Hello all,
I am trying to build a dual guitar circuit using the same three "Volume", "Tone" and "Bass" knobs for both active/passive circuitry.
to that end i need the pots to have a dual stacked structure but with different values - let's say 500K for the passive and ~25K for the lower impedance active circuit, ON THE SAME KNOB.
1. Am i in a direction towards a total failure? or does my logic make any sense?
2. any pointers to where can i find such parts?
a few notes:
- I am not trying to merge these two circuits. I intend for them to be selectable (basically having the passive as some sort of fallback)
- First post here, so if i'm violating any policy or rule, please be mercyfull
thanks!
Roi
I am trying to build a dual guitar circuit using the same three "Volume", "Tone" and "Bass" knobs for both active/passive circuitry.
to that end i need the pots to have a dual stacked structure but with different values - let's say 500K for the passive and ~25K for the lower impedance active circuit, ON THE SAME KNOB.
1. Am i in a direction towards a total failure? or does my logic make any sense?
2. any pointers to where can i find such parts?
a few notes:
- I am not trying to merge these two circuits. I intend for them to be selectable (basically having the passive as some sort of fallback)
- First post here, so if i'm violating any policy or rule, please be mercyfull
thanks!
Roi
Check specific Guitar Parts suppliers and also browse mainstream ones such as Mouser or Digikey.
OR get "stereo pots" such as, say, 500k + 500k and design Active circuit around those values, no big deal.
The Active circuit can always have a final buffer which isolates 500k circuitry from output cable driving it from a low impedance.
Or you can do some "surgery" and transplant a 25k track to one half of a dual 500k pot; not too hard to do if all same brand and type.
Or you can build just the passive circuit, and either send it straight to output jack (standard passive mode) or through a buffer which might also add some gain, say 2X which is ample.
In fact many "active circuits" are just that.
OR get "stereo pots" such as, say, 500k + 500k and design Active circuit around those values, no big deal.
The Active circuit can always have a final buffer which isolates 500k circuitry from output cable driving it from a low impedance.
Or you can do some "surgery" and transplant a 25k track to one half of a dual 500k pot; not too hard to do if all same brand and type.
Or you can build just the passive circuit, and either send it straight to output jack (standard passive mode) or through a buffer which might also add some gain, say 2X which is ample.
In fact many "active circuits" are just that.
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Buy a dual 500K and add (in parallel) to one of them a 27K fixed resistor. You may have to mess with the taper a bit. See the GEOFX document for ideas: The Secret Life of Pots
Stph
Blore-Edwards Limited will make just about any pot configuration possible including ganged pots...
2-45 Dual Gang Series 45 Metal Shaft (3 - 5 gang options) | blore-edwards-ltd
Pricing is very reasonable and quality is excellent, but they are UK based so USA customers pay a fairly steep shipping fee which may be a deal breaker for anyone outside of the UK. Still, if you don't desire to roll your own a suggest by others in the above replies, for $20 or so you will be provided with a well built pot built to your exact specifications.
2-45 Dual Gang Series 45 Metal Shaft (3 - 5 gang options) | blore-edwards-ltd
Pricing is very reasonable and quality is excellent, but they are UK based so USA customers pay a fairly steep shipping fee which may be a deal breaker for anyone outside of the UK. Still, if you don't desire to roll your own a suggest by others in the above replies, for $20 or so you will be provided with a well built pot built to your exact specifications.
Here we are;
HIGH QUALITY ALPHA STACKED DUAL CONCENTRIC POTENTIOMETER LINEAR OR AUDIO 25-500K | eBay
I knew I had seen some.
HIGH QUALITY ALPHA STACKED DUAL CONCENTRIC POTENTIOMETER LINEAR OR AUDIO 25-500K | eBay
I knew I had seen some.
Not to rain on anybody´s picnic but that simply is wishful thinking.
Parallel resistors are a VERY crude way to somewhat approach a desired curve, sort of, lots of limitations, work only on "volume controls" and only if driven by a low impedance; a mess on tobe controls, and even so, only within a narrow range.
In the needed 20:1 range (500k:25k) it´s absolutely useless.
Sorry.
PS: do a little Math yourself: as a simple example, a 25k pot set to "5" (halfway rotation) will show the next stage an impedance of 6.25k ; a 500k pot with a 25k resistor in parallel will show a 125k one ... a 2000% error.
IF it were that easy nobody would use Log/Audio pots any more and/or a 500k pot plus a couple resistors would be theb"universal" pot.
And that´s not happening
Sorry for this post, don´t want to sound harsh, but sadly bad or incomplete data is copypasted thousands of times.
> What if you used 2 resistors?
Then what can the pot do? The result is essentially the two (fixed) resistors, until the pot is turned so close to the end that its end resistance is comparable to the fixed resistance. To "make 20:1 change", only the last 5% of the pot "does something", all the rest is "fixed" by the two resistors.
Also: R.G.'s essay is for POTS, potentiometers. There are several common situations where we use a 3-leg "pot" as a 2-leg RHEOSTAT. Often "reverse taper". Notably microphone amps and tremolo speed knobs. Slugging a resistor across a 3-leg pot won't do what you want here.
Then what can the pot do? The result is essentially the two (fixed) resistors, until the pot is turned so close to the end that its end resistance is comparable to the fixed resistance. To "make 20:1 change", only the last 5% of the pot "does something", all the rest is "fixed" by the two resistors.
Also: R.G.'s essay is for POTS, potentiometers. There are several common situations where we use a 3-leg "pot" as a 2-leg RHEOSTAT. Often "reverse taper". Notably microphone amps and tremolo speed knobs. Slugging a resistor across a 3-leg pot won't do what you want here.
So basically the low impedance load will tend to cut the highs out of the signal, right?
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