Too much gain loss in passive lo-pass

[spinoza]

I've built a little passive tone control but I guess I didnt do it right. I have massive gain loss.

I've searched and some seem to connect a resistor/pot with a cap in series, ground unaltered. Some plug a cap and a res/pot in parallel, ground unaltered, some plug a res in parallel with a pot to ground. I've tried several combinations but always get massive gain loss. I've also tried a couple caps and resistors.

So, what's the good way? Anyone has a schematics? What cap and res should I use?

[spinoza]

*bump*

What I actually want to know is how to make a passive tone pot, like the ones in the guitars. My bass does not have one and I'd like to add it. Problem is I get almost no gain when I do this.

-g

[Mark Hammer]

1) What you really mean is "signal loss", not "gain loss".  What you really want to ask is how to roll off highs without losing too much signal through passive loss.

2) A treble-cut style of tone control is a simple matter of having a 500k pot, wired as variable resistor (wiper and one lug) and a cap in series going to ground.  For bass, a cap value of .022uf is probably appropriate though some might find .033uf or even .047uf more suitable for their needs.  The higher the cap value the more mute-sounding the maximum treble cut will be.

3) My personal fave these days is to use a 1meg pot wired up as a bi-directional tone control, such that you get one kind of rolloff in one direction, a different one in the other direction, and no rolloff in the middle setting.  To do this, the pot wiper goes to the input of the volume control, and two different caps get soldered, one to each outside lug.  A 5 to 1 proportion in value is probably about right (e.g., .01 on one side and .047 on the other).

[Roobin]

Mark, this is different to your Stupidly Wonderful Tone Control, is it not?

Or am I a dumbo to the highest degree?

[Mark Hammer]

No you are not dumbo.  You are observant.  This IS different.  The SWTC is also a lowpass filter, but it is intended to be placed in an effect such that it maintains a constant series resistance in front of the volume pot.  What I suggested here mimics having two separate "normal" tone controls (the kind that bleed treble to ground as the pot resistance is made smaller) crammed into one pot.  The pot is wired up as two variable resistances to ground going through two different caps.  The reason it is 1meg is that when set to the midpoint, it's like having two 500k pots set to no rolloff at all.  Rotate it in one direction and that resistance gets lower while the resistance on the other side of the wiper goes higher.  The two caps are different values such that in one direction you get degrees of "rounding" of the tone, while in the other you get more dulling or muting of the tone.  Sometimes you like one, and sometimes you like the other.

Alternatively, if a person had a suitable inductor, you could use a cap and inductor on one side such that one direction gave you treble cut while the other side of midpoint gave you mid-cut.

As another alternative, old two knob (volume/tone) Fender amps would use a nifty tone control that was a combination brite switch and treble cut.  The tone pot wiper is soldered to the volume pot input lug.  One side lug of the tone pot goes to a medium-value cap and then ground, and the other pot lug goes to a small-value cap and then the wiper of the volume pot.  Rotate the tone pot one way, and the resistance in series with the treble bypass cap (the small value one) gets big while the other resistance gets small.  So, more treble bleed and less treble bypass.  Rotate the other way and the resistance to ground gets big while the bypass resistance in series with that sm,all cap gets smaller.  So, less treble bleed and more treble bypass.

[Roobin]

Wow. I love your detailed answers Mark. So considerate and benifical.

I am intrigued by your first idea, with two caps, one on each side. I must try this. The idea of 'rounding' with the option of 'muting' looks cool. I am one for versitility, so this sounds like a plan.   

[Jeremy]

As another alternative, old two knob (volume/tone) Fender amps would use a nifty tone control that was a combination brite switch and treble cut.  The tone pot wiper is soldered to the volume pot input lug.  One side lug of the tone pot goes to a medium-value cap and then ground, and the other pot lug goes to a small-value cap and then the wiper of the volume pot.  Rotate the tone pot one way, and the resistance in series with the treble bypass cap (the small value one) gets big while the other resistance gets small.  So, more treble bleed and less treble bypass.  Rotate the other way and the resistance to ground gets big while the bypass resistance in series with that sm,all cap gets smaller.  So, less treble bleed and more treble bypass.

Dang!  That's what that Fender tone control does!  I've been wondering how that think worked for quite a while...  I need to read up on my AC circuit analysis.  Thanks, Mark!

[Mark Hammer]

I am intrigued by your first idea, with two caps, one on each side. I must try this. The idea of 'rounding' with the option of 'muting' looks cool. I am one for versitility, so this sounds like a plan.   

You're welcome.  I have an ongoing grudge against most tone controls on guitars and basses.  It seems like most people (with the possible exception of Hartley Peavey) just stopped thinking about them around 1954 or so.  Tone controls for bridge pickups ought to be "voiced" differently than those for neck because we want different things from each of those pickups, yet most guitar tone controls are "designed" (and boy is that a strong word to use for something consisting of a pot and a cap) as if every pickup had the same things you wanted to accentuate or adjust.

Although it has less utility for bass than it does for guitar, having the bidirectional tone control means that only half as much pot rotation (from the mid-point) is required to make major adjustments, and a mere 40 degrees of rotation can change a lot.  That lets you use the tone pot as a "wah" controlled by your pinky finger.

[spinoza]

1) What you really mean is "signal loss", not "gain loss".  What you really want to ask is how to roll off highs without losing too much signal through passive loss.

Exactly. Sorry for my newbie-ness.

2) A treble-cut style of tone control is a simple matter of having a 500k pot, wired as variable resistor (wiper and one lug) and a cap in series going to ground.  For bass, a cap value of .022uf is probably appropriate though some might find .033uf or even .047uf more suitable for their needs.  The higher the cap value the more mute-sounding the maximum treble cut will be.

Cool! I'll go for the .047uF since the bass is used mostly for reggae and needs to sound really muted.

3) My personal fave these days is to use a 1meg pot wired up as a bi-directional tone control, such that you get one kind of rolloff in one direction, a different one in the other direction, and no rolloff in the middle setting.  To do this, the pot wiper goes to the input of the volume control, and two different caps get soldered, one to each outside lug.  A 5 to 1 proportion in value is probably about right (e.g., .01 on one side and .047 on the other).

I'm not sure I understand. Do you mean like this?

[Mark Hammer]

No.  The wiper is the middle lug.  One cap goes from outside lug to ground, and the other goes from the opposite outside lug to ground.  The wiper is connected to the input on your volume control.

This gives you two paths to ground for the signal to "bleed" through from the wiper (which is also the volume input lug).  When the pot is absolutely dea center, the signal has a 500k path through one cap, and a 500k path through the other cap.  That's like having two separate tone controls, both of them turned "off" (500k is common value for a tone control pot).  If you move the tone pot a little in one direction, the resistance from wiper to ground on that side is reduced a bit, and the resistance on the other side is increased by the same amount.

Does that give you a better mental picture now?

[spinoza]

More like it?