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Impedance for Musicians

Only what the home studio operator needs to know

 

under construction

Ebtech LLSXLR Line Level Shifter (2-Channel)
The Ebtech Line Level Shifter is designed to increase or decrease signal voltage (-10dBV and +4dBu) without adding noise. The Line Level Shifter also converts back and forth between balanced (+4dBu) and unbalanced (-10dBV) signals automatically. The Line Level Shifter also contains Ebtechs' Hum Eliminator technology to break ground loops that cause AC hum.
Radial JDI MK3 Passive Direct Box
The Radial JDI is considered by many to be the world's finest direct box. It is a passive DI that employs a Jensen isolation transformer for optimum audio performance offering outstanding linearity at all frequencies, combined with extraordinary level handling without introducing distortion.

Behringer DI100 Active Direct Box
Behringer's Ultra-DI DI100 is your ultimate friend and helper on stage and in the studio. With this active DI box, hum and impedance problems are a thing of the past. Every conceivable sound source - electric guitar, bass, keyboards, or what have you - can be "tamed," sending it balanced and noise free into your console. You can even connect a guitar amp's speaker output with up to 3,000 Watts. No sweat.

Radial Pro RMP Reamping Device
The Radial ProRMP, a transformer coupled reamping device that allows a prerecorded line level signal to drive a guitar amplifier and pedal effects by properly matching impedance and allowing the user to determine the appropriate level.

 Imagine a world where pieces of gear could talk to each other.  Here we have Mr. Microphone, Mrs. Mixer, and Master Newb. I know this sounds stupid, but listen, I am going to make sure this gets through your newbly head.  See, I am going down to YOUR LEVEL where you understand me.  That is what IMPEDANCE is all about; it's getting things on the same playing field, where they can talk and hear each other.  Then when the electrons flow they do so in the way we want them to.  The two pieces of gear fit, electrically speaking, like a glove. And they sound like they were made for each other. Forget all that stuff about 0.775 volts @mW into 600 ohms.  OK?  Save it for when you go pro.  But even then you can get by without it.  You can't get by without the info on this page.  Ready?  ;)

 

Master Newb in his first act with his home studio tries to connect his shiny new XLR mic into the line input of his mixer.  He already knew his mixer did not have an XLR input, so he bought an XLR to 1/4" line cable. Newb smiles ready to firmly insert the 1/4" into a line in. 

 

"Uh, uh, you're Not sticking me in the Line input with THAT monstrosity you ...you...Yahoo!",  scolds Mrs. Mixer.  "You gotta go into a HiZ Mic input for that awful cable hack job to work, and even then its going to sound weak and noisy like crap!

 

Then Mr. Mic shouts out "Dude, you idiot, I'm a LoZ!!  You just about destroyed my balanced signal with that cable and now you want to put me in a LINE input--it won't even know I'm there!"   Newb retorts:  "Silence, my studio tools!  It says right on the package 'XLR to LINE level".  Mic shouts back "It doesn't matter what the package says, what matters is the electricity flowing between me and that lovely mixer."

 

Mixer chirps up: "Dweebo, +1 on what Mic said.  See, I don't have a Low Z XLR input, so take that stupid cable back to the store and get a mic preamp with a line level output.  Then Mr. Mic and me will be a perfect match and we'll sing all night!"  Of course, Master Newb does not listen.  He jacks up the mixer and screams into the mic.  Hmmm. Nothing....wait, lets turn the amp all the way up...and then turn the mixer all the way up....yeah, there it is I can barely hear it but it works, kinda, er...sorta....

 

Lol. :)  Sticking a mic into a line input is one of the worst impedance mismatches you can do.  Don't do it.

 

This article is deliberately and unashamedly non-technical.  Do you need to know about impedance to run a home studio?  Yep!  But what we need to know is far different from what an electronic technician or equipment designer needs to know.  Do you need to understand the theoretical basis for impedance?  Nah, don't buy the hype.  Not for a home studio. All we need to cover here are the types of impedance our gear offers so we can correctly connect one piece to another.  We need to know how to match impedance from one piece to another and avoid mismatches that will deteriorate the sound quality we have worked to achieve.  We also need to know how to interpret the jargon in sales info about products so we clearly understand the exact nature of the inputs and outputs of the gear we want to buy and know with certainty whether it will interface properly with our other gear. 

 

Impedance is about creating an environment for signal integrity inside electronic circuits.  Don't think of pieces of gear in isolation when you think of a circuit.  Think of the flow of electrons between two or more pieces making up ONE circuit.  If you correctly match the impedance of 2 pieces of gear that share an circuit of electrons, those electrons will ideally conform to intended signal the and not create unwanted noise.  Likewise, if you mismatch impedance in a circuit, you may be creating an extra layer of noise.  The source signal may sound to loud and noisy or at a much lower level than you desire. 

 

There are two basic levels of impedance and variations within each that we need to know.  They are Mic Level and Line Level.

 

 Mic Level

 

When an input is at Microphone level, it is often said to be either High Impedance (HiZ) or Low Impedance (LoZ).  Typically, high impedance Mics are those that are connected to cables with a 1/4" unbalanced output, though keep in mind I said typically.  Usually these are cheap mics designed to work with guitar amplifiers, pocket recorders and toys.  Most electric guitars and basses are also outputting at high impedance.  Sometimes a high impedance input is called "instrument level".    If you plug one of these hi impedance sources into a line level input what happens?  The source is very weak, and as you add gain to bring it up to useable level, you find you are adding a lot of noise. 

