Questions about impedance, volts, current, etc.

[ahrenba]

Hey Guys,

I am a total recording newb, but I have been trying to learn the ways of how recording equipment and signals work over the last few days. It has involved A LOT of research and many questions. Get ready for a beast of a post.

Ok. I know that Guitars output a high impedance signal (hi-z), while something like a mic outputs a low impedance signal (lo-z). Hi-z signals have a high voltage, but low current. Lo-z signals have a low voltage, but high current.

I am going to list what I know...let me know if I am right. Then, I have a couple questions.
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So, I understand that power equals voltage times current.

Same impedance
If you plug a device with, let's say, 600 ohm's into an input (load) of 600 ohm's, you will match the impedance. This allows for the most power to be passed on, however nothing is boosted.

Lower impedance into a higher impedance
If you plug a device with, let's say, 10k ohm's into an input (load) of 100k ohm's, the voltage will be boosted, but the current will decrease. This is called "bridging" because the load impedance is 10 times that of the output's.

Higher impedance into a lower impedance
Not sure on this: If you plug a device with 10k ohm's into an input (load) of 2k ohm's, the current will be boosted but the voltage will decrease. Could someone verify this?

Am I correct with my logic so far?
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Ok, now the questions:

Situation: You have a Hi-z input on a mixer (let's just say it's 100k ohm's). Then you have a guitar that outputs 10k ohm's. When you plug the guitar into the hi-z input, the notion is that the lower impedance of the guitar going into a higher impedance of the input will boost the voltage, but lower the current. This is where I am confused.
Question #1: If the voltage is increased, but the current is lowered, doesn't the overall power output remain the same (not boosted), since one variable is increased, and the other is decreased?

Situation: In this article at Impedance FAQ , they say this:
Nowadays, nearly all devices are connected bridging -- low-Z out to high-Z in -- because we want the most voltage transferred between components.
Question #2:With audio, why do we want the most voltage transferred, and not the most current? Why not both? Why is voltage more important than current? I am not really sure what the difference is in terms of signal.

Question #3: What exactly does a pre-amp boost, the voltage or the current? Aren't these two variables bound? By this, I mean how can you increase one without affecting the other?

Question #4: What exactly happens when you plug a higher Z item into a lower Z input? Was I right in my logic above?

Question #5: This information is from a Shure Mixer (inputs):
Line: Designed for use with - / < 10k ohms, 50k ohms internal, +36 dBV clipping level
Is the "50k ohms internal" the impedance of the load (input)? If so, this unit must not be bridging capable, correct? The reason I ask is because it says it is designed to work with a max of 10k ohm source, but it's input ohm level is 50k, which is only five times that of the output. This is not true bridging, is it?

Question #6:Is power (voltage x current) how "loud" a signal is coming from let's say a guitar into a mixer? By this I mean when someone says the signal is not strong enough, are they referring to the power coming from that output? Is power measured in dBu's?

Question #7: What is the typical ohm (impedance) rating on a standard line level input? Also, decibel level?

Question #8: How much power output is to be expected from a Lo-z mic, a hi-z guitar, line level output, and a speaker?
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Wow, that's a lot of stuff. I realize that this stuff isn't really necessary for home recorders, but I am the type of guy who like to know the technical details of things I work with. Thanks guys!

[.tom]

well, you've opened a subject that is far too complex to thouroughly explain without writing a book and requiring you earn a degree! i'll try to explain this in terms that are common and more applicable to your tasks...

1) impedance is an AC characteristic that can only be measured using an AC source and associated measurement equipment. it is expressed in "ohms" but it is not measured using DC. hence, an ohm meter cannot be used to measure the "8 ohm impedance" of a speaker or a "150 ohm" microphone, as this meter will simply apply a DC voltage to the circuit and read a DC value back as a result. If you were to apply this device to a 8 ohm speaker, you'd get a reading near 1 ohm... you'll also notice these specs are in reference to 1kc or 1 kilo herts -1,000 cycles or herts. this means your souce voltage needs to be AC @1kc as i said in the beginning. it's "impedance", not "resistance" as this would define a DC measurement.

2) the rule of input impedance and microphones is this; the input impedance should be at least 10 times the microphone impedance. so, a 600 ohm mic should be mated with a preamp that features a minimum 6k ohms input impedance. but, just like all rules, it can be broken with little or no degradation... i don't recommend breaking rules to beginners tho.

3) output impedances are a different animal. we concern ourselves with power devices when we discuss output impedance. speakers and power amplifiers. so, if a power amp is rated 400 watts @8ohms, 800 watts @ 4 ohms, etc. this is fairly straight forward since halving the speaker or "load impedance" doubles the maximum output power of the power amp.

4) line drivers or line inputs are typically hi impedance inputs -20k ohms to 100k ohm. these inputs are designed to accept a "small signal" amplifier output such as the L-R outputs or sends of a mixer.

5)bridging should be avoided at all costs. don't parallel plug mics, it's usually gonna be a better capture if you simply use one mic, strategically placed to pickup two sources. you're not just fuking up impedances there, you're also risking lf filtering... you can move sources close to eachother for this "one mic capture" and also match the volumes nicely too.

