Where to connect a pot

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carlmart
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Joined: Mon May 19, 2014 6:11 am

Where to connect a pot

Post by carlmart »

In a way it's unfortunate that I took the decision to go for a single supply preamp project, instead of the usual +/- supply.

There's very little or no information around, not even on electronics books, on how to deal with most situations. Should I connect this to battery ground or to V+/2 ground?

A major point are the level pots, particularly when they precede an opamp. But even if they don't, I can't figure the rule to what ground a level pot should be connected to.

On Walt Jung's book, who gathers more examples on single-supply application, sometimes he uses a high value resistor, like 470k, to isolate the V+/2 point from the input. But there are no examples or explanation on how to wire a pot with single supplies.

Can anyone throw some light over this matter?
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JR.
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Re: Where to connect a pot

Post by JR. »

I guess I could repeat my suggestion to use split supplies. :D

So much of old design used single supplies. To connect a level pot to ground you generally cap couple the audio going to it and coming from it. Take care to pay attention to polarity of electrolytic caps.

Gain pots configured as variable resistor to ground generally need one cap in series.

For applications using more complex chip sets, look for manufacturer's app notes for specific advice. If they do not show a single supply app note, that may be a hint.

JR
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carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Where to connect a pot

Post by carlmart »

Yes, using blocking caps in and out of a pot should solve the DC issue, as no DC should go through it, only the AC signal.

A THAT 1510 using a single supply demands an output blocking cap, even if that hand-drawn design circling around on this forum, having a 1512 at the input and a 1646 didn't have any cap in between stages.

Problem is in that case, the first stage level was primarily done using a multi-position switch or an anti-log level pot. In my case I'm more traditional and prefer an attenuating pot to ground.

As I'm also adding a series lo-cut filter, the problem is how to do all that and still use a single-supply.

That's why I asked opinions on why should I connect one side of the pot to battery ground or to V+/2. What do you think?

The way I see it, if I connect it to battery ground I will certainly have to cap block the pot on both sides, but the pot would really get to 0v. If I connect one side of the pot to V+/2, that wouldn't be 0v and I don't think I would have full attenuation on my AC signal.

Is something wrong in this reasoning?
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JR.
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Re: Where to connect a pot

Post by JR. »

carlmart wrote:Yes, using blocking caps in and out of a pot should solve the DC issue, as no DC should go through it, only the AC signal.

A THAT 1510 using a single supply demands an output blocking cap, even if that hand-drawn design circling around on this forum, having a 1512 at the input and a 1646 didn't have any cap in between stages.

Problem is in that case, the first stage level was primarily done using a multi-position switch or an anti-log level pot. In my case I'm more traditional and prefer an attenuating pot to ground.

As I'm also adding a series lo-cut filter, the problem is how to do all that and still use a single-supply.

That's why I asked opinions on why should I connect one side of the pot to battery ground or to V+/2. What do you think?
In case I have been unclear I do not think it is a great idea to operate these from single rail.

I do not have any personal experience trying with these chips. I have designed many single supply circuits, including battery powered, but in this case would not.
The way I see it, if I connect it to battery ground I will certainly have to cap block the pot on both sides, but the pot would really get to 0v. If I connect one side of the pot to V+/2, that wouldn't be 0v and I don't think I would have full attenuation on my AC signal.

Is something wrong in this reasoning?
I will repeat something I already posted... your v/2 needs to look like ground at AC for whatever you connect to it. That will mean different things for different connections.

Maybe build one and see what happens.

JR
Cancel the "cancel culture", do not support mob hatred.
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