Low impedance microphones would be even more faint plugged into a line level device.  They require a preamplifier designed to work at extremely low signal levels and be able to boost these weak signal cleanly.  What happens when you take a line level synth and plug it into a low impedance mic preamp?  It overdrives the signal past the breaking point of the circuit and extreme distortion can be heard. 

 

Line Level

 

Much of our gear is said to be at Line level.  Audio interfaces, mixers, effects boxes, synths, keyboard workstations, preamp outputs are generally all at line level.  However, there are 2 standards, often referred to as +4 and -10.  It used to be, over a decade ago, that "professional" gear was set to +4 and "consumer" gear was set to -10.  This does not hold up as well as it did.  Many pieces of consumer and professional gear today can be switched from -10 to +4 via a switch in the piece's firmware.  But it is not always the case.  And this is why you need to be careful. 

When you plug a +4 output into a -10 input its very easy to overload the input and bring in noise.  Likewise a -10 output into a +4 input seems a little faint, which makes one want to boost the volume and add noise that way.  Often, you can make a mismatch work, but you will know its not ideal and that the integrity of the signal was blemished. 

The practical application of this knowledge is as follows;  if you know you have consumer -10 gear you want to use, but you also want to use better +4 gear, make sure your audio interface or mixer can handle both +4 and -10.  A lot of DJ mixers and DJ audio interfaces will only take -10 impedance at line level.  Unfortunately for the industry, DJ rigs tend to fall back on a standard of RCA cables at -10 impedance and unbalanced gear, which can be a problem rejecting ground loops, using long cables and driving a loud and clean signal to the amp and speakers.

 

Common Impedance Issues and their solutions

 

Perhaps the most common mismatch is when someone plugs a guitar or other instrument level device direct into a mixer.  Mixers typically only have LoZ and line level inputs, so plugging in the HiZ guitar output is a bad recipe.  In this case the direct box is often used.  The Direct box or DI box as it is sometimes called changes the unbalanced HiZ signal to a balanced LoZ signal.  This allows 3 things to happen.  1) the impedance is matched and gain can be applied cleanly to the signal.  2) longer cable lengths can be used.  3) the circuit  resists ground loops (hum).  There may even be a "ground lift" on some DI boxes that will kill any ground loop that has formed. The audio result using a DI box is often clearly superior. 

Some indications you may have an impedance mismatch are when you say:

"I turned the channel up all the way but I can barely hear the track"  

"When I plugged in the outboard gear, it was way louder than any other piece and had hums and buzzes and  radio stations leaking in"

 

Questions and answers:

 

Q) Tweak, I think I have a mismatch, what do I do?

A) Look for ways to change the impedance of the gear in question, either at the source or the destination.  Look in the control panel for your audio interface.  If it has impedance switches, try them out.  Also look in the global parameters of the source gear.  There is often an impedance switch buried in a menu.

 

Q) Is +4 "better" than -10?

A) The important thing is to have the gear match up for optimum signal quality.  If I were forced to choose one or the other I would choose +4.

 

Q) Is +4 always balanced and -10 always unbalanced?

A) No.  You can switch impedance, yet retain the balanced nature of the connection.  -10 balanced is possible as is +4 unbalanced.  Since I have a large rig I strongly prefer balanced connections and matched impedance.

 

Q) Is it a good idea to have some audio interface channels set to -10 and others to +4.

A) That is fine if that is what the gear requires.

 

Q) I have an old synth at -20 impedance.

A) Two ways to deal with that.  Plug into -10, turn up the front panel volume all the way, and boost the gain (and deal with the noise) or plug it into HiZ instrument level and cut down the front panel volume a bit. 

 

Q) Tweak, I think my love life is one big impedance mismatch.  Every time I say something, I get crap back.

A) Hmmm that is a different kind of impedance.   To that I say...

 

"Have fun and Let nothing impede your path to greatness!"

Tweak

January, 2009

 

 

Links

Microphone Impedance by Rupert Neve Designs

 

Threads

Lo to Hi impedance adapter?

Re: How important are balanced inputs on a soundcard?

Re: a brainless +4 dbu AND -10 dbV question!!

speaker impedance question

Behringer DI800

Speaker cab 8 ohms vs 4 ohms out amp..please help the stupid

Hi-Z: Grrrrrr........

 

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Tweak's Articles on Essential Studio Concepts

Hooking Up Audio
MIDI Basics
The Many Functions of MIDI Data
The Audio Interface
Signal Flow Computer-based Studio
Signal Flow of an MPC Hip Hop Studio
Signal Flow of a MultiTrack Studio
Assembling Your Studio Rig
Studio setup in a Nutshell
5 Hot Tips
Building a Quiet Room
Understanding MIDI Interfaces
The War on Hum
Multiple Video Displays
Latency and how to Deal
Word Clock
TimeCode
Everything About Cables
Digital Audio Converters
Bit Depth and Sample Rate
Studio Monitors
Impedance for Musicicans
How to setup a Patchbay
Room Acoustics Basics
Studio Monitors Price List
Acoustic Products
Catalog of MIDI Interfaces

 

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