins

i always thought if you used a single sided opamp youd be better off than biasing a dual supplied amp

thanks for the correction !!!