What does it mean to "connect a potentiometer between a positive and negative voltage

diymixer

Member
2010-06-05 5:28 pm
What does it mean to "connect a potentiometer between a positive and negative voltage

I was looking at the spec sheet for an SSM2164 VCA and I saw that to produce gain, I need to connect the control voltage pot between positive and negative, as opposed to positive and ground. What does this mean?
 
The way I read it, you only need to connect the control voltage pot between positive and negative when you want both gain AND attenuation.

In other words, if you just reference between positive and ground, the VCA will go from unity gain (when the pot's wiper is at ground), to some positive gain. Going between positive and negative lets you go from some positive gain, to unity gain, to some negative attenuation.

se
 
Ground is not negative. The chip needs a split supply. With batteries, you could do it like this...
 

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so would I get the full 20 dB of gain when the wiper is connected to the positive side or the negative side?

My bad. I read the datasheet a little too hastily.

When the pot is connected between positive and ground, you only get unity gain and attenuation with unity gain being when the control voltage is 0 volts.

So if you want gain, you'll need to connect the pot between a positive and negative voltage (relative to ground). So in that case, you'll get gain as the control voltage goes negative.

To be honest, I don't know just how the amount of gain is established. I know the datasheet says it has a maximum of 20dB, but I'm not sure if that's only the case when the negative control voltage is equal to the negative supply rail voltage.

Perhaps someone else can chime in on that issue.

se
 

diymixer

Member
2010-06-05 5:28 pm
well, I would like the fader to operate exactly like the Yamaha PM4000 I've fallen in love with: negative infinity attenuation all the way down, unity gain 80% of the way up, and then +10 dB gain (before assignment to the VCA groups) at the very top of the fader's travel
 
well, I would like the fader to operate exactly like the Yamaha PM4000 I've fallen in love with: negative infinity attenuation all the way down, unity gain 80% of the way up, and then +10 dB gain (before assignment to the VCA groups) at the very top of the fader's travel

Ok, if you want both gain and attenuation, you need to connect the pot between a positive and negative voltage.

se