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Find me! Call DAP at 214.350.7678 or email rene@dallasaudiopost.com. Also check out echocollectivefx.com for custom sfx, and tonebenders.net for my podcast.

Sunday, April 22

building a ribbon mic part 2: listening test

One of the best pieces of advice I got after building my ribbon mic was to test it through different preamps (thanks John Sanacore).

At my house I noticed some RF buzz, which is actually typical because my house is an RF nightmare.  I decided to ping the designer, Rick Wilkinson, and ask his advice.  Rick responded almost immediately with a comprehensive and detailed troubleshooting list that was incredibly helpful.  The problem ended up being a faulty cable, but I learned that the brass grille was part of the faraday cage from the exchange.

I took the mic to work the next day and ran it through the John Hardy M1, but wasn't very impressed with the sound.  It actually had sounded louder and clearer at the house through the RF buzz.  I then plugged into a sound devices pre similar to the one I used at home and was much happier with the output.  I also began looking into getting a cloudlifter to help with impedance loading and output gain from the mic.  My research was showing that ribbons in general tend to want pretty loud sources, but I have designs on using in more delicate situations and needed get it up to par there.

I emailed Rick again, and again he was again super responsive and detailed.  here's a quick excerpt of what he said to me regarding my experiences to this point:

High end response is determined by the input impedance of the preamp.
Generally, a higher input impedance delivers better high-end response in ribbon mics.  A REALLY high impedance preamp (10k ohms or higher) like the AEA TRP, or my DIY Preamp Kit made especially for my mics (available in another month or so) will really help the top end.  
"loading" is the term for the interaction between the output impedance of a microphone and the input impedance of the preamp.  The correct description of this interaction is something like: "The microphone has to work harder, if the input impedance of the preamp is lower."  This makes sense if the microphone is overly-simplified to a current source: It takes more current to drive a low-impedance load.

A simplified version of this is:

A lower input impedance requires the mic to work harder to deliver a signal. Thus, low-energy, high frequency soundwaves do not get transferred to a low impedance preamp as efficiently as to a high-impedance preamp.

You can see this interaction in my own design, by looking at the impedance curve on the specification PDF on my website.  As the measured frequency goes past 10kHz, the impedance soars - literally off the chart - lowering the effective output of the mic in those frequencies.
That's probably why an Austin Mic through your Hardy sounds like it does... I just looked at the specs, and their Jensen input transformer is 150 Ohms. The current flowing through the ribbon is trying to keep the foil inside the gap, not allowing it to move with low-energy, high-frequency soundwaves.
Supporting that theory, the Sound Devices input shows an input impedance of 2000 ohms - 13x higher than the Hardy.  Better high-end, right?

 And of course, he was correct.  The thing that the cloudlifter provided me was a good impedance load (3000 Ohms) in addition to the extra 20db of gain.  Here's a quickie listening test of the mic through the three different configurations.  All of this audio is 100% as recorded - no gain or eq adjustments of any kind. Note that the gain settings are described in the recordings, and in the first two examples its all the way open, and in the last clip its around 12 o'clock.

So, the impedance loading clearly makes a huge difference with regards to the tonality and the output level of the mic.  IMO it's not very usable straight into the John Hardy M1 unless you're talking about very loud and bright sources, but the sound devices pre and the cloudlifter/M1 combo make the mic much more versatile.

So, given an input chain that I liked (cloudlifter/John Hardy M1) I did a few more little tests with bright, transient things and the mic performed extremely well.

To my ears the mic is certainly mellower on the top end than my usual LDC, the Audio Technica 4050, and its certainly got its own personality.  It handles transients like a dynamic, which is to say that its a little jumpy on the loud stuff and it drops off on the low level stuff more quickly than a condenser.  To me that generally means that its not going to catch as much detail on a highly transient source, and will be happier with something relatively consistent.  It's also got low end for days.

With all of that said, that little strip of aluminum leaf I bought at hobby lobby, cut, corrugated and mounted into that chassis is clearly capable of capturing frequencies well above 20k, which is impressive.  I only ran the instrument tests at 44kHz, but check out how easily it handles all of the frequencies up to the top of that 22k spectrum.

This is the spectrogram of the instrument file linked above:

And here's the zoom in on my voice slate and the tambourine.

This mic is clearly capable of capturing ultrasonic frequencies.

I'll run another test soon where I use it as the S in an MS config on some thick metal movements and see how she does.

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