Red LLama, Stupidly Wonderful Tone Control problem

[Andrekp]

This MUST be something simple, but I'm just flummoxed...  At least you might be able to tell me that what I am doing just won't work, or whatever:

Built a Red Llama on vero.  Made a couple of small component value changes, but largely as you'd expect.  Works great, sounds great.  No problem there.

Then I wanted to add a simple tone control to eliminate some of that prodigious high-end.  I came across the SWTC and thought it would just the ticket.

As the value of the output pot is 10K, I figured that the SWTC pot should be 1K and the in-line resistor 100R to minimize effect on overall volume.  Using that as a starting point I calculated the cap of the RC filter should be .22uF, giving a rolloff of 658Hz to 7235Hz.  I think all of that is correct - in theory.

I created a small vero layout with the two components (100R, .22uF) and the pot.  Basically, output from the 10uF on the main board (that used to go directly to the Vol Pot, now goes to one side of 100R, then out the other side of the resister to Pot1, Pot2(wiper) is connected to one side of the cap.  Other side of cap connected to GRND at output jack.  Pot3 is connected to the output volume pot.  The only signal continuity is through the 100R, then across the entire 1K SWTC Pot, then out to the Vol Pot.  This all seems right and is according the the SWTC schematic.

Here's the result:
The Pedal works fine and has the same basic sound as before, but with some treble rolloff.  If I rotate the 1K SWTC pot, the amount of rolloff does not seem to change.  It sounds like it's at full value everywhere.  If I change the caps for different values, the sound changes overall, as I would expect, but the Pot still has no effect.

I've double checked the layout a bunch of times.  I've checked the resistance at the top of the cap as I rotate the pot, it's about 100R to 1K1, as expected.  I've checked the series resistance at the top of the volume pot, it's about 1K1, unchanging, as expected. I've checked continuity where relevant, it's fine.  The pedal still sounds fine generally, it's just that the SWTC Pot seemingly has no varied affect on things.  (It shouldn't be THIS subtle should it?!)  This thing is SO simple, either I'm missing something obvious, or it just doesn't work in its original form used in a Llama.  I'm banking on the former.

I did some searching and while I find a few references to tone controls on the Llama, I fund nothing helpful to me.  And only one mention that someone specifically used a SWTC successfully - meaning it's possible.  Does the SWTC need to be wired in differently in this circuit that as found at http://www.muzique.com/lab/swtc.htm ?  Or...?  Any ideas?

Any ideas appreciated,
Andre

[Hatredman]

Any pics so we can try to spot something?

Scarlett Johansson uses a Burst Box with her Telecaster.

[Mark Hammer]

Your math seems correct, and your description of the connections also seems correct.

I should say that the treble rolloff is not profound.  You may find that you need to rotate the pot between the 7:00 and 5:00 positions to hear something obvious.

It can happen that pot lugs become loose for whatever reason, and continuity is interfered with.  So, if, for instance, the wiper lug had been loosened, such that the solder lug was not making good contact with the wiper, your path from outside lug to outside lug would be just fine (which is why you have output at all), and the lug-to-ground connection through the cap would be good, but moving the wiper around wouldn't result in any varied lowpass filtering.

Admittedly, this is a longshot, but it will be helpful to know that's NOT the problem.  If you have suitable needlenose pliers, give that centre lug rivet a little pinch, just to make sure it's making good contact.

[Plexi]

I had a similar trouble when placed the tone stack after the volume pot.
I would use a simple treble roll off here

[Andrekp]

Not sure pics would be easier to see than my written description.  It's obviously a very simple circuit.  Just checked it over again, and I honestly see nothing and it is as described by me and on the AMZ website...

It has been placed BEFORE the Volume control (actually between the main board of the Llama and the volume control.  I understand this to be the intended placement.

Thanks, Mark, for confirming my values.  Not sure about the Pot suggestions for two reasons:
1). I just checked the Pot about as thoroughly as I can.  Checked resistance and continuity between each and every point that should and should not have it - it all checks out.
2). I built TWO of these things (the Llamas) and built two identical SWTC and installed one in each.  They BOTH manifest the same symptom.

I wonder if there is any possibility that the SWTC Pot is somehow acting as an RC filter in both directions (from the main board, and from the Volume Pot), giving a total RC filtration of 1K1 + .22uF, no matter where the pot is set?  Maybe having something to do with the low value of the SWTC Pot...?

I just DON'T see anything wrong with it.  I'm about to go over it again.  The fact that the signal DOES flow all the way through and I get the expected, though rolled-off-treble output, suggests to me that the circuit is working fine, it's just not varying.  Barring two identically faulty Pots, which are also undetectable as faulty, I just don't SEE anything.

I guess what I am wondering is: Has ANYONE actually built a Red Llama with a 10K volume pot and then added a SWTC and made it work?  What values were used?  How did you insert it?  Maybe it just doesn't WORK in my situation.  Has anyone done it before?

[Andrekp]

OK, here's some images.  The first is the schematic and layout, just so we all know what I intended.  (didn't show the wiper of the Volume Pot)



Now here's the front and back side of the board:





(The cap is just one I was most recently experimenting with, not the .22uf I intend to use.)

