Question About Variable Power Supply Resistor

[Paul Marossy]

I don't quite understand why, but apparently the "stab" control on a certain Z. Vex effect (5K variable resistor between +9V and circuit) will induce an oscillation when turned down to a certain level in conjunction with the settings on the other knobs.

Another example is the 3-legged dog on a low battery. That sucker will squeal like a pig, but you don't have any control over it as on that certain Z. Vex effect.

So, my question is: Why is that?! That doesn't make any sense to me.  :oops:

[Hal]

for the 3 legged dog, its a function of internal chip functioning, as far as I know.  The oscillations occurs becuase the hex inverter does not function correctly under low voltages.  Don't know too much about it, though.

[Paul Marossy]

I know that comparing those two isn't really apples to apples. I also know that the CMOS hex inverter chip in the 3-Legged Dog will squeal with too low a voltage, but I wonder just why that is. And why would a transistor based circuit do it? That is more mysterious to me than the CMOS chip. I mean other electronic gadgets won't squeal with a low battery, so why those types of circuits? Is it because they are high gain circuits and if the voltage drops too low, it tries to amplify but goes into oscillation instead? This is a very curious thing to me.  :?

[Paul Perry (Frostwave)]

Some amplifiers will oscillate when there is resistance in the power supply, because of positive feedback.
Where does this come from? The first time I saw this, it was a two transistor amplifier, where the current drain through the power supply resistance caused the voltage at the amplifier power input to ripple in sympathy.
And, depending on the design of the circuit, this ripple can get to the input stage & if it is the right phase there, cause feedback. Note that the effect can be present without being strong enough to cause oscillation, in whihc case you wonder why none of the maths work!! too much gain, input impedance nothing like you expected etc.

And that is why a "flat" battery can make a different sound (as well of course as the voltage beign lower).

[Paul Marossy]

Thanks, Paul. That makes sense to me now.