Potentiometers and Volume Control

[Tucson Annie]

Hi Guys,

This is a general question about potentiometers and my understanding of them. Let's see, how to ask this.....okay, say I make a little audio amp circuit with an LM386 chip (I've been experimenting with them lately)....If I designed the circuit to have a 10K pot on the input....as the pot is turned thus changing the resistance it would also change the voltage across the pot....so, more voltage and more resistance would lead to less current and therefore less signal going into the chip therefore limiting volume....

Is my thinking correct? I just downloaded LTSpice and have yet to find the time to learn how to use it so I haven't done any simulations.....

Thanks,

Annie

 

[picbits]

Thats generally how they work.

One end of the pot to the signal input, the other end to ground and the wiper to the amp input.

 

[audioguru]

The voltage across the pot is not changed. It is the voltage of the source signal.The pot is a variable voltage divider. Its divided signal is its output to the amplifier.

 

[MikeMl]

Here is a LTSpice sim of two 10K pots, showing how the voltage at the wiper varies as a function of the position of the wiper (Parameter called "pos"). Note how the pot resistance above the wiper gets smaller at the same rate that the resistance below the wiper gets bigger. The independent variable of the simulation is Pos, which varies from 0.001 to 0.999

I included two cases; one where the pot wiper is not loaded by an external resistance, and the other where the pot wiper is loaded with 5K to ground. Note the linearity of the unloaded pot (i.e. a "linear" pot) and how the output voltage vs wiper position is shifted by having the pot drive a low resistance.

 

[bountyhunter]

Hi Guys,

This is a general question about potentiometers and my understanding of them. Let's see, how to ask this.....okay, say I make a little audio amp circuit with an LM386 chip (I've been experimenting with them lately)....If I designed the circuit to have a 10K pot on the input....as the pot is turned thus changing the resistance it would also change the voltage across the pot....so, more voltage and more resistance would lead to less current and therefore less signal going into the chip therefore limiting volume....

Not if it's designed right. The input impedance of the gain stage fed by the pot wiper is usually high enough that it doesn't load the pot much so the voltage across the top of the pot to ground stays pretty constant. The point that the pot's wiper taps off is a changing voltage.

 

[audioguru]

The input impedance of an LM386 amplifier is 50k ohms. It barely loads down a 10k pot.

 

[Tucson Annie]

Thanks to all of you! I tried a 'real world' experiment last night with a 386 chip and 10K pot and didn't have any luck...I wish I could attach a schematic but I need to sit down and figure out LTSpice (they don't even have a model of a 386 chip)...

I built this siren from a Forrest Mims book on a breadboard....then, I took the output of that and stuck it on the left pin of a 10k pot....the middle pin of the pot went to the input of the 386 and the right pin of the pot went to ground....well, the amp definitely cranked up the volume of the siren circuit, but the 10K pot has no effect on the volume whatsoever....

I'll draw up the schematic of what I did later today after spending a few minutes learning LTSpice....

Thanks again to all....I know these are basic questions, but you all are really helping me get a grasp on what is going on! And I really appreciate it!

Annie

 

[MikeMl]

The 386 is funky. You have to look at the data sheet to figure out how to bias the inputs, while reducing the voltage gain with a pot. This shows some basics.

There is a down-loadable model for the LM386 in the FILES section of the LTSpice Yahoo group.

 

[MikeMl]

I redid the pot sim to include different load resistors by adding a second variable (the load resistance in binary steps from 500 to 8K Ohms) to show a family of output curves.

 

[crutschow]

The connection that MikeML shows with a resistor to ground generates an approximation to an "audio taper" or log pot which is often desired for a volume control.

 

[flat5]

If the pot did not change the gain then the ground on the pot was not right. The siren, pot, and 386 must share the same ground.

 

[audioguru]

The 386 is funky. You have to look at the data sheet to figure out how to bias the inputs

The inputs are already biased. You simply connect them to 0V.

... while reducing the voltage gain with a pot.

The datasheet shows how to reduce its gain of 20 at high audio frequencies resulting in bass-boost. It explains that the LM386 will oscillate if its gain is reduced below 10.