On “anti-mud” caps in series with pickups: a side note...

freefrog

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Over times, I’ve seen series caps repetitively recommended as an anti-mud solution for neck humbuckers. Let’s dedicate a separate topic to this subject instead of polluting another thread. It has been wrote hastily @ 6AM before a working day so, take it accordingly. :)

Recommending a cap in series as a way to tighten a bass heavy PU is certainly meaningful, IMHO: Rickenbacker already mounted 4.7nF series caps in some guitars decades ago and it’s a good idea to extrapolate on this cheap easy recipe to solve quickly some issues with muddy bass (albeit a 4.7nF cap might contribute to a kind of “banjo tone” in some cases. LOL).

Now, IMHO, there’s a few things to keep in mind with series caps.


1-BEWARE WITH THE LOCATION OF A SERIES CAP.

If you try this in a guitar: any tone pot would better be mounted BEFORE the cap. In other words, you must have pickup(s)> volume & tone controls > cap > output.

If the cap comes first, the tone pot will behave a bit like a second volume control.

It’s not an indifferent advice: the Burns Bison 62 RI, for example, has been designed with a series cap before the tone pot, making this tone control unusable when the guitar is played on its “Wild Dog” setting. I had to correct this wiring once and found it rather problematic for a stock guitar.


2-LET’S THINK TWICE ABOUT EFFECTIVE CAPACITANCE.

The thing with series caps is that physics make them behave in an opposite way compared to parallel caps.

When a capacitor is wired to ground in parallel with a pickup, the more capacitive it is, the more it affects the sound.

When a capacitor is wired in series with a pickup, it’s the contrary: the more capacitive it is, the less it changes the tone.

Whatever is the inductance and resistance of a pickup, a series cap of 0.1µF (100nF) would cut the frequencies below 20hz, which would be pretty useless. Even a 22nF (0.022µF) would hardly modify the lowest fundamental note of an electric guitar… It’s below 15nF (0.015µF) that things get useful in their action on fundamental frequencies produced.

That’s why the treble control of most guitar amps is based on a very low value cap in series (250pF for Fender amps, 470pF for Marshall, and even 47pF for Vox amps, whose “bright cap” is typically of the same incredibly small capacitance: 47pF = 0.000047µF).

Regarding the series cap mounted in Duncan Invader neck models and sometimes evoked online, the “56n” mentioned once by a member on the Duncan forum appears to me as a mistake: 56nF would cut the bass below any fundamental note and wouldn’t be efficient on a guitar… Maybe the cap mentioned “562”, which is a mark for 5600pF = 5.6nF = 0.0056µF.

But apparently someone found a 10nF cap in the Duncan neck Invader, as recalled in the topic below (where the post number 20 illustrates the mistake that I humbly try to prevent: the contributor appears to believe that 10nF is not a much effective value for a series cap while in fact and according to the laws of physics, it has MORE effect as a series component than a higher capacitance one).

https://forum.seymourduncan.com/forum/the-pickup-lounge/263198-what-does-the-capacitor-in-the-neck-invader-do/page2#post5251955


FWIW: the goal of this topic is essentially to avoid a waste of time and money to other members who would try caps in series.

Below a measurement: it shows the electrical response from 20hz to 100hz of a neck humbucker direct (brown), with a 47nF cap in series (red) and with a 4.7nF one (blue). For the record, the lowest fundamental note of an electric guitar is above 80hz.

I’ve cut the vertical dB scale: I most often amputate experimental results of essential information, for questions of intellectual property. But even without dB scale, anybody can see that electrically, the effect on the bass range of a 47nF cap in series with a pickup is close to… none. LOL.
 

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cooljuk

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@freefrog, good material, as always. Thank you very much for your contributions.

When playing around with small caps in series, such as you are discussing, I ran into a situation where I was watching the phase relationship of two pickups (one with a cap placed in and out of circuit in series, another without) shift back and forth on a four channel scope analyzing the input signal and the output of multiple coils at once.

Now, when I noticed this, I was actually working on something unrelated so I didn't dig too much into it, beyond just noticing it was interesting.

