"Bright" cap...again

[Chuck H]

OK... Has any one come up with a circuit that actually sounds like your guitar when turning down the volume pot.

Currently I am using a 470pf cap across a 500k pot with a H-S-S configuration. It's a bit too boomy with just a tiny bit rolled off, boomy and bright lacking mids in the middle and too bright when down low on 2 or 3.

I've considered using a dual ganged pot to implement a gradual circuit But I don't have the freedom to experiment that I used to have (when I was young and single ).

Does anyone here know of some tried and proven RC neworks that will give a more predictable result?

Any input is appreciated.

Chuck

[David Schwab]

You can mess with RC values and get close, it's probably different for each pickup/pot combination.

The only thing you can do to get no change in tone when you turn down is install a buffer preamp.

[Mark Hammer]

The bright cap provides a kind of shortcut-for-treble, but the fundamental question to ask is "Shortcut relative to what?".

Some Fenders will use a cap plus fixed resistor for the volume pot bypass/compensation so as to introduce a different "taper" to that compensation. In other words the compensation will exhibit a more consistent degree of change over more of the pot's rotation.

A bit of theory will help here.

Look at it this way. The volume pot is made up of two complementary resistances which we will call R(i) and R(g), each on the other side of the wiper. The overall volume is a function of the proportion of the overall pot resistance that sits between the wiper and ground. If your 500k is turned up full, then 500k of that resistance is between the wiper and ground - R(g) - and zero ohms is between the wiper and the input - R(i). So, no attenuation. Move the pot a bit and maybe R(i) = 80k, and R(g) = 420k, introducing attenuation.

But here's the thing. Any bypass cap straddling the input and wiper provides a parallel path for high frequencies where R(i) = 0 for those frequencies high enough. How high is "high enough"? That depends on R(g). So if the cap is 470pf, and R(g)=420k, then the frequency where that cap starts to pose a zero-ohm path is F = 1/(2*pi*R*C) = 1/(6.28*.42M*.00047uf) = 541hz. Turn the pot down more such that R(g)=350k and that frequency moves up to 967hz. So, as the volume gets turned down, the point where the "treble advantage" kicks in moves upwards.

But how much advantage? Well, that will depend on the volume pot setting. If I have dropped the obverall output level for the entire spectrum by half, but I still have a treble advantage so that any content above frequency X gets a free pass without any attenuation, then the relative volume of that treble content will be double what it was when the volume was up full.

The value of the added fixed resistor in Fender's scheme is that the fixed resistor (which subs for R(i) just for the highs), in tandem with R(g), also turns down the highs a bit, though not quite as much as the rest of the volume pot turns the remainder of the signal down. The net result is that you don't encounter a zone where the bypass cap introduces disproportionate harshness or bite over too much of the pot's range.

I can't tell from here what portion of the top end you want to keep (or even HAVE to keep), so it is difficult to recommend values, but here is a start. First off, I think you may want to reduce the value of the bypass cap to 390pf or even 220pf. That will move the rolloff point upwards. With 390pf your treble advantage at the R(g) = 420k and 350k marks are 971hz and 1166hz respectively. 220pf at those same points gets you 1722hz and 2067hz respectively. Maybe a value of 270pf would be bang on (1403hz and 1179hz). The other thing would be to install a fixed resistor of perhaps 22k-47k in series with that cap so that the treble advantage applied is gentler at the top end of the volume range.

Try that and see if it makes a useful difference. Certainly there should not be any need to opt for dual-ganged pot solutions.

[Chuck H]

Great post Mark. Good info on the subject that I've never seen discussed here in full tech.

But I guess I misrepresented myself in my post. I design guitar amps so I do have an understanding of how the pot and bypass cap function together. But it's more of an intrinsic understanding as I am formula impaired (read: mid 80s HS education). The formulas go over my head, but not the implementation. If you get my meaning. I have landed on the 470pf value as the best "macro" compromise, and was hoping for some practical experience on the matter to save time and solder.

But, when you consider the effects of the pot value and specific pickup influencing impedance, and therefor shelf frequency, it's becoming clear that there is no "one size fits all" solution. Too bad. I wouldn't have thought this would be finicky circuit.

I'll be re reading your post to take what I can from it and continue experimenting.

Thanks for the lesson.

Chuck

[Chuck H]

David, I'm seeing your posts all over the place. Here and sometimes on the amps forum where I hang. It's clear that your alot like me. That is to say, you have enough rationale and experience to know what your talking about...But no EED to back it up.

How'd I do?

