Recording - March 2014 - 8

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Three Wise Men on Impedance And Tone
Hi! I read Paul Stamler's article about how impedance affects
the frequency response of the Shure SM57 at tinyurl.com/
RecMagSM57Gizmo and I found it a very interesting subject.
I'm studying audio engineering at SAE and now it's time for me
to write an essay; I thought that the impedance effect on frequency response would be a great thing to talk about, but I can't
find much information about the subject.
I'm wondering if Paul or anybody else there could give me
some tips on where to find "deeper" information (books or any
other article) about impedance and its effect on frequency
response of dynamic or ribbon microphones? Cheers!
Victor Raso
via email

The Three Wise Men (Paul J. Stamler, Scott Dorsey, and
Mike Rivers) reply:
Victor, there's almost nothing on the subject in the technical literature, aside from an article from the early 1950s by an RCA engineer; I think I cited that in my article "When Mikey Met Ohmy"
(available online at tinyurl.com/RecMagWhenMikeyMetOhmy).
There's also a brief mention of the topic in the chapter on mic preamps in Glen Ballou's book, but it never really gets developed.
Both of these references are about purely electrical behavior. What
no one seems to have written about in the refereed literature is the type
of electromechanical damping effects I discussed in my article on the
Gizmo. Scott Dorsey discussed a similar effect on ribbon mics in his
review of an AEA preamp designed specifically for ribbons and having a higher-than-average input impedance. (There may be some discussion hidden in patent filings or the RCA and Western Electric technical journals, to the latter of which I don't have access.)-PJS
Actually, there is a little discussion of this in Music Physics and
Engineering by Harry F. Olson, which is available used on Amazon
and available for the Kindle, and which every audio person ought to
read at some point. Some of Olson's research was extended in
"Analysis of the Interaction between Ribbon Motor, Transformer, and
Preamplifier and Its Application in Ribbon Microphone Design", an
AES paper by Julian David from ribbon-mic maker AEA.
Mark Zaim from Audio-Technica has an engineering brief,
"Phantom Powering the Modern Condenser Microphone II: The
Effects of Load Impedance on Microphone Performance", which
is available free to AES members and which talks on how electronic output circuits also suffer from loading issues.
I don't know of anyone who has done a formal study on loading of dynamic mics, and maybe someone should.-SD
I've read what Paul's read and written and can only add anecdotal evidence that I've attempted to explain in a series of articles that were published in Recording about 10,000 years ago
(and are available online at recordingmag.com; start in the
Resource Library under How Stuff Works > Electronics).

I recall reading, I believe from the writings of James Boyk (pianist,
recordist, author, professor of music at Cal Tech, designer of the all
tube Ampex-based Magnesaurus recorder, and conductor of
detailed listening tests), that he and others recording classical music
at the time removed the input transformer from their recorders and
connected the ribbon mic directly to the grid of the first amplifier
tube. This presented the mic with a very high impedance load and
they liked how it sounded better than when going through the transformer. It's not clear whether this proved that the mic sounded better
practically unloaded, or if the transformer created a sound that they
preferred not to hear. I suspect the latter.
It's only after some marketing genius came up with the idea of
matching the input impedance of the preamp to the mic, ostensibly to
provide maximum power transferred from mic to preamp, that we had
preamps with selectable input impedance like the Groove Tubes VIPRE
(which actually selected taps on an input transformer to provide four
choices between 300 and 2400 ohms) or modern preamps that generally just hang resistors across the input terminals à la Paul's Gizmo.
Mics with output transformers (all tube mics and most dynamics) tend to be affected more by changing their load impedance
than transformerless mics typical of modern condenser designs.
Generally lowering the load impedance cuts the high end,
which some people describe as "more vintage sounding." I suspect but can't prove that the load has a greater effect on the
transformer than it does on the microphone element itself.
One more data point, again based on experimentation rather than
theory, is the Cloudlifter from Cloud Microphones. This is a "pre-preamp" like what's built into some of today's active ribbon mics. It presents (presumably) the optimum loading for Cloud's ribbon mics. The
standard Cloudlifter has an input impedance of 3000 ohms, contrasted with AEA's ribbon mic preamp designs which have an input
impedance greater than 15000 ohms. Both Cloud and AEA ribbon
mics are based on, but not identical to, classic RCA designs.
However, one Cloudlifter model has a pot across its input terminals, providing a continuously adjustable load impedance of about
300 to 3000 ohms. Roger Cloud says it's to be used like a "tone
control." I guess it's for those who believe that you should get the
sound you want straight out of the mic rather than use EQ. So again,
adjusting the load on a mic is just another recording technique.
Engineers tend to find certain combinations of mic and preamp that they like for specific applications and consider the pair
as a system to be used together. And the rest of us just use what
we have, however we can, to get the best results without an
unreasonable amount of fooling around in session.-MR

Correction
Antelope Audio has asked us to clarify one comment in our
review of the Orion32 audio interface (February 2014). In our
review we stated that "The Orion 32 can only achieve 24 channels of high sample rate operation." Antelope Audio points out
that this is really only true for 192 kHz; the Orion32 can deliver 32 channels of 24-bit/96 kHz audio.-MM

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RECORDING March 2014

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Recording - March 2014

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