Questions about pre-amps, signal, and audio level

[ahrenba]

Hey guys,

I have had another thread going ( https://homerecording.com/bbs/showthread.php?t=256701 ), but it was getting extremely long and hard to follow. In order to make it easier, I created a new thread with my remaining questions.

If you could, please notice how I numbered my questions. If possible, please number your answers accordingly. Thanks!!

1. What does a pre-amp boost? Voltage, current, or power? How does it actually boost anything in the first place? By this I mean, how does it work? Does it actually ADD voltage and/or current? If so, how??
EDIT for question 1: From Wikipedia: "The preamplifier provides voltage gain (about: 10millivolts to 1volt) but no significant current gain." Ok. So it boosts voltage.... How exactly does it physically do this?? Also, this doesn't seem to make sense, because from what I have seen, pre-amps have very high impedance inputs (about 1Mohm) for instruments, but the output is about 600 oHm. If a preamp provides voltage gain, how in the world can it output an extremely lower impedance since the lower the impedance, the lower the voltage, and higher the current. Can someone explain this?

2. I realize that if you plug a higher impedance source into a lower impedance load (ex. guitar into mic input), than the guitar will be loaded down. What does loaded down actually mean, and does it physically do anything to the source?

3. Since it is recommended to plug a source into a load that is has at least 10x the impedance. This ensure maximum voltage transfer. Why don't manufacturers make all inputs extremely high impedance? That way it will always be 10 times the impedance. That way you wouldn't have to worry about the effects of high z to low z. Why don't they do this? Does it create problems?

4. Let's use a guitar as an example. I realize that the pickups produce a voltage. However, do a guitar's pickups (or anything for that matter) produce a current? If not, where does it come from? How is it determined? I have heard that a current is a draw, but am not really sure what this means.

5. Ok, I was informed that voltage is what determines an audio signal. Does voltage also determine the level of the signal (like with mic level, instrument level, and line level which are measured in dB's or dBV's)? I would assume the answer to this would be yes since dBV is converts to 1.0v.

6. What role does current play in the audio signal? I realize it is how much "flow" there is, or how many electrons are moving through the circuit, and I know it is essential to maintaining voltage, but what does it actually do? I am confused, because if voltage is what determines the level of the signal, what role does current play? Also, if a preamp boosts voltage that much, and doesn't boost the current, is current even significant?

7. If I were to use an external preamp for my guitar, which would then get plugged into my current guitar amp (which I assume has its own built-in preamp), is there a way to bypass the effects of the amp's internal preamp and just use my external one?

8. Is an electrical circuit defined as only having two resistors (one output, and one input)? It can't have like a chain of input/outputs, can it? The reason I am trying to visualize this is because, if you plug an output from a guitar into the input of a pre-amp, that input obviously has to take that signal somewhere, presumably to another circuit, correct? If not, and the inner workings of a preamp are actually part of the same circuit as the guitar -> input, then how does the step down of impedance inside the preamp not load down the guitar?

Thanks for your time, and users: dementedchord, drossfile, and Oancient1 have been amazing in my other thread. Thanks guys!

 

[rayc]

Q7. If you have an effect loop you could try that but it would all depend on you amp. Post the brand, model & any modifications done to it.

As to the other questions - they're good so I'll come back looking for answers too!

 

[boingoman]

1. The preamp is a voltage amplifier. It uses the input voltage from the mic or instrument to control an output device capable of creating a larger voltage, like a power tube or transistor. That larger voltage is an exact copy of the input voltage, just with more amplitude. That is the basic way all amplifiers work. A small input voltage controls a larger output voltage.

You can't relate the input impedance and voltage to the output impedance and voltage of something like a preamp by applying Ohm's law. The input and output are separate sections of the device, and can have completely different characteristics determined by the designer.


2. Loading means the source doesn't have enough current to maintain it's output voltage into an impedance. As a result, the output voltage drops, which in a guitar means less signal. Another bigger problem is that the signal will degrade.

The other problem is that the output and input form a voltage divider, something else for you to learn about. A guitar plugged into an amp generates 99.999% of it's signal across the amp's input. Plug that same guitar into a mic pre, and you get only about 20% of that value. That's roughly 14db of loss.

Also realize that at the impedances we deal with that signal loss from loading isn't a big factor. The degradation and resistive losses are a bigger pain.


3. Something first- when you plug something in, you are making a circuit formed by the source, cable, and destination. Guitar-cable-amp, mic-cable-pre, for example.

Low-impedance circuits are less susceptible to what is known as capacitive roll-off. Cables have capacitance. If the impedance of the circuit is high enough, the capacitance of the cable will form a low pass filter in the audio band, rolling off the high end response.

This is why you can have a 300 foot mic cable or 10,000 miles of phone cable that sound fine but if you use a 50 foot guitar cable it sounds like ass, and why they don't make everything hi-z.

4,5,6- I'll get to these later! But basically, yes, we ignore current. We aren't looking for current, but voltage, which carries the audio information we want.

 

[ahrenba]

Q7. If you have an effect loop you could try that but it would all depend on you amp. Post the brand, model & any modifications done to it.

It is a very basic amp that came with my guitar. The model of the amp is the Ibanez IBZ10. Anyone know of a way to bypass the pre-amp in that baby?

1. The preamp is a voltage amplifier. It uses the input voltage from the mic or instrument to control an output device capable of creating a larger voltage, like a power tube or transistor. That larger voltage is an exact copy of the input voltage, just with more amplitude. That is the basic way all amplifiers work. A small input voltage controls a larger output voltage.

You can't relate the input impedance and voltage to the output impedance and voltage of something like a preamp by applying Ohm's law. The input and output are separate sections of the device, and can have completely different characteristics determined by the designer.