That considered, I'll have to believe you about a preamp being the only truely transperant solution. Not an option though. However, the passive pickups and pot values we all use just aren't that damn dissimilar. I guess I can't expect perfection, but I intend to experiment some more. I'll let you know if I come up with anything cool.

Thanks

Chuck

[Mark Hammer]

Thanks, and apologies for underestimating your chops.

I think the challenge you face is that of identifying a bypass cap value which is suited to both a SC and HB pickup; a bit like trying to plan a family dinner for the meat-and-potatoes uncles and goth-vegan nieces and nephews.

In some respects, maybe one way to address your challenge (without routing and installing extra switches) is by means of a pickup selector switch that includes additional poles which can be used to changeover bypass components when the HB or SC pickups are selected on their own. So, perhaps 180-220pf when using SC pickups only and 470pf whenever the HB gets added to the mix.

The other thing is that, if you are sporting a SC-SC-HB configuration, might I assume we are talking about a Strat-alike of some sort? And if so, might I assume we are talking about one volume and two tone pots? If that is the case, one of the tone pots can be easily co-opted to perform some other function that may be exactly what you need.

[David Schwab]

David, I'm seeing your posts all over the place. Here and sometimes on the amps forum where I hang. It's clear that your alot like me. That is to say, you have enough rationale and experience to know what your talking about...But no EED to back it up.

How'd I do?

Yeah, I spent too much time on the computer! I've worked in electronics, and read a lot over the years... but I'm totally self taught in most of the things I do.

I'm actually supposed to be working on a website for someone, and I end up here...

That considered, I'll have to believe you about a preamp being the only truely transperant solution. Not an option though. However, the passive pickups and pot values we all use just aren't that damn dissimilar. I guess I can't expect perfection, but I intend to experiment some more. I'll let you know if I come up with anything cool.

A preamp opens another can of worms though. Once you remove the loading from the pickup it doesn't sound the same as it did passive. I like preamps in my basses, but I had one in a Tele style guitar I built, and it was too bright. But you can adjust the preamp's input impedance, or add some RC loading to compensate.

For a passive setup Mark has the right ideas. I was thinking along the same lines, but without the math! I'm too ADHD for the math anymore, so I just grab parts and experiment. Well I do a little math....

Hey Mark, I have a situation I'm going to work on when I get a chance, but I thought I'd run it past you and see if you've done anything like this. I put a 500K audio M/N taper blend pot in a bass. It works fine as far as giving full output on both pickup when in the middle, and at the ends works fine. But the in-between settings are too close to the middle. So you go from both pickups, to the one, almost like a switch. You can get some blending, but it's a very small range.

So I'm thinking of adding a couple of resistors to linearize the taper slightly. I'm thinking 20% of the resistance, or 150K should get me started. Have you ever tried anything like this? I know you can fake a log pot with a linear this way, but I'm trying to add a bit of anti log to the log I guess.

[BackwardsBoB]

With some of my guitars, I find it handy to be able to turn the volume down to "9' from "10" and roll off the brightness, kind of like an amp presence control. The volume doesn't go down much, but the tone changes dramatically. We want the volume knob to do volume, and the tone knob to do tone, but you ain't gonna get all the way there with a passive circuit and a high-impedance pickup output.

In a perfect world, the guitar wouldn't have a tone control. It would be located in the amp after the first tube stage, and there would be a pretty lady (that wifey couldn't see) to read your mind and adjust it for you.

[David Schwab]

In a perfect world, the guitar wouldn't have a tone control. It would be located in the amp after the first tube stage, and there would be a pretty lady (that wifey couldn't see) to read your mind and adjust it for you.

I don't agree. A passive tone control gives a real nice resonant hump at the cutoff frequency. If you use a smaller value cap, you can get a great "wah pedal in one spot" tone. Tone controls in amps don't do the same thing at all. If the tone control is clobbering your tone when it's turned down, use a smaller value cap, like 0.01, or even 0.005. I only have one bass with passive controls, and I use a 0.02 instead of the usual 0.05 or 0.1. Just rolls the high end off and leaves all the chewy midrange!

I'm a big fan of the Clapton "woman tone", and you need a tone control for that. And when you switch both pickups on, it changes the frequency and Q of the tone control, so it's very interactive. The tone control combines with the pickup to form a resonant circuit.

If I had to have one or the other, I'd have a tone control and no volume on a guitar! I only use the volume to mute the sound most of the time.

The only pretty lady I would want to touch my amp is my wife. If I didn't like her I wouldn't have married her! I don't change my tone settings on my amp once I'm set up for the room I'm playing in.

