Wnat to make a 25k pot into a 50k pot, Just add resistor?

[JustMike]

 This is for a Seymour Duncan Pickup booster pedal that has too much gain even when turned all the way down. So I believe I could accomplish this by putting a 25k resistor in series with the wiper. Correct?
 I read on another forum that it has a reverse audio taper 25k pot and when it's turned all the way down, there is still 6 dB of gain. I want to get it down to 3 dB.

[sluckey]

What you will have is the equivalent of a pot that can adjust from a minimum of 25K up to a maximum of 50K. It will never go to zero ohms. Is this OK for your circuit?

[eleventeen]

Though it is free to try, without seeing the surrounding circuit, I would say probably not. But it depends upon whether the following stage is high or low impedance. Usually, audio signals going to high impedances are not much affected by series resistances. Again, without seeing your circuit, the gain stages of the booster pedal (if it was made in the last 10-15 years) are almost certainly made up of of op amps. (TLO32, TLO84, several other popular types) And why not? They are very cheap, very quiet, and very flexible. Usually they have enormously high input impedances---with JFET inputs.


On the input of a Fender amp, we have those two 68K ohm resistors. They are not there to reduce gain (though they might, slightly) they are there so that if two guitars are plugged into one channel, diddling one volume control on one guitar tends not to affect the gain of the other guitar. In other words, they are there for isolation.


It seems to me that you would want to install an "input trimmer" across the input jack on your pedal and whack down the incoming signal. That's where your pot should go.

[JustMike]

I found a circuit diagram. I just want to be able to turn this down to unity gain. Right now it can only go down to +6.

[HotBluePlates]

... I just want to be able to turn this down to unity gain. Right now it can only go down to +6.

It is a booster... Just flip the switch off?

If you never want to get the full boost of the circuit, add your 25kΩ resistor between the yellow wire and the 25kΩ pot. That will cut maximum boost by half.

If you add the resistor between the wiper and black wire, you will never be able to turn the boost control down to zero.

[tubeswell]

If you want to attenuate more, you can either:


1) put the 25k in series with the (50k) pot's input lug and the source. This decreases the AC load on the source, but automatically makes a 25k/50k voltage divider with the pot on full CW rotation, so it will be quieter.


2) put the 25k between the pot's input lug and ground lug (25k||50k = 16.7k). This increases the AC load for the source (so it will be even quieter than 1) above)


If you want to improve the log taper of the pot, put the 25k between the pot's ground lug and pot wiper. You will have a 25k between source and ground on full CW rotation, but as you turn the pot CCW, the 25k will be in parallel with whatever resistance is between the wiper and the ground (for any given wiper setting) and you will have some corresponding series resistance  between the pot wiper and the pot's input lug.  If you want to then boost mids or highs, you can bypass the pots input lug to wiper with a cap of whatever desired value you want (smaller cap will give more mid to bass cut as the pot rotation moves more CCW).


If you want to add anti-log taper, put the 25k between the pot's input lug and pot wiper.

[JustMike]

Thank you all. I misspoke when I said I wanted to be able to turn it down to unity gain. I want a little boost but being able to have unity gain would give me a range to play with. I'm using this pedal in the fx loop as a lead boost, but 6dB is too much.

[HotBluePlates]

This may sound silly, but you have a volume control on the guitar. You could just turn it down a little when stepping on the boost...

If you have a schematic for the pedal (and not just a layout), someone can probably recommend a circuit change to reduce the amount of boost. Layouts can be harder for that task because it takes a lot of time to mentally interpret the physical position of parts to their functional role. At a glance, there appears to be some feedback between Q1,2 to Q3 and to follow what's going on, I'd have to draw a schematic from the layout to to make sure I see what I think I'm seeing.

But maybe I'm just not familiar enough with pedals...