The purple wire coming in at the upper left is the output from the main board.  The signal then passes down that 100R resister.  next the red wire at the bottom left on the board connects to Tone Pot 1.  You can see on the backside view that I have separated the top and bottom rows at this point, so this is a discrete section of the SWTC circuit.

Pot 2 connects to the red wire coming in on the middle bottom of the board.  this is connected to one side of the yellow cap.  beyond the yellow cap, that section of the vero is isolated as well, as you can see on the back (though it really doesn't need to be).  The other side of this cap is connected to the middle top row, and that middle top red wire.  That red wire is connected to the sleeve of the output jack. (GRND).

Pot 3 is connected to one red wire in the middle row on the right of the board.  The other red wire, electrically connected to the previous red wire from Pot 3) is connected to the Volume Pot.

Previous to me installing this SWTC, the purple wire mentioned above as the input to this SWTC board, was connected to the Volume Pot lug just mentioned.

As I said, EVERYTHING measures out as expected.  the Tone Pot measures out as expected when you turn it.  It just doesn't work as expected as a circuit.  It ACTS as if the RC filter is always seeing 1k1 (100R + 1K Pot).  You can change the cap to an appropriate value for 1K1 and get the rolloff that you'd expect to see with that value + 1K1.  The Pot just doesn't change the sound AT ALL when turned in circuit. 

One more pic:


If you can make it out, you can see that the purple wire does come off the main board, as I said.  And you can see the three wires to the tone control pot underneath the SWTC dughterboard.  You can see the red wire ground connection to the sleeve of the output jack, and you can see the red wire that you'll have to trust me goes to the output lug of the Volume Pot, under the main board.

As I really don't think I have two identically bad Pots that have both failed in some way I can't measure, (remember, I made two of these and they are both acting the same) the problem MUST be one of two possibilities:

1). There is something obviously wrong with my circuit layout of this SWTC, or
2). The circuit, for whatever reason, is incompatible with the Red Llama.

Not sure what can go wrong, but clearly SOMETHING is.

[Mark Hammer]

Others HAVE done what you did, and reported success here, although I can't remember what pot values they used.

Given that the actual effect of a working SWTC is modest, you might want to consider just using a 3-position toggle for some fixed tone presets.  So, omit the 1k pot and 100R resistor, and replace with a 1k fixed resistor.  From the junction of the 1k resistor and input to the 10k volume pot, run a wire to the centre lug (common) of a SPDT on-off-on toggle.  Now, from the outside lugs of the toggle, run two different cap values to ground.  Let's try .22UF and .047UF.  That will mimic what you were aiming for with the SWTC in the fully treble-cut position, and around halfway or so, in addition to full treble.

That might be simpler and less frustrating.

[Andrekp]

Others HAVE done what you did, and reported success here, although I can't remember what pot values they used.

Given that the actual effect of a working SWTC is modest, you might want to consider just using a 3-position toggle for some fixed tone presets.  So, omit the 1k pot and 100R resistor, and replace with a 1k fixed resistor.  From the junction of the 1k resistor and input to the 10k volume pot, run a wire to the centre lug (common) of a SPDT on-off-on toggle.  Now, from the outside lugs of the toggle, run two different cap values to ground.  Let's try .22UF and .047UF.  That will mimic what you were aiming for with the SWTC in the fully treble-cut position, and around halfway or so, in addition to full treble.

That might be simpler and less frustrating.

It may well come to that, Mark. In was just trying to understand what could possibly go wrong in this simple circuit.

As I'm sure you saw, photos have been attached, so unless someone sees something wrong with my ad-on, in have to presume this circuit simply doesn't work in this particular case. 

I'm hoping it'll wind up being a big "DOH!" On my part when something is spotted.  [emoji1]

Sent from my XT1575 using Tapatalk

[Kipper4]

I dunno why it ain't working. I can't see where you went wrong yet.
I think your layout looks good.
I can't gather anything wrong from the pics.
It's a pity you can't check some stuff.

This should work.

I never thought I'd hear myself ask this but. Show us the lama please ?

[amz-fx]

First, use the multimeter and verify that the ground on the stripboard is 0 ohms when measured to the ground on one of the jacks or to the volume pot's ground.

Cmos inverters used as amplifiers do not like capacitive loads. They lose gain and bandwidth. This might not be the right tone control for it. 

Best regards, Jack

[Andrekp]

First, use the multimeter and verify that the ground on the stripboard is 0 ohms when measured to the ground on one of the jacks or to the volume pot's ground.

Cmos inverters used as amplifiers do not like capacitive loads. They lose gain and bandwidth. This might not be the right tone control for it. 

Best regards, Jack

Ground connections at the output jack from both the main board and the SWTC board read continuity, so that's not it.

I'm really starting to believe that this is something to do with the interaction of the 10k volume pot and the 1k tone pot causing the RC filter to be always full on.

Maybe someone will spot something though.

The capacitive load theory is interesting, except that this supposedly has worked before for some people. (and isn't the 10uf output cap found in the Llama effectively a capacitive load on the 4049?)