Do you suppose it's typical to see some frequency-dependent phase shifting with this type of filtering or was I just experiencing some anomaly of having my equipment set up for entirely different types of measurements, at the time?

I've been wanting to revisit what I was doing that day on my own time but it's hard, lately, to find time to dive into things that aren't prime concerns.
 

freefrog

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@freefrog, good material, as always. Thank you very much for your contributions.

When playing around with small caps in series, such as you are discussing, I ran into a situation where I was watching the phase relationship of two pickups (one with a cap placed in and out of circuit in series, another without) shift back and forth on a four channel scope analyzing the input signal and the output of multiple coils at once.

Now, when I noticed this, I was actually working on something unrelated so I didn't dig too much into it, beyond just noticing it was interesting.

Do you suppose it's typical to see some frequency-dependent phase shifting with this type of filtering or was I just experiencing some anomaly of having my equipment set up for entirely different types of measurements, at the time?

I've been wanting to revisit what I was doing that day on my own time but it's hard, lately, to find time to dive into things that aren't prime concerns.
Wow, James... I answer during a short laps of free time and if I understand your post correctly, I owe you a light bulb moment.

What happens when a pickup 1 is in parallel with a pickup 2, itself in series with a cap?

Pickup 2 is an inductor in series with a cap, placed between pickup 1 and ground.

IOW, it forms the kind of notch filter used in a Varitone (!)...

Consequence: an anti-mud cap would NOT protect the characteristic sound of pickups in parallel and should be disabled as soon as the de-mudded pickup is not used alone (?)...

It never came to my mind because I don't use series caps: I prefer parallel inductive filters because they sound more natural and are more flexible IMHO.

So, thx a lot to give me access to this discovery! :cool:

I'll try to do some ultra quick test(s) later today to check my hypothesis above.
 
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jvin248

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.

You'll want to have a series cap on the muddy neck pickup hot lead, such as where the hot lead was previously attached at the switch, because you may have a master-volume/master-tone guitar with a bridge pickup you like as-is. If you put the series cap after everything then you'll hit the pickup you like as well as the one you don't.

In practical situations .. I haven't found a problem with the remaining controls on a guitar when one pickup gets the series cap -- and it's not always the neck pickup, some bridge pickups need help instead.

It would be helpful to see chart runs for different layouts of HH with 2 and 4 knobs plus SS with 2 knobs and different series caps.

Generally, some brands of pickups have higher internal capacitance than other brands. Machine wound vs boutique hand-scatter-winding creates a clear difference but some machine winding is different than other machine winding. And, of course, some players want 'warm' (muddy) pickups because they have very bright amps.

The impact of short vs long guitar cables and their capacitance problem may be influenced by a pickup series cap too since that series cap is in-line with that extra long high capacitance cable -- which may be a useful finding.

.
 

freefrog

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I thank other members for any past or future contribution.

The goal of this topic being mainly to share useful info and not to argue, I'll avoid to discuss and will focus on providing data. Maybe I should have started with less words and more data, anyway (not easy @ 6 AM).

Below are pics explaining my BEWARE WITH THE LOCATION OF A SERIES CAP.

Pic 1 shows the electrically induced frequency response of a pickup with tone pot @ zero. Pink line = the expected and "normal" response of the pickup in such cases. Black line = the response of the same pickup with the same tone pot @ 0/10 but this time, this tone pot is "crippled" by a series cap located between pickup and pots. The difference speaks by itself when it comes to frequencies AND relative levels.

Pic 2 shows the frequencies produced by the same (neck) humbucker with its tone pot @ 0/10.

The guitar was plugged direct to the board through a standard 1M input and has been played in chords from unfretted strings to 12th fret (carelessly but with an even pick strenght).

In this case, the blue line is the response of the pickup without series cap. Orange line is the response of the pickup with a series cap located AFTER the tone pot. There's very little difference because the tone pot is properly located (before the series cap).

Pic 3 shows the same thing but this time, the orange line is with the tone pot AFTER the series cap. The drastic difference of output volume can clearly be seen and explains why I've evoked a tone pot behaving like a volume control.

If you want to avoid the disparity shown in pic 3 (translating itself pic 1), DON'T wire your tone control after a series cap.



