Pitch Adjustment
Pitch control is one of the most common and useful features that can be added to a majority of electronic, sound making device. It’s also one of the simplest to find. MOST simple audio circuits use a resistor mounted on the circuit board to determine the clock speed. Clock speed is the rate at which the circuit processes information. Basically the clock speed is the pitch.
Some circuits use other means of setting the pitch (clock speed) such as crystal or inductors. These are harder to modify. The following examples only apply to circuits which use a resistor to set pitch.
The first step in installing a pitch adjustment is to find the pitch resistor.
Below are a few rules and simple steps that will help you find the pitch resistor.
THESE ARE JUST GENERAL SUGGESTIONS. THERE ARE ALWAYS EXCEPTIONS AND ANOMALIES.
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LOOK AT THE BOARD:
The following technique most often works with very simple circuits like the example circuit to the right.
-Find the “BRAIN” of your circuit. This is usually a black blob. This blob is a cheap form of integrated circuit (or IC). These are also called “gum drop” ICs. An IC is a collection of microscopic electrical components working together to perform complex functions…like making sound. Some circuits use different styles of IC which look like flat black boxes with lots of metal legs. If this is the case, look for the box with the MOST legs. This will probably be the audio processing IC.
-Once you’ve found the brain, look for the resistor placed closest to the brain. 9 out of 10 times this will be your pitch resistor.
-Test your theory by touching the leads of the pitch resistor while the circuit is making sound. If you’re right, the pitch should jump up or down.
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LICK THE BOARD:
If you have a more complex circuit with lots of components you can try the “lick and poke” method.
-Lick the tip of your pointer finger.Turn on your circuit and activate a sound.
-While it is making sound, start touching the board with your pointer finger. Eventually you should hear a jump in the pitch when you touch a certain part of the board. Narrow down your search until you can identify a single point or two that effect the pitch. Maybe use your pinky instead.
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IDENTIFY THE PITCH BASE:
In most cases there is one lead coming off of the brain that is the pitch base point. By connecting this point to another point on the board (usually the positive or negative power supply) through a resistor you can set the pitch.
-Once you have found your pitch resistor you will need to determine which end of the resistor is connected to the pitch base and which is connected to the power supply.
You can usually do this by looking at the board to see if one of the leads goes to the power supply.
OR you can use a continuity tester to test both ends of the resistor to see if one is connected to the power supply. Or you can link your fingers and hold a metal pointer, like a jewelry screwdriver” and touch both leads of the resistor. Touching the end connected to the pitch base should result in a noticeable jump in pitch. Once you have found the pitch base, follow the diagram to the right labeled “voltage divider method”.
-It is less common, but sometimes the case that the circuit has two pitch base points. In this case both points will be sensitive and effect the pitch when touched. If that is the case, follow the diagram labeled “attenuator method”
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INSTALLING PEAK BUFFERS:
It is of utmost importance when installing a pitch adjustment that you install peak buffering trim pots or resistors. A buffer will limit how high and low the pitch can go.
I usually use potentiometers to find the correct buffering value, then replace the pots with resistors.
A buffer on the high end is most important. Turning the pitch too high for too long can burn out the circuit. Sometimes turning the pitch too high for just a second will fry it. In some cases no buffer is needed at all, but this is very uncommon.
Ryan McKay said:
September 16th, 2009 at 8:00pm #
Is there any easy way to figure out which points on a circuit are connected to ground? Been having trouble figuring this one out. Thanks a lot!
casper said:
September 17th, 2009 at 12:56am #
Ryan,
I use a continuity tester. This is a function on my multi-meter. Touch one end of the tester to ground and then poke around the board to see what else is connected to ground.
If you don’t have a multi meter and you do electronics work with any amount of regularity (more than once every few months) you should really get one. It doesn’t have to be fancy, but you should make sure it has a continuity tester on it. Some cheaper ones don’t. Expect to spend $20-$50 for a “good enough” model.
JP said:
September 17th, 2009 at 12:06pm #
Hi
I have been trying to bend a vtech phonics little smart alphabet teacher. Seems to be the only one that i cant find any schematics or tutorials to help me to find pitch.
If you could post a picture of a circuit board, would be very good.
Thank you even if its not possible
All the best
JP
yoni said:
September 19th, 2009 at 1:22pm #
Excuse what may seem a stupid question,
but the circle in the square icon in your diagram, looks like a screw head or something, is that your ground point?
I got this rigged up with alligator clips right now, it’s waaaay too fun.
thanks for the tutorial my friend,
yoni
casper said:
September 20th, 2009 at 1:38pm #
Yoni, the circle in the square is a trim pot. It is used to set the pitch depth. I usually use resistors to do this, but sometimes it is easier to use a trim pot so you can tune the depth by adjusting the trim. Make sense?
yoni said:
September 23rd, 2009 at 11:23am #
hmmm,
correct me if I’m wrong, but the advantage gained from the trimpot is that it makes the base pitch variable? the resistor that’s already there sets the clockspeed, but a trimpot would allow you to add resistance if you so desire? if this is the case,why not simply replace the resistor with the trim?
also, are there methods for pitch shifting up? maybe that has something to do with the trim pots.
answer any or none of the above questions,
either way, thanks for sharing your knowlege,
yoni
casper said:
September 23rd, 2009 at 3:36pm #
Yoni,
One trim pot can be used to set the base pitch and another can be used to set the peak pitch. I put the word “OR” in between the trim pot and the resistor. You can use either one, but it’s not necessary to use both.
casper said:
September 23rd, 2009 at 3:37pm #
Yoni,
Both of the methods shown above should allow you to shift the pitch up AND down with one potentiometer.
John said:
October 5th, 2009 at 12:50am #
What is a good device to bend for a first timer?
casper said:
October 7th, 2009 at 2:10pm #
John, The speak&spell is a classic for a reason. It’s easy to bend and makes lots of crazy sounds. They’re not super cheap, but for $20 or so you’re guaranteed to do some quality bending.
Casio Keyboards are good too, but can be a little confusing for your first time.
jamie woody said:
November 16th, 2009 at 7:29pm #
on question #4, i assumed that is a trim pot?
Ian said:
December 1st, 2009 at 2:43pm #
Hi, I’m modifying a Casio SA-8 as per the diagram on Casperelectronics.com.I have two crystals in the circuit now but it’s not working - does it matter which way round that i mount the crystals, i.e. is one leg an input and another an output? Sorry, new to this! Thank you!
Sha said:
December 25th, 2009 at 4:56pm #
Hello,
I really like your bends and I have a question :
How do you know the value and the type of the resistor used to limit the pot ?
Thank you.
ryan said:
January 11th, 2010 at 9:27pm #
this site just blew my mind.
thank you thank you thank you
i will be back with lots of questions!
casper said:
February 18th, 2010 at 1:44am #
@Ian: Sorry for not getting back to you sooner. It shouldn’t make any difference which direction the crystal is oriented. Unfortunately the problem is something else.
@Sha: I usually just do trial and error until I find a good value. One method to help you make an informed guess is to take the value of the original pitch resistor. Lets say it’s 50k. The high end pitch buffer is usually a quarter of that value although it will vary from circuit to circuit.
sean said:
March 8th, 2010 at 4:40pm #
i’m looking to bend my original crybaby wah pedal, any suggestions?
casper said:
March 11th, 2010 at 1:25pm #
@sean: hmmmm. I’ve never worked on a wah pedal before. My first thought is that there probably isn’t a whole lot to find. I think the most you’ll be able to get would be some feedback effects