[jrfrond]

I don't agree. A passive tone control gives a real nice resonant hump at the cutoff frequency. If you use a smaller value cap, you can get a great "wah pedal in one spot" tone. Tone controls in amps don't do the same thing at all. If the tone control is clobbering your tone when it's turned down, use a smaller value cap, like 0.01, or even 0.005. I only have one bass with passive controls, and I use a 0.02 instead of the usual 0.05 or 0.1. Just rolls the high end off and leaves all the chewy midrange!

Which is why all of my personal guitars have a rotary switch with 5 switchable caps. Sometimes you want to really squash the highs, and sometimes, you just want to give it a "haircut". Also the reason why I prefer passive controls. My basses have two caps, one at either end of a 500KL center-detent control going to ground.

I'm a big fan of the Clapton "woman tone", and you need a tone control for that. And when you switch both pickups on, it changes the frequency and Q of the tone control, so it's very interactive. The tone control combines with the pickup to form a resonant circuit.

Which is why the bright cap mod doesn't work well in an H-S-S axe, because the inductance is doubled on the humbucker. You'd have to compromise the value to work for both. Hey, here's an idea: if you use a Super Switch in a Strat, you can use one of the remaining two sections to change the value of your bright cap.

If I had to have one or the other, I'd have a tone control and no volume on a guitar! I only use the volume to mute the sound most of the time.

Totally agree!

[Mark Hammer]

I like your thinking.

First, we both agree on the potential for a Super-switch/Mega-switch and adapting the bypass cap to suit the pickup/s in question ( http://music-electronics-forum.com/s...0811#post70811 ).

Second, I also use a "bidirectional tone control", except I haven't been able to score a pot with a centre detente. I also use a 1M pot, since in the centre position it equates to two 500k tone pots in parallel, which provides a certain amount of treble bleed-off.

On my guitars, I tend to use my volume control AS a tone control. I use a 500k volume with SC pickups, and a larger-value bypass cap, with the net result that when I turn the neck pickup down to maybe 3 or 4, I'm looking at Nile Rodgers tone on my neck pickup.

David is spot on in mentioning the manner in which passive controls on the guitar can serve interesting roles in tone-shaping that are hard to mimic on the amp. I'll just point out that having dual volume pots that interact also provides a means for altering tone.

[jrfrond]

The interesting thing here is that we have established that, while passive electronics aren't perfect, they do have certain sonic advantages and organic responses over opamp filter and buffer schemes. Personally, I love the way passive guitars and basses interact with the 1st preamp stage (no wireless, thank you!) or even EFX boxes. David mentioned that "resonant hump" at the cutoff frequency on a passive control. Man, I LIVE there, especially on bass. This is where basses growl and guitars sport woman-tone. You just can't get that from active. It's more sterile, and too clean. It's all about feel, really.

[David Schwab]

Hey, here's an idea: if you use a Super Switch in a Strat, you can use one of the remaining two sections to change the value of your bright cap.

That's a good idea.

[David Schwab]

The interesting thing here is that we have established that, while passive electronics aren't perfect, they do have certain sonic advantages and organic responses over opamp filter and buffer schemes. Personally, I love the way passive guitars and basses interact with the 1st preamp stage (no wireless, thank you!) or even EFX boxes. David mentioned that "resonant hump" at the cutoff frequency on a passive control. Man, I LIVE there, especially on bass. This is where basses growl and guitars sport woman-tone. You just can't get that from active. It's more sterile, and too clean. It's all about feel, really.

Yeah, I had a buffer in my Tele for a while, but it was too brittle sounding... it's already plenty bright. I tend to leave my guitars passive, and I LOVE my 0.02 tone control cap!

I had been using active basses for a long time until I removed the preamp to test my new pickup designs, which was the first time I've used a passive bass in about 25 years. I liked it, but also missed being able to adjust the tone from the bass, so I put the preamp back in. I like that too, but missed the passive tone... so I installed a push/pull volume pot to bypass the preamp.

Now it's perfect.

I'm working on a low pass filter preamp.. that's a good way to get the best of passive and active tone.

[Chuck H]

I also use a "bidirectional tone control", except I haven't been able to score a pot with a centre detente. I also use a 1M pot, since in the centre position it equates to two 500k tone pots in parallel, which provides a certain amount of treble bleed-off.

Hey Mark, try this:

Take the back off a 1M linear pot and scratch a big divot in the middle of the resistance track. Then it will be completely out of the circuit at the newly created "detent" position and there will be no interaction.

Chuck