Invader Jim

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Wednesday, January 15, 2014

Tone capacitor myths

See also: p?t=1666193

Many people like to think that the type of capacitor used in their guitar's tone control makes some sort of tonal difference, so they will toss the ceramic or poly film caps that came stock and replace them with expensive audiophile caps like Orange Drops, Black Beauties, or any of several other types.

Obligatory Disclaimer: This is an attempt to debunk the "mojo" surrounding tone capacitor types as used in guitars (that is NOT to say that they don't have such properties in other circuits). If you are someone who fervently believes that your expensive new-old-stock audiophile tone caps sound better than your OEM tone caps, and that anyone who says differently speaks heresy, this is your opportunity to stop reading this and leave. Again, I am not implying that audiophile caps have no "mojo" properties, I am suggesting that these properties can't possibly be perceived in any shunt-to-ground application, such as a guitar's tone control circuit.

First we need to look at how a guitar's tone control works. Here is a schematic of a typical guitar (for simplicity, only one pickup and its associated volume and tone controls is shown). Also keep in mind that a guitar signal is not a single frequency, but is made up of many frequencies all superimposed on top of each other.

Notice that the tone control circuit is parallel to the signal path (it is strapped across the signal and ground). When the tone pot is rolled all the way up (the wiper rotated all the way to the unconnected terminal), there is a 500k resistor in series with a 47nF cap to ground present across the signal. A cap's reactance varies with the applied frequency (reactance is basically the AC equivalent of resistance) with it being the lowest at higher frequencies, causing them to pass through the cap very easily. The pot's resistance does not change with frequency. So the total impedance (resistance+reactance) of the tone circuit will be 500,000+Xc where Xc is the reactance of the cap *at a specified frequency*. Put simply, this puts a very light load on the signal (ignoring the parallel loads of the volume pot and whatever the guitar is plugged into) and only the highest frequencies in the signal are bled off because the load gets heavier as the frequencies get higher.

When the tone pot is rolled all the way down, it is out of the circuit (being unconnected at one end) and only the reactance of the capacitor is present across the signal. This loads the signal quite heavily--but again, the load varies with frequency. All the highs are dumped

from the signal, along with a good chunk of the mids depending on the cap value. The cap's reactance is highest at lower frequencies so the lows stay in the signal. With a big enough cap the tone circuit could let all the frequencies in the signal pass and the tone control would then become a terrible volume control. Conversely, very small tone caps will subtly shave off very high frequencies to smooth out the sound without it sounding like a useless, muddled, treble-dump.

It should now be obvious why the tone cap type is insignificant in this circuit: the output signal doesn't even pass through it. Whatever bits of the signal do pass through the cap are shunted to ground and out of the signal path, never to be heard ("ground" can usually be thought of as "oblivion"). Since this is a passive tone control (it uses no amplifying devices) it can only cut frequencies out of the signal. The circuit is not boosting lows, but is just throwing away highs.

Having said that, there are most certainly tonal differences among the various capacitor types. Any application where the main signal, or at least part of it, passes through the capacitor will let the distinct tonal properties of that capacitor to become apparent. The best way, by far, to exploit the "mojo" of a capacitor is to use it as a coupling cap in an amp or effect.

So, in closing, any shunt-to-ground circuit, such as most guitar tone controls, will not display tonal differences between capacitor types. Some people will refuse to believe that, and that's just fine, though people should use their ears and experiment and not blindly buy into hype and parroted myths. Question everything! But no matter what your opinions and prejudices, remember that golden rule of guitar gear:

If it sounds good, it is good.
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Tuesday, November 05, 2013

Capacitor Primer

This is intended to give people a good working knowledge of capacitors in general, not just as applied to guitars. It is linked to from the Wiring Thread.


