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Microphone impedance <200 ohms... too low?
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wm_b
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1st December 2007
Old 1st December 2007
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Microphone impedance <200 ohms... too low?

I have several AKG D12 microphones. One, which is more recent than the others but still a D12 and not a later D12E measures about 220 ohms where the others are in the 50 ohm range. If I'm not mistaken, many preamps don't spec down to 50 ohms do they? Seems like the 200 - 600 ohm is the most common range. Is there something I should do to make accommodattion the extra low impedance.

I tend to use these on kick drums, bass cabs, upright bass and the odd trombone/baritone sax.

They sound fine to me but the 220 ohm version seems a little more extended in the bottom. Commonly I will couple the mic with a portico 5012 or BAE 1272. Does this stuff make a difference down that low?

Cheers, wm
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1st December 2007
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I'm not sure about the ohmage.

The only difference between the d12 and the d12e is the d12e has an XLR. The e stood for export. Maybe the mics measuring low are in need of repair.
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1st December 2007
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I was just trying to clarify that they are older than the E series. The E model came along during the 60's where the D12 was introduced in 1954 or so.

The specs from AKG say that there is a 60 ohm and and 200 ohm versions. There is no explanation as to why there are different impedances just that there are.

I had the night off and just started going through the mic cabinet and listening. It's been awhile since I've done that. My favorite D12 for kik happens to be one that measures around 56 ohms. The 216 ohm version sounds slightly more extended in the lows but the 56 sounds a little tighter to me. I was just wondering if the load could be optimized.
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You want the input imp. to be far more than the mic imp.
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1st December 2007
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I think my old one has the impedance selection available by swapping a different color wire in the output connector. I haven't seen the manual in years.
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1st December 2007
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Low impedance (50 ohm nominal?) mics were offered to mitigate against HF losses when driving very long mic cables, due to the pole formed with source impedance and cable capacitance. The 50 ohm version will typically have lower output voltage so may not deliver optimal S/N when used with typical preamps.

You should also be careful about measuring impedance with a DC ohmmeter which is measuring resistance not impedance. If the mic has active electronics on board the source impedance will often be a resistive build out.

JR
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1st December 2007
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I've been advised to strap a 50 ohm resistor across pins 2 and 3 on my old rca 44bx since it has too low an imp. to work with some pres. haven't tried it yet, because i'm nervous to cross pins 2 and 3 on an old ribbon. But it comes from a very reliable source, so I imagine it's fine.
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3rd December 2007
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50 and 200 were common standards in Europe. 50 (Western Electric) and 250 (RCA) used to be standard here in the U.S.
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3rd December 2007
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Quote:
Originally Posted by beats workin' View Post
I've been advised to strap a 50 ohm resistor across pins 2 and 3 on my old rca 44bx since it has too low an imp. to work with some pres. haven't tried it yet, because i'm nervous to cross pins 2 and 3 on an old ribbon. But it comes from a very reliable source, so I imagine it's fine.
David
I see NO reason for loading down the mic with a 50 ohm resistor...People have loading and matching confused.
Ribbons mics have a low output as it is, why would you want to make it worst???
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RCAs were designed to be run unterminated which meant at least 500-1500 Ohms. I can imagine 50 Ohm resistors in series with each side but 50 Ohms across would probably be a muffled sonic disaster.
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I've thought you typically would load an output with 8 to 10x it's output impedance. While this isn't the most efficient power transfer, it tends to even out the frequency response of a microphones output. This is part of the reason the old Neve modules had switchable impedance, from 300 Ohm, to 1.2kOhm. 600 Ohm input Impedance is more normal for line level equipment. A 300 Ohm mic pre would probably be ideal for for most 50 Ohm mics, but I almost always leave mine on 1.2k. Most of the Mics I use are 150 to 300 Ohms.
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4th December 2007
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"People have loading and matching confused."

you mean Bridging?
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Quote:
Originally Posted by mike_mccue View Post
"People have loading and matching confused."

you mean Bridging?
No, matching, as in matching impedance; 600 ohm to 600 ohm...
Matching and bridging is different, He was thinking if a mic has 50 ohm outout it needs to SEE a 50 ohm input(matching)...
Yes bridging is correct for a low impedance into a higher impedance such as a mic into a pre, a amp driving a speaker ect...
I could have been more clear in my comment...
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Mics like to "see" a load that is AT LEAST ten times their impedance. That is why most modern mic gear offer source impedances of at least 2.5k or so...to reflect 250 back to the mic. I have discovered while building and racking mic pre's that I like an input of 3.3 to 5 k depending on the mic but that is my own personal taste, not from a data sheet. There are some notable exceptions..I have some old sm57's that live and breathe at a true 500 ohm input..a bunch of really experienced engineers and producers (but young guys who have grown up in the 10k input era could not believe the tones I was getting with these old work horses. I simply racked 2 old 312's with a switch that changed the loading resistor on the input transformers secondary....low for 57's and other old dynamics and high for modern mics and condensors...also explains why some folks hate dynamics while they are jamming them into higher impedances than can't work well with...there is alot more to being an engineer than knowing how to copy, cut and paste.

Cheers,
Ray
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Quote:
Originally Posted by rsolinski View Post
Mics like to "see" a load that is AT LEAST ten times their impedance.
Unfortunately the situation is much worse than that.

It depends on what sort of load the mic was designed to work with. Most are designed to work with loads as you wrote. Some, are not. It is difficult to get this information on a mic by mic basis.

