Dear Colleagues,
Today I set up a comparison of ways of connecting the 183 mics. They
call them preamps, and they do have a little gain, but I'm going to
call them power modules to avoid confusion with the mic preamps that
they have to be used with.
The MX power module can be ordered separately as the MX183PK. The
battery module can be ordered as the MX1BP.
For my first test I compared my Sharp MD-MT90 recorder with the Shure
MX183 phantom powered module. I put two 183 mics next to each other
on the bench, and viewed the results on a Macintosh "Spectrafoo" FFT
spectrum analyzer. All my tests were comparisons, there were no
absolute measurements. I swapped the mics to make sure I wasn't
looking at differences between the two mics. They matched within
about 1 dB.
Test 1:
Left: WR183 -> Sharp MD-MT90 gain MIC L 20, phones level 30, phones
output -> Sound Devices USBPre line input L, gain max.
Right: MX183 (gain jumper +12) -> Sound Devices USBPre mic input
(48V), gain knob 1:00.
I looked at room ambience, which had a lot of low frequencies since
I'm near a freeway. Gain controls when set as above matched the
spectra exactly in the range 200 Hz - 1 KHz.
Observations:
Low end: below 60 Hz the Shure MX module starts rolling off a little.
It's about 3dB lower than the Sharp at 30 Hz, 5 dB at 16 Hz.
Treble: the Shure module showed an approximately 2 dB depression
between 2 KHz and 8 KHz, followed by a rise at the high end, up about
5 dB at 18 K. I suspect the hf rise was noise mixing with the
ambience.
Test 2:
Same wiring setup, with a pile of pillows on the mics. This damped
out the ambience above around 1 KHz so I could see the high-frequency
noise level.
Observations:
The 2 KHz - 8 KHz depression disappeared. The MX183 preamp had more
noise than the Sharp, starting from about 2 KHz, to +3 dB at the top
end.
Test 3:
Same wiring setup. Changed the gain jumpers in the MX preamp to 0 dB.
Brought up the gain at the USBPre input to match the Sharp.
Observations:
The high frequency noise stayed about the same, with a 1 dB midrange
rise between 500 Hz and 2 KHz. Couldn't tell if that was a change in
noise or a change in frequency response.
Test 4:
Instead of using the USBPre mic input, I changed to a Sound Devices
MP-2, feeding balanced to the USBPre's right line input. The MP-2
gain was 12:00.
Observations:
No change. Since the MP-2 is supposed to be a little quieter than the
USBPre, this indicates the preamp noise probably isn't dominant in
this situation.
Test 5:
Keeping the MP-2, I substituted the Shure battery pack version, the
MX183BP, without battery (using phantom). The output was about 7 dB
lower, so I raised the MP-2 gain to around 2:00 to match it to the
Sharp again.
Observations:
The HF noise was higher, up from the Sharp about 1.5 dB at 8 KHz, and
+4 dB at 16K. The extreme low frequency response came up several dB,
however, higher than either the Shure MX or the Sharp.
Conclusions:
The noise level of the Shure 183 mics with good preamps is determined
by the mics and their power modules. No improvement should be
expected with expensive preamps.
The mics were the quietest on the cheap little preamps in the Sharp
MD recorder! The noise (hiss) is about 3 dB higher at the top end
with the MX power module and a pro preamp, and an additional dB
higher than that with the BP power module.
I'm thinking about tearing apart the BP preamps and kluging together
a phantom supply that just turns phantom into p-i-p, and passes the 3
Kohm impedance mic signal right through. Whether this is too high a
source impedance for professional inputs remains to be seen.
-Dan Dugan
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________________________________________________________________________
"Microphones are not ears,
Loudspeakers are not birds,
A listening room is not nature."
Klas Strandberg
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