A question for a Friday afternoon. --- Hi Z / Mic etc explained

[johnnymegabyte]

A question for a Friday afternoon.

I play guitar --- I'm no tech-wiz or know anything about what I'm about to ask --- hence, I hope someone here can shed some light, supply an explanation, reading guide or link.

A guitar input is HI Z from what I read on guitar amps inputs.
If I plug my guitar or via Boss GT6 into a 4 track cassette mixer, it "seems to work", bt is it really efficiently?
Plug my SM57 into same channel, too low, you have to plug it in a specific mic channel with "gain control"

What is the "impedence" for :
* Hi Z
* mic ( Shure SM57)
* Synth Line Out ( or any LINE device )
* CD player
* phono pre-amp. (this brings turntables to "line")

Why do they have different impedence (know the word, don't know how impedence works, what is high or low, or plus or minus (dunno) )

I have read about Phantom Power for Mics. I think its to boost the level of the mic output to "line" level ( whatever that amount actually is ? +48 what?)

Also, what can plug what into what ?
Example: Can I plug a mic 1/4" with RCA adapter into a phono pre-amp to bring it to "line" level ?
Maybe a phono pre-amp might give a guitar overdrive ( or get a major zap or blwo up your speakers )

Any help, advice, etc, would be appreciated.

 

[suntzu1982]

https://homerecording.com/impedance.html
https://homerecording.com/mics.html

This has some good initial basics on your questions.

A short disclaimer first, is all this info I drop will be based on very limited research. But trying to explain it will help me learn too.

'Impendance' does not apply to any one particular piece of equipment. It basically is a measure of how much power is needed to get a certain signal strength through (to your computer, or whatever the destination is). So, what's important is to make sure that the impendance levels on your inputs match or is higher (higher impendance means lower current needed to pwoer signal) those of the machine you are going into (meaning the impendance that the machine can 'handle'.)

Hi-Z is a term for high impendance. I believe the guitar is considered that because it uses an unbalanced line (don't quote me on that one.)

Phantom power is 48 VOLTS. Microphones don't put out a 'loud' enough sound so they need a power supply to 'amplify.' This is what bringing to 'line level' is. However, mics like your shure 57 is a dynamic mic, which means it is self powered, sending a line level signal already. Condenser mics are not, and thus need phantom power.

I am still trying to understand it all, but like everyone quickly learns on this forum, do a search. Most the imformation is on this site (I found on google in like 5 seconds). Seems to explain it well without getting into the theory of ohms, resistors and physics equations:

http://arts.ucsc.edu/EMS/Music/tech_background/TE-15/teces_15.html

 

[bennychico11]

suntzu-
you're close on some of this, but wrong on a few things.

First off, the phantom power (also referred to as 48V) has to do with how the microphone itself works...before signal is allowed to be sent down it. In the diaphram of a condenser microphone there are two plates that vibrate to incoming signals. These plates are charged with a constant voltage...approx. 48V. The output voltage of the condenser mic is so low that this 48V is needed in order for it to be brought up to even use in a preamp. The preamp itself is what brings up the microphone signal to line level...not the 48V. Dynamic microphones are permanently magnetized so they don't require the 48V...but the still require a preamp in order to bring it up to line level.
So remember, condensers need phantom power...dynamics do not. But BOTH need preamps.

Impedance is a device's natural resistance to electrical power and is measure in ohms. Almost all typical microphones are low impedance (150-300 ohms). It's very important to understand the impedance of the gear you are connecting, otherwise some of the sound can be ruined and frequencies can be lost! Impedance used to be traditionally matched at 600 ohms. But it was found that a source device with a low output impedance connected to a destination device with a higher impedance (twice as high) was more efficient for studio applications. In fact, when purchasing microphones you should keep this general rule in mind...a lot of times the impedance of the two devices matching or not matching can give the microphone a different sound.

A typical guitar can be around 20,000 ohms. You need to find some way to plug this into a device who's input is higher than that. Some audio interfaces have Hi-Z inputs for this reasons. However, a lot of times you don't have that input so you need a direct box to convert it for you (and it also will convert it to a balanced connection at the same time)

Phono preamps are for turntables that require it...don't plug your guitar or mic into there. It does things differently to the signal than a normal mic preamp does.

Microphones go into microphone inputs (preamps)
Guitars go into Hi-Z inputs
CDs, effect processors, etc. go into line level inputs.
Digital connections go into digital inputs

Balanced and unbalanced connections also have nothing to do with the above...they are just ways of transporting our signal so that noise isn't introduced while it is being transported.

A great book for the beginner/intermediate that I recommend is "Recording Tips for Engineers" by Tim Crich. Lots of visual aids and very easy to understand.

HTH

 

[suntzu1982]

you're close on some of this, but wrong on a few things.

Thanks, benny, I was hoping someone would correct me. I couldn't put that discrepancy between preamps and phantom power. I couldn't figure out why a mic needs power other than for amplification.

One question though. You say that the output source should be higher impedance than the input source. I thought it was vice versa (Higher impedance on the input source) so that the output source so that it can handle the signal from the input.

Thanks.

