Stupid resistor tricks

[Gaz]

All the time I see schematics (especially modern ones) where there is a resistors connected across potentiometers.

On common one is a resistor across a gain/volume pot, from outer lug to outer lug (fig. A). I can understand that it's in parallel with the whole pot, and would lower the overall load pot value and load to the preceeding stage. Is that correct? Does it change the taper/voltage division of the pot in any way? I'm wondering if this is a useful tool, or just a way for amp companies to save money on pots

Sometimes there is also a resistor 'ahead' of the resistor forming a voltage divider before the pot (fig. B), where the bottom leg is in parallel with the pot.

The next trick I often see on gain pots, is a resistor from the wiper to ground (Fig. C). If I imagine the pot as a voltage divider, the added resistor would form a voltage divider with the top leg of the pot, and be in parallel with the bottom leg. I'm not sure why this would be done, but see it a lot in high gain amps. Does it make the taper more exaggerated? It seems like this would also screw with the impedance at different settings.

Thanks for any thoughts.

[Firestorm]

Google "The Secret Life of Pots". I can't figure out how to copy the link on my phone. It's a wonderful article by the great R. G. Keen.

[Gaz]

Google "The Secret Life of Pots". I can't figure out how to copy the link on my phone. It's a wonderful article by the great R. G. Keen.

Thanks, I've read the article before, but I am curious about why a designer might do any of these tricks in practice, like example C when they are using log pots already, or example A if they could just use a lower value pot, but still use a resistor across.

[Gaz]

Well, I did something novel... just measured the pot with the resistor across as in example C. With a 1M pot and a 220k resistor from wiper to ground the overall pot's resistance varied from 1M all the way counter clockwise to 183k when full clockwise. So it seems that when you would turn up the gain control the load would get lower. I'm just not sure how this interacts with the previous and preceding stage, or if both would see the same load...

[pdf64]

That's a useful method for mitigating blocking distortion.
In conjunction with the coupling cap from the previous stage, as the pot (ie gain) is increased, the high pass turnover frequency also increases. This facilitates a 'tight' low end at high gains without the tone seeming thin at low gain settings.
Pete

[Gaz]

That's cool! How does the highpass turnover increase, just by lowering the load?

Another dumb question (but I have a reason for asking): If this was used as a master volume in front of a long tail pair with 2 x 1M grid resistors, would the impedance of the master be in seen in parallel with 2M? I guess this would be the same with any master before the LTP.

[roberto]

That's cool! How does the highpass turnover increase, just by lowering the load?

You obtain two effects lowering the load at higher gain settings: you raise the hipass frequency and you lower the gain of the previous stage by lowering the load.

would the impedance of the master be in seen in parallel with 2M?

Which two Megs?

[Gaz]

That's cool! How does the highpass turnover increase, just by lowering the load?

You obtain two effects lowering the load at higher gain settings: you raise the hipass frequency and you lower the gain of the previous stage by lowering the load.

would the impedance of the master be in seen in parallel with 2M?

Which two Megs?

Sorry that was unclear, I mean the two 1M grid resistors. I read in an Aiken article that without feedback, the input impedance of the LTP with two 1M grid load resistors is about 2M.

I'm asking generally about this, but it initially crossed my mind because I noticed I had a 220K from wiper to ground on my master volume in the overdrive channel. I attached a schematic of what I mean...

I wanted to have a permanent 220K/220K divider for the clean channel only coming from the tonestack, and a 220K forming a voltage divider with the 1MA pot on the OD channel. I thought I was being clever using minimal parts, but realized that the 220K bottom leg of the divider was also going from wiper to ground.

I can't really notice any bass reduction as the master is turned up, and I'm wondering why. I think I'm missing something about the loading.

[roberto]

Ok, got it the 2 Megs, but they are not.
In that schematic you have 1M going to 47k//(1M+Zc) going to ground.
So you'll always have around 1040 kOhm to ground.