beginner questions

[Andy]

I wrote of some questions.  Hopefully, I won't get flamed for my ignorance.

I have been reading this forum for a long time now.  I have read most of the “Technology of the….” From RG’s site.  I have formulated some questions I decided to itemized and list in one topic.  I hope some other newbie might find of value as well.  I hope I don’t ask too many simple questions.  Just be easy on me.


1.   what do the letters “LFO” stand for and what does this (apparent) circuit do?  What is it used in primarily?
2.   same question but.. what is “OTA” mean?
3.   why does a higher value pot let more sound(for example) through?  I would figure that a 500K pot would RESIST MORE than a 250K.
4.   what does input and output impedance mean?  And how does that affect the circuit?  What does “loading” the pickups mean in regards to this?
5.   I don’t seem to understand about the “4.5 volt” part of some circuits.  I think it has something to do with biasing?  What does that mean also?  What is this used for and how is it generated from the nine volt source?
6.   when a sine wave is produced, it is from one frequency, what about when a chord is struck?  Are there MANY sine waves at once or is it something entirely different?  Where can I go to get more info on this subject?

[aron]

Disclaimer - These are simplified answers - not that I could give detailed answers anyway!  There are probably errors, so please correct me.

1. what do the letters “LFO” stand for and what does this (apparent) circuit do? What is it used in primarily?

Low frequency oscillator - the low frequency oscillator usually produces a waveform at low than auible frequencies. When applied to amplitude (loudness) you hear tremolo that corresponds to varying volume in the shape of the waveform. When applied to pitch, you hear vibrato with pitch corresponding to the shape of the waveform.

Common uses:

Phase/flange speed, tremolo speed, vibrato etc...

2. same question but.. what is “OTA” mean?

Operational Transconductance Amplifier - I have no idea how to use it. It is used in the MXR DynaComp for compression duties.

3. why does a higher value pot let more sound(for example) through? I would figure that a 500K pot would RESIST MORE than a 250K.

Yes, you are right, but in this case it's probably used to "resist more from ground". Think of ground as "shorting out" the signal, so a 500K will add more resistance against ground.

4. what does input and output impedance mean? And how does that affect the circuit? What does “loading” the pickups mean in regards to this?

Related to question 3. If the impedance is low, then the circuit is more prone to being "grounded out". This is BTW in regard to high impedance circuits - which your pickups are. In any case, if a circuit is "low impedance" and you connect your pickup/guitar to it, there will be loss of volume and fidelity.

Zachary had a good write up on impedance in the archives. As soon as they are up, you will be able to read a lot more on this topic.

5. I don’t seem to understand about the “4.5 volt” part of some circuits. I think it has something to do with biasing? What does that mean also? What is this used for and how is it generated from the nine volt source?

Think of a sine wave. Now draw a horizontal line through the wavefrom from left to right - right in the middle. That's what biasing does. It sets where that line is in a circuit. If the line is too high or too low vertically, it can chop off parts of the wave if the amplitude of the waveform exceeds the bounds. The bounds BTW are set by the power supply and the device itself. This "line" or bias is set when the circuit is idle - i.e. no signal is passing through. The 4.5 or bias point is typically created by use of a voltage divider i.e. two resistors in series with the middle point where they connect as the "tap" for 4.5V.

The reason you use a voltage divider is because you want to have the "line" or bias at half the power supply voltage or 4.5V when the power supply is 9 volts.

6. when a sine wave is produced, it is from one frequency, what about when a chord is struck? Are there MANY sine waves at once or is it something entirely different? Where can I go to get more info on this subject? :?  :?  :?  :?  :?  :?

Yes, there are many sine waves each corresponding to the number of harmonics in the signal.

You can read up on additive synthesis since it relates to how waveforms can be created by adding sine waves.

[Andreas]

First of all, I'm a beginner, too, so my help will be a bit limited.

1.   what do the letters “LFO” stand for and what does this (apparent) circuit do?  What is it used in primarily?

LFO stands for Low Frequency Oscillator. As the name says, it's an oscillator (i.e. it varies it's output voltage) running at a low frequency (some Hz). It's used for modulations effects. For example it may used in a tremolo to control the amplitude of the signal.

3.   why does a higher value pot let more sound(for example) through?  I would figure that a 500K pot would RESIST MORE than a 250K.