Capacitor construction is very simple. They are just two metal plates put really close together, never touching, with an insulator of some kind (the dielectric) between them. Consider a vintage tuning capacitor. It has two sets of metal plates, one stationary (the stator) and one connected to a rotating shaft (the rotor). Each individual set of plates is connected to a common point, putting the plates in parallel (to increase effective plate area; capacitance adds in parallel) but the stator and rotor themselves are separated by air, never touching. The more enmeshed the plates, the higher the capacitance because of the higher shared surface area. For variable caps this value is very low, usually around 500pF or less because physical size becomes a factor (RF circuits don't require high values anyway). Modern tuning caps are much smaller because they have mica sheets between the plates, which are extremely thin and so close there's virtually no air between them, allowing a far smaller size. Putting the plates closer together increases the strength of the electrostatic field (where the energy is stored) but has a lower breakdown voltage than a cap with the plates farther apart for the same dielectric material. Note that the shape of the rotor plates determines the "taper" of the variable cap. Also note the vintage cap's segmented outer plates; these can be bent to adjust the tracking of the low-end of the band (they adjust the maximum attainable capacitance). Modern tuning caps have them too, but you can't see them. Lastly, note the screws on the caps; these are called padders, as they are used to "pad" the value of the main cap (they are in parallel) to help alignment and tracking of the top-end of the band (they adjust the minimum attainable capacitance). On old caps they are two metal plates with a mica sheet between them; the top plate bows outward and is compressed with the screw, increasing the plates' proximity and thus capacitance. On modern caps they use the stator/rotor arrangement like the main cap. The value of the padders is very small, only a few pF.

Fixed capacitors are folded up tightly inside a container of some kind, like a ceramic disc or epoxy. They are not polarized. In the old days, the 'outer foil' side of a non-polarized cap was marked on the case and usually treated like the negative terminal. This is for noise-reduction purposes, as the outer foil sort of becomes a shield. I've read in old texts that it also "increases the life of the capacitor slightly". If you want to see which lead of a particular cap is connected to the outer foil, all you need are your guitar amp and a cable. Wire the capacitor across the two wires of the cable, noting which lead is on the hot wire, and squeeze the capacitor tightly between your thumb and index finger. Reverse the connections and squeeze it again. Whichever lead was on the hot wire when you heard the loudest hum/noise is the outer foil side. Protip: you can trim the value of a ceramic disc cap down by filing away part of it. This also works with carbon comp and ceramic paste resistors, but the value trims upward.

Electrolytic capacitors are a different beast. They are made of aluminum plates with electrolyte-impregnated paper between them. The leads are welded to the plates at certain points to minimize inductance (the plates are coiled up, after all) and the assembly is rolled up into an aluminum can and sealed with a rubber plug. The paper is not the dielectric, but is actually part of the negative plate. At the factory a voltage is applied to a newly-made cap to start a chemical reaction, forming an extremely thin aluminum-oxide layer on the positive plate (a process called anodizing). The magnitude of this voltage and how long it is applied determines the voltage rating of the cap. As such, electrolytic caps will "un-form" if left unused for long periods. A voltage must be present to maintain the oxide layer. It will deteriorate, sometimes in only a year or two, slowly damaging itself while it sits on a shelf or in your junk box. Over many years the electrolyte will also evaporate and can even corrode the plates. The oxide layer makes electrolytic caps polarized. If you hook it up backward it will break down causing the plates to short, destroying the capacitor; the anodized plate must always be more-positive than the other plate or the forming will be undone. Exceeding the voltage rating of the cap will also break down the oxide layer, by ripping electrons from it, and will heat up the electrolyte, causing a pressure build-up until it explodes. On larger caps you'll see score marks on the top of the can to control the explosion. Note that you should never use a capacitor whose case is bulged, cracked, or dented, or whose plug is cracked, bulging, or has leaked (the leads look corroded). Protip: to prevent premature failure of the electrolytic caps in electronic equipment, just turn the device on and let it run for an hour or so every month.

Non-polarized electrolytic caps, like those used to start some motors (such as a drill press), have both plates anodized, allowing them to work with either polarity (ever wonder what that hump on the motor's case was?). You can make a non-polarized cap by wiring together the negative leads of two identical polarized caps. Since they are in series the total capacitance value will be half that of a single cap. Note that in reverse-series their voltage ratings don't add. The oxide layer prevents significant leakage in the forward-direction, but acts almost like a short circuit in the reverse- direction. Using a polarized cap with AC slowly breaks down and partially reforms the oxide layer as the voltage swings, leading to eventual failure. Using two caps in reverse-series prevents excessive leakage in both directions, acting sort of like an electro-mechanical diode. You can still improve on this, however.

Capacitors are classified according to their dielectric material. These materials include air, mica, pulverized ceramic, polyester, polystyrene, and Aerogel, a nearly-invisible ultra-light material that's a near-perfect insulator. It's what made very small-size, very high-value caps ("supercaps") practical. In the old days, wax paper was a very common dielectric material. Paper isn't very robust so these old caps are incredibly unreliable. "Black beauties", tubulars, and "postage stamps" are some types of paper caps. It's a great idea to replace any paper caps you find in old electronic gear.

