microphones/preamps and ohms

[moresound]

Would someone explain matching mic and mic preamp ohms. How necessary is it and how much should one be aware of this.

 

[mshilarious]

It is of very little importance with modern mics and preamps because the hard work has already been done for you. First, you generally don't match impedance, you want to bridge impedance. Professional microphones are usually less than 300 ohm output impedance, professional preamps are 1K or more. Ideally, you'd have a 1:8 source:load ratio, but even 1:3 works fine. Higher ratios are not a problem.

Some dynamic mics might sound better with a heavier load, but that's a more advanced topic. Vintage mics and preamps can get a bit confusing as well. But for modern stuff (say less than forty years old) the general rule is plug an XLR output mic into an XLR input mic preamp and you don't have to worry about it.

 

[moresound]

Ok thanks! But what about preamps that have adjustable omhs is that for vintage mics ?

 

[dgatwood]

Ok thanks! But what about preamps that have adjustable omhs is that for vintage mics ?

Predominantly, and particularly for vintage ribbons. As MsH said, you can change the sound of dynamic mics (and really, to a limited degree, transformer-coupled condenser mics) by changing the loading on the mic's output transformer. Usually there's no good reason to do this except with vintage mics, but there's nothing wrong with experimenting.

 

[darrin_h2000]

My vintage shures Ive collected have switchable impedence.

 

[moresound]

Predominantly, and particularly for vintage ribbons. As MsH said, you can change the sound of dynamic mics (and really, to a limited degree, transformer-coupled condenser mics) by changing the loading on the mic's output transformer. Usually there's no good reason to do this except with vintage mics, but there's nothing wrong with experimenting.

OK thanks. It's just one of those questions that has always bin in the back of my mind when I look at mic ohms and preamp ohms.
For nothing has ever gone wrong during my recordings due to this (I was hoping) I just wanted to be sure that I wasn't missing any thing that would help tailor the sound in future recordings.
I always try to pick the right mic be it after two or three switch outs then positioning of said mic knowing that this is half the battle.

 

[moresound]

My vintage shures Ive collected have switchable impedence.

You mean from high to low impedance right darrin ?
Now dose this transfer to ohms?

 

[darrin_h2000]

You mean from high to low impedance right darrin ?
Now dose this transfer to ohms?

Both low impedence...the 535 has 3 settings...one high 2 low.

 

[moresound]

And a preamp with adjustable ohmage works well with this mic or dose the switch on this mic handle all of that?

 

[darrin_h2000]

I just go with what sounds better.

 

[mshilarious]

OK, you had to ask. Vintage mics with switchable impedance have several taps off of their transformer secondary. It might be low, mid, high; or just low and high. Low and mid aren't that different, usually 50 and 250 ohm. High is much higher (often 25K) and is intended for an instrument input, like a guitar amp. Don't use high into a mic preamp.

Between low and mid, you'll get a bit more output from the mic on mid (because you're tapping more turns on the secondary--transformer voltage step-up is a function of turns ratio). There will be some tonal variation as well, but less so as the preamp's input impedance gets higher.

There are a couple of reasons why a dynamic mic's character changes with load. First reason is a transformer-output mic has a complex source impedance (resistance + reactance). A pure resistive source doesn't vary its output impedance with frequency. A reactive source does. So first, the frequency response of the mic can change in complex ways as it gets loaded down.

Second, the loading on the transformer is seen by the mic capsule as well, and that can affect its impulse response. Good article on that here:

http://www.recordingmag.com/resources/resourceDetail/330.html

So if you have variable input impedance, try a 600 ohm load on dynamics for fun and pleasure.

Generally, you want the highest possible setting for condenser mics; transformerless condenser mics are nearly pure resistive sources, so all you will do by increasing the load is shed some level and increase distortion.

 

[Shadow_7]

Preamps like the Art MPA Gold have variable impedence. Those who use it seem to suggest that you can dial in a mic. And otherwise alter it's EQ characteristics. Kind of moot in the digital age where you can do all that digitally in post. But with analog gear... Or if you just want one less step in post... Or for live use... Perhaps useful.

The TRP has high impedence (18k?) and it advertises that it's to allow for long cable runs without signal degredation. What actually is, I don't really know, I'm not an electrical engineer.

 

[bonezy]

OK, you had to ask. Vintage mics with switchable impedance have several taps off of their transformer secondary. It might be low, mid, high; or just low and high. Low and mid aren't that different, usually 50 and 250 ohm. High is much higher (often 25K) and is intended for an instrument input, like a guitar amp. Don't use high into a mic preamp.

Between low and mid, you'll get a bit more output from the mic on mid (because you're tapping more turns on the secondary--transformer voltage step-up is a function of turns ratio). There will be some tonal variation as well, but less so as the preamp's input impedance gets higher.

