Variable gain control - boost and cut

[Aentons]

I am trying to recreate my Boss LS-2 line selector in it's A+B mix mode which is basically a 2 channel parallel looper. It has a knob on each return. I have not examined the circuit, but it appears that noon is unity, turn to the left and volume gets cut, turn to the right and gain is increased. The knobs are labeled "Level" and the extreme positions are labeled -infinite and +20. I assume it's using opamps for the boost.

Can anybody point me in the right direction on how to do that? Or is that what it's even happening?

[sonnyboy27]

Look up info on summing amplifiers. That should put you on the right track.

[Aentons]

Thanks for that. I looked it up and I think I understand what's happening. Correct me if I am wrong, but it sounds like, when summing, you use the opamp to boost to make up for any loss and then you attenuate the boosted signal as needed. So the knob on the return of the LS-2 is working like a mixer fader. That sounds easy enough to do.

I guess where I confused myself is I'm also trying to do a pre type control with real boost/cut before the send. Is that possible or even something that is nirmally done? I have the pre boost worked out by putting a 10k pot in the negative feedback loop but it only starts at unity. Its sounds pretty good that way but would like the flexibility of cutting if ever needed. I understand the pot in the loop method may not be the best way for various reasons. Am I thinking of it all wrong? Is there a better way to do the pre?

[diablochris6]

For a pre boost/cut, you could implement what you mentioned in your first paragraph. Create a op amp boost that boosts your signal however much you want and use a pot after it as an attenuator. You might need to tweak the impedance with a couple passive components or a buffer afterwards.

[Aentons]

Thanks but I was trying to stay away from cutting an already boosted signal.