Notes & Cautions:
Electrolytic caps have an average service life of 20-30 years. If you get an old electronic device, a cap job should be high on the to-do list, even if it still works. Power supply filter caps take the most abuse so you should replace them in equipment older than 20 years (or less). If your vintage gear has multi-section filter caps and you want to retain the original look (important when servicing vintage amps), you can "re-stuff" the cans. This just means removing the old guts and soldering modern discrete replacements to the old terminals. You could also try "re-forming" a vintage electrolytic by slowly bringing it up to full rated voltage and keeping it there for some length of time based on how long it sat unused, but this is tricky,  time-consuming, and requires certain equipment. Sometimes it works, even if only temporarily, and sometimes a cap just can't be re-formed. Personally, I think it's better to just go through a circuit and shotgun all the electrolytic and paper caps. Why prolong the inevitable? New caps can bring a tired old circuit back to life.

In audio gear, weak or bad filter caps cause a pervasive hum with the volume and tone controls usually having little or no effect. In extreme cases this can damage the power supply or other circuits. Tonally, worn-out filter caps cause bad bass response--the entire amp literally lives off the first filter cap (the reservoir cap) most of the time. The rectified AC becomes DC pulses that charge the cap up to the peak voltage, while the load (the amp) pulls the charge from the cap at all other times, so the reservoir cap powers the whole amp by itself (thus, filter caps smooth the DC voltage from power supplies). The speaker is just a paper cone connected to a moving electromagnet, and it takes more current to push the cone out on lower frequencies. Worn-out filter caps have a harder time supplying this current due to high leakage and other damages resulting in lowered capacitance, so the lows sound weak.

Some Theory & Practice:
Put simply, caps store a charge because any voltage across the plates causes one plate to lose electrons (become more-positive) while the other gains electrons (becomes more-negative). Since the plates never touch, the electrons have nowhere to go and remain on the negative plate (ignoring leakage current through the dielectric since there's no such thing as a perfect insulator), forming an electrostatic field. Increasing the distance between the plates two-fold will lower the capacitance between them four-fold, thus capacitance is inversely proportional to the square of the distance between the plates. Any two adjacent conductors will have capacitance between them as long as they are insulated from each other. Even two adjacent wires can become capacitively coupled.

Capacitance is the measure of how much charge a capacitor can hold and is measured in Farads. A Farad is a huge amount of capacitance, so most caps are rated in micro- (millionth), nano- (thousandth of a millionth), or picofarads (millionth of a millionth).

Capacitance adds in parallel because the total surface area of the plates adds up. When in series, the separation distance adds up, lowering the total capacitance. Parallel caps are like series resistors and series caps are like parallel resistors. Like many components, you can wire identical caps in series to increase the total voltage rating. The catch is that the total capacitance is reduced (the math is the same as with parallel resistors). It's good practice to include divider resistors across each cap to divide the voltage equally across them and ensure that their leakage currents, which increase with age, won't cause any one cap to take more voltage than it is rated for. The typical value is from 100k-470k across each capacitor. They also serve to bleed off the caps' charge when the power is shut off.

A capacitor can block DC and pass AC, though with DC a pulse always gets through. If you wire a battery, lamp, and capacitor all in series, you'll see that the lamp glows briefly when you apply voltage. While the capacitor charges, electrons move through the lamp to one plate of the capacitor and the other plate loses electrons to the battery. This gives the illusion of current flow. When the cap is fully charged, no more current can flow (because the plates are "full" and there's a gap between them) so the lamp goes out. Small values don't hold much charge so the effect isn't as noticeable. If you replace the battery with a closed switch you'll see that the same thing happens as the cap discharges, though the current flows in the opposite direction.

This pulse can be heard as a pop or thump in an amp when switching effects in and out. The reason "pull-down" resistors across the input and output get rid of this pop is that they provide a DC path to ground for the floating end of the input and output caps to discharge. When one end of a cap is open, the leakage current through the dielectric lets the voltage on the connected side bleed onto the open side. When the signal is switched back onto the open side, the voltage previously on it doesn't match the average ground level of the signal and this voltage difference equalizing itself is heard as a pop. Typically a value of 1M is used on the input and the volume pot itself is the pull-down on the output, if it's a master volume with no caps after it (sometimes you'll still see a pull-down resistor on the output anyway--this is redundant and serves no real purpose). The value used depends on the value and leakage of the cap; smaller resistors do a better job but cause more signal loading and treble-loss. If using grounded-input true-bypass, you can do away with the input pull-down because the switch shorts the circuit input to ground in bypass mode.

