Mastodawn

The wildest and most "out there" feature in this audio processor is, of all things, the master stereo gain control which is the 10k multiturn pot in the middle of this schematic. Yes, a single track variable resistor and no, it isn't a DC volume control.

Philip Bragg Mar 16

The circuit is credited to W. Stephen Woodward and was originally from Electronic Design magazine's Ideas section. I might see if I can find that…

Rue Mohr Mar 16

Rue Mohr Mar 16

@synx508 by which I mean, how the heck did they control the volume of the other channel...

Philip Bragg Mar 16

@RueNahcMohr When the unmolested side gets dimmed by the balance control, the inverted "not balance" or "getting louder" side is sent to the diff amp of the other channel which also makes it quieter.

@synx508 @RueNahcMohr is it shorting the input side of each op amp slightly to ground, but on a divided basis?

Philip Bragg Mar 16

@u0421793 @RueNahcMohr Yes, the series resistors feeding the gain control mean it can't ever turn the volume down to nothing but if you took them out it could. The extra bonus weirdness is that if you adjust the 100kΩ calibration pot incorrectly the right side will turn down and then turn up again in antiphase, which is quite exciting.

Philip Bragg Mar 16

@u0421793 @RueNahcMohr When it's set correctly it's perfectly symmetrical, which is actually better than you'd ever get with a stereo 2 gang variable resistor.

@synx508 @RueNahcMohr due to IC54A?

Philip Bragg Mar 16

@u0421793 @RueNahcMohr Yes. It's a IC54A doing the difference between the "getting louder" signal from the not-really-balance control and the original signal, which turns "getting louder" into "getting quieter".

Philip Bragg Mar 16

@RueNahcMohr Yeah, but it really does work.

sleepy duck pond appreciator 🦆✨🏳️‍🌈Mar 16

@synx508 the missile knows where it is, because it knows where it isn’t 🤣

Graham Sutherland / Polynomial Mar 16

@synx508 doesn't this get fucky if left and right aren't identical signals?

Graham Sutherland / Polynomial Mar 16

@synx508 or... wait, does it subtract the scaled inverse of the original subtraction? this is hurting my head.

Philip Bragg Mar 16

@gsuberland Yes, it does that. I don't know why the circuit scrambles my brain, schematics can do that if they're drawn in strange ways but this is as clear as it can possibly be and my brain just shouts "NO THIS IS IMPOSSIBLE" at it. But yeah, just scales and subtracts from that signal that's getting bigger as the other one gets smaller. Like a conjuror.

Graham Sutherland / Polynomial Mar 16

@synx508 clever, but also... augh!

@gsuberland @synx508 Uses (gain) for top and (1-(1-gain)) for bottom to let you use the same gain pot for both. Bet it sounds really weird when the pot gets dirty.