Hello all, I'm about to embark on a (re) design mission after coming across some problems in a circuit.
I've been using 100k tone pots typically in my circuits but have been simulating 25k, and 10k pots in the Duncan Tone Stack, and I like what they're showing me.
Short of just a more limited sweep with a smaller value tone pot, are there any hard/fast rules to why someone would opt for a 250k, 100k, over a 25k or 10k pot? I presume something like impedance comes into play downstream but this is where things get fuzzy for me.
Quote from: FingerBlisters on January 25, 2021, 04:06:57 PM
I presume something like impedance comes into play downstream
You presume right.. :icon_wink:
A 100k pot driven by a 10k output impedance (e.g. a Collector/Drain resistor) loads it by 10% where a 10k pot loads it by 50%..
Quote from: antonis on January 25, 2021, 04:10:57 PM
Quote from: FingerBlisters on January 25, 2021, 04:06:57 PM
I presume something like impedance comes into play downstream
You presume right.. :icon_wink:
A 100k pot driven by a 10k output impedance (e.g. a Collector/Drain resistor) loads it by 10% where a 10k pot loads it by 50%..
Alrighty. For the dumb kids in the back what does "load" mean in this instance? Reduce treble? How does this loading play out and is it a good thing? Do you want more of it or would less of it be handy.
I'm not able to draw a schematic for the moment so let's consider it as a (non ideal) Voltage source feeding a load (resistor)..
Source's internal resistance (ZS) is considered in series with whatever it feeds (ZL)..
https://en.wikipedia.org/wiki/Input_impedance (https://en.wikipedia.org/wiki/Input_impedance)
So, for +9V (VS) source with 10k internal resistance and 100k load, voltage acros the load (VL) is 8.18V where for 10k load voltage across it is 4.5V..
Another way to face it in real world ciruits is to consider load in parallel with the output impedance of the preceding stage..
tone pot resistance is totally dependent on the surrounding circuit.
i have build pedals with anything from 15k to 1M tone pots, but the main thing was the surrounding circuit.
that 'loading' might reduce treble or bass, it might make a chip have more crossover distortion (sounds really bad) or it might lower the gain of a transistor... what loading does depends on the circuit.
i suggest you post the tone stack in question, circuits preceding and following it.
cheers, Iain
Quote from: iainpunk on January 25, 2021, 04:50:43 PM
tone pot resistance is totally dependent on the surrounding circuit.
i have build pedals with anything from 15k to 1M tone pots, but the main thing was the surrounding circuit.
that 'loading' might reduce treble or bass, it might make a chip have more crossover distortion (sounds really bad) or it might lower the gain of a transistor... what loading does depends on the circuit.
i suggest you post the tone stack in question, circuits preceding and following it.
cheers, Iain
Well the thing that got me thinking about it was the circuit I posted a few days ago with which I was having problems. You commented that the impedance heading out the tone stack was around 68k or so due to the use of a 100k pot and 100k resistors either side in a big muff style stack.
That got me thinking. I ran the numbers on a 10k and I like the look of the graph on the tone stack calculator but whether a 10k pot is a good choice is something I'm wondering about how to ascertain.
in your situation, its totally no problem to use a 10k pot based BMP tone-stack, you can also up the resistors of the next stage, a factor 10 larger does a lot, also the cap should be a factor 10 smaller for the same frequency response
cheers, Iain
Quote from: iainpunk on January 25, 2021, 09:07:16 PM
in your situation, its totally no problem to use a 10k pot based BMP tone-stack, you can also up the resistors of the next stage, a factor 10 larger does a lot, also the cap should be a factor 10 smaller for the same frequency response
cheers, Iain
Ok cool thanks man. I actually did what you suggested with the 470k/1m combo and it seemed to limit the action of the tone pot itself which was interesting. I tried different combos/ratios in there but that whole rabbit hole led me to asking this question :)