With AC things are a bit different. As the voltage rises and falls, the cap charges and discharges, giving the illusion of current flow. The voltage across the capacitor lags the current through it by 90 degrees because caps resist voltage changes across them. This is why capacitors respond differently to different frequencies (it's called capacitive reactance). The ability of a cap to block DC and pass AC makes it perfectly suited for coupling amplifier stages, where it passes the AC audio signal while blocking the DC supply voltage, which could upset the operation of the following stage. They're perfect for bypassing gain stages because they act like an open circuit to DC (not upsetting the stage's DC bias) and if the cap is arbitrarily large, as is usually the case, act like a short circuit to AC (maximizing stage gain). Protip: partially bypassing gain stages (using small cap values) creates a treble-boost effect. I've rarely seen this done with guitar amps and effects; it's a great opportunity for tone-shaping rather than indiscriminate gain-pumping.

A look at this equation will illustrate how frequency affects capacitive reactance. Try working it with a fixed capacitance and different frequencies and graphing the results ("Where's my TI-83??"). Xc is capacitive reactance in Ohms; f is frequency in Hertz; C is capacitance in Farads; 6.28 is 2(pi).

You'll notice that caps aren't like "walls" blocking frequencies, rather they have a center-frequency where they have the least reactance, and the reactance tapers up on either side of it. It has to do with the voltage lagging the current. Caps pass highs more easily than lows because lows don't cause the cap to charge and discharge fast enough for a particular capacitance--remember when I said RF circuits don't need large capacitances? Large caps end up acting like short-circuits at radio frequencies.

Like a battery, capacitors can hold a charge--even for years--but unlike a battery, a capacitor can dump it's entire charge almost instantly. The xenon flash lamp in a camera uses this principle. A high DC voltage across the flash tube is required to sustain ionization and a very high-voltage trigger pulse is needed to actually fire it. A capacitor dumps all its charge into a coil almost instantly, creating a very high flyback voltage which is used to fire the flash tube. This characteristic makes high-voltage caps dangerous. If you were to end up across the cap's terminals, you'd get a nasty shock. It could even kill you, especially if you have a heart condition you're unaware of. Tubes, unlike transistors, stop conducting when their cathodes cool off. Semiconductors discharge their own circuits and don't use such high voltages anyway.

A related phenomenon exists known as dielectric absorption. It's a strange but common occurrence where a capacitor that has been fully discharged, even removed from the circuit, starts to charge up again. No, your voltmeter isn't lying to you; there really is a voltage there. Put very simply, the molecular dipoles in the electrolyte become aligned with the electrostatic field produced by the voltage across the plates. When the voltage is removed, not all the dipoles are "reset" to a random orientation. The remaining aligned dipoles give the illusion of a voltage across the plates so the cap charges up again to a percentage of the voltage that was across it. Consider a TV picture tube. On the back is a conductive paint called an aquadag (it's the same paint used to shield guitar cavities). It forms one plate of a capacitor. An internal dag coating and the CRT's second anode form the other plate, with the thick glass between them the dielectric. Together they make up the filter cap for the flyback voltage. This cap can be up to 10nF with up to 30kV on it depending on the physical size of the screen. Even if you discharge the tube you'll get a horrible shock unless you wait a minute or so and discharge it again, and maybe a third time. Dielectric absorption is especially common with old, worn-out caps and high-voltage caps, though it occurs to some degree with all caps, especially electrolytics.

Well, that's about it. Hope you found this informative.

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Tuesday, November 05, 2013

Alternate wiring for effect LEDs

Here's a nifty way to wire LEDs for guitar pedals that doesn't have the LED/resistor solder junction floating in mid-air where it would cause undue stress on the components and look shitty. It's so simple it's stupid, but I've never seen or heard of anyone doing it. I'm all for the propagation of tips, tricks, and knowledge so I want to share it. Enjoy.
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Tuesday, November 05, 2013

GCB-95 mods tures/gear/484555/657204

The mods are as follows:
*6n8/10n/22n/47n sweep caps on 4-way rotary switch
*Whipple inductor and stock Dunlop inductor
*Wah/Yoy mode switch
*Q1- BC109 NOS metal can
*Q2- 2N3904
*2k2 mids resistor
*68k Q resistor
*Grounded-input true bypass
*Volume/wah switch
*RGB check LED
*The switch at the heel selects Wah/Yoy and turns the LED Blue or Green, respectively.
*The switch at the toe selects Volume/Wah (or Yoy, depending on the heel switch) and turns the LED Cyan (Blue+Green).
*The rotary switch on the side selects the value of sweep cap, which changes the Wah and Yoy sounds as well as the timbre of the Volume setting.
*The toggle switch on the other side is the bypass switch. It provides grounded-input true bypass and cuts the pedal's power source so I don't have to unplug the input cable when I'm done.