There are a couple of reasons why a dynamic mic's character changes with load. First reason is a transformer-output mic has a complex source impedance (resistance + reactance). A pure resistive source doesn't vary its output impedance with frequency. A reactive source does. So first, the frequency response of the mic can change in complex ways as it gets loaded down.

Second, the loading on the transformer is seen by the mic capsule as well, and that can affect its impulse response. Good article on that here:

http://www.recordingmag.com/resources/resourceDetail/330.html

So if you have variable input impedance, try a 600 ohm load on dynamics for fun and pleasure.

Generally, you want the highest possible setting for condenser mics; transformerless condenser mics are nearly pure resistive sources, so all you will do by increasing the load is shed some level and increase distortion.

Now what about the sennhieser 421 with the 5 position switch between M and S...(what do those two letters stand for)...is that just a low end roll off and why so many positions? If so are ohms affected?

 

[Richard Monroe]

OK MSHilarious! I've got a couple of seriously oddball mics, and I'm still contemplating what they do, or don't. The first one is a 1939 Shure 55C, the high impedence version of the original unidyne. The manual recommends a load of 1,000,000 Ohms (no shit). I do have an amphenol to 1/4" cable. How will that bridge to a high-z instrument input? The second one is more problematic. It's a Shure SM82 line level broadcast mic (built in preamp). Unfortunately, it requires phantom power, which you usually don't get from line level inputs. I've discovered it can be just barely used with my Avalon AD2022 with the variable impedence set for 50 ohms. Does this pose a threat to the preamp? If so, I might have to use a dedicated phantom power supply and run it into a line level input.-Richie

 

[mshilarious]

Now what about the sennhieser 421 with the 5 position switch between M and S...(what do those two letters stand for)...is that just a low end roll off and why so many positions? If so are ohms affected?

It's a low cut. M and S are first letters in the German words for music and speech, which coincidentially happen to start with M and S in English too. M = no low cut, S = low cut.

Or I hope I got that right, I sold my 421s some years ago . . .

Does not affect impedance to my knowledge (well, it would technically affect impedance at low frequencies, but never mind that).

 

[mshilarious]

OK MSHilarious! I've got a couple of seriously oddball mics, and I'm still contemplating what they do, or don't. The first one is a 1939 Shure 55C, the high impedence version of the original unidyne. The manual recommends a load of 1,000,000 Ohms (no shit). I do have an amphenol to 1/4" cable. How will that bridge to a high-z instrument input?

Just fine, plug it into an instrument input or a DI.

The second one is more problematic. It's a Shure SM82 line level broadcast mic (built in preamp). Unfortunately, it requires phantom power, which you usually don't get from line level inputs. I've discovered it can be just barely used with my Avalon AD2022 with the variable impedence set for 50 ohms. Does this pose a threat to the preamp? If so, I might have to use a dedicated phantom power supply and run it into a line level input.-Richie

Doesn't threaten the preamp, but probably causes excess distortion in the microphone. Use with a 25dB inline pad into a regular mic preamp.

 

[Richard Monroe]

Thanks for the suggestion MS- I never thought of using a pad. Good rep for you, even if it is disabled.-Richie

 

[bonezy]

It's a low cut. M and S are first letters in the German words for music and speech, which coincidentially happen to start with M and S in English too. M = no low cut, S = low cut.

Or I hope I got that right, I sold my 421s some years ago . . .

Does not affect impedance to my knowledge (well, it would technically affect impedance at low frequencies, but never mind that).

And the other clicks in between M and S are diferent low end roll off settings?

 

[PhilGood]

M = S = hilarious!!!

 

[moresound]

OK, you had to ask. Vintage mics with switchable impedance have several taps off of their transformer secondary. It might be low, mid, high; or just low and high. Low and mid aren't that different, usually 50 and 250 ohm. High is much higher (often 25K) and is intended for an instrument input, like a guitar amp. Don't use high into a mic preamp.

Between low and mid, you'll get a bit more output from the mic on mid (because you're tapping more turns on the secondary--transformer voltage step-up is a function of turns ratio). There will be some tonal variation as well, but less so as the preamp's input impedance gets higher.

There are a couple of reasons why a dynamic mic's character changes with load. First reason is a transformer-output mic has a complex source impedance (resistance + reactance). A pure resistive source doesn't vary its output impedance with frequency. A reactive source does. So first, the frequency response of the mic can change in complex ways as it gets loaded down.

Second, the loading on the transformer is seen by the mic capsule as well, and that can affect its impulse response. Good article on that here:

http://www.recordingmag.com/resources/resourceDetail/330.html

So if you have variable input impedance, try a 600 ohm load on dynamics for fun and pleasure.

Generally, you want the highest possible setting for condenser mics; transformerless condenser mics are nearly pure resistive sources, so all you will do by increasing the load is shed some level and increase distortion.

Thank you for answering.. It's o so clear now.I have a base in which to do some experimenting with the knowledge that you've passed on.