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awjoe



Joined: 08/03/11
Posts: 126
ribbons and input impedance
      #1017075 - 05/11/12 04:32 AM
I don't have a ribbon mic, but I want one for my guitar amp to see what all the fuss is about. (If it turns out not to be better than the Shure SM58 I'm using now, at least I'll be wiser...)

The question I have is about input impedance for a ribbon. The input impedance on my RNP is 5k ohms. (That's what FMR told me.)

The switchable input impedance for a unit like the Focusrite ISA Two is as follows:

Low = 600 Ω
ISA 110 = 1.4 kΩ
Med = 2.4 kΩ
High = 6.8 kΩ

What's the significance of those various impedance settings on the Focusrite compared to the single 5k ohm setting on the RNP? Is 6.8 better than 5? (The SOS reviewer said he preferred the highest setting on the Focusrite.) Or is it just another case of 'it all depends on how the unit sounds and what sort of sound you like'?

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https://yousemusic.bandcamp.com/


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ef37a



Joined: 29/05/06
Posts: 6851
Loc: northampton uk
Re: ribbons and input impedance new [Re: awjoe]
      #1017080 - 05/11/12 06:19 AM
Ooo! I would say almost any ribbon is going to be better on ac' guitar than a 58! (that said, my son prefers the sound of a 57 to either my AKG P150s or a Sontronics STC-2!).

Five k is on the high side for a mic amp input Z anyway, usually around 1.2-2k, so that would be close to (accepted?) optimum. If you wanted to experiment with lower resistances simply make a switch box, XLR-XLR and slap other values across the input (pins 2 and 3). If you do not have a noise problem with a 58 on guitar then a modern ribbon at the same distance should be no bother with the RNP.

The Top Men will surely be along post brekkie, but I suspect your room is going to be a bigger concern than a few K either way!

Dave.


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James PerrettModerator



Joined: 10/09/01
Posts: 10824
Loc: The wilds of Hampshire
Re: ribbons and input impedance new [Re: awjoe]
      #1017097 - 05/11/12 10:11 AM
It always used to be common practice to use an input transformer with a ribbon mic in order to match the impedances and bring up the voltage level. I've not kept up with what happens with modern ribbon mics but I would have thought that they would still need a transformer for optimum performance.

James.

--------------------
JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 22046
Loc: Worcestershire
Re: ribbons and input impedance new [Re: awjoe]
      #1017114 - 05/11/12 11:33 AM
Quote awjoe:

I don't have a ribbon mic, but I want one for my guitar amp to see what all the fuss is about. (If it turns out not to be better than the Shure SM58 I'm using now, at least I'll be wiser...)




I'd be very surprised if you didn't end up preferring the sound of a ribbon -- althoguh I guess it will depend on the type of guitar music you play and the quality of the ribbon. However, the ribbon won't have the huge presence peak of the SM58, so will inherentrly sound a lot smoother and more natural.

Quote:

The question I have is about input impedance for a ribbon. The input impedance on my RNP is 5k ohms. (That's what FMR told me.)




Sounds about right. There is a trend to using higher input impedances on mic inputs, and Mr Neve is a big fan of 5K as the optimum for most situations!

Quote:

What's the significance of those various impedance settings on the Focusrite compared to the single 5k ohm setting on the RNP?




It's really just a gimmick, but occasionally a useful one. Some old-school mics -- like the SM57 -- were designed back in the day when mic preamps all used input transformers and 600 ohms was a standard interface impedance. Consequently, some elderly moving coil designs were optimised for 600 ohm input Z and sound a tad smoother and more natureal when they see that relatively low load.

More modern mics are designed to work with input impedances of at least 1K, and typical values are between 1.5 and 2.5k ohms. European mics are generally designed with a 150 ohm source Z and need to see around 1.5k; US mics generally have a source impedance of 200 ohms and need something a bit higher like 2.5k.

However, we are talking about a constant voltage interface design here, and the higher the input impedance the less the mic capsule is loaded and the better it works and sounds -- usually. hence the inclusion of the 6k8 option on the ISA. The highest setting works best with mics with electronic transformerless outputs, again because it minimises the load.

In practice, you can think of the variable impedance as a kind of EQ control -- play with i to get a sound you like -- you won't damage the mic in any way regardless of impedance setting. Having said that, with the exception of the SM57/58 and a few other historic mics I rarely use any setting other than the highest.

Some ancient ribbon mics were designed to work with very low input impedances of 30, 60, or 600 ohms, but most modern ones like as high an impednace as you can find. AEA -- the specialist American ribbon mic manufacturer -- designs its own dedicated ribbon preamps with input impedances of around 18kohms. This is extremely high, but the ribbons seeem to appreciate the minimal loading and sound all the better for it!

Quote:

Is 6.8 better than 5?




It's a bigger number so it must be better...

Of course, if you buy an active ribbon, the internal buffer amp will ensure the ribbon itself sees the optimum impedance (usually very high), while the active output circuitry really won't care what the inpedance at the preamp is (although the higher the better, in general). And you'll not have to worry about low output levels either!

H

--------------------
Technical Editor, Sound On Sound


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awjoe



Joined: 08/03/11
Posts: 126
Re: ribbons and input impedance new [Re: Hugh Robjohns]
      #1017215 - 05/11/12 07:02 PM
Do you have any idea how useful that was? Thanks very much.

--------------------
https://yousemusic.bandcamp.com/


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Hugh RobjohnsAdministrator
SOS Technical Editor


Joined: 25/07/03
Posts: 22046
Loc: Worcestershire
Re: ribbons and input impedance new [Re: awjoe]
      #1017218 - 05/11/12 07:24 PM
We do our best!
Buy the magazine and I can keep on being helpful...

hugh

--------------------
Technical Editor, Sound On Sound


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awjoe



Joined: 08/03/11
Posts: 126
Re: ribbons and input impedance new [Re: Hugh Robjohns]
      #1020023 - 24/11/12 07:25 AM
Quote Hugh Robjohns:

Quote awjoe:

I don't have a ribbon mic, but I want one for my guitar amp to see what all the fuss is about. (If it turns out not to be better than the Shure SM58 I'm using now, at least I'll be wiser...)




I'd be very surprised if you didn't end up preferring the sound of a ribbon




You were right. Thanks!

--------------------
https://yousemusic.bandcamp.com/


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