--Another mod you could try is replace the 220n cap on the wiper of the wah pot with a 330n. This will closely simulate the famous ICAR pot taper. Cap type affects the tone.
--Something else you could do is put a 1k pot in series with the inductor. This is the Q control on the 535Q wah, or so I've read.

The switch wiring is rather complex but it is very easy and intuitive to use.

Positioning of the Wah/Yoy switch is somewhat critical. You want it directly under the rubber pad on the treadle. To install it, you must remove the "column" on the treadle where the rubber pad is attached to. Drill it out then use a Dremel cut-off wheel (or several) to remove it completely. You want it to be trimmed flush like it was never there. When you're done, glue on a rubber or felt pad to cushion the top of the switch. Reassemble the pedal and adjust the switch height so that it activates easily.

Since the shell is so short at the heel, you may want to try the Alpha #107-SF12020-L DPDT stomp switch (available from smallbear, stock # SF12020-L). I used a normal-sized DPDT stomp with the terminals coming out the side and it was a bit too tall. I had to bend the bottom cover plate (and insulate it) to try to clear it.

The stock transistors are MPSA18. I changed the second transistor to a 2N3904, but in hindsight it should probably be left alone. Since it is just an emitter follower, the super-high Hfe of the MPSA18 better approaches ideal characteristics. In a perfect world, emitter followers would have a gain of exactly unity (one). This world isn't perfect so the gain is always less than unity. Higher Hfe= that much closer to unity.

The BC109 tames some of the harshness of the wah because its lower Hfe dulls the resonant peak of the circuit. The stock MPSA18's Hfe is WAY too high and it causes a sharp resonance which can be harsh to listen to.

Here is how it sounds:
The sweep caps are a great combo. 6n8 is a trebly sweep that's great for fast single-note riffs and tapping, or when you don't want the heel-end of the stock sweep muddy-ing the sound. I liked the stock 10n cap so I left it. The 22n is very throaty. The 47n is really low. It almost turns the pedal into a bass wah (a stock bass wah uses 68n). The location of the switch knob alows me to switch with my foot on-the-fly.

Wah mode sounds a little beefier than a stock Crybaby because of the second inductor. In Yoy mode it has a "Woy" or "Eeyoy" sound depending on the sweep cap.

The Whipple sounds alot like the stock Dunlop inductor (which isn't bad at all), but all the reviews made me expect the Whipple was the best wah inductor made. It's alot cheaper than a Fasel, though. I rewound it (scatter-wound) and it actually sounds better now.

The Volume switch gives the signal a real nice warm tone and a bit of a volume and mid boost as compared to the bypassed sound. The value of the sweep cap, in volume mode, sets the range of volume (smaller values make the toe setting louder) and the amount of mids in the volume signal (smaller values for more mids). It's a nifty side-effect. Staying in volume mode and hitting the heel switch to Yoy mode makes a "weh"-like sweep that has little effect on volume.
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Saturday, May 22, 2010
Friday, June 26, 2009

Rules of teh GB&C.

Current mood: blank

1.  If it exists, there is an ULTIMATE thread for it.
2.  If an ULTIMATE thread for it does not exist, it will soon.
3.  Check the stickies.
4.  Google is your friend. No exceptions.
5.  Kit builds are not builds. No exceptions.
6.  Don't name your edits.
8.  Finish the break, apply titebond original, clamp, sand flush, refinish.
9.  Yes, it's possible.
10. New amps are not for everyone.
11. LEDs are over-rated.
12. Killswitches are over-rated.
13. It's not necessarily a bad ground.
14. Build planning threads are lame. Period.
15. Parroting is stupid.
16. Drain your caps or your head will implode.
17. Ending a sentence like this is so annoying!?
18. Parroting is indeed stupid.
19. ^I agree, parroting is so stupid.
20. Thanks in advance!
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