RE: [Harp-L] Fireball and Impedance Matching Transformers

["John Balding" <John.Balding@xxxxxxxxx>]

In over 15 years of playing, I have used Low impedance SM57s, SM58s,
Shure and EV omnidirectional TV reporter-type mics, as well as many
other Low Z mics directly into various guitar amps, including one with
an 8 Meg input impedance. The only thing I ever had to do differently
with the Low Z mics as opposed to the High Z mics, was to turn the amp
volume up more. For a hypothetical example: If you normally play an SM57
into a Fender Bassman using a line transformer, you could probably play
the amp at about 3 or 4 on the volume control. Without the transformer,
the amp would have to be played at 6 or 7 to obtain the same volume
level and tone. 
Of course, if you have a tube amp set up with very low-gain tubes, you
may not be able to get the volume of the Low Z mic up to the same level
you could achieve with a High Z mic or using a Low Z mic with a
transformer. You could change the tubes and try it that way, if you are
seriously thinking of using the Fireball as your main mic. All of the
amps in music stores are set up with high-gain tubes. While you are in
the showroom looking at the Fireball, plug it into the clean channel of
a tube amp most like your own, and give it a whirl. See what you think.

I have researched this topic extensively in the past. I am not claiming
to be an authority on the subject, but my research has led me to some
factual, proven conclusions:

1. Putting a Low Z mic (source) into a High Z amp input (load) will not
induce distortion or any change in frequency response. In fact, in this
scenario, there is actually maximum voltage transfer occurring. This is
called bridging impedance, and is practiced on all high-quality mixers
and studio gear. The source signal should always go into a load higher
than its own in order to obtain maximum voltage transfer. (That is why
we have separate gain and level controls on the individual channels of a
quality mixer, as not all Low Z microphones are 150 ohms.)
2. Most High Z mics have an impedance of a few thousand ohms to about
100,000 ohms. Most of the Fender amps popular with harp players have an
input impedance of 1Mohm, or 1 million ohms. 
3. Microphone manufacturers list minimum load impedances or optimum load
impedances for their mics. As of this writing, I have yet to see a
maximum load impedance given for a microphone. Because a high impedance
load will not damage a microphone, I doubt a maximum number will ever be
stated.
4. The old Astatic crystal mic elements were rated at 5Mohm, or 5
million ohms. Electrically speaking, running an old Astatic JT30 in to a
'59 Bassman is a BIG No-No. That would be a 5 million ohm source into a
1 million ohm load. Technically, a step-DOWN transformer should be used
to match this mic with the amp! Do we do it anyway? Does it sound good?
Has anyone's mic or amp ever blown up using this combination?

All I am trying to say here is that if you don't like carrying around a
transformer, or you think the equipment purists down at the local open
mic will make fun of that big metal tube hanging out of you mic or amp,
then don't use it! Put the high-gain tubes back in your amp, turn it up,
and get on with it!

John Balding





-----Original Message-----
From: harp-l-bounces@xxxxxxxxxx [mailto:harp-l-bounces@xxxxxxxxxx] On
Behalf Of Greg Heumann
Sent: Wednesday, August 23, 2006 10:32 AM
To: harp-l@xxxxxxxxxx
Subject: [Harp-L] Fireball and Impedance Matching Transformers

Richard is right - my only experience with the Fireball was with the
pre-volume-control, old element. I withhold further judgement until I
can try one of the new ones. I still expect it to be a very smooth clean
mic.

But as for adding an impedance matching transformer, just do it.  
Plenty of vintage mics have them (the 545 come to mind), as do modern
ones like the Shure 520D - they're just built in to the mic itself.  
They ARE impedance matching transformers, nevertheless.

/Greg
http://www.blowsmeaway.com
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