Frustrated:
Andre
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The D12e has a 260ohm impedance.
I've got a couple of questions or thoughts if you will on this subject. I tend to use mostly 150 to 300 ohm mics. I've always heard the rule of thumb was 5x was optimal hence why most consoles Neve, trident etc have 1200 ohm inputs.
The word I've heard is, when the inputs load is similar to the mics impedance you lose freq response.
My questions are:
1.What happens when you are using say a new Neumann transformerless mic(most are 50 ohm) and go into say a 1200 ohm input...is there some sort of reaction this way as well? If there weren't I'd guess most manufacturers would make 10k ohm inputs
2.Some dynamics are 600ohm (audio technica atm-25's for example) How do we bring those in line with a 1200 ohm load? If you go by the standard formulas we'd need an input of 3000-6000ohm. I know how to pad down impedance via a resistor network in some xlr cans..but can you "pad up" the inputs in some simple way.
Thanks all...a good discussion. Good ol' basic knowledge being handed down by the greats that find time to post in here.
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4th December 2007
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If the mic is lower say 50 ohms that's better, will load the mic less.
But just because a mic is say 150 ohm does not mean it will drive the same load,say a 1500 ohm as well as another mic that is also 150 ohm, example; a SM57 is a 150 ohm but loads easily with a 1500 ohm input, I measured around a 6db drop in level compared to several other dynamic's that were 150 ohm.
Maybe that's why its a favorite for snare, has a built in 6db pad...
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Quote:
Originally Posted by nosebleedaudio View Post
If the mic is lower say 50 ohms that's better, will load the mic less.
There has to be more to it than that, there has to be a downside or manufactures would have made all mic inputs 10k ohms etc.
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4th December 2007
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Quote:
Originally Posted by studio1117 View Post
There has to be more to it than that, there has to be a downside or manufactures would have made all mic inputs 10k ohms etc.
It's also a noise figure, and since mics are very low level devices you need to transfer as much energy as possible, its a trade off like alot of things in electronics.
For those who don't all ready know this, a pre with 60db of gain is amplifying the signal 1000 times, so you want as much energy as possible and as little loss as possible.
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Not to jump off topic, but there's something very funny about you posting underneath me in these forums...my picture...your name......
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Quote:
Originally Posted by studio1117 View Post
Not to jump off topic, but there's something very funny about you posting underneath me in these forums...my picture...your name......
Yeah, I thought of changing it to my name but it's too late...
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4th December 2007
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Quote:
Originally Posted by avare View Post
Unfortunately the situation is much worse than that.

It depends on what sort of load the mic was designed to work with. Most are designed to work with loads as you wrote. Some, are not. It is difficult to get this information on a mic by mic basis.

Frustrated:
Andre
Not really..poke around for data sheets..email or call manufacturers, ask old engineers ....

If you want to really understand how a mic drives things you need to look no further than ohm's law. See how current is related to resistance. All dynamic mics are designed differently. Modern dynamics are much "hotter" than shure's because of the capsule, magnet and transformer, not the loading per se. Shure's Beta series is on par with modern designs. If you really want to warp your mind the AEA ribbon mic pre, which gets good reviews, has an 18k input impedance. The reason Neve's and Trident's set their impedances at 1.2k were to split the difference between dynamics and condensors. It is relatively easy to change impedances in a transformer based mic pre since the transformers themselves don't actually have an impedance. They reflect impedance based on their turns ratio. Change the secondary loading resistor and volila, different input impedance. I like to put a switch in that changes the resistor. All this aside...experiment! Sometimes a mic running into a wierd impedance sounds cool. Lots of advaned to techniques were made by guys thinking outside the box

Cheers,
Ray
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The rule of thumb for mic preamp input terminations of 10x is not cast in stone and basically trying to avoid loading down the microphone in such a way that deteriorates its sonic character while also maximizing voltage transfer for best S/N ratio. A simple inspection of the voltage divider formed by mic's source impedance, and preamps input impedance will reveal how much signal is lost in the input termination. Signal lost here reduces the "S" in S/N.

There is no S/N penalty from terminating a microphone with higher than 10x but quickly diminishing benefit. Further if the microphone is designed expecting a nominal termination to deliver specified frequency response, variation significantly high or low from that could cause deviation from that expected response curve.

Another poster asked about 600 ohm mics. These AKA "high impedance" mics can use simple matching transformers that will step down the 600 ohms to be a better match for typical low impedance mic preamps. I've seen these small matching transformers built into simple adapter cables.

JR
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4th December 2007
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Quote:
Originally Posted by JohnRoberts View Post
The rule of thumb for mic preamp input terminations of 10x is not cast in stone and basically trying to avoid loading down the microphone in such a way that deteriorates its sonic character while also maximizing voltage transfer for best S/N ratio. A simple inspection of the voltage divider formed by mic's source impedance, and preamps input impedance will reveal how much signal is lost in the input termination. Signal lost here reduces the "S" in S/N.

There is no S/N penalty from terminating a microphone with higher than 10x but quickly diminishing benefit. Further if the microphone is designed expecting a nominal termination to deliver specified frequency response, variation significantly high or low from that could cause deviation from that expected response curve.

Another poster asked about 600 ohm mics. These AKA "high impedance" mics can use simple matching transformers that will step down the 600 ohms to be a better match for typical low impedance mic preamps. I've seen these small matching transformers built into simple adapter cables.

JR
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