6) a guitar coil is a hi impedance source. you could go direct but, if you have a guitar amp that has a "pre out" you might like that more. this would be the preference unless you've got a preamp designed to accept guitar direct input.

hope i covered enuff here for you to see this from a different angle. it's not about matching power. it's about keeping frequency response level across the audio spectrum of 20<->20kc... if you mess up your impedance matching, you'll most likely hear a rolloff in hi freq response or sometimes you get lo freq rumbling for no reason, depending on what devices you've connected to make a mismatch in impedances.... if there's something i've not addressed just come on back and i'll try to answer without too much trouble

[ahrenba]

Wow! Thank you for you awesome response. I appreciate your time.

My notion of bridging was messed up. I now see what it means.

One of the main things I fail to understand is this:

1. Ok, if I were to plug a mic into an input, and the resulting signal is too "low" to work with or get decent frequency response, what causes this? Is it a low voltage, low current, or the combination (power [voltage x current])?

2. Now, in direct relation to the question above: Obviously this signal would need to be pre-amped (boosted) in order to obtain a more workable level. What does a pre-amp actually boost? Voltage? Current? By this I mean, if you have a output going into a pre-amp, what part of the signal will be higher once the signal leaves the pre-amp?

3. Why exactly do you want an input (load) impedance to be approx. 10 times higher than that of the output? I mean, I know that this results in maximum voltage transfer, but doesn't this degrade the amount of current?

I guess the main area where I am confused, is what actually carries the audio signal? The voltage or the current? I may be totally off here since I don't know much about electricity, etc.

Thanks!

[.tom]

well, i'll limit this to solid state preamplification since this is primarily whatz used these daze.. there are two types of ssl preamps; transitor type and FET type.

1) a transistor type preamp is commonly integrated circuit based, unless you're spending on the good stuff -which is built using discrete transistor technologies. all the same tho, in terms of your question. this technology is current based. by electrical engineering design rule and practice, a transistor is a "current gain" device. tho this technology reacts to changes in current and amplifies accordingly, there will also be changes in voltage too. larger voltages as we see current gains increase. but from a electrical engineering point of view, transistor gain is achieved thru current gain.

2) a FET type preamp is a "voltage gain" preamp. so, tho voltages increase as change in amplification occurs, there is also a change in currents. again, from a electrical engineering point of view, FET gain is achieved thru voltage gain as a FET device is a "voltage gain" device.

so, to get some bottom line action going here, current is the main player in all this garbage -tho we as engineers will approach the design from alternate rules of electrical theory. the reason we want the source impedance to be no greater than 1/10th the input impedance is complex to answer but, it does have to do with the fact these specs are based on 1 test frequency -1khz. so, this should tip you to the answer having to do with not power gain but, frequency response.... across a band starting at 20hz, all the way up to 20khz. when an amplifier begins to distort, it's noticable at certain freqs right? not ALL freqs... certain freqs. upper and lo lo's... so, i hope this gives you a better understanding of this (w)hole audio amp thingy

[ahrenba]

Ah! Ok, cool, thanks for the response!

Can you tell me if my logic is right on this:
1. If you match impedance, you get maximum power transfer, which is like saying that voltage and current will nearly stay the same.

2. If you plug a lower impedance item into a higher one, the current decreases. What happens to the voltage here? Does it actually become higher than it originally was?

The reason I am not sure about this is because of the hose analogy:

Garden hose (low pressure (voltage), high current), which is like a lower impedance item, goes into a smaller hose (smaller diameter), which is like a high impedance input (load). Wouldn't the smaller hose now decrease the flow (current), and increase the pressure (voltage) to an amount higher than it was in the garden hose?

If number 2 is actually true wouldn't the voltage actually determine the audio signal, not current?

[.tom]

1. If you match impedance, you get maximum power transfer, which is like saying that voltage and current will nearly stay the same.

no. first, you don't match impedance when connecting devices cuz every manufacturer has different impedance specs. all mics are different impedances. it's the trik of preamp designers to have a product that will work with the majority of mics out there. and b) current/voltage changes as the mic pix up your voice etc...

2. If you plug a lower impedance item into a higher one, the current decreases. What happens to the voltage here? Does it actually become higher than it originally was?

no. but, some preamps are more sensetive to current/voltage change than others. this has less to do with input impedance and more to do with preamp gain structure and just the overall design philosophy of the preamp. some preamps are designed for better ribbon mic matching -which is typically a lower output mic, so they can get more of that market etc.


my advice to you would be to start googling AC circuit theory... wait. my advice to you is to make music and let the electrical designers take care of all that theory shit 4u... if you must, google. the answers to the questions you are asking are waiting for you in a university bachelors degree in electrical engineering. it's 4 brutal years dude. you can try to condense it in one internet thread but it just wont happen... it's kinda complex stuff. the way you're thinking, and the hose analogy is just the beginnings of DC circuit theory... AC theory is far advanced beyond this. it involves frequency response. and that's just not coming into your part of this conversation. it's all good but, you're only seeing one dimension of wutz really 3 demensions trust me, itsa fuking lifetime of learning -this electron flow thingy