inverting opamp question

[ElectricDruid]

Rob, the Arachnid normally runs chorus, delay, phaser, flanger, reverb programs

And this circuit snippet blends the wet with dry

Is there a better way to mix, to have perceived similar volume while adjusting the blend pot ?

The snippet shown blends from 0%/100% through to 100%/0%, so the only effect of different levels into the two inputs, or differing degrees of correlation is that either the ideal 50%/50% (if that's what we've even after) won't be exactly in the centre (so...big deal. we can simply adjust to taste) or that the overall volume might be somewhat different from the dry-only signal (and we've got a "volume" pot to adjust the make-up gain for that too). So while the circuit as shown might have theoretical defects, in practice it's highly versatile and does everything you need it to do.

[Rob Strand]

Rob, would a different circuit architecture lead to better blend results ?

Like maybe ganged pots ?

MN ?

(I have always appreciated your mathematical posts !

Thanks! )

In practice circuits that don't keep the same level (more or less) bug me a bit.   I'm sure it bugs other people in professional audio circles as well.  That's why companies like Peavey (dead link other thread) have investigate it and put up articles.  The Peavey case is not targeted for delay-based pedals but the underlying problem is the same.   So (IMHO) pushing the original circuit towards what you want is a good thing. 

I know I've looked at this in the past but I can't remember the winning configuration.  The result might even be on this forum.  Suppose a dual a pot was the winner then it's only a good solution if you want to give-up the single pot.   

In its basic form a dual pot will give more than 6dB change from center to extreme, worse than the existing circuit.  It doesn't matter if you mix resistively or use a virtual ground (inverting opamp mixer).   The centered (linear) pot must divide by more than half at the center.    Keeping the pot resistance low compared to the mixer resistors will help reduce the drop.   Here's the trick:  the way to change the loss at the center is to add a resistor in parallel with the pot wiper and non-grounded pot terminal.  Here's a few examples designs (please check),

dB change      -2dB             -1dB           -2dB        -1dB
Rmix              75k              75k             75k           75k
Rpot              47k              47k             10k           10k                ; dual-ganged pot
Radded          5.77k            2.41k          1.60k         0.646k
Rpot/Radded   8.14             19.5            6.24         15.5

where,
        Rmix = input resistor opamp, Rpot = mix pot, Radded is added resistor across top pins of pot

(Circuit B here but Radded not shown
https://www.diystompboxes.com/smfforum/index.php?topic=118967.0)

The thing to note here is the added resistor is *somewhat* smaller than the pot.  What that means is  the pot isn't going to do much in the center and will only do something near the ends.   To help stop that, using a lower pot value helps, as expected, and using a less aggressive a fix (ie. -2dB) helps.

Basically we get into the same dead zone in the middle problem that the previous circuit had.   As far as it being a winner or not we would need to compare the two circuits with equal center to end gain change then look at which one
has the most effect when the pot is set to 25% and 75%.

Special pot tapers would help but we that means we have gone from a single pot to a dual pot with a special taper.   You have to question is it worth going that far.   Nudging the values on the original circuit is easier.

FYI: A low value of Radded will load the previous circuit when the pot is set to minimum.

[ElectricDruid]

Was this the thread where we discussed this last?

https://www.diystompboxes.com/smfforum/index.php?topic=128576

There's a few posts with Ben/Processaurus and I talking about the blend/mix circuit and tweaking it to account for the hump in the middle with correlated sources. Executive summary: You can either have the hump, or you can have almost no hump, but more noise.
Later on Antonis fixes the OP's *actual* problem! Thanks Antonis! Lol!