Impedance Knob - what is it for?

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mattkw80

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I have a ART TPS 2 Pre Amp, which has a "Input Impedance" knob on it.

I have never understood what it is for.

It goes from a range of 150 to 3K.

I have read the instruction manual for the unit, but still do not understand what I would do with this knob, so I have always left it at 150.

Can anybody explain this to me?

I use the following 2 mic's with this unit.

1. Shure SM57

2. Behringer B-1 Large Condensor

and sometimes I "line in" devices into it.

Any help is always appreciated.
 
Variable impedance can be used to change the tone of a dynamic mic, or a condenser mic with an output transformer. This occurs as the mic gets loaded down by the heavier load of a lower impedance.

150 ohms is a very low impedance, and will considerably load down a mic. For a condenser like the B1, that will simply degrade its performance, with lower sensitivity and possibly lower SPL handling. For the B1, there is really no reason to run it less than 3K ohms.

For the SM57, I would recommend setting it between 600 ohms and 1.5K ohms.

If you don't feel like experimenting, you could leave it at 1.5K ohms for all mics.

The ART's impedance control only affects the mic input, not the instrument or line inputs.
 
I wonder it the knob is generally useless for many average users...

:confused:
 
Awesome.... thanks for clearing that up.

Do you own this pre-amp ?

If so, what is your opinion of it ?
 
Actually I was considering one at one point. I ended up going with twinQ (just got it, but still waiting for my converter)... But then I see that some expensive and well known preamps have that knob as well...

Personally it is something I'd like to do some research on. However, I'm not sure if it the knob provides an obvious difference... Unless maybe you know what you are listening for... Again, I'd personally like to research/play around with one of those knobs....
 
Yeah..... well I need things simple.

I'd rather not have to deal with an "Impedance" knob at all...... so I guess I'll set it to 3K and forget about it.
 
mattkw80 said:
Yeah..... well I need things simple.

I'd rather not have to deal with an "Impedance" knob at all...... so I guess I'll set it to 3K and forget about it.
Click to expand...


I think mshilarious recommended 1.5k setting...


I guess you can experiment with it... :D:D:D:D:D:D:D:D:D:D:D
 
oops........ yes 1.5K.

You are right.


I should have bought a pre-amp with 3 controls on it.

Input, Output, and On / Off.
 
mattkw80 said:
I just looked up the specs for my B-1 Mic.

http://www.behringerdownload.de/B-1/B-1_A_Specs.pdf


Impedance is listed at > 1K.

What does this mean to me ?
Click to expand...


Uhm... pretty much nothing. Personally I'm not even sure if an average user even needs to worry about this. Our audio is not RF where one may want to worry about reflections...


Generally speaking I think the whole concept is academical...

"The noise from the electronics is generally white above the 1/f corner of
the FET used. In modern design this is usually below 100 Hz and seldom a
problem, and the resulting A-weighted noise is around 1 microvolt RMS, or
-120 dBV. Unlike most mic and console makers, I don't use dBm. dBm refers
to noise _power_ (it is dB ref 1 milliwatt into 600 ohms) and is only
relevant when a matched transfer of power is taking place. The output
impedance of a microphone is seldom, if ever, matched these days into the
load impedance of the preamp which would make a power measurement
meaningful. If you ask the manufacturers about this, they will shuffle
their feet and say, "oh yes, we mean dB referenced to the voltage level
(0.775V) that 1 milliwatt from a 600 ohm source makes across a matched 600
ohm load." Arrgh."
http://www.harmony-central.com/Other/mic-faq.txt

Although there seems to be a bit of science behind impedance concept in general...
http://sound.westhost.com/impedanc.htm
http://www.freepatentsonline.com/4922547.html
http://www.answers.com/topic/microphone?cat=biz-fin
http://www.rane.com/note159.html
 
I've twisted and turned this thing, but my ear cannot hear the difference.

I guess I will leave it pointed to 7:00 o' clock, which I am hoping is 1500.
 
However, I'm not sure if it the knob provides an obvious difference
Click to expand...

it can make a massive difference, depending on the mic

although it doesn't necessarily make it true, i've heard many people state that the optimal impedance on a preamp will lie somewhere between 1.5-2 times the output impedance of the mic - so if you have a mic with an output load of 150, you would most likely get the best performance with the impedance set between 225-300 ohms.

of course, this is shit that a bunch of people have blurted out, and may not be true at all. i usually just twist the knob and listen, and leave it wherever is sounds best.
 
Ironklad Audio said:
it can make a massive difference, depending on the mic

although it doesn't necessarily make it true, i've heard many people state that the optimal impedance on a preamp will lie somewhere between 1.5-2 times the output impedance of the mic - so if you have a mic with an output load of 150, you would most likely get the best performance with the impedance set between 225-300 ohms.

of course, this is shit that a bunch of people have blurted out, and may not be true at all. i usually just twist the knob and listen, and leave it wherever is sounds best.
Click to expand...


Does it sound best at about the same place it used to be before you had to tweak it?

:p:D
 
You may find, as your ear develops, that the mostly subtle change that the variable input impedance provides is a good thing to have. There are times when it isn't even optimizing the load that I'm after, which Ironklad correctly points out is not a case of matching input/output impedance. There are times when I want the slightly softer sound that can sometimes come with selecting the "wrong" input impedance.
 
Load does make a pretty big difference to the sound a mic reproduces. Pay particular attention to the low end...when a mic is loaded properly the proximity effect becomes more pleasant and less muddy...I don't know how ART accomplishes it's loading in this pre as I don't have a schematic. Transformer based pre's sound vastly different with different loading.

Cheers,
Ray
 
Hope I'm not too way off topic here........ but speaking of my ART Pre-Amp.........

I also have the ART TCS Compressor. (These units are painted almost identical, so I'm pretty sure they meant to be run together).

Should I hook it up like this :

*Pre-amp* XLR *Compressor* XLR *MIXERBOARD*

OR

*Pre-amp* 1/4" jack *Compressor* 1/4" jack *MIXERBOARD*


I've have never know what cables would work best. I've tried it all hooked up with XLR but then it seems like it's too hot to use. (Would XLR make it hotter?)

Between that problem and the mystery of the Impedance knob, I've never got any use out of this rack-mounted $600 waste.
 
The reason the xlr seems hotter is because the xlr input on your mixer is expecting a mic level signal and you are feeding it a line level signal.

The correct way would be:

*Pre-amp* XLR *Compressor* 1/4 jack *MIXERBOARD*

You could use a 1/4 between the preamp and the compressor as long as it's a trs cable to keep it balanced.

There is no difference is sound or function between an XLR cable and a 1/4 TRS cable.
 
Farview said:
The reason the xlr seems hotter is because the xlr input on your mixer is expecting a mic level signal and you are feeding it a line level signal.

The correct way would be:

*Pre-amp* XLR *Compressor* 1/4 jack *MIXERBOARD*

You could use a 1/4 between the preamp and the compressor as long as it's a trs cable to keep it balanced.

There is no difference is sound or function between an XLR cable and a 1/4 TRS cable.
Click to expand...


Good tip. Thank you.
 
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