I'm not sure if I know what you mean. There are several ways to wire up a pot and in some cases it may be that it "let's more sound though" by presenting a smaller load to the output it is connected to. If you could explain in which way the pot you are talking about is wired, I might perhaps be able to explain why it is so (though as I said, I'm a beginner, too, so there may be some uses which I don't understand).

4.   what does input and output impedance mean?  And how does that affect the circuit?  

Impendance is AC resistance. As a general rule, if the input impendance of one stage is smaller than the output impedance of the one before it, you're getting some trouble, see below.

What does “loading” the pickups mean in regards to this?

Pickups have quite a high output impedance (i.e. a high AC resistance). Now if they are fed into a low impedance input, a lot of current is drawn from them. The more current is flowing, the bigger the voltage drop across the pickup itself is. As the pickups impedance is frequency dependant, this changes the frequency characteristics of the signal (usually towards less treble, which makes it sound dull).

6.   when a sine wave is produced, it is from one frequency, what about when a chord is struck?  

Before we look at a chord, let's first consider a single note picked. A single note is not just a sine wave, but rather a bunch of, i.e. the fundamental frequency sine wave plus multiples of this frequency, called harmonics. These sine waves are all added up, so that the resulting wave looks different from a sine.

When playing a chord, each note of the chord consists of it's fundamental frequency and the harmonics, and all note signals are again added up. The resulting wave looks even less than a sine, but it's just the equivalent of all these sine waves added up.

I'm sure there are more experienced users here you can explain all of this better, but I hope this at least helps a bit  :wink: ...

[Transmogrifox]

The guys above covered the guts of impedance.  The primary difference between "impedance" and "resistance", is that resistance is a subclass of impedance...for instance, you could say "resistive impedance."

Impedance is a current voltage relationship, for example, a resistor relates an applied voltage to a resulting current in a linear way:

V = IR.

If R is very large, then there is not very much current, therefore a voltage source not capable of delivering much current will likely maintain its voltage across the resistor, however if the resistor is small, the voltage will decrease to a maximum value of the current that can be supplied.

So what if it's not a resistor?

Resistors have this property because they dissipate energy as heat.  The input of a transistor, however, depends on more than pure resistance.
The current relationship of a transistor is this:

Collector current = (Beta)x(base Current)

Beta is also known as hFE

And the collector current also depends on the voltage from the base to emitter (this is an exponential relationship that I will spare you of).

so....

More voltage at the base means more current in the collector, and more current in the collector means more current at the base.  The voltage source must supply this current, or there is an attenuation of the signal.  This current voltage relationship is not a "Resistor", but for small changes about a "BIAS" point, it LOOKS like a resistor from a mathematical approach.

So you can see how this "impedance" thing gets more complicated when this is no longer DC Voltage and current and only resistors and transistors.  Capacitors and inductors change "resistance" for different frequencies.  This is the impedance of a capacitor:

I(t) = C(dV/dt)

In non-mathematical words, the current is largest when you change the voltage really fast.   Really fast voltage changes are high frequencies.  

This is why coupling capacitors are put on the input of circuits.  The DC currents and voltages are "blocked" from other circuits, but voltages that change quickly can pass through the capacitor.  So for DC, the impedance of the capacitor is nearly infinite, so no matter how large the input impedance of the next stage is, it's nothing compared to infinity.  When you apply a guitar signal, with changing voltages, then the capacitor impedance looks very small compared to the input of the next stage, so for those frequencies, it is as if it isn't even there.

Now I hope I haven't confused you.  If you're getting the idea, EXCELLENT!



I hope now you can see why you want a very large input impedance,
and small output impedance.

Large input impedance:  A source that supplies a limited amount of current maintains a signal the best if it doesn't need to supply anywhere near it's maximum current supplying capacity.

For example, a guitar doesn't supply much current.  This is described by "high output impedance".  Therefore, you need the input impedance of the amplifier to be much much larger than the output impedance of the guitar.  This should usually be about 100 times the output impedance, as a loss of 1/100 (ish) of the signal is hardly noticeable in most cases.

Small output impedance:  This means that the output CAN supply much more current than the input of the next circuit requires, therefore delivering most of the signal to the "load" and not losing it in the output.  This is why many pedals have another emitter follower stage after the Pot wiper.  This maintains constant output impedance while the pot merely controls the signal amplitude level.