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Old 11-16-2007, 11:27 AM   #109
kenn
Human being with feelings
 
Join Date: Aug 2006
Location: Toronto ON
Posts: 314
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John,

Thanks alot for the interesting thread. I've just added a touch of book larnin' to your mojo, from my aborted attempt at a degree in Electrical Engineering:

Quote:
Originally Posted by J Kennedy View Post
Capacitors, rolled layers of foil and a paper dielectric in between. No direct connection, but reactance is set up that let higher frequencies thru. The larger ones can hold a near lethal charge from a 9 volt transistor radio battery.

Larger the farad rating, the lower the frequencies that are passed on, when a capacitor is connected in series. Lower the values and less LF gets transmitted, with the lower frequencies being blocked with progressively smaller capacitance ratings. The useful range in a guitar for tone control or different filters will be usually between 0.1 and 0.0001 microfarads. Starting from 0.1, a lot of frequencies get passed thru. Using 0.0001, most of the low frequencies are blocked but the hi frequencies still get thru.

Tone controls use the capacitor, usually a 0.01 mcf to shunt the high frequencies to ground with the pot restricting how much can be sent to ground. Turning down the tone control opens the capacitor to ground and the sound gets dull. Turn tone to 10 and you have mainly isolated the filter from the circuit and the tone is brite. Here is where you can experiment with various values to change the nature of the tone control. 0.005 mcf to 0.001 give a punchy midrange. 0.1 mcf gives almost pure bass.

Get a bunch of cheap waxed capacitors (Radio Shack deluxe again) and just start experimenting.

Fender Telecasters have their signature twang partly from using a small capacitor, not to ground, but to bridge the two non-grounded prongs of the volume control. The small capacitor keeps passing the higher frequencies as the master volume is turned down. This is why the sound gets more tinny as you turn the volume knob down. The Fender amps use the same principle with a capacitor on a switch across the volume control. This is what the brite switch is and how it works. Here is another area for stage amp modeling, by going in and installing or changing the volume bypass filter to different values.

Guys, correct me if I'm wrong, but one 0.01 mdf capacitor will flip the phase of what is passed thru 180 out. Series two 0.005 caps and you get the original 0.01 with the bypass back in phase with minimal damage. This has never been a problem in my experience regardless.

What a capacitor does to a signal is entirely dependent on how the capacitor is connected (inline or as a shunt) and the net reactance (resistance/inductance/capacitance) of the rest of the circuit. So it's really hard to generalize, like the above about phase. Not so useful anyways. Also, net capacitance increases when capacitors are hooked in parallel (two .005 capacitors in parallel = .01) and is the "inverted sum of the inverses" when you hook them in series (Two .005 capacitors in series = .0025)

Experimenting with different capacitors across the guitar master volume knob opens up a lot of neat rolloff possibilities and also can be used to offset guitars that tend to lose treble as the volume is turned down. You can use a resistor in series with the capacitor across the volume control to moderate the amount of capacitance in the circuit. Use a test potentiometer in series to adjust the right amount of capacitor bleed and then replace with a fixed resistor close to the value you want. (Use the ones with the gold band convention for higher precision, just a few pennies more). Small "trimmers" may work if they are within range and fit under the pickguard as permanent hardware. Gibsons have an extra knob to play with in the Gibson mod, so a variable rolloff knob can be used very nicely.

Rickenbackers get their characteristic sound partly with filters also. There is a fixed capacitor in series with the bridge pickup, essentially filtering out most of the bass response. When this is mixed with the neck pickup, you end up with a resulting sound that is heavy on the hi end and low end but not a lot of middle. Downside to this is that the filter makes the bridge pickup weak in volume and you have to be careful blending in the neck pickup without blowing a speaker. I had my bridge pickup on a switch to override the filter when needed. This approach can get some awesome sounds out of about any guitar. It simply requires an inline series capacitor on the hot lead of the bridge pickup and a switch to bypass the capacitor. Each guitar is entirely unique and so the choice of capacitors to give the best sound will be different for each.

As far as tone controls and Telecaster filters, these can be auditioned without taking the guitar apart first. A metal box with two potentiometers, two 12 position rotary switches and a bunch of capacitors is a must have tool if you do much of this. In the box, have one pot connected to a rotary switch that selects several gradients of capacitor values (0.1 to 0.0001). Run the output of the rotary to ground (capacitor and pot in series). The pot will now allow you to test different values of the tone control. This pot is set up (minus the rotary) identical to your tone control in the guitar, just outside. On the second setup, wire the pot just like your guitar volume control. Use the rotary and second set of capacitors to shunt across the two ungrounded leads of the volume control. This will allow you to test different rolloff filters and get the best choice for the specific guitar.

Rickenbacker filters can’t be put in an external box since you are not globally affecting the signal. Got to get inside for that one.

John K
A great one-stop electronics reference is The Art Of Electronics - probably the single best electronics book out there, with lots of practical info, but maybe more depth than most people need. More basic books like Getting Started In Electronics make great starting points. Also search for Craig Anderton books like DIY Projects for Guitarists.
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