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[sadworld]

some things i've read recently prompt me to ask this question... what is the input impedance switch on pres for? for example , my x73i is switchable between 1.2 and 300. the manufacturer basically said i will never have to use the 300 setting. what is that for and what will it do for me if anything... thanks, matt.

[Phillip Graham]

sadworld wrote on Sat, 08 May 2004 01:16

some things i've read recently prompt me to ask this question... what is the input impedance switch on pres for? for example , my x73i is switchable between 1.2 and 300. the manufacturer basically said i will never have to use the 300 setting. what is that for and what will it do for me if anything... thanks, matt.

Hey Matt,

There are two types of electric circuit designs we must discuss to understand this question.  The first is the impedance MATCHED circuit, and the second is the impedance Bridged circuit.

In a circuit where the impedances (input and output) are matched, it can be shown that maximum POWER transfer takes place.  This is a classic problem in any introductory circuits textbooks, and some introductory physics texts.  There are additional advantages of this topology when you talk about transmission lines, but audio frequencies are far too low to worry about them.

On the other hand, most (all?) audio interconnect circuits work in the bridged topology, where the input impedance is substantially higher than the output impedance (usually aroud 10:1).  This results in the majority of the voltage being transferred from output to input, and keeps the amount of current that must be sourced by the output devices small.

On your device, 1.2 probably refers to an input impedance of 1.2kilo ohms, and 300 to an input impedance of 300 ohms.  if the output impedance of the FET inside your condensor mics is nominally 150ohms, 1.2kohms is a bridging impedance.

Somewhere out there in the world of audio past there are probably microphones (likely tube microphones) that have output impedances of around 300ohms, and need a matching impedance.  One of the microphone guys will have to pipe up here, as I am simply too young to know.

Switching over to the 300 ohms switch with your average run of the mill microphone will cause it to have to source more current.  For a condensor this could me the FET leaves its linear range sooner, which will produce harmonics that you may or may not find pleasing.  Your condensor mics electronics may not take kindly to this lower input impedance.
On a dynamic microphone you will be changing the effective electrical q component of diaphragms damping.  This also may, or may not prove auditorially interesting.  

There are other subtleties of the effects of input impedance, but this probably answers your question.

As an aside, impedance matching is one of the most important concepts in science.  The optical analog (index of refraction) the mismatch determines the amount of reflected and transmitted light.  The acoustic analog (radiation impedance) determines how well a source reflects sound, or loudspeaker couples to the air.  Finally, in the electrical world, matched impedances are important at higher frquencies (Mhz+) to reduce the amount of reflection back down the cable, which now acts as a transmission line.

Whew!

[sadworld]

whew is right!   ... now how about a lamens version of that reply!  no really, thanks, i understood most of that, i'll probably read it a couple more times....

[magicchord]

All Phill is saying is that switching the preamp to a low input impedance will change the sound of some mics.
Just experiment with the mics you have; you may hit upon a combination that you find pleasing.

Patrick Bryant
MagicChord Music

[JGreenslade]

Matt,

The Vintech has taps for 300 and 1200 as it is a "clone" of the Neve 1073, and uses the same type of input transformers, similar to the St Ives / Marinair / Carnhill VT22670 / 10468

The 1200 tap is typically suitable for say, a Neumann capacitor mic which may usually have an o/p impedance of around 150 Ohms. The 300 tap is typically for ribbon mics, which can have an o/p impedance of anything down to 30 Ohms (I have a '60s one in front of me here, it provides taps at either 30 or 50 Ohms). I have yet to hear of a mic from any era that is designed for the archaic 300 Ohm impedance-matching concept.

A basic understanding of impedance is essential for ANY audio engineer, and I would suggest scrolling down this link to the impedance explanation (and also click on the "reactive load" link): http://www.rane.com/par-i.html

Cheers,
Justin Greenslade

[Phillip Graham]

thermionic wrote on Sat, 08 May 2004 13:48

The 1200 tap is typically suitable for say, a Neumann capacitor mic which may usually have an o/p impedance of around 150 Ohms. The 300 tap is typically for ribbon mics, which can have an o/p impedance of anything down to 30 Ohms (I have a '60s one in front of me here, it provides taps at either 30 or 50 Ohms). I have yet to hear of a mic from any era that is designed for the archaic 300 Ohm impedance-matching concept.

Thanks Justin, that bit fills it right in.  My audio is 90% in the live world, where ribbons are generally too fragile.

Quote:

A basic understanding of impedance is essential for ANY audio engineer, and I would suggest scrolling down this link to the impedance explanation (and also click on the "reactive load" link): http://www.rane.com/par-i.html

yup!

[ted nightshade]

a good thread on this subject

http://www.3daudioinc.com/cgi-bin/ultimatebb.cgi?ubb=get_top ic&f=14&t=001513

some great stuff from gear designers there.

[JGreenslade]

Had a brain-fog yesterday, in the interests of accuracy I should add that people were using impedance matching in mic lines, but probably only up until the '60s when they started thinking in terms of voltage transfer rather than power, I wasn't around in the '60s so I can't comment exactly.

