correctly setting the mic impediance on my ACMP81/73

[jasonbmerrill]

hello!

my two neve clones have switchable mic impedance. Sure its only 2 settings, but better than no choice at all

what i am wondering, is what to use for my mics?

it is switchable between 1200 and 300 ohms(?)

....

my mics are:

Electrovoice RE20
audix i5
shure sm57
shure beta 52
Studio Projects C1/LSD2
MXL 2003/603s
Nady SCM900
Sm7b (soon)
Fathead II Ribbons


is this normally in some manual? or is there a rule, like, dynamics get lower impedance, condensors higher?

so for instance, it says here:

http://www.gbaudio.co.uk/data/re20.htm

Impedance: 50, 150 and 250 ohms

"The RE20 is supplied wired for 150 ohms impedance. "

that would seem to imply lower impedance is better in this case, no?

same thing with the i5:

"Output Impedance: 150 Ohms "
__________________

 

[dgatwood]

No. Input impedance should always be significantly higher than the output impedance of the mic. Otherwise you're loading down the output transformer on the mic (assuming it has one, of course). You would almost always use the higher setting. In fact, if I were designing something, it would be switchable between about 1k-ish and about 10k-ish.

I can't imagine why anyone would ever use something as low as a 300 ohm input impedance. Anybody have any ideas?

 

[antichef]

Yeah - leave it on high all the time. If you're ever using a mic and it seems unexpectedly noisy (hissy, I guess), try the low impedance setting. Seems unlikely.

As to when you would want to use it, um... a really low impedance mic with a *really* long cable?

 

[Weasel9992]

Leave it on high impedance, just like the other guys said.

Frank

 

[jasonbmerrill]

Yeah - leave it on high all the time. If you're ever using a mic and it seems unexpectedly noisy (hissy, I guess), try the low impedance setting. Seems unlikely.

As to when you would want to use it, um... a really low impedance mic with a *really* long cable?

how long? I do have some long cabling going on with my RE20... possibly about 25 or more feet (not 50)

 

[antichef]

how long? I do have some long cabling going on with my RE20... possibly about 25 or more feet (not 50)

Somebody else needs to answer, because my premise itself may be false. But generally speaking [from my copious reading ] no one seems to complain about cable length until it exceeds 100 ft. My longest cable is about 50', and I can't really tell any difference between it and my 3' cables.

 

[Weasel9992]

Somebody else needs to answer, because my premise itself may be false. But generally speaking [from my copious reading ] no one seems to complain about cable length until it exceeds 100 ft. My longest cable is about 50', and I can't really tell any difference between it and my 3' cables.

100' easy. Anything over that and noise induction is usually an issue to some degree anyway, impedance notwithstanding.

Frank

 

[dgatwood]

I was going to sarcastically suggest a quarter of a mile, but your phantom power voltage drop over that distance would probably render it unusable.

With properly shielded cables, you ought to be able to go at least a hundred feet without any significant degradation. With longer runs, even with perfect shielding, you'll get high frequency loss due to the cable capacitance. The dividing line between "good enough" and "not good enough", of course, is somewhat a matter of personal preference....

 

[mshilarious]

I was going to sarcastically suggest a quarter of a mile, but your phantom power voltage drop over that distance would probably render it unusable.

Not really, the cable resistance in that case would only be around 100 ohms, whereas the resistance of the phantom supply is 3400 ohms.

With properly shielded cables, you ought to be able to go at least a hundred feet without any significant degradation. With longer runs, even with perfect shielding, you'll get high frequency loss due to the cable capacitance. The dividing line between "good enough" and "not good enough", of course, is somewhat a matter of personal preference....

True, but it's important to note the relevant spec there is microphone source impedance, not preamp input impedance. With a typical low impedance microphone, 400ft should be the approximate length before any high-frequency loss is experienced.