 

[bennychico11]

One question though. You say that the output source should be higher impedance than the input source. I thought it was vice versa (Higher impedance on the input source) so that the output source so that it can handle the signal from the input.

reread what I wrote again, I added the word "destination" to clarify what I was saying (I think it was a little confusing the way it was)

but you're right. A good rule of thumb is the destination (input) has to be higher than the impedance of your original source (output).
Examples:
Guitar output impedance: 20kohms
Direct box input: 80kohms

Microphone output impedance: 150 ohms
Console mic preamp input: 1,500 ohms


It's suggested you plug guitars into inputs that are at least twice the impedance...the above example is 4x, not a problem.
For microphones it's suggested you plug it into something 10x the impedance. So when you're buying a new microphone (or preamp) for your studio, it's important to also compare these numbers. Some mic pres even have switchable impedance on them.

 

[sjjohnston]

Impedance is really just a description of the relationship between current and voltage. V = IR, where:

V=Electromotive force, or how hard the current is being pushed, measured in volts. If you want to use a electricity=water analogy, it's water pressure.

I=Current, measured in amps. It's like the actual volume of water flowing.

R=Resistance (with DC) or impedance (with AC), measured in ohms. It describes the relationship between how hard you push and how much current flows.

Back to the water analogy, think of wires as big pipes full of water, connecting boxes. If the hole through which water flows into a particular box is really small, it has a high input impedance. Not very much water flows through it, even if you push really hard. If it has a low input impedance, it has a big hole, and a lot of water flows through it. That's okay, but the box that's pushing the water into it needs to supply enough water (aka amps) to keep the pipe full, or the "pushing box" won't push as hard as (create the voltage) it intends to.

Back to electricity: if a box has an input impedance of 1000 ohms, that means that when the incoming signal is 1 volt, 1/1000 of an amp is supposed to flow into it. The guy who's sending it a signal needs to produce 1/1000 of an amp of current to "keep the pipe full of water" ... if it can't do this, it won't push with the 1 volt of force, like it's trying to.

If the box has an input impedance of 10,000 ohms, at 1 volt only 1/10,000 of an amp is supposed to flow into it. The sending box has an easier task, as it needs to produce much less current.

Looking at it from the point of view of the signal-generating-thing that's sending the signal to the box: say the box has an input impedance of 1000 ohms. The signal-generating-thing needs to be able to produce 1/1000 of an amp of current, if it wants to send a 1 volt signal. If the signal-generating-thing has an input impedance of 10,000 ohms, the signal-generating-thing only needs to be able to produce 1/10,000 of an amp of current to send a 1 volt signal.

The rated output impedance of a device just describes how much current it is capable of producing at a particular voltage. If a box has an output impedance of 50 ohms, it can produce 1/50 of an amp of current at 1 volt. Put another way, the output impedance is the figure where -- if the next box in the line has the same input impedance -- there's the maximum power transfer between them (power being equal to volts times amps). In the ordinary world of audio, power transfer doesn't matter. The next box in the line has its own power supply, and doesn't need any power. It just needs a signal.

The numbers above are in the ballpark (more or less) for signal-generating-things that are sending line level signals, and boxes that are receiving line level signals. Most items of equipment have low output impedances (they can push a lot of water into the pipe at a particular pressure), and very high input impedances (they don't take a lot of water out of the pipe at a particular pressure). No harm is done: everyone can supply a lot more water than they need to. Boxes can produce far more current than the next box in the line needs.

When you get to mics, particularly dynamic mics (or, at the other end, speakers), the world gets tougher. They're electro-mechanical systems, whose actual function is affected by how much current flows out of them. If a lo-Z mic (ratio of voltage-to-current is low, or to put it in a way that maybe is more intuitively meaningful, ratio of current-to-voltage is high) is connected to a mic preamp with a very high input impedance (very little current flows into it at a particular voltage), not very much current will flow out of the mic, and it may not perform the way it's supposed to. Typically, the way you hear it is in bad frequency response.

 

[suntzu1982]

Thank you so very much for that. It took a lot of patience to write that, and I feel so much more satisfied. I hate when I don't understand something completely.

So, then, let's say we have a low input and a high output impendance. Therefore, the signal-thingy-sender is sending a lot of water through that pipe, but the box only has a little pipe. What would happen? Will the input try to force the current to the box and overload it? Or will it just be bad frequency response (which I'm assuming means, that because of the high output impendance, not all of the current (signal) will get into the box and recorded. Meaning lots of noise to fill the gaps?)

 

[dementedchord]

$ .02

you still dont understand.... if the output impedence is high it cant/wont be able to drive a lower imp input.... remember there's an inverse proportion law in effrect here... the lower the impedence the more the current whether that is required by the recieving unit as in your example of high output low input or the way its supposed to be low out put to higher input....ands ultimately the result is .... if the sending unit cant provide enough current to drive the next stage the power supply starts to collapse so you dont get the full output ( instead of +/- 15 volts for the sak of argument you might only get +/- 12 volts) and it will clip early and sound distorted....

 

[sjjohnston]

Right.