Anyone is free to verify my statements. It's relatively easy to do and to reproduce consistently, since it has to do with physics...


EDIT - BTW, that's why such caps are generally mounted on controls giving hi pass OR low pass filters and not both. Such a dual control is easy to mount on dual Fender TBX pots, for those who would want to.

& I've added the screenshot of a 5spice sim showing the response of a pickup with tone pot @ 0/10, while a cap is in series before or after this tone control. This pic 4 should confirm that my experimental results are not due to the gear that I've used for my tests.

FWIW. :)
 

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freefrog

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Wow, James... I answer during a short laps of free time and if I understand your post correctly, I owe you a light bulb moment.

What happens when a pickup 1 is in parallel with a pickup 2, itself in series with a cap?

Pickup 2 is an inductor in series with a cap, placed between pickup 1 and ground.

IOW, it forms the kind of notch filter used in a Varitone (!)...

Consequence: an anti-mud cap would NOT protect the characteristic sound of pickups in parallel and should be disabled as soon as the de-mudded pickup is not used alone (?)...

It never came to my mind because I don't use series caps: I prefer parallel inductive filters because they sound more natural and are more flexible IMHO.

So, thx a lot to give me access to this discovery! :cool:

I'll try to do some ultra quick test(s) later today to check my hypothesis above.
Ok.

Done some ultra hasty tests and it didn't deny my hypothesis above.

As soon as it’s in parallel with another pickup, a.pickup in series with an anti-mud cap seems to behave like a notch filter (and is itself influenced as a transducer by the other pickup).

Pic 1 below is about 2 single coils in parallel, through normal pots and cable in a 1M input, without series cap. The crude resonant peak of each pickup has been measured while they were constantly in parallel. Below their respective resonant peaks is a zoom on their phase response in this case.

Pic 2 shows the same thing while the second PU is in series with a cap: The curves are now drastically altered.

Of course, this drastic difference is partly due to the experimental conditions involved here: in my test, each pickup was excited separately while in a real playing situation, both would be active voltage sources...

... BUT... playing the guitar a few minutes with these two kinds of wiring still reflected my screenshots to my ears: the presence of a series cap on ONE of the two pickups resulted in a narrowed frequency response exhibiting some audible comb filtering but perverting the typical sound of two pickups in parallel… a bit as if one of the two pickups had been moved under the strings to produce different harmonic peaks.

So, the anti-mud cap appears to be a solution with downsides: it affects the pickup itself but also its relationship with the other transducer when they're played altogether, apparently.

FWIW...
 

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freefrog

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A few adds about the action of series caps with a standard low output humbucker (8k, 4H) paired with 500k pots and a 370pF cable plugged in a 1M input.

I've used the typical simplistic spice model with a voltage source + LRC filter.

It's not that I find this model realistic (for ten years, I largely prefer our theoretical models based on transformers) but it will do the job here.

Pic 1 shows the hi-pass filter a.k.a "bass cut" effect due to the kind of caps mounted in guitars as standard tone capacitors.

Pic 2 shows the same thing with lower capacitance components.

EDIT: apparently, some guitars host a bass contour control with a cap of 2.2nF for more "bass cut".
Here is the related schematic, with the usual low pass tone control wired BEFORE the hi pass filter, as it has to be done (does it comes from Reverend guitars? G&L? Not enough time to check it):



FOOTNOTES:

-I've not shown what it would give with 300k, 500k and 1M pots, with 2 or 4 pots and so on. Reason: a change in the related resistive load has a negligible action on a hi-pass filter of several nF.

-I could have shown what happens when the amp has a non typical input impedance. If you use a Marshall JVM with a 470k input impedance, any hi-pass capacitor will have a greater effect than through a 1M input (and things will get worse with a tone pot after the series cap. LOL).

-it's 7 AM here while I write this before a working day. I've not enough time to refine my posts.... and no desire to be totally accurate scientifically. As a matter of fact, this topic was not meant to be a petty lesson : I just try to share what I consider as useful. Thx to read my contributions accordingly... and thx for any useful correction if ever my lack of free time made me do an error (errare humanum est and we are all human beings). :)
 

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