How To Add An Output Level Pot

[railhead]

I have another newbie-ish question.

Let's say I want to add an output level/volume pot to an effect. Would I just take the "hot" signal and tap it to the pot lug before I pass it to the output jack?

[zhx]

Basically yes

Effect output signal to lug one, wiper (lug 2) to output jack tip and lug three to ground usually works fine. Often 100K log does the trick.

[railhead]

Sweet, thanks.

I'm assuming that the jump to a 250k pot would really do nothing but allow a bit more fine tuning of the output level, right?

[zhx]

Hmm... I'm not an expert on the subject, but ...

When the pot is "full open", then there's xxxK resistance to ground and zero resistance to output.
When it's "full closed", then its vice versa, right?

I think the resistance to ground determines the output. When there's no resistance, all of the signal goes to ground, thus no sound. The bigger the resistance -> more output. So I guess you're right. There could also be bit more output volume, not shure though. I recommend you try it and see how it works.

Thanks for letting me clear this out to myself also. When I started writing this a few minutes ago, I was about to say the opposite, but it seems I came to another conclusion. Thinking with my keyboard, cool!

EDIT: Now that I think of it again, I'm not sure my conclusion was solid. If we assume 100k to ground is enough to let full volume to output, then increasing that value would not do anything but shorten the range.

Like this: (turning the pot)
250k to ground  -> full output
200k to ground -> full output
150k to ground -> full output
100k to ground -> full output
90k to ground -> sligth decrease to output  --- at this point we have turned the knob over half of its cycle.

Then again I left out the resistance to output. That has an adverse effect. So I'm saying that I'm not sure. Sorry.

[railhead]

I'm not sure, either -- but the Fuzz Clone kit I bought from BYOC used a 500k vol pot. 

[Cardboard Tube Samurai]

BOften 100K log does the trick.

...but not in EVERY situation. Try a couple on for size and flavour! Get adventurous 

[GibsonGM]

A few things could happen with larger pots.  You could start to notice the filtering effect of output cap and pot...that cap & pot do make a filter.  Increasing the pot value will have a similar effect to increasing the output cap size, making your signal 'bassier'.   This is probably why the BYOC fuzz has a 500k pot.  Doubling the pot value halves the cutoff frequency.

Sometimes I will reduce the cap if I use a larger pot.  That said, you can get more control by using a larger pot, sometimes. Not always. It's also taper-dependent. What's going on is the pot wired this way is a voltage divider.  So, you can have something like 10k to ground, and the signal flowing thru 90k.  Or, say signal going thru 10k with 90k to ground, the opposite rotation.  Signal to ground = no volume, so having more resistance there will mean more signal output.   Try experimenting with pot size and measure the value as the signal is decreasing, it will give you more of an 'intuition' about that!  And read "the secret life of pots' over at GEOFX, it's a good article.

[slacker]

At its simplest a volume pot works as a voltage divider when the pot is on full it's just like having a resistor to ground of the same value as the pot. If you turn the pot down then it's like having 2 resistors wired like this.

In ---Res--------Out
                  |
                 R
                 E
                 S
            Ground

The ratio of the resistors sets the output voltage so if you turn the pot to the middle of its resistance the output voltage is half the input voltage, if you turn it to 75% of it's resistance then the output voltage is 75% of the input voltage. As it's the ratio that's important, in theory the value of the pot doesn't matter half of 10K gives the same voltage as half of 1Meg.
In practice depending on the circuit the value of the pot can affect the sound by loading down the output (tone sucking), this can either lower the volume or change the tone of the effect. It can also interact in other ways, this is why some people use 500k pots on fuzzes and some use 100k pots as they can sound brighter.
A 100k pot will work fine most of the time for transistor circuits, for opamp circuits you can probably go lower, maybe down to 10k without any ill effects.

[Barcode80]

Basically yes

Effect output signal to lug one, wiper (lug 2) to output jack tip and lug three to ground usually works fine. Often 100K log does the trick.

this is backwards. signal should go to 3. ground to 1.

[Mark Hammer]

Many circuits will end with a terminating resistor to ground; for example, a 10k resistor.  You can simply replace the 10k fixed resistor with a 10k pot, wiring up the two outside lugs of the pot to where the 10k resistor used to be on the board.  Your output now becomes the wiper (middle lug) of the pot, that "taps" different point along that 10k resistance to achieve different degrees of attenuation of the original signal.  As that last statement suggests, this addition will get you the original volume and less, but not more.

Sometimes, you want a bit of trimming but not the capacity to reduce volume to nil.  There, you would use a combination of a fixed resistor and pot.  For example, following our 10k case, you could replace the 10k fixed resistor with a 5k pot and a 5k1 fixed resistor (pots are often just a bit below stated value so we'll compensate by a slightly higher resistance).  The fixed resistor goes between ground and the ground side of the pot.  This combination now will behave like a 10k pot that can never be turned below halfway.  So, you'll be able to dial in shades of attenuation without having a twitchy pot with a zone where you can (too) easily go from audible to inaudible.

Conversely, sometimes a circuit is TOO hot on the output, and you want the capacity to achieve bypass/effect unity-volume...easily.  In that instance, you need to replace the fixed resistor at the "end" of the circuit with a pot that has a fixed resistor going to its input.  If we flipped our 5k pot plus 5k1 resistor around such that the pot was now wired to ground and the 5k1 resistor went to the input lug, that combination would now behave like a 10k pot that could never be turned up past halfway, and all control available would be shades of attenuation below that.

You can invent combinations too.  For instance, say a pedal is a little too hot, but you don't really want to turn down THAT much.  Following our 10k example, we could stick a 1k2 fixed resistor between the effect output and the input lug of a 5k pot, and a 3k9 fixed resistor going from the ground side of the pot to ground.  This would mimic a (roughly) 10k pot that could never be turned above a certain maximum or below a certain minimum, but still provide some measure of variation between those two end-points.

Note that this general principle can be applied to pots in ANY application, not just volume pots on the output.

[jakenold]

Basically yes

Effect output signal to lug one, wiper (lug 2) to output jack tip and lug three to ground usually works fine. Often 100K log does the trick.

this is backwards. signal should go to 3. ground to 1.

There's that age old confusion - how to number the lugs of a potentiometer 

Put it like this; when you turn the pot all the way up, the lug that is closer to the knob's direction should be connected to the output of the circuit. The middle lug should be connected to the output of the effect itself, and the last lug should be connected to ground. Think of it as having to use the darn knob, and it ain't that hard 

mfg Jake

[railhead]

So I need a resistor between my signal and the pot lug? What I'm wanting to do is add a output level control to a switch box, so that I can set volume at the pedal, not the amp.

[railhead]

Bump to see if I can get a response to whether or not I need a resistor between my signal and the pot lug...

[GibsonGM]

You should look at the circuit, railhead...what is in there at the output right now?  What Mark Hammer was getting at, is that you can do things to 'replace' the output resistor that (I bet) is there now.   Either lower it and add a pot, remove it and add a pot, use 1 before and 1 after to simulate a taper.   It depends what you want to do.   

For just a simple application, replace the resistor at the output with a pot near that value.  If there is no identifiable output resistor, just use a 100k audio taper pot, the standard volume-type pot. This will allow you to get Quieter or Equal to (unity) what the box is already doing, but not louder.  You won't hurt it.    What is the circuit you're talking about?

[railhead]

Heh, I guess knowing what I'm building would help, no? I thought I mentioned that -- sorry!

Anyway, all I'm wanting to do is add volume pots to a channel switcher so that I can control the output level of each outgoing channel.