Ah! Ok, so a preamp, take the voltage from the guitar, "reads it", then copies and amplifies it. So the original signal basically gets thrown out? Thanks for the explanation about the impedances, that makes sense.

Loading means the source doesn't have enough current to maintain it's output voltage into an impedance. As a result, the output voltage drops, which in a guitar means less signal. Another bigger problem is that the signal will degrade.

Ok, so "loading down" basically just results in a loss of voltage? Also question: current is not created by the source, it is drawn by the load correct? This concept is still a little fuzzy for me.

Low-impedance circuits are less susceptible to what is known as capacitive roll-off. Cables have capacitance. If the impedance of the circuit is high enough, the capacitance of the cable will form a low pass filter in the audio band, rolling off the high end response.

Ah! So this is why you want the shortest cables possible with a hi-z circuit!

If you guys have the time, my original questions 4, 5, 6, 7, and 8 remain. Thanks!

 

[dementedchord]

4. Let's use a guitar as an example. I realize that the pickups produce a voltage. However, do a guitar's pickups (or anything for that matter) produce a current? If not, where does it come from? How is it determined? I have heard that a current is a draw, but am not really sure what this means.

5. Ok, I was informed that voltage is what determines an audio signal. Does voltage also determine the level of the signal (like with mic level, instrument level, and line level which are measured in dB's or dBV's)? I would assume the answer to this would be yes since dBV is converts to 1.0v.

6. What role does current play in the audio signal? I realize it is how much "flow" there is, or how many electrons are moving through the circuit, and I know it is essential to maintaining voltage, but what does it actually do? I am confused, because if voltage is what determines the level of the signal, what role does current play? Also, if a preamp boosts voltage that much, and doesn't boost the current, is current even significant?

7. If I were to use an external preamp for my guitar, which would then get plugged into my current guitar amp (which I assume has its own built-in preamp), is there a way to bypass the effects of the amp's internal preamp and just use my external one?

8. Is an electrical circuit defined as only having two resistors (one output, and one input)? It can't have like a chain of input/outputs, can it? The reason I am trying to visualize this is because, if you plug an output from a guitar into the input of a pre-amp, that input obviously has to take that signal somewhere, presumably to another circuit, correct? If not, and the inner workings of a preamp are actually part of the same circuit as the guitar -> input, then how does the step down of impedance inside the preamp not load down the guitar?

Thanks for your time, and users: dementedchord, drossfile, and Oancient1 have been amazing in my other thread. Thanks guys!

4. perhaps you're getting hung up on the idea of producing a current... remeber that voltage is a potential not an absolute... and can be viewed as "pressure" so when the pressure is applied to a resistence then current flows...the electrons simply exsist not produced... and are forced to move between pressure zones...


5. yes those are for the most part expressions of voltage... atleast within the system... db can also be an expression of sound pressure as well...


6.current is what actually does the work and is released as heat...


7. that would be specific to what model you have...


8. the idea they were trying to get across to you with the 2 resistors was that often in analyizing circuits we try to think of them in both the macro and micro sense.... everythin in your pre-amp can be seen as just one resistor (the total circuit)... two resistors (an input and an output) ... or many resistors (all the different subcircuits)....

 

[mixsit]

4. perhaps you're getting hung up on the idea of producing a current... remeber that voltage is a potential not an absolute... and can be viewed as "pressure" so when the pressure is applied to a resistence then current flows...the electrons simply exsist not produced... and are forced to move between pressure zones...

Voltage as pressure, current as flow works great with the 'water/pipe/hole' analogy:

2. I realize that if you plug a higher impedance source into a lower impedance load (ex. guitar into mic input), than the guitar will be loaded down. What does loaded down actually mean, and does it physically do anything to the source?

The high impedance source is a small pipe with it's voltage being its pressure (potential only). This pipe into an even higher impedance load is a small' hole (relative to the pipe) for the water to flow.
If that same source sees a low impedance load -a hole as big or bigger than the pipe, the source loses it's pressure -the voltage is drained down.
Outputs are low impedance so they can feed one or several high impedance flows w/o degrading the 'pressure.

 

[ahrenba]

Thanks for the replies guys.

the idea they were trying to get across to you with the 2 resistors was that often in analyizing circuits we try to think of them in both the macro and micro sense.... everythin in your pre-amp can be seen as just one resistor (the total circuit)... two resistors (an input and an output) ... or many resistors (all the different subcircuits)....

So ok, I understand that a whole circuit acts as a single resistor (in a macro sense). When you have a pre-amp, how are the inner workings separated from the input?

Bare with me here:

1mV -----> 20 koHm's --------> 1 MoHm -----> 600 ohm ----->>>>

explation:
Guitar output of 1mV at 20 koHm's connects to a preamp with an input impedance of 1 MoHm, and has an preamp output impedance of 600 ohm. This isn't all considered one circuit, with three resistors, is it? If so, wouldn't the extremely low output impedance load down the initial source (guitar).

If I were to guess, the guitar output to preamp input is one circuit. THEN the preamp has separate wiring which has it's own impedances. Is this right? If so, what creates the "separation" from the guitar -> preamp input circuit and the internal preamp -> output circuit?

Thanks for the info mixsit, I appreciate it!

 

[dementedchord]

to go any deeper i'm starting to think you're gonna need to take a class at the local jr college or the like... you're starting to get to a point where some actuall circuit theory is going to be needed...

 

[dementedchord]

to go any deeper i'm starting to think you're gonna need to take a class at the local jr college or the like... you're starting to get to a point where some actuall circuit theory is going to be needed...

 

[ahrenba]

to go any deeper i'm starting to think you're gonna need to take a class at the local jr college or the like... you're starting to get to a point where some actuall circuit theory is going to be needed...

Alright. My bad. I am getting to into it. haha. Thanks for the help though.