21 July 2012

Wiring DIY - Part 02

Potentiometers (pots)

Well, in Part 01 we saw that the sound of our electric guitar is being generated by the pickups. If such a signal is directly routed to the jack nothing happens at all but, we are loosing the possibility to control the exact volume level that we want at every moment.

Potentiometers (Pots) are variable resistors. They have three legs (connections). The  middle leg (cursor) has a movable inner piece that sweeps an strap covered with some resistive material. The other two legs are linked to the both extremes of such a strap. The cursor is being moved rotating the post (rod or edge). When fully rolled off, the cursor is in direct contact with one leg and, when fully rolled on is in direct contact with the opposite leg. When the cursor in in an intermediate position, it presents different resistive values for each of the other two legs.

When used as a volume control, one of the external legs is the input and, the other leg is connected to ground. In that way, the volume is at full when the cursor is fully rolled to the input leg (no resistance) and absolutely off when the cursor is fully rolled to the grounded leg (all the input goes to ground). When intermediate positions, part of the signal is derived to ground, while the rest of signal is being send to the rest of circuit by the middle leg.

When used as a tone control, only one of the extremes is being used as an input, while the middle one (cursor) is linked to a capacitor, which is also linked to ground. The idea here is to gradually throw to ground some high frequency range (that depends on the capacitor value).

You can think on pots as if they were faucets that can be opened in more or less degree to let flow the exact amount of water (signal) that you wanted at any instant.

As any resistor, pots are identified with a value that specifies their maximum resistance. So, if a pot has a value of 250 KOhm, it means that it will have a resistance of zero when "open" and a resistance of 250 KOhm when "closed). Which intermediate values the pot presents when the cursor is sweeping the resistive strap will depend on the type of potentiometer.

There are Linear pots, where the increase of its resistive value (when rolling on the cursor) is linear so, you can expect half resistance (125 KOhm) in the middle point or a quarter (62.5 KOhm) when the cursor is on a quarter of its sweeping range. Linear pots are identified with letter B.

The most common pots are Logarithmics. Logarithmics start very slowly to increase the resistive value, until they reach the middle of the sweeping range, more or less, then they increase very fast the resistive value with way shorter sweeps. Logarithmic pots work analogically equal to the human ear and, therefore are felt as working in a more natural way. This is way they are called also Audio pots and, are identified with letter A.

Usually, electric guitars have Audio (logarithmic) pots but, a linear pot can be of help if you wanted a better control over tone, by example. You should noticed that when rolling off the tone of your guitar, at the beginning of the rotation there is a few loose of trebles but, when reaching the middle position, more or less, the loose of trebles is way dramatical and, it is really difficult to set up an intermediate treble content between one position and the following one on the dial of your tone knob.

There are even more types of pots (anti-logarithmic, etc.) but, I never saw one of those in a guitar so, we can forget those by the moment.

Independently of if they are logarithmics or linears, we have some special pots. We have stacked pots that are two pots stacked one over the other and actuated with a central rod that can be pulled up (to reach the upper pot) or pushed down (to reach the lower pot). Imagine that old Juke Boxes that had a lot of disks stacked inside and a single pickup to reproduce them. Stacked pots usually have two pots with different resistive values (by example 500K and 250K) and, they are useful if you wanted to have the volume and tone controls under the same knob, by example.
The famous Fender TBX tone control is a sort of stacked pot wired in a tricky way.

Fender introduced no-load pots. Regular pots have some resistance (greater than zero) when completely open. The no-load pot has a resistance of zero (no resistance) when the pot is fully open and, it begins to present some resistance as soon as the cursor is being rolled off.

We have also pull/push and push/push pots. Those are really two passive electronic components assembled together. Above, we have a regular pot and, below we have a switch (usually a DPDT on/on switch). The two positions of the switch can be selected by pushing down or pulling up the rod of the pot and, the values of the pot are controlled by the rotation of the rod. The famous Fender S-1 volume pot, usually seen in Deluxe models is a push/push pot with a 4PDT switch below.

So, under the common aspect of a control knob, a guitar can hide some special features, depending on the type of pot that we are using. Usually guitars come from factory with standard logarithmic pots, with resistive values of 500K (for humbuckers), 250K (for single coils) or 10K (for active pickups). The highest the resistance of a pot, the brighter the pickup will sound, because it raises the resonant peak of the pickup.
That's why greater values are being used for humbuckers (darker by nature) and lower values of single coils (brighter by nature). Active pickups are an special case and the value is more related to the load that active pickups want to see.

I recommend you to see this video if you are curious about what is inside a pot and how it works:

And, this one if you want to see the differences between Linear and Logarithmic pots:

Capacitors (caps)

In the case of guitar electronics, a cap is mainly being used to roll off trebles. You can search any electronics page to have a deeper knowledge about caps but, for our needs, just think on a cap as if it was a device to capture just a certain range of high frequencies. The cap allows just a certain range of frequencies to cross it and, the range of such frequencies is being determined by its capacitance value. The starting frequency in the range that can cross the capacitor is being called Cutoff Frequency.
The higher the capacitancy value is, the lower the cutoff frequency so, the wider the range of high frequencies that are crossing the capacitor.

So, we have a "net" to catch high frequency "fishes". What we finally do with those catch frequencies depend on what do we want to achieve.

The regular use of a cap is to used it together with a pot, as a tone control, allowing to remove part of the higher frequencies present in the sound of our guitar. So, the pot is sending to the cap part or the full signal and, the cap throws the range of high frequencies that are able to cross it to the "trashcan" (ground).

But caps are also used for Treble Bleed mods. We are bridging the input and output of a volume pot with a cap and, in this way, we are ensuring that a certain range of high frequencies are being directly sent from the input to the output, even if the signal volume is decreasing (what usually ends in a darkening of tone, also).

Usually, humbucker pickups work with capacitors with a value of 0.022 mF (micro Farads) to remove less high end and, single pickups with higher capacitance values, as 0.047mF or even 0.1 mF. But lately, practically all guitars are using 0.022 mF capacitors, independently of their pickups because this value is just enough to dark guitar's tone (it can go excursively muddy if a very high capacitance value is used).

Capacitors are constructed in a different way and using different materials for their plates and dielectric. The cheapest is the ceramic disk capacitor,.
Lately, people seems to be very happy using Sprage Orange Drops, because they are an slight improvement respect of cheaper caps but they still have a very reasonable price.
There is some other people that prefers PIO (Paper In Oil) capacitors (that's my personal case).

You can or cannot hear some slight difference using some or other type of cap. Most of people hears no difference and every man schooled in electronics will say that a cap is just a cap, the only thing that matters is its capacitancy, that's all. But, surprisingly, most of those people ends loading their axes with Sprage Orange Drops, swapping the cheap caps and, usually they rave against PIOs.
The famous Bumblebee cap found in vintage Gibson LPs is a PIO cap. Vintage caps can have an abusive price on market but, there are some PIO caps at reasonable price (Mojotone Vitamin-TSprage Vitamin-Q, etc).

In any case, the improvement in sound is always negligible, compared to the improvement that many other changes can do to your guitar. To be honest, a change of pick (plectrum) can be more noticeable than a change of cap in your tone (usually, always open) control but, you know, everything sums up together for the big bill.

If you are in the doubt about if it makes sense to swap your capacitors, I fully recommend the following two videos to you:

It's an awesome test and it's a very helpful information for your decision, whichever it will finally be.
This man has several other videos of interest, related to wiring topics. They worth the time. He has an Oriental patience, IMHO.

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