Thanks for the ideas so far...
Andre


Sent from my XT1575 using Tapatalk

[dschwartz]

What if you use a 100k for volume and 10k for swtc?
The cmos would be more comfortable.
Although as you have shown, it should work.. I'll bet there's a hidden mistake somewhere...let it res for a day and check it with a fresh pair of eyes

[reddesert]

I'm really starting to believe that this is something to do with the interaction of the 10k volume pot and the 1k tone pot causing the RC filter to be always full on.

Maybe someone will spot something though.

The capacitive load theory is interesting, except that this supposedly has worked before for some people. (and isn't the 10uf output cap found in the Llama effectively a capacitive load on the 4049?)

That - the RC filter being always full on - doesn't happen.  The SWTC is basically a voltage divider that controls what signal gets to the top lug of the volume pot. But it's a frequency dependent divider. If you think of the capacitor in terms of its reactance - for a signal at frequency f, the reactance is like a resistance = 1/(2*pi*f*C) - you can draw it as a resistor network and calculate how much of the input signal gets to the volume pot.

I think the problem could be that by making all the R values about 10x smaller than Mark's original SWTC and the capacitor 10x larger, you're making the impedance very small for modestly high frequencies and loading the output of the 4049. You could try replacing the volume pot with a 50K or 100K and going back to Mark's original SWTC component values.

[Bill Mountain]

When ever I build with CMOS I keep in mind that it is not an opamp and does not have a low output impedance.  For this reason, I scale up any filters after a CMOS stage and I assume the CMOS has a 1k ohm output impedance (not measured - just assumed) in my calculations.  So, DSM's suggestion of a 10k and 100k pot would be my first choice.  And then go higher if need be.

As it stands the filter cap is seeing the output of the CMOS inverter  (not a perfect amplifier), the 100R, and the 1k POT.

Good luck.

[Andrekp]

@reddesert:

I think I was expressing that it sounds like the filter is always on (i.e. the treble is clearly rolled off JUST AS IF the had maxed out the treble roll off on the tone control).  So effectively, it IS always fully on, the question on the table is why?

I fully understand the calculations involved, and in fact if you look at Mark Hammers original posting on this control, as I did to get the idea in the first place, he is somewhat agnostic about the "correct" values to use, simply stating that ratio is important to control gain loss (and of course the cap value must be chosen appropriately.

I think you are likely correct in your second paragraph and that the values in use in MY pedal are not capable of working correctly.  Either because they are objectively (not relatively) low, or because of some interaction with the 4049 chip, or some combination of the two factors.

@Bill Mountain:
This is likely to be true.  Unfortunately, I do not HAVE a 100K audio pot on hand.  So I think, until I need to make another order in which to slip two in, I may just have to pass for now, and do as Mark suggested earlier and just rig up a switch instead of a pot, with some predefined values.

I DO have a pairing of 500K Audio and 50K linear though, I wonder if that would work, or if the 500K might be too much...?  Since this is just a voltage divider network to ground does it just not matter, of will the 500K have an adverse effect on something?  Looks like, 100R into a 50K tone pot into the 500K volume pot, using a .0033uf cap should put me in the ball park.

I remain wonder has ANYONE here actually put a successful SW tone control into a Red Llama (supposedly they have), and how?

[Kipper4]

Have you considered it could be you hearing.
I know mine is wonky and some regions stand out more than other.
This may account for why you didnt hear as much of a change as anticipated.
I could be off beam with this. Ignore me at will.
I go.

[Andrekp]

LOL, believe me, I've thought of that.  But I just hear NO change at all.  I think I'd here SOMETHING, since the treble rolloff is so obvious when I swap caps to "simulate" a working TC.

[Andrekp]

OK, Success:

I changed the 1K SWTC pot for a 50K linear, and the 10K Audio Volume pot to a 500K Audio.  Cap is a .0033uf for now.  I left the in line resistor at 100R, because, who cares?  When I turn the SWTC I now hear a definate rolling off of the highs.  As suggested it IS subtle, but it's definite.  You can consider my above layout verified, with appropriate values, if you like.

I would have preferred to start with a 10K/100K combo, but parts not in my stock.  Next order, I might get some and try that combo.    As it is I had to use salvaged guitar pots for the 500K, which are rather larger and make it a VERY tight fit to get my SWTC in place next to it.

Anyway, it works.  Thanks to everyone who added wisdom here.

I still would very much like to see red LLama tone controls that OTHERS have implemented.

-Andre

[Mark Hammer]

Mazeltov!  I like hearing about triumphs. no matter how small.

Ultimately, you may well decide down the road that a 3-position toggle gives you all you need.  It's like that sometimes.  People think they want a continuous control, and over time find that all they really make use of is the 7:00 and 5:00 positions, and occasionally something in the middle.  But, like anything, you don't know until you try.

[Andrekp]

Thanks, Mark.

Modded the twin build as well to the new specs.  Works fine.

I may leave it continuous for now.  My amp is a rather bright 18watt type with EL84's, so being able to roll off that piercing high is nice, but I don't always play that same amp, so variability might be nice.  I don't know, it works, so...

Thanks again,
Andre