Taken from: http://www.rane.com/note124.html

Quote:

Impedance matching went out with vacuum tubes, Edsels and beehive hairdos. Modern transistor and op-amp stages do not require impedance matching. If done, impedance matching degrades audio performance.

Not sure I agree with the comment on vacuum tubes though!

Justin

edit: I should add that the Rane link provides some useful info on interfacing cross-coupled o/p stages / inverting op-amps / transformers etc - for anyone unsure I would recommend giving it a few reads so you understand what losses are involved when you connect unbal-bal and vice-versa, essential stuff if you don't want to damage electronic o/p stages, and maximise noise-floor.

[sdevino]

Magichord had it right. Impedance matching is something for designers and installers to worry about.  As far as your mic pre goes, you will not harm anything in either setting, the idea was to allow you the user to play with the impedance which will change the way many older mics sound quite a bit.

Switch it to which ever way sounds better with that particular mic and recording circumstance and don't worry about it.

Steve

[dwoz]

Gtphill....what a great response!  kudos.

dwoz
david wozmak

[Erik]

sdevino wrote on Sun, 09 May 2004 10:25

Magichord had it right. Impedance matching is something for designers and installers to worry about.

I disagree... it's something every engineer should be aware of.  You need to understand the science if you're going to make intelligent decisions in the control room instead of just cramming patchcords into holes like a monkey.

Or put another way: anytime you patch a signal path together, you're effectively an 'installer.'  Especially given the odd mixure of semi-pro gear combined with pro gear--and new stuff combined with very old--in modern studios.

Another particularly obvious example is the use of a direct box.

So even if you're not designing mic preamps, it's important to understand what changing the impedance is doing so you know what to listen for when you make the tradeoff.

--Erik

[micguy]

I can answer this question -

Impedance matching is important in things like telephones, where the line is so long that the wire's impedance comes into play, and to make it work without a lot of ringing, you need to match the line's characteristic impedance at both ends.

Microphones work with much shorter cables, so what you're up against with a mic pre is just how much the preamp loads the mic's output (usually a resistive load for all intents and purposes). However, a lot of telephone trained engineers were around in the early part of the last decade, so a lot of terms and practices (like dBm and 600 ohm lines) got "transferred" into pro audio, even when they don't appply as much as they do in telephony.

For a condenser mic, loading it more (lower impedance) takes more current out of the mic's output stage for a given signal level. As a result it'll go non-linear at a lower level - occasionally useful, but usually a drawback.

For a dynamic mic, the impedance is not constant with frequency, so what you get is more loading of the output at frequencies where the mic's output impedance is high - usually at resonance (somewhere between 40 Hz and 200 Hz on a typical mic) and at extreme high frequencies. Usually the low frequency loading is the more severe. It won't kill a dynamic mic to load it heavily, but it will usually thin out the sound - may or may not be something you want.

As far as this "loading" affecting the physical system of the microphone - damping the diaphragm or whatever, there is some calculable effect, but in reality the coupling between the electrical and mechanical systems isn't all that strong - mostly it's just an electrical system phenomenon.

[egg_]

micguy

As a result it'll go non-linear at a lower level - occasionally useful, but usually a drawback.

I re-amped some guitar stuff lately - I had recorded direct to DAW with a DI box, then just plugged an analog output from the DAW into the amp. It sounded bad, no matter what I did. So the amp is drawing too much current from the DAW output (or rather, the amp isn't providing enough of a load to the DAW output), making things non-linear, is that what's wrong?
How to fix it - I know you can buy a re-amp box ... would a passive DI box in reverse also work? It essentially does the same thing, right

[magicchord]

Also, with mics that have output transformers:
The transformers are usually designed to be critically damped (flattest high-frequency response) when connected to a pre that has a 1 to 3 kilohm input impedance.
A 200 - 300 ohm load will overdamp the mic's output transformer, resulting in reduced high end.
Of course, you may desire that effect in certain situations.


Patrick Bryant
MagicChord Music
My avatar is an actual picture of me.

[micguy]

egg_ wrote on Thu, 20 May 2004 10:56

micguy As a result it'll go non-linear at a lower level - occasionally useful, but usually a drawback. I re-amped some guitar stuff lately - I had recorded direct to DAW with a DI box, then just plugged an analog output from the DAW into the amp. It sounded bad, no matter what I did. So the amp is drawing too much current from the DAW output (or rather, the amp isn't providing enough of a load to the DAW output), making things non-linear, is that what's wrong? How to fix it - I know you can buy a re-amp box ... would a passive DI box in reverse also work? It essentially does the same thing, right

There's a lot going on here that could have contributed to "bad sound" - most likely not that the guitar amp is to heavy of a load for the DAW, though. Guitar amps have input impedances in the range of 1 megohm (a really light load)- not going to load any reasonable DAW output. My guess is you're overloading something somewhere - either recording too hot, or you're hitting the guitar amp with too much level from the DAW. Can you see clipped waveforms on the track in the DAW?