awjoe
Joined: 08/03/11
Posts: 104
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ribbons and input impedance
#1017075 - 05/11/12 04:32 AM
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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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ef37a
Joined: 29/05/06
Posts: 5629
Loc: northampton uk
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Re: ribbons and input impedance
[Re: awjoe]
#1017080 - 05/11/12 06:19 AM
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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 Perrett
Joined: 10/09/01
Posts: 9660
Loc: The wilds of Hampshire
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Re: ribbons and input impedance
[Re: awjoe]
#1017097 - 05/11/12 10:11 AM
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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 Robjohns
SOS Technical Editor
Joined: 25/07/03
Posts: 18399
Loc: Worcestershire
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Re: ribbons and input impedance
[Re: awjoe]
#1017114 - 05/11/12 11:33 AM
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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: 104
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Re: ribbons and input impedance
[Re: Hugh Robjohns]
#1017215 - 05/11/12 07:02 PM
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Do you have any idea how useful that was? Thanks very much.
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Hugh Robjohns
SOS Technical Editor
Joined: 25/07/03
Posts: 18399
Loc: Worcestershire
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Re: ribbons and input impedance
[Re: awjoe]
#1017218 - 05/11/12 07:24 PM
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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: 104
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Re: ribbons and input impedance
[Re: Hugh Robjohns]
#1020023 - 24/11/12 07:25 AM
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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!
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