Impedance is defined as the ratio of voltage over current (not the other way around), so "high" means less current for a particularly voltage, and "low" means more current for a particular voltage. Which seems sort of backwards in this context, though not in other contexts.

So, just making an edit to the question two posts back:

So, then, let's say we have a high-impedance input and a low-impedance output ..."

Generally speaking, nothing bad should happen. The sending box will have more water available than the receiving box wants. But, so long as the sending box is properly designed, it should always create the appropriate pressure (voltage), and only send the amount of water (current) that flows out the other end of the pipe at that pressure.

In the opposite situation (high-Z output to low-Z input), exactly what dementedchord describes will generally happen.

Mics are different from the abstract "sending box" I've talked about. The signal (in a dynamic mic) is created by a coil of wire bouncing around a magnet. If you vary the input impedance of the thing it's connected to, it affects the mic, and the signal it produces, in various weird ways. Usually, these would present themselves as oddness in frequency response (e.g. loss of high frequencies, so the mic sounds "muddy" and "dull"). This most commonly arises when you connect a low-Z dynamic mic (most of the "good" dynamics mics are low-Z, they create a lot of current but not much voltage) to a high-Z input (most inputs intended for things other than low-Z mics are high-Z). The reverse also comes up, e.g. when connecting a phono cartridge (very high-Z, creates very little current) to a mic input, or even to an ordinary input (which may be pretty high-impedance, but not high enough).

 

[sjjohnston]

Another thought on the topic in the immediately preceding post:

If connecting a low-Z output (supplies lots of current) to a high-Z input (takes very little current) were a problem, consider what would happen if you ever turned on a box with nothing connected to its output: it would be trying to push current into an open circuit (infinite-Z).

Of course, exactly that situation (running a piece of equipment with nothing connected to the output) is a problem with some tube guitar amplifiers. As most people (I hope) know, when you're connecting a power amplifier to a speaker, it becomes important to match the impedance. This is an example of how what I've said above about abstract "boxes" doesn't always hold true at the far ends of the signal chain, when you're not just dealing with line-level audio-signal-using and -outputting "boxes," but real electro-mechanical devices like microphones and speakers.

 

[dementedchord]

something about that doesnt jibe....

the amp problem asside..(its a special case because of the output tranny on the amp needs to be loaded) its no big deal driving a high imp from a low because the sending unit simply "swamps" the unused current remember it does not have to be used it is a potential...nothing more let that set in.... a potential.... it exists incase its needed thats all... as for driving an open out put...(not connected) to consider as a problem is laffable....what are the power requirments of a piece not connected.... " this is the police put down the bong and step away....." and im not sure what your trying to describe with the low mic to high in example...it is a current problem not a voltage problem....perhaps thats the missunderstanding they are ALL current problems.... and most of the time the low to high mic problem is a matter of most dont know what to do with the other side of the output.... differential drive remember....

 

[sjjohnston]

the amp problem asside..(its a special case because of the output tranny on the amp needs to be loaded) its no big deal driving a high imp from a low because the sending unit simply "swamps" the unused current remember it does not have to be used it is a potential...nothing more let that set in.... a potential.... it exists incase its needed thats all... as for driving an open out put...(not connected) to consider as a problem is laffable....what are the power requirments of a piece not connected.... " this is the police put down the bong and step away....." and im not sure what your trying to describe with the low mic to high in example...it is a current problem not a voltage problem....perhaps thats the missunderstanding they are ALL current problems.... and most of the time the low to high mic problem is a matter of most dont know what to do with the other side of the output.... differential drive remember....

I can't tell if you're disagreeing with me (or maybe just don't understand the use of the subjunctive case), because most of what you say isn't different from what I said.

Here it is more simply:

A - Main point: Low-Z output -> High-Z input = not a problem, in the case of line-level audio.(1)

B - But, relevant note: Low-Z mic output -> High-Z input = can create a fidelity problem.(2)

C - But, before you go off and think "A" is always the case: Power amp output -> unmatched speaker load = can create a fidelity problem, or even damage equipment in some cases.

_______
(1) I tried to illustrate, as a sort of interesting and intuitive example, why the original poster shouldn't be worried about hurting the equipment in this situation, by noting that you don't worry about running an output into an infinite-Z input (i.e. leaving it disconnected).

(2) Really, it does. Try it. That's, in part, why people with 4-tracks with no low-Z inputs use (or used) those little inline transformers. It's not just "a current problem:" the mic is lower-Z than the input. And it's not just a question of balanced vs. unbalanced.

 

[dementedchord]

understood

although i would still quibble about thelowimp mic problem only in that the transformers you mentioned are really about termination....without them the coil of the mic is floating and as such there's no signalto amplify.... no??? to some a minor point but to us......LMAO see ya round the playground....bob

 

[suntzu1982]

Eventhough I didn't originally post this...thanks guys for a very informative thread. Like I got a free training class.

 

[boingoman]

although i would still quibble about thelowimp mic problem only in that the transformers you mentioned are really about termination....without them the coil of the mic is floating and as such there's no signalto amplify.... no???

The circuit is complete if a proper cable is used, the transformer is for impedance bridging.