Pick Ups and Capacitors

[Will C]

Golly, I feel nervous about saying this but I am worried about Mark's answer. Q is not "gain". There is no "gain" in a pickup. There will be self-resonant frequency, and with inductance and capacitance in parallel, that resonance will produce a maximum response at a certain frequency.

What that frequency is, I can't measure, nor even offer an educated guess at, as my inductance meter won't register my guitar's pickup as an inductor. Without any doubt, adding the parallel capacitor will lower the resonant frequency of the combination, but I can't comment on where it will resonate before or after adding it. Adding a few hundred pF will not shift it by much. It would be swamped by the existing parasitic capacitance already present in the pickup.

It may be a matter of shifting the pickup's response away from some inner mechanical resonance, but I doubt it. You notice that Mark is careful to say that he has made an assumption. Knowing how keen manufacturers are to reduce production costs, he asks us to believe that a manufacturer would fine-tune a guitar, using ear tests. I don't buy that, it just doesn't seem likely.

When in doubt, go for the obvious answer first. In view of the prevalence of high power local AM radio stations during the production years, I'm sticking with the view that the cap is probably to shunt away radio frequency signal pickup. Knowing how easy it is for ham radio operators to be plagued by pickup in nearby audio amplifiers even nowadays, it seems a whole lot more likely as an answer.

Removing such a low value capacitor is unlikely to make the signal "brighter" - it's a hundred times too small. The effect would be too small for human ears to notice in our part of the spectrum.

I would not regard this as an issue on which any definitive answer has been given. What we need is hard facts. Since we will never know what the manufacturer was thinking, we will have to get some measurements made. Mark, what kit have you got that would help us clarify the issue?

[Mark]

Will, I seriously doubt someone in your profession would be nervous to question... LOL! No need to be nervous

I used the words gain and Q not in their technical sense, but as words equipment users might generally recognise. Most users would understand gain to mean volume or level and many users would understand Q to mean the width of a frequency band being affected. Of course when engineers use these terms with their greater knowledge it suggests a tighter definition - I also referred to it as a peak or bump in the overall response, by gain I mean the level of the peak or bump and Q the width of the peak or bump.

Yes, it is difficult to measure the inductance of a pickup! Pickup research seems to be a costly business... Telonics production pickups are tested and checked to see if all of about 15 measurements fall with the tolerance values. This level of testing is necessary to ensure consistency between pickups. The equipment used to perform this degree of testing isn't cheap and challenges even the measuring equipment manufacturers! When development pickups are tested even more parameters are evaluated. I must admit my own expertise in this area is far below Dave's, Telonics has about 40 years of experience dealing with wound component technology for various applications.

The exchange of information between guitar and pickup manufacturers has been going on for years. In my experience of dealing with six string guitar builders and listening to pedal steel builders talk, the builder/designer of instrument and some trusted players with 'good' ears typically over a period of time home in on the weaknesses and feed information back to the pickup manufacturer. This is a long term process, so guitar builders often use the capacitor method of shifting the self-resonance as a 'quick fix'. I should say, the fitting of a capacitor across the pickup to shift the self-resonance frequency has been used almost as long as the pickup itself has been about. One of the famous players to use this method of tweaking the pickup response is none other than Hank Marvin.

With all this said, why not try adding a capacitor across your pickup, experiment with values and use your ear? You might be surprised how small capacitor values affect the higher frequencies Keep in mind the terminating impedance also affects the self-resonance. When I've looked at pickup and speaker curves it never fails to amaze me what looks like a small change on a graph has a significant affect when you hear it.

To date I've never heard a guitar or steel guitar builder suggest they fit a capacitor across the pickup to shunt away radio signal pickup. You might also question the logic shunting away RF at the pickup when there is no demodulator diode and the pickup is connected to the amplifier via a long screened lead... However, I have had one situation where a customer had a noise coming from a Mullen guitar... it turned out that just a few hundred yards away was a huge radio station transmitter! The strange thing, it didn't affect an Emmons guitar when connected to the same rig Obviously the solution was to sell the Mullen LOL!

Cheers Mark

[Donny Johnston]

Mark said :" Keep in mind the terminating impedance also affects the self-resonance."
I bow to your superior knowledge, and admire how such long messages can be left regarding something of which I know absolutely nothing. How can you have all that information in your brain and still play pedal steel? I don't have room for both. Are you aliens? Donny.

[JohnDavisStringsHere]

Donny when these nerds get going anything can happen we are better off not knowing cos its easier to ask and gives us more time to play

[Will C]

Well Mark, we are talking about steels built in the 60's I think? The equipment available then - and production control on tolerances - would not have been a patch on what is available these days.

Nervous? Well yes, given your standing here and in the steel world generally. Nobody has any reason to think I know enough to ask these questions or cast doubt on your thoughts. I finished with engineering over 20 years ago, but the fundamentals are valid still. Sorry about the pun.

It's a pity the original poster failed to answer my question about the capacitor value. It would have helped to know. But ok, I'll certainly try a small cap across my pickup, just out of curiosity. Regardless of any theory, nothing beats observation and measurement!

[RonMc]

This is all most interesting, however, I personally prefer the proven, tried and tested, engineering principle of 'suck it and see', theory is great and fits well in it's place, like the theory of 'aerodynamics' according to which, the 'bee' cannot fly, however the 'bee' knows nothing about aerodynamics and continues on his merry way. A happy New Year to all and success in the coming year.

[Mark]

Thank you for those kind comments Graham, Will and Donny.

Donny apologies for the term terminating impedance, it's just what's connected to the pickup, i.e. the guitar wiring, the tone control circuitry that Ron mentioned, cable to the volume pedal and the volume pedal itself etc. Every component in the chain affects the pickup output / frequency response.

Just for interest, if you own or you've ever looked at a Telonics FP-100 volume pedal you might have noticed a small hole in the casing labelled 'Input Impedance'. This hole allows a preset potentiometer to be adjusted so the input impedance of the volume pedal itself can be set. And yes, this affects the self-resonance of the pickup.

Players going back to the 50's twigged changing the value of the potentiometer in the pedal and the length of the leads to and from the volume pedal (capacitance) affected the tone. But they also twigged changing the tone this way didn't just roll off the high frequencies like an amplifier tone control. I know of one, possibly two named pedal steel players that still today like to use a specific cable and pot value to help them achieve their sound. I'm sure Baz could name one or two lap players who he's seen using the same method to tweak their tone.

Will, you raised a good point, it would be interesting if Graham could recall any specific guitar he's noticed a capacitor fitted too. And of course if anyone has a guitar with these capacitors fitted, details of markings on the capacitor would be great. Note, it's NOT the tone control capacitor that's of interest, it's a capacitor directly across the pickup.

M

[zbplayer]

Hi Mark,Will and all. Mark,your very last sentence has now made me wonder if I,m leading you up the garden path. Now you,ve said......"NOT the tone control cap" I reckon I got it wrong then. I used the term connected across the pick up loosely. I,ll be totally honest and say I have not seen a cap directly across the pick up. But re the original question about caps,I think I meant the pF cap seen around the pick up/wiring area. Guessing now it must be to do with the tone control. Dunny,call me dummy! Regards Graham

[bob adams]

Graham you are not alone in the knowledge of controls and wiring, I've just finished a steel with volume and tone controls.. having finished the mechanics I checked the electrics.... wrong value Pots, both A pots,should be A+B.... soldered in the wrong places and the cap soldered right up to the body of the cap chances are it was fried.. needless to say nothing was working so well! Sorted now .. but never assume because it's a PSG the wiring will be right! for what it's worth I put a 22 orange drop cap on it; theory states it should be a 47 on a single coil pick up but it is sounding good with the 22 so it can stay! ... in saying all that I would rather go straight to the jack and bin tone control altogether........ plenty to mess with at the amp end!

For those technical 0.022µf

[Will C]

Bob, thanks for weighing in. I'd been hoping that we would get a contribution from you. Those orange drop capacitor values are exactly what I'd expect for a tone control circuit. What sparked this whole discussion off was a suggestion by the OP that the cap "across the pickup" was a low value in the pF range. The values you have quoted (22 or 47 nF) are exactly what you'd expect to find in a tone control circuit, but are 1000 times larger than originally posted, so that was what got me wondering whether such a small value cap could make any discernible difference to the tone. But as Ron says, there's nothing like actually trying it, and seeing the effect.

Mark, I do have a Telonics FP-100 pedal. I'll try to do a test on the impedance setting and see if I can hear any difference.