General Blend Control for Custom Designs

[ElectricDruid]

Here's a simpler way, 100% wet to 100% dry:

Only down side is the output impedance is a little higher, but at 2.5k max, nobody is going to notice.

Nice! I wonder if anyone can work out a way to do one of these with only a single op-amp? We've got two approaches with a dual now...</challenge>

Tom

[merlinb]

Nice! I wonder if anyone can work out a way to do one of these with only a single op-amp?

You mean blend with one opamp and one pot? Yes you can (see the first page of http://www.geofex.com/Article_Folders/panner.pdf), but you have to give something up, either input impedance or noise performance.

[katatoncidean]

Hey,

So sorry for not really posting the design, but life and work has really gotten in the way. But I am delivering pics like I said I would:
https://drive.google.com/open?id=0B4oaw_Fq8VbHcmJPRjA0LTc1aE0 <----- Gut shot of the PCB
https://drive.google.com/open?id=0B4oaw_Fq8VbHMlJVajh3VUhzb00 <----- Love me those light up Pots
https://drive.google.com/open?id=0B4oaw_Fq8VbHeE9mMk1JV2R5RDQ <-- Empty Case
https://drive.google.com/open?id=0B4oaw_Fq8VbHaFNFVWVoVGU2dUE <-- Fully built pedal
https://drive.google.com/open?id=0B4oaw_Fq8VbHQUlWRmMycnpRUEk <-- 4 that I sold off

The only issue it really has, is that it will self oscillate if the gain and all the EQs are at max, and the volume is half way up. Not THAT big of an issue (because this thing is LOUD), but it is there and is noticeable. I will try and get audio clips, but that depends on how much time I have coming up. I ended up going with Electric druids approach for the blend. It isn't overly usable for guitar, but for bass it is perfect. You retain your low end, but still get the growl of the overdrive. Hope you all like how it came out.

[PRR]

> do a trick inverting one of the signals and then use cancellation to do the blend with a single pot.
> input impedance of the op-amp stages needs to be big enough that the pot resistance isn't that significant

Otherwise the "half rotation" is not quite 50:50 blend.

You can relax that second requirement by adding one resistor:



If all resistors are 100K (a good value) and pot is 1Meg (a bit extreme), a wide range in the center of knob rotation will be 45:55 to 55:45 blend, "expanded center". Pot 100K or less will give blend more like knob position.
________________

The plan Merlin shows (which is older than he is) is far simpler but does need good low impedance on both ends of the pot to minimize the unwanted side. When cap-coupled, this can mean relatively monster caps to suppress the bass bleed.

[katatoncidean]

> do a trick inverting one of the signals and then use cancellation to do the blend with a single pot.
> input impedance of the op-amp stages needs to be big enough that the pot resistance isn't that significant

Otherwise the "half rotation" is not quite 50:50 blend.

You can relax that second requirement by adding one resistor:



If all resistors are 100K (a good value) and pot is 1Meg (a bit extreme), a wide range in the center of knob rotation will be 45:55 to 55:45 blend, "expanded center". Pot 100K or less will give blend more like knob position.
________________

The plan Merlin shows (which is older than he is) is far simpler but does need good low impedance on both ends of the pot to minimize the unwanted side. When cap-coupled, this can mean relatively monster caps to suppress the bass bleed.

I ended up using 100K resistors and a 10K pot as suggested by ElectricDruid. The only values I changed are the two on Input B (being my wet signal). I used those to level shift the wet signal so at the midway point I could actually hear the dry signal. For the in line resistor, I chose 270K, and the line to GND resistor was 39K. At the input to the blend pot, Input B was dropped down by about a quarter making the blend a little bit more even. This is very "application based" though. Not all designs will need to drop the wet signal; I just forgot to level shift after the active EQ. I'll post the schematic in a bit. I